LEAN REPORT. Prepared for Scottish and Southern Energy Power Distribution Ltd. Report Number: MAN-01. Revision C

Transcription

1 LEAN REPORT Prepared fr Scttish and Suthern Energy Pwer Distributin Ltd. Reprt Number: MAN-01 Revisin C 13 TH JULY 2015 S&C Electric Eurpe Ltd, 6B Albertn Huse, St Mary s Parsnage, Manchester, M3 2WJ, UK

2 Reprt: LEAN Reprt Page 2 f 107 Reprt Revisin Histry: Issue Date Revisin Cmments 12 th June 2015 A First issue 10 th July 2015 B With incrprated client cmments 13 th July 2015 C With transfrmer switching related current traces and related text included in reprt recmmendatins Prepared by: Reviewed by : SARAT CHANDRA VEGUNTA Senir Cnsultant S&C Electric Eurpe Ltd DAVID HAWKINS Partner LIG Cnsultancy Services LLP Apprved by: STEVE STAPLETON Cnsultancy Services Manager S&C Electric Eurpe Ltd DISCLAIMER THIS DOCUMENT WAS PREPARED AND PROVIDED PURSUANT TO A CONTRACT WITH CUSTOMER AND/OR END USER THAT DEFINES THE PARTIES RESPECTIVE RIGHTS TO WORK PRODUCT AND INTELLECTUAL PROPERTY. ANY OTHER USE OF THIS DOCUMENT IS STRICTLY PROHIBITED. ALL OTHER RIGHTS ARE RESERVED AND NO LICENSE OR RIGHTS TO THE SUBJECT MATTER OF THIS DOCUMENT ARE GRANTED BY POSSESSION THEREOF.

3 Reprt: LEAN Reprt Page 3 f 107 TABLE OF CONTENTS EXECUTIVE SUMMARY INTRODUCTION Scpe f Wrk Prject Organisatin LOSSES Quantificatin Perfrmance Csts Breakdwn Regulatin Management Opprtunities Feeder Tie Open-Pint Optimisatin Cnservatin Vltage Reductin (CVR) Demand Side Respnse (DSR) Transfrmer Aut Stp-Start (TASS) Meshed Netwrk Operatin Reactive Pwer Cmpensatin Energy Strage Distributed Generatin Active Netwrk Management (ANM) Netwrk Reinfrcements Other Opprtunities Prjects Distributin Netwrk Lsses Management Western Pwer Distributin, UK... 36

4 Reprt: LEAN Reprt Page 4 f Isle f Wight Netwrk Lsses Reductin Study ( ) SSEPD, UK Capacity t Custmers (C 2 C) Prject Electricity Nrth West Ltd., UK Smart Urban LV Netwrk UK Pwer Netwrks, UK Reactive Pwer Dispatch Using Distributed Generatin UK Pwer Netwrks, UK Distributin System Lsses Reductin Mazn Electricity Cmpany, Oman Distributin System Lsses Reductin Electricité du Las, Las Cnclusins and Lessns Learnt TRANSFORMER AUTO STOP-START TASS Methd Risks Inrush Currents Harmnics Overvltages Sympathetic Interactins Current Chpping Health and Lifetime Mitigatin Cntrlled Switching Cmplete Inrush Reductin r Eliminatin Strategy Other Methds Analysis Data, Assumptins, and Mdelling Methdlgy Results and Discussin Cnclusins and Lessns Learnt RECOMMENDATIONS REFERENCES...87

5 Reprt: LEAN Reprt Page 5 f GLOSSARY APPENDIX A. EMBEDDING OF SOLUTION FOR LOSSES IN THE MARKET SYSTEM B. REGULATORY INCENTIVES TO REDUCE LOSSES C. DSR TRIAL AVERAGE REDUCTION IN PEAK DEMAND SUMMARY D. A REVIEW OF TASS IMPACT ON TRANSFORMER S HEALTH AND LIFETIME

6 Reprt: LEAN Reprt Page 6 f 107 EXECUTIVE SUMMARY This dcument summarises a literature review n technical lsses and related reductin methds based n Great Britain and internatinal experience, a literature review f varius transfrmer switching related aspects and available mitigatin ptins, an electr-magnetic transient analysis assessing the impact f cntrlled pint-n-wave switching f substatin transfrmer n substatin s vltage and current perfrmance, and finally an in depth literature assessing the impact f repeated switching and prlnged de-energisatin f a substatin transfrmer n its health and lifetime. The initial literature review n technical lsses and related reductin methds revealed the fllwing lessns: Transmissin and distributin lsses estimatin is a difficult exercise t establish a reasnably accurate verall figure fr technical lsses in Great Britain at bth distributin netwrk peratr level and natinally, the cmparisn with lsses f ther cuntries is even mre prne t errr due t different bases fr assessment. Several factrs affect the validity f cmparisn f electrical lsses perfrmance; therefre, due care needs t applied when cmparing them acrss natinal r glbal electrical netwrk systems. Arund the wrld, n average, arund 9% f ttal electrical energy that enters the transmissin and distributin system is lst befre it reaches end cnsumers. In Great Britain, distributin electrical lsses represent abut 6% f the ttal energy supplied t cnsumers; these lsses represent an aggregate value f arund 1 billin a year. The transmissin and distributin electrical lsses in the United Kingdm accunt fr abut 1.5% f all greenhuse gas emissins. The breakdwn f technical lsses vary significantly with netwrk tplgy and peratin, netwrk vltages, asset management and prcurement plicy, level and types f netwrk lad and generatin, gegraphical area, climatic and seasnal cnditins, cnsumer use behaviur, lsses and breakdwn calculatin methdlgy, netwrk peratr/wner, etc. The need fr reductin f technical lsses is in respnse t the grwing glbal cncern arund climate change, and grwing energy csts that utilities and custmers are frced t grapple with. Acrss the Eurpean Unin, including in the United Kingdm, there are currently energy efficiency targets t be achieved; in the United Kingdm, these are distributed acrss varius industrial sectrs, including the pwer sectr. In the United Kingdm, OFGEM requires distributin netwrk peratrs t ensure that lsses n their netwrks are as lw as reasnably practicable, and t maintain and act in accrdance with their respective published lsses strategies.

7 Reprt: LEAN Reprt Page 7 f 107 A brad set f netwrk lsses reductin methds and pprtunities were reviewed; lessns learnt and applicability f sme f these methds was evaluated using a sectin f SEPD s wn netwrk. SSEPD s wn studies have shwn that a dynamic tie pen-pint ptimisatin scheme, when slely applied t reduce netwrk lsses, may be expensive and may have limited return n investment. Hwever, studies elsewhere shw that when nn-dynamic penpint ptimisatin is emplyed, 1.4 MW (i.e.17% f ttal technical lsses at peak lad) f instantaneus technical lsses savings r 12.1 GWh f annual lsses savings can be achieved with a payback perid f 0.3 years. Althugh, amng studies and tests undertaken in the USA and ther places have shwn reductin in lsses using Cnservatin Vltage Reductin implementatin up t 1% t 3%, practical studies t cnfirm the apprpriate Cnservatin Vltage Reductin factrs fr SEPD/United Kingdm custmer and lad grups are required t quantify the benefits ffered by the Cnservatin Vltage Reductin methd in the United Kingdm. Depending n the day-in day-ut r 'critical peaks' based Demand Side Respnse implementatin, the reductins culd vary between frm 0% t 22% fr peak demand and frm 5% t 38% fr critical peaks respectively; hwever, there are several challenges assciated with the Demand Side Respnse implementatin, which are being researched and trialled acrss the wrld. SSEPD s wn studies have indicates that abut 9% reductin in verall 11 kv netwrk lsses (including 33/11 kv primary and secndary lw vltage transfrmer lsses) culd be achieved thrugh the use f Transfrmer Aut Stp-Start scheme. Hwever, there is n evidence f similar schemes applied r trialled in the United Kingdm r glbally. Althugh, meshed netwrk peratin is implemented primarily fr the purpse f increase netwrk reliability and availability, reductin in lsses are achieved as a secndary benefit. SSEPD s wn studies have indicated that abut 1.7% reductin verall 11 kv netwrk lsses (including 33/11 kv primary and secndary lw vltage transfrmer lsses) culd be achieved thrugh the use f netwrk meshed scheme, with a payback perid f abut 4 years. EDL s (La Peple s Demcratic Republic s state enterprise cmpany) netwrk medium vltage lad in La was knwn t have pr pwer factr. Studies undertaken n EDL s netwrk in Las have shwn that the ptimal placement f capacitrs can reduce instantaneus technical lsses by 2.5 MW (i.e. 30.4% f ttal technical lsses at peak lad) r 21.5 GWh with a payback perid f 0.7 years.

8 Reprt: LEAN Reprt Page 8 f 107 SSEPD s wn netwrk lsses reductin study has shwn that battery based energy strage purely frm an electrical lsses reductin pint may nt be cst effective; hwever, this result may change with expected future reductins in per kwh cst and increase in battery density. Inclusin f distributed generatin in distributin netwrk can either imprve r wrsen electrical lsses; the level f electrical lsses als depend n several factrs, such as its prximity t lad, level f cnsumptin, distributed generatin technlgy, distributed generatin daily and seasnal generatin patterns, distributed generatin penetratin dispersin level in the netwrk, spillage f excess generatin t ther netwrk vltage levels, netwrk circuit cnductr sizes and selectin criteria, level f distributed generatin s cntributin and participatin in the external netwrk reactive pwer cmpensatin, etc. With renewed fcus n energy efficiency at bth United Kingdm natinal and Eurpean Unin level, electrical netwrk lsses reductin pprtunity, as an additinal desired netwrk peratinal cnstraint, within Active Netwrk Management methdlgy is currently being explred. Studies have shwn that the applicatin f advanced active management techniques that wuld maximise the utilisatin f existing netwrks may increase lsses in the lcal netwrk very significantly; hwever, the increase in lsses may be efficient when traded against the facilitatin f lw-carbn generatin cnnectins and avided netwrk reinfrcements. SSEPD s wn netwrk lsses reductin study has als shwn that while reinfrcement f existing netwrks ffers significant reductin in lsses, the upfrnt high capital csts utweigh any cumulative benefits. A number f netwrk lsses reductin prjects bth in the United Kingdm and glbally were als reviewed and the lessns learnt frm these prjects are detailed belw. Netwrk lsses shuld be included within netwrk design plicy and that netwrk energy efficiency shuld rank alngside safety and security f supply in the bjectives f verall netwrk management. There are several ecnmically viable interventins n existing netwrks. Tgether with new design plicies, these will enable a mve t higher netwrk energy efficiency. There is evidence f technical and practical slutins t harvest and use waste heat generated by electrical lsses in rder t imprve verall energy efficiency f running the netwrk. Hwever, mre wrk is required if the develpments are t be deplyed in thse

9 Reprt: LEAN Reprt Page 9 f 107 cases where there may be a match between heat demand and heat generatin frm the netwrk. Distributin netwrk peratrs may develp a lng-term plan t discver new knwledge f netwrk lsses and t develp netwrk plicies, standards and netwrk designs fr future netwrks which may be demnstrably best practice in electrical pwer distributin. The Transfrmer Aut Stp-Start methd acting alne, r its cmbinatin with Alternative Netwrk Tplgy (r meshed netwrk peratin) methd, was fund t be the ptimal interventin slutin (accunting fr bth netwrk technical and cst-benefit metrics ffered by these interventins) in reducing distributin lsses. The level f lsses reductin achieved using meshed netwrks is expected t be marginal. The level f lsses reductin achieved using lw vltage netwrk recnfiguratin is currently under trials in the United Kingdm. Fllwing this, the reprt prvides a review f varius risk and mitigatin aspects related t Transfrmer Aut Stp-Start methd based n a brad set f literature gathered frm nline surces and frm industry best practice guidance dcuments. This review has cvered the fllwing aspects: transfrmer energisatin related inrush currents, inrush current related harmnic impact n the external netwrk and vltage harmnic impact n pint-n-wave switching ability, transfrmer switching related vervltages, sympathetic interactins with ther series r parallel cnnected already energised transfrmers, series circuit breaker current chpping, and impact n transfrmer s health and lifetime. Due t high frequency f transfrmer switching peratins expected as part f Transfrmer Aut Stp- Start methd implementatin in Lw Energy Autmated Netwrks Prject, up t 3 times a day n average annually, significant reductin r cmplete eliminatin f inrush currents achieved by cntrlling the switching times f the energizing circuit breaker was better suited, and therefre was studied in detail. The analysis undertaken using electr-magnetic transient simulatin sftware assessing the impact and effectiveness f cntrlled pint-n-wave was analysed; principal findings frm these studies are listed belw: It may be pssible t energise a primary substatin transfrmer withut adversely affecting the pwer quality within the substatin s 33 kv netwrk and it s cnnected 11 kv lad. It is likely that ple-independent cntrl and peratin f the Switched Primary Transfrmer 33 kv circuit breaker will be required. The use f cntrlled pint-n-wave switching appears t be a feasible strategy fr effectively limiting transfrmer energisatin related inrush currents. Hwever, this was based n several study assumptins, such as cnsideratin related t circuit breaker pint-n-

10 Reprt: LEAN Reprt Page 10 f 107 wave switching abilities, its current chpping, transfrmer s remnant flux decay, external netwrk cnditins, etc. A cntrlled prir line-t-grund vltage pint-n-wave de-energisatin at 0º fllwed by energisatin f the transfrmer at the same line-t-grund vltage pint-n-wave angle and phase is expected t draw an inrush current that is typically less than 10% f transfrmer nminal current; this result was als fund t be valid when substatin transfrmers sympathetic interactins were cnsidered. In additin, the impact f such pint-n-wave transfrmer energisatin n substatin s high vltage and medium vltage RMS vltages is expected t be negligible. Based n the simplified mdel f external netwrk used in the study, during cntrlled instances f pint-n-wave transfrmer de-energisatin and energisatin, significant levels f transient vervltages were bserved. Switching vervltages may nt be a prblem due t the presence f netwrk lad and resistance in DNO netwrks; this shuld be verified via additinal studies r site r cntrlled labratry based transfrmer energisatin/de-energisatin tests. In additin, a sensitivity study assessing the effect f transfrmer stray capacitances (including any errr in calculatin f their magnitude). Prvided these are kept within tens f nf, they may have negligible impact n cntrlled switching methd s ability t minimise transfrmer remnant flux during the de-energisatin prcess; hwever, use f very large transfrmer stray capacitances values were fund t have significant impact n the transfrmer remnant flux values. These effects may als require calibratin by physical testing. Additinally, HFDE s reprt, attached t this reprt in Appendix D, prvides additinal cnclusins related t transfrmer health and lifetime impact risks and their mitigatin with implementatin f Transfrmer Aut Stp-Start Methd. The LEAN Reprt here recmmends the fllwing listed tasks t be undertaken by SSEPD/SEPD: Based n cnsidered primary substatin transfrmer parameters and simulatin test cnditins, substatin peak current traces (with superimpsed transfrmer inrush peak current traces) detailed in Figure 1 can be expected theretically depending n if a substatin transfrmer was switched either using a uncntrlled POW mechanism (such as Optin 1 in LEAN prject) r a cntrlled POW mechanism (such as Optin 2 in LEAN prject); it is, hwever, recmmended that these inrush currents and their decay trends are validated (fr studied substatin transfrmers) based n parameters btained frm actual nsite transfrmers where TASS Methd may be implemented.

11 Reprt: LEAN Reprt Page 11 f 107 High transfrmer peak inrush current superimpsed n t lad current due t uncntrlled substatin transfrmer switching Negligible transfrmer peak inrush current superimpsed n t lad current due t cntrlled transfrmer switching Figure 1 Substatin Peak Inrush Current Traces with Type f Transfrmer Switching Detailed technical risk evaluatin using electr-magnetic transient simulatin based analysis assessing the impact f the fllwing aspects is recmmended: Three-phase single- r three-ple circuit breaker transfrmer de-energisatin related chpping current impact n transfrmer s remnant flux and resultant inrush currents during its energisatin. Netwrk capacitance, and series circuit breaker and switched transfrmer s stray capacitances, n the vervltage transients that may ccur n the system during transfrmer energisatin. Opprtunities fr timed pint-n-wave switching using existing and new high vltage and medium vltage breakers. Transfrmer phase-cupling n inrush currents and cntrlled pint-n-wave switching ability in reducing r eliminating inrush currents. Substatin medium vltage side lad transfer switching impact, which will ccur prir t and after switched transfrmer switching actin, n substatin s vltages and transfrmers. Vltage harmnic distrtin n vltage pint-n-wave angle detectin, and therefre impact n cntrlled switching mechanism ability in reducing transfrmer remnant flux and inrush current. External netwrk harmnic impedance resnances f significant impedance magnitudes (typically due t lw netwrk resistances) can significantly amplify netwrk vltages, even

12 Reprt: LEAN Reprt Page 12 f 107 n small harmnic current injectins. Althugh cntrlled switching f transfrmer switching is expected t draw lw inrush current related harmnic currents frm the netwrk, their impact n harmnic vltages due t impedance resnances can be significant. While switching vervltages due t transfrmer switching are nt usually a prblem, it is a recmmendatin f this reprt t verify this with additinal studies r site r cntrlled labratry based transfrmer energisatin/de-energisatin tests. In the studies presented here, the impact f variatin in transfrmer s effective HV s side surge capacitance n transfrmer s remnant flux was investigated fr ne pint-n-wave de-energisatin angle (i.e. 90º); it will be useful t understand such impact fr ther pintn-wave de-energisatin angles and further studies are recmmended. Frm the abve listed studies it is recmmended that SEPD undertakes the fllwing set f tasks: A review f available ptins fr existing 33 kv and 11kV switchgear in SEPD s netwrk that may be able t prvide pint-n-wave based cntrlled transfrmer switching. Explre characteristics f varius switchgear types (e.g. vacuum, air, il, etc.), their pintn-wave switching capabilities, and perfrmance (ple scatter, chpping current values, rated switching peratin numbers, etc.) that may be best suited and cst effective fr Transfrmer Aut Stp-Start Methd applicatin. Undertake a cntrlled site r labratry based testing f principal and critical cmpnents (i.e. equipment and peratinal philsphy) f Transfrmer Aut Stp-Start Methd cncept. Results btained frm these tests can assist with calibrating a simulatin mdel f the same test system. HFDE s wn reprt (attached t this reprt in Appendix D) includes cnclusins and recmmendatins related t TASS methd impact n switched transfrmer s health and lifetime aspects which shuld als be cnsidered during the Lw Energy Autmated Netwrks prject. The Lw Energy Autmated Netwrks prject business case may need t be re-evaluated t see if Transfrmer Aut Stp-Start Methd, with incrpratin f any risk mitigatin required, remains a viable and cst-effective slutin t reduce netwrk lsses and related carbn emissins and cst t cnsumers, while prviding a reasnable return n investment t the distributin netwrk peratrs.

13 Reprt: LEAN Reprt Page 13 f INTRODUCTION Suthern Electric Pwer Distributin plc (SEPD) has submitted Lw Energy Autmated Netwrks (LEAN) hardware demnstratin prject (referred t in the remainder f this dcument as LEAN Prject ) pr-frma [1] in the third quarter f 2014 t Great Britain s (GB) Office f Gas and Electricity Markets (OFGEM) fr funding t the value f 2.67 millin 1 ; the prject was frmally apprved by OFGEM in Prject Directin pen letter [2] in Dec Accrding t SEPD s LEAN Prject pr-frma [1], it prpses t demnstrate and deply the Lw Energy Autmated Netwrks slutin. The LEAN Prject will cnsist f demnstrating tw methds: Transfrmer Aut Stp Start and Alternative Netwrk Tplgy. The Transfrmer Aut Stp Start methd will switch ff ne in a pair f transfrmers in selected substatins t reduce fixed lsses. The Alternative Netwrk Tplgy methd will be deplyed alngside the afrementined methd where apprpriate, t further reduce lsses and maintain netwrk supply integrity [1]. Tgether, these methds, accrding t [1], are expected t save ver 31,000 MWh f electricity (r 6.42 kt f CO 2 ) ver 45 years, which is valued ver 40 millin t GB custmers; hwever, the trial described in the LEAN Prject prfrma has never been deplyed in GB r verseas and pses an element f risk, which may deter DNOs frm integrating LEAN int business as usual activities. S&C Electric Eurpe Ltd. (S&C) was cmmissined by Scttish and Suthern Energy Pwer Distributin Ltd. (SSEPD), as part f SEPD s LEAN Prject, t deliver a reprt (called the LEAN Reprt ) summarising a literature review f lsses reductin methds based n GB and internatinal experience, and a literature review and analysis summarising the impact f repeated switching and prlnged deenergisatin f a distributin transfrmer n its health and lifetime Scpe f Wrk The LEAN Reprt here cvers the fllwing listed aspects: Task 1: Review f GB and internatinal High Vltage (HV) netwrk lsses reductin prjects and experience. Task 2: Review, assessment, and prspective inrush current mitigatin ptins t reduce repeated transfrmer switching peratin impact n the HV netwrk. Task 3: Review, assessment, and prspective mitigatin ptins t reduce repeated transfrmer switching peratin impact n the transfrmer asset health and life. 1 A 90% prject cst funding request submitted t Britain s Lw Carbn Netwrks Fund and remaining 10% funding t be prvided by SEPD.

14 Reprt: LEAN Reprt Page 14 f 107 Task 4: Prvisin f recmmendatins t supprt the LEAN prjects successful implementatin Prject Organisatin The set f tasks undertaken by each prject partner in preparing this dcument included the fllwing: S&C was the prject main cntractr t SSEPD; it undertk cmplete respnsibility fr LEAN Reprt prject; this included prject management f tasks, cmpletin f aspects cvered under Task 1, Task 2, and Task 4, and delivery f LEAN Reprt t SSEPD. High Frequency Diagnstics & Engineering (HFDE) Ltd was a subcntractr t S&C and undertk aspects cvered under Task 3. Althugh, key findings frm Task 3 wrk are included in the main bdy f LEAN Reprt here, HFDE s cmplete wrk and reprt n Task 3 is detailed in Appendix D. LIG Cnsultancy Services LLP (LIG) was a subcntractr t S&C and has prvided an independent review f wrk undertaken by S&C and HFDE and high level expert pinin n aspects cvered under Task 1 thrugh Task 4.

15 Reprt: LEAN Reprt Page 15 f LOSSES Electrical lss (r electrical lsses) is the amunt f energy lst during generatin, transmissin, and distributin f electricity, including plant and unaccunted fr use [3]; these lsses are typically classified as technical and nn-technical in nature. The technical lsses cmprise f electrical energy lst due t the physics and ecnmic decisins applied t electricity distributin plant s design, engineering and peratin, while the nn-technical lsses represent electrical energy lst in the electricity distributin that is unaccunted fr use, such as, theft, measurement inaccuracies, etc. [4, 5] This sectin, as detailed in Task 1, prvides a review f electrical energy lsses, specifically cvering technical lsses in the distributin netwrk systems, and its management within GB and glbally Quantificatin Estimatin f electrical Transmissin and Distributin (T&D) lsses is a very challenging exercise t establish a reasnably accurate verall figure fr technical lsses in GB at bth DNO level and natinally, the cmparisn with lsses f ther cuntries is even mre prne t errr due t different bases fr assessment [6]. Several factrs affect the validity f cmparisn f electrical lsses perfrmance (listed belw as per [4, 6]), and therefre, due cnsideratin is required t these factrs when accunting fr lsses estimate figures r when cmparing them acrss natinal r glbal electrical netwrk systems: Definitin f Distributin. There is a diversity f perating vltages acrss Eurpe and the distinctin between Transmissin and Distributin is drawn at different vltages, e.g. distributin in England and Wales includes 132 kv but nt in Sctland. Transmissin lsses. Sme cuntries reprted lsses d nt disaggregate transmissin frm distributin lss. Input versus utput. Sme cuntries recrd their lsses as % f netwrk input and sme as % f netwrk utput, requiring adjustment in making direct cmparisns. Nn-technical lsses. The simple difference between input and utput als includes nntechnical lsses which include metering errrs, theft and data errrs, all f which are difficult t determine (e.g. theft cannt be fully assessed unless it is detected). Nature f cmpany and its service area. The sizes f the distributin cmpanies vary, with sme cuntries having a single cmpany and thers have multiple cmpanies f different sizes and custmer types (e.g. urban netwrks cmpared with rural netwrks). Embedded generatin. Sme cuntries reprts d nt take accunt f the impact n lss calculatins f embedded generatin.

16 Reprt: LEAN Reprt Page 16 f Perfrmance Arund 65% f all energy that is used t prduce electricity (as detailed in Figure 2) is lst in the electrmechanical-chemical cnversin prcess and cnsumed by pen-cycle thermal generatin plants fr their wn use, and f the remainder f electrical energy that is distributed frm generatin plants n T&D systems, arund 9% f it is lst in the electricity distributin prcess befre it reaches end cnsumers fr use. Accrding t reference [7], the IEA has shwn in their 450 Scenari that imprving energy efficiency is the least cstly abatement ptin. Efficiency accunts fr half f the cumulative glbal CO 2 emissins abatement share relative t the New Plicies Scenari, r 73 Gt, between 2011 and Figure 2 Energy Flws in the Glbal Electricity System (MTOe) [8] S&C has fund that, based n Wrld Bank data in [9], the glbal average T&D lsses f arund 9% (as detailed in Figure 3 2 ) have sustained 3 fr ver 20 years between 1990 and ; during this time, the United Kingdm s (UK) T&D lsses (arund 8% annually) were cnsistently lwer than the glbal average; hwever, when the UK T&D lsses were cmpared against ther 28 Eurpean Unin (EU) member states fr the same perid, the UK cnsistently ranked arund 15 th amng the EU member state list. These findings are als supprted by (accrding t [6]) the Recent surveys f use f electricity such as Natinal Energy Plicies by Enerdata fr ABB and the Energy Statistics Yearbk frm the IEA include an estimate f the ttal Transmissin and Distributin lsses fr each cuntry. This data indicates a large difference between the best (Luxemburg) and the wrst (Rmania). These surveys shw GB lsses as 8% (Enerdata) and 7.1% (IEA) cmpared t a Eurpean average f 6.7%. The GB values [i.e. fr UK as detailed in Figure 4] are higher than cmparable EU cuntries with the same GDP such as 2 Prduced fr this reprt using data btained frm Wrld Bank s website in [9]. 3 With grwth in annual electricity demand glbally and related netwrk upgrades r reinfrcements made. 4 T&D lsses data beynd 2011 was nt available frm The Wrld Bank Grup s website when this reprt was prepared.

17 Reprt: LEAN Reprt Page 17 f 107 Germany and France. The higher UK lsses values culd be due t varius factrs, including custmer density, variatin in lad demand, LV, MV, HV tplgies and respectively share f custmer supplied at each vltage level, netwrk design and peratin, natinal plicies, etc. cmpared t ther EU cuntries. Figure 3 EU and Glbal Average T&D Lsses frm 1990 t 2011 [9] Figure 4 EU Cuntry Lsses Vs GDP Relative t EU Average [6]

18 Reprt: LEAN Reprt Page 18 f Csts Based n the factsheet published by OFGEM in 2010 [10], electrical lsses represent abut 6% f the ttal energy transmitted in GB distributin (nt including transmissin) system alne; these lsses (as detailed in Figure 5a) are wrth (as detailed in Figure 5b) abut 1 billin a year. The transmissin and distributin electrical lsses in the United Kingdm accunt fr abut 1.5% f all greenhuse gas emissins [11]. a. Distributin Electrical Lsses [10] b. Value f Electrical Lsses in Figure 5a Figure 5 GB Electrical Distributin Lsses Althugh the value f technical lsses are cuntry specific, a backgrund paper fr the Wrld Bank Grup energy sectr strategy n Reducing Technical and Nn Technical Lsses in the Pwer Sectr [5] suggests that in the next tw decades, glbal prices f primary energy resurces (il and ther fssil fuels) will be rising 5 in real terms and prices f equipment in the electricity sectr (generatin, transmissin and distributin) steadily rse this decade [ ] until the glbal financial crisis that began in the 3 rd quarter f The backgrund paper als predicts that against these price trends, the ttal csts f technical lsses tend t exceed investment csts f transmissin and distributin equipment required t reduce them t their ptimum value, mre s where a significant prtin f generatin is based n fssil fuels. This tendency is accentuated if envirnmental csts f pwer generatin (harmful lcal pllutants as well as greenhuse gas emissins) and increasing difficulties in achieving scial acceptance f new pwer plant cnstructin (regardless f fuel type and technlgy) are taken int accunt. 5 In its Wrld Energy Outlk 2008, the Internatinal Energy Agency frecasts wrld il prices rebunding t abut US$130 (2007 U.S. dllars) per barrel in 2030 [12].

19 Reprt: LEAN Reprt Page 19 f Breakdwn The electrical technical lsses breakdwn by electrical distributin system plant is dependent n several factrs; these include, netwrk tplgy and peratin, netwrk vltages, asset management and prcurement plicy, level and types f netwrk lad and generatin, gegraphical area, climatic and seasnal cnditins, cnsumer use behaviur, lsses and breakdwn calculatin methdlgy, netwrk peratr/wner, etc. In GB, accrding t the UK s Department f Energy & Climate Change s (DECC) Digest f United Kingdm Energy Statistics (DUKES) reprt in [13], f the ttal 27 TWh electricity lsses in 2013, distributin netwrk lsses alne represented abut 73% f the ttal T&D lsses (technical and nn-technical lsses cmbined), this was fllwed by 24% lsses in the transmissin netwrk, and the remainder, between 3% t 4%, due t electricity theft and meter fraud; these figures are als detailed in Figure 6. Transmissin and/r distributin electrical lsses breakdwn by electrical distributin netwrk equipment in GB and ther develped cuntries in the wrld is shwn in Figure 7; it can been that, due t varius factrs affecting lsses breakdwn, the share f lsses in each equipment is fund t vary between different cuntries, DNOs within the same cuntry (such as, GB), lad and netwrk types. As a general trend, with GB electric distributin feeder lengths lw cmpared t ther larger cuntries, such as USA, Canada, Australia, etc., electrical lsses were fund t be cncentrated in the distributin netwrks in the fllwing rder: Lw Vltage (LV), secndary, primary, and HV netwrks. Cnversely, in larger cuntries, where lng distances f primary netwrks exist and secndary transfrmers are lcated clse t the lad centres, electrical lsses are typically cncentrated in the primary distributin netwrks, fllwed by secndary netwrks. Figure 6 GB s Ttal T&D Lsses Breakdwn [13]

20 Reprt: LEAN Reprt Page 20 f 107 a. Central Netwrks in GB [6] b. YEDL/NEDL in GB [6] c. SEPD in GB (% f ttal lsses) [14] d. Ausgrid in Australia (% f ttal lsses) [15] e. Hydr One in Canada (% f ttal lsses) [16] f. EPRI Study in the USA (% f ttal lsses) [17] g. GB Lsses by Netwrk Type (% f ttal lsses) [6] Figure 7 Distributin Netwrk Lsses Breakdwn

21 Reprt: LEAN Reprt Page 21 f Regulatin A 2009 backgrund paper fr the Wrld Bank Grup Energy Sectr Strategy [5] has suggested that Many electricity utilities in develping cuntries succeeded in significantly reducing [in sme cases mre than 50% f ttal lsses, e.g. Nrth Delhi Pwer Limited (NDPL), India, between 2002 t 2008] r eliminating nn-technical lsses [e.g. fraud, theft, etc.] in electricity supply n a sustainable manner, but thers cntinue t shw high lsses ; utility activities in reducing electrical lsses were als supplemented by refrms made in pwer sectr and establishment f regulatry framewrk fr electricity generatin, transmissin, and distributin in these cuntries t drive change and imprve services t electricity custmers. The paper als lays ut fur axims f refrm, extracted frm the handbk prepared by PA Cnsulting Grup fr USAID in Nvember 2004 [18], the critical cnditins fr success in sustainable reductin f lsses in electricity distributin; these are accuntability, wnership thrugh participatin, hlistic apprach, and enhancement f scial develpment and equity. Different cuntries within the EU have adpted different regulatry lsses reductin incentive mechanisms and slutin embedment fr lsses in their respective market systems; these mechanisms are summarised in Appendix A and B. Lenard Energy in respnse t a ERGEG Psitin Paper fr public cnsultatin n Treatment f Lsses by Netwrk Operatrs has cmmented in [19] that the current tariff systems in mst cuntries are nt favring netwrk efficiency imprvements. In several Eurpean cuntries (France, Pland, Spain, Germany), there is a price cap n the netwrk tariff, in which the term fr netwrk lsses is nt included. This means that the cst f netwrk lsses can be entirely charged thrugh t the custmer. This tariff system prduces a strng disincentive fr investing in netwrk efficiency. The EU as part f its Energy Strategy fr 2020 [20] has set ut targets in the EU area t reduce greenhuse gas emissins 6 by 20%, rising t 30% if the cnditins are right, t increase the share f renewable energy 7 t 20% and t make a 20% imprvement in energy efficiency 8, these were then translated t natinal targets fr each EU member state; fr the UK, an EU member state, these targets fr 2020 are set t 16%, 15%, and Mte (primary energy cnsumptin levels in 2020 expressed in Mte) respectively. The EU s Renewable Energy Directive 2009/28/EC [22] target fr the UK t achieve 15% f its energy cnsumptin frm renewable surces by 2020, in additin, will result in large amunts f Distributed Generatin (DG) cnnected directly t the distributin netwrk. On the demand side, hwever, the UK Gvernment s Carbn Plan [24] acknwledges that in GB With the ptential electrificatin f heating, transprt and industrial prcesses, average electricity demand may rise by 6 Emissin reductin target frm EU Emissins Reductin Decisin 2009/406/EC [21]. 7 Renewable energy integratin target frm the EU Renewable Energy Directive 2009/28/EC [22]. 8 Energy efficiency target frm the EU Energy Efficiency Directive 2012/27/EU [23].

22 Reprt: LEAN Reprt Page 22 f 107 between 30% and 60%. These cnditins, include frecasts in [24], shw that the transitin t a lw carbn ecnmy may lead t significant increases in electricity demand and a crrespnding rise in electric netwrk lsses. And Reducing shrinkage [a term used fr wastage in gas sectr] and lsses is recgnised as the mst effective methd f imprving energy efficiency f netwrks [25]. Traditinally, DNOs in GB have reduced lsses thrugh lng-term asset management, replacing end f life assets with energy efficient mdels; hwever, with the intrductin f EU s Ecdesign Directive 2009/125/EC [26], this has becme mandatry fr utilities. In additin, OFGEM in GB, as part f its RIIO- ED1 ( Revenue = Incentives + Innvatin + Outputs fr Electricity Distributin) Strategy Decisin in March 2013 [27], has utlined a mechanism fr DNOs t place an apprpriate level f fcus n lsses reductin activities [28]; the cre cmpnents f this mechanism includes a licence bligatin, lss reductin expenditure in the business plans, a lsses strategy and the Lsses Discretinary Reward (LDR). Fllwing this, OFGEM in an pen letter t Western Pwer Distributin [29], dated 30 th July 2014, has specified that Frm 1 April 2015 DNOs licences will require them t ensure that lsses n their netwrks are as lw as reasnably practicable, and t maintain and act in accrdance with their published lsses strategies. A cnsultatin n the draft RIIO-ED1 Lsses Discretinary Reward was als cmpleted 9 by OFGEM n the 7 th May Management DNO electrical lsses management and plicies are driven by bth internal and external factrs. The internal factrs include aspects such as prviding the best value t stakehlders (e.g. custmers, sharehlders [31]) and sustainability (accunting fr ecnmic, scial, and envirnmental factrs) [24, 27, 32]. External factrs include cmplying with statutry (e.g. [33]) and regulatry (e.g. [28]) bligatins, and lcal and internatinal plant design standards [26]. These plicies drive nn-technical and technical appraches t electrical lsses reductin. A wide variety f ther reductin strategies are als currently emplyed tday by the DNOs. These include training f utility staff, custmers, and general public in the use f electricity and practical ways t reduce cnsumptin (e.g. [34]); prmtin f sustainability and envirnmental cnservatin (e.g. [35]); raising awareness f related utility service fferings t custmers (e.g. [34]); implementatin f lw lsses asset prcurement strategies [26]; etc. These appraches are cmplimentary t DNO s technical lsses reductin appraches applied in peratins, design, majr prjects, and netwrk reinfrcements [36]. Typically, frm a netwrk design perspective (which will naturally assume the ptimal day t day peratin f the netwrk with regard t verall efficiency and security) fr a electricity utility, accrding t UK Pwer Netwrks (UKPN) in [37], ptimising lsses is essentially a trade-ff between up-frnt 9 As per Ofgem s Cnsultatin n the draft RIIO-ED1 Lsses Discretinary Reward Guidance Dcument web link in [30].

23 Reprt: LEAN Reprt Page 23 f 107 investment (fr example in lwer lss equipment and/r additinal netwrk capacity) and the lnger term cumulative benefits f reduced lsses. In pure business terms, the ptimum design frm a lsses perspective is that which delivers the highest NPV [Net Present Value] f incremental cst-benefit in terms f initial investment and lnger-term benefits arising frm reduced lsses. With changes in GB regulatry apprach t electrical lsses brught in thrugh RIIO-ED1, all DNOs within the UK have submitted respective lsses strategies as part f their business plans in [32, 38-42] t OFGEM fr 2015 t Amng these DNO lsses reductin strategies, the fllwing listed strategies remained cmmn fr cnsideratin during RIIO-ED1 perid: Reduce technical lsses thrugh transfrmer, verhead, and cable size selectin based n ecnmic assessment. Reduce nn-technical lsses and increase revenue prtectin by addressing supplier side abstractin, theft in cnveyance, and unmetered supplies. Other additinal lsses reductin strategies prpsed by GB DNOs t be implemented either during r beynd RIIO-ED1 perid als included the fllwing: Netwrk plicy, design, peratin, and ecnmics: Bring imprved understanding f lsses. Netwrk design plicy accunting fr lsses reductin. Prmting the efficient use f electricity. Investment decisins based n pr-active replacement and pprtunistic interventin accunting fr lsses reductin. Bring in peratinal measures t reduce lsses. Impact f future time f use tariffs. Netwrk design accunting fr future lads, such as increased electrificatin f transprt and heat, and related lsses reductin. Implementatin f existing prven and implementatin f new lsses reductin technlgies. Impact f embedded generatin and its management n lsses. Netwrk vltage upgrade and vltage design ratinalisatin fr lsses reductin. Pwer quality management, including vltage imbalance, pwer factr and harmnics. Demand side participatin and respnse. Innvative measures t alter netwrk pwer flws.

24 Reprt: LEAN Reprt Page 24 f 107 Active netwrk management. Vltage regulatin and ptimisatin. Netwrk meshing. Heat recvery frm electricity netwrks. Energy strage. A 2013 white paper in [43] by Schneider Electric suggests the fllwing staged apprach t reducing utility electrical lsses: Within the next 3 mnths, identify areas where waste can ccur: primary substatins, MV feeders, secndary substatins. Cnsider parameters such as density f ppulatin, pwer f existing and frecasted Distributed Energy Resurce (DER), strategy arund smart metering, and existing cmmunicatin ptins. Within the next year, install sensrs and applicatins that can accurately assess the magnitude f the efficiency lsses. Begin t identify areas f imprvement. Within the next tw years, implement pilt prject t demnstrate feasibility, quantify the gains, and estimate the deplyment csts. Within the next 10 years plan and implement the staged rllut Opprtunities This sectin prvides a summary f varius technical lsses reductin pprtunities, and studies prjects that have been carried ut in GB and internatinally Feeder Tie Open-Pint Optimisatin Optimal feeder sectinalising, r lad balancing between feeders, is ne f the cheapest lsses reductin measures [36]. This is currently achieved by DNOs in GB by selectin f 11 kv feeder pen pints assessed using internal pwer flw studies, engineering judgement, and accunting fr variatin in seasnal lad and lad grwth. Currently there are n examples in GB where this is implemented t peridically recnfigure the netwrk (e.g. every half hur) thrugh a centralised (r lcal) lsses reductin ptimisatin algrithm; hwever, similar schemes in cmbinatin with ther netwrk aspects are prpsed in several studies (e.g. [44-46]). SSEPD s studies in [47] shw that a dynamic tie pen-pint ptimisatin scheme, when slely applied t reduce netwrk lsses, may be expensive and may have limited return n investment. This is due t tie pen-pint Circuit Breakers (CB) requiring replacement every few years. Hwever, ther studies in [48] have shwn that the nn-dynamic netwrk ptimal re-cnfiguratin can reduce instantaneus technical

25 Reprt: LEAN Reprt Page 25 f 107 lsses by 1.4 MW (i.e.17% f ttal technical lsses at peak lad) r 12.1 GWh with a payback perid f 0.3 years Cnservatin Vltage Reductin (CVR) Accrding t 2010 US Department f Energy s Smart Grid reprt in [49], End-use energy cnsumptin has been shwn t drp when the electric service vltage is reduced. This strategy, r CVR, ccurs primarily because the energy cnsumptin f certain end-use lads such as incandescent lights and certain electrnics g dwn as the vltage is decreased. A field study in [50] invlved studying 31 feeders at 10 different substatins and 11 utilities in the Pacific Nrthwest regin. The study shwed that a 1% change in distributin line vltage prvided a 0.25% t 1.3% change in the end-user energy cnsumptin, and that vltages culd be reduced frm 1% t 3.5%. A review wrk undertaken in [50] suggests that the CVR and advanced vltage cntrl scheme tgether when applied universally in the US culd deliver a 2% reductin in 2030 electricity demand. Accrding t reference [49], hwever, Accurate determinatin f the CVR effects n any given feeder must include analysis f the electrical lad as well as the design f the distributin system. The design f the distributin feeders includes everything frm line and cable types, line and cable cnfiguratins, use f vltage crrectin capacitrs, and use f tap-changing vltage regulatrs fr transfrmers. Thus, extraplating the CVR results t estimate the natinal ptential is difficult. An ESB Netwrks published paper in [51] has identified a CVR factr (dp/dv) f 0.35 fr dmestic lad that cnsisted f significant lighting. At the time f this study, the majrity (arund 92%) f dmestic lighting identified by ESB Netwrks was incandescent rather than CFL r LED, neither f which are vltage dependent. Practical studies t cnfirm the apprpriate CVR factrs fr SEPD/GB custmer and lad grups are required t quantify the benefits ffered by the CVR methd Demand Side Respnse (DSR) Traditinally, electricity utilities have designed their systems fr maximum electricity demand; maximum electricity demand, due t cnsumer behaviur, hwever, ccurs during certain times f the day and seasn, e.g. at arund 5pm n a winter weekday in GB [52]. With increasing lad demand and new types f lad and generatin cnnecting t the electricity distributed systems, there are likely t be mre peaks and valleys in the daily lad demand prfiles varying with time, seasns, cnsumer behaviur f electricity time f use. Hwever, the actual substatin lad factrs may, hwever, nly marginally increase r even decrease with mre DG added t the electrical distributin netwrks [53]. One apprach t help utilities defer netwrk reinfrcements, and therefre limit r reduce cst f electricity t cnsumers, is t implement DSR scheme, where custmers are incentivized financially t lwer r shift their electricity use at peak times [54]. Electrical technical lsses in the electrical netwrk are prprtinal t square f current flw (r applied lad) in the netwrk; therefre, reductin in peak lad (via DSR fr example) can

26 Reprt: LEAN Reprt Page 26 f 107 enable significant reductin in electrical technical lsses. The key drivers fr the intrductin f DSR in existing electrical netwrks, as detailed in [55], include the fllwing: The ptential fr mre efficient use f (generatin and transmissin) assets and increased efficiency f electricity system peratin. Recent develpments in Infrmatin and Cmmunicatin Technlgy (ICT), which culd facilitate the deplyment f further DSR slutins. With ageing electricity infrastructure assets in the UK, the pprtunity exists t integrate DSR technlgies in the strategy fr asset replacement. Greenhuse gas emissin reductin gals leading t a fcus n distributed generatin (e.g. Cmbined Heat and Pwer (CHP)), and renewables (in particular wind pwer). DSR culd play a useful enabling rle fr such technlgies. Currently, DSR and its varius schemes are being trialled in GB and elsewhere. The UK DECC s 2012 reprt in [56] prvides a cmprehensive review f majr trials (30 DSR trials) in dmestic electricity sectr glbally f DSR schemes that aim at delivering a reductin in electricity use at peak time n a day-in day-ut basis and 'critical peaks' in electricity demand. The reprt suggests that dmestic Cnsumers d shift electricity demand in respnse t ecnmic incentives (such as the applicatin f higher prices during peak demand perids) even if these incentives are accmpanied by nly basic infrmatin n the prices being applied, hwever the size f the shift can vary significantly. And, depending n the day-in day-ut r 'critical peaks' based DSR implementatin, the reductins culd vary between frm 0% t 22% fr peak demand and frm 5% t 38% fr critical peaks respectively. Element Energy s 2012 reprt n demand side respnse in the nn-dmestic sectr t OFGEM in [52] cncludes that the nn-dmestic buildings (excluding industry) cntribute apprximately 15GW t peak demands n Great Britain s natinal grid and the ttal technical ptential fr peak demand reductin via DSR measures in nndmestic buildings is estimated t be frm arund 1 4.5GW (r 0.6 2GW if n flexibility can be prvided frm lighting). The three sub-sectrs that may cntribute mst t delivering this ptential are retail, educatin and cmmercial ffices. Hwever, there are als several challenges assciated with implementatin f DSR; these are discussed in detail in references [52, 55, 56] Transfrmer Aut Stp-Start (TASS) Utility distributin transfrmers (ranging frm tens f kva t tens f MVA) typically have peak efficiencies higher than 98% [57]; the remainder f the transfrmer s transfer energy (i.e. less than 2%) is lst as transfrmer fixed (r irn) lsses and variable (r cpper) lsses. The peak transfrmer efficiency, as per

27 Reprt: LEAN Reprt Page 27 f 107 [57], ccurs when lad lss and n-lad lss are equal, hwever, efficiency (as detailed in Figure 8) als vary with ther factrs, such as transfrmer s applied lad, pwer rating, lsses specificatin, etc. a. Efficiencies 10 fr Typical Distributin Transfrmers f Varius Pwer Ratings b. Efficiencies fr Several 1000 kva Distributin Transfrmers Figure 8 Transfrmer Efficiencies in a Typical GB Distributin Netwrk [58] Althugh transfrmers are efficient at higher lad factrs, the average annual lad may be much lwer, leading t verall cpper lsses being lwer than verall irn lsses. This phenmenn was bserved in several feeder lsses evaluatin studies: SSEPD cmmissined Isle f Wight Netwrk Lsses Study in [47] and [59], EPRI s KCP&L Green Circuits Analysis in the USA [60], Pwer System Engineering, Inc. study [61]. In [47], fr example, amng nine 33/11 kv substatins during the year 2012, the demand at five substatins is less than 40% f their firm capacity fr abut 95% f the time, the demand at tw ther substatins is less than 40% fr abut 40% f the time, and the demand at the remaining tw substatins is less than 50% fr abut 40% f the time. In additin, transfrmers may suffer internal cre damage, due t clse-up feeder faults. Damage may include shrting f laminatins, disfigurement f laminatins causing increased flux infringement, etc., leading t increase in transfrmer irn lsses; this is reprted in [62]. Switching ff ne f a pair f under-utilised distributin transfrmers was suggested by ESB Netwrks in [36], which bserves that, Such switching is usually nly practical in SCADA r remtely cntrlled statins, where the cst f carrying ut the switching is minimal. Althugh, variants f Stp-Start schemes have been applied befre t Arc Furnace related transfrmer frequent switching (frm 50 t 100 times per day) applicatins (as detailed in [63]), the TASS applicatin fr the purpse f distributin transfrmer lsses reductin has nt been fund in GB r elsewhere in the wrld. 10 Althugh, transfrmers d nt typically perate beynd their thermal rating, it was assumed by S&C that efficiency curves prduced in [58] and presented here, fr transfrmer ratings <1000 kva, were extraplated t 1000 kva fr cmparisn purpses.

28 Reprt: LEAN Reprt Page 28 f 107 SSEPD s cmmissined netwrk lsses reductin study in [47] indicates abut 9% reductin in verall 11 kv netwrk lsses (with inclusin 33/11 kv primary and secndary LV transfrmer lsses) culd be achieved thrugh the use f TASS scheme. a. Typical Summer (tp rw) and Winter (middle rw) Lad Prfiles and Annual Substatin Lad Distributin (bttm rw) Curves with Incremental Additin f Existing Lad, EV Lad, and DG Penetratin Share t Each Scenari (frm left t right clumns) fr Varius Lad-DG Scenaris b. Annual Substatin Lad Lsses withut TASS (slid bars), Lss Savings with TASS (checked bars), and Percentage Lss Savings with TASS (% numbers in bars) fr Varius Lad-DG Scenaris Figure 9 Evaluatin f Lsses Savings with TASS Methd [53]

29 Reprt: LEAN Reprt Page 29 f 107 In additin, in a recent paper in [53], prepared by S&C, LIG, and SEPD fr CIRED 2015 cnference, evaluating the level f lsses reductin that culd be achieved using TASS methd at a primary substatin fr future levels f lad and distributed generatin mix (as detailed in Figure 9a), has shwn that the lsses savings achieved with TASS methd can be substantial; based n the data and assumptins used in the paper, any frm 8% t up t 26% in lsses savings (as detailed in Figure 9b) frm the base case culd be achieved Meshed Netwrk Operatin The primary benefit f the Alternative Netwrk Tplgy (r meshed netwrk peratin) is the maintenance and/r imprvement f netwrk reliability. Accrding t The Brattle Grup reprt in [64], distributin systems are frequently radial in design, whereas transmissin systems are nrmally meshed. Netwrk meshed tplgies are als typically applied t high lad-density urban distributin netwrks, and radial tplgies t lwer lad-density rural distributin netwrks. In urban meshed netwrks, accrding t [64], the Netwrk systems are designed with redundant supply paths, althugh lines t individual custmer premises are typically stand-alne. This enables utility s persnnel t visually identify the fault lcatin, identify the best fault islatin switches, islate the fault, and re-establish supply t custmers. Althugh, the manual prcedure t restre electrical supply may be lng, it reduces the number f custmers withut supply fllwing a system fault. With advancements in switching technlgy and new cntrl algrithms (e.g. S&C s IntelliRupter PulseClser and IntelliTEAM II Autmatic Restratin System [65, 66], distributed autmatin using reclsers and sectinalizers [67]), netwrks are able t deply c-rdinated distributed intelligence, enabling fast autmated fault islatin and sectinalizing schemes; this significantly reduces the electrical supply restratin time fllwing a fault. Sme f these technlgies are currently deplyed as part f a technlgy demnstratin prject at the SEPD s Isle f Wight regin in GB [66]. At the cmpletin f the prject pilt, SEPD in [66] has cncluded that the technlgical benefits f pulseclsing and ecnmic benefits f reducing CML were repeatedly demnstrated. The secndary benefit achieved using netwrk meshed tplgy is the reductin in an verall netwrk impedance, and as a cnsequence [68, 69], it reduces the verall netwrk lsses and vltage drp, maintains higher fault levels, imprves netwrk pwer quality, etc. Frm studies undertaken in [47, 59, 70], it can be drawn that the netwrk lsses reductin is generally achieved via implementatin f netwrking meshing, hwever, the verall gains (with a psitive return n investment) culd be marginal. A review undertaken by Parsns Brinckerhff in 2014 in [71] n business plans submitted by GB DNOs in t OFGEM have identified areas where a DNO s prpsed innvatin is targeted twards intercnnectin [r netwrk meshing] f HV and LV distributin netwrks thrugh the next price cntrl perid. Under this review we [Parsns Brinckerhff] have als identified key n-ging prjects which present evidence that there is a clear interest, n a [GB] natinal scale, in develping intercnnected distributin netwrks.

30 Reprt: LEAN Reprt Page 30 f 107 Hwever, the netwrk meshing pprtunities currently being studied, sme via GB s Lw Carbn Netwrk Fund (LCNF) prjects, by GB s DNOs are primarily related t imprvement f netwrk reliability, increase in DG cnnectin capacity, etc. with reductin in verall netwrk lsses seen as a by-prduct f their implementatin Reactive Pwer Cmpensatin Where there are lw pwer factrs, DNOs typically install reactive cmpensatin equipment t imprve lcal r verall netwrk pwer factr, which cnsequently reduces the netwrk current flws. As All current flw causes lsses bth in the supply and distributin system [72], such installatins may help reduce the netwrk electrical lsses. There is a variety f reactive pwer cmpensatin equipment available tday; they range frm simple passive equipment t advanced custmer pwer devices [73]. Deplyment and cntrl f these devices in the netwrk culd be lcal and standalne, r be part f the verall distributin netwrk multi-parameter (pwer factr, vltage, lsses, etc.) ptimisatin and c-rdinated cntrl (e.g. [48, 72-74]); the devices may als be placed strategically t enable such cntrl (e.g. placement f distributin capacitrs, DSTATCOMs, etc.) Currently, the average pwer factr at SEPD s substatins (based n Isle f Wight 11 kv lad demand) is better than 0.96; the pprtunity fr the use f reactive pwer cmpensatin t reduce netwrk lsses in SSEPD s studies in [47, 59] was therefre nt cnsidered. Hwever, studies in [48] have shwn that ptimal placement f capacitrs can reduce instantaneus technical lsses by 2.5 MW (i.e.30.4% f ttal technical lsses at peak lad) r 21.5 GWh with a payback perid f 0.7 years Energy Strage Currently, energy strage slutins are primarily emplyed t: prvide fast frequency respnse supprt t the grid [75, 76], act as a back-up supply t a site during lss f mains, reduce netwrk reinfrcements, enable grid stabilisatin by accmmdating distributed generatin (e.g. wind and PV) [75, 77], prvide pwer flw peak shaving and cngestin management, etc.; with reductin in netwrk lsses achieved cnsequentially f its primary applicatin. Fr energy strage t be effective in pwer flw peak shaving applicatin, which enables cnsequential reductin in electrical lsses, sme level f feeder lad frecasting is needed. Currently, there are distributin lad frecast algrithms that are emplyed currently in the US (such as EPRI s Artificial Neural Netwrk Shrt Term Lad Frecaster (ANNSTLF) [78]) that have shwn t maintain the lad frecast errrs t less than 2%. SSEPD s 2014 netwrk lsses reductin study in [47] has shwn that battery based energy strage purely frm an electrical lsses reductin pint may nt be cst effective; hwever, this result may change with expected future reductins in per kwh cst and increase in battery density. A 2012 reprt by

31 Reprt: LEAN Reprt Page 31 f 107 Element Energy in GB n cst and perfrmance f EV batteries in [79] estimates a significant reductin in cst f battery and increase in verall densities [47], these are summarised in Figure 10. Figure 10 Csts and Weights fr a 30 kwh EV Battery [79] Distributed Generatin Embedded generatin s impact n the lcal and verall netwrk electrical lsses depends n several factrs, such as its prximity t lad, level f cnsumptin, DG technlgy, DG daily and seasnal generatin patterns, DG penetratin dispersin level in the netwrk, spillage f excess generatin t ther netwrk vltage levels, netwrk circuit cnductr sizes and selectin criteria, etc. Frm the cntext f lad demand, a study cnducted by Strathclyde University fr the Electricity Netwrk Strategy Grup in [45] has shwn that the embedded generatin may enable a reductin in lsses nly when netwrk lad is greater than abut 70% f its peak value. OFGEM s 2007 reprt n the integratin f DG int the UK pwer systems in [80] suggests that the Micr CHP utput is cincident with the system peak lads (i.e. winter 5.30 pm) and s has the ptential t make a significant cntributin t reductin in lsses during this perid. The same reprt als mentins f a similar study in [81] that cncludes that in the rural netwrks, as detailed in Figure 11a, the level f lsses can be reduced by ver 40% (frm 8.5% t 5%) by micr CHP with installed capacity f 50% f the ttal Grid Supply Pint (GSP) peak lad. In urban areas, the same capacity f micr CHP reduces the level f lsses frm 4.5% t 3% (reductin f 33%). Similar trends were als shwn fr Phtvltaic (PV) DG. The same later study in [81] has als shwn that with increase in DG in the MV netwrk, as detailed in Figure 11b, the lsses initially may reduce until the minimum level f netwrk lsses is achieved, but thereafter, the lsses can increase substantially. SSEPD s netwrk lsses reductin study n the Isle f Wight 11 kv netwrk in [47] and [59] has shwn a similar result that the embedded generatin may increase 11kV netwrk electrical lsses; hwever, the system wide electrical lsses (400 kv t LV) may reduce.

32 Reprt: LEAN Reprt Page 32 f 107 a. Impact f Micr CHP and PV n Distributin Netwrk Lsses fr Rural and Urban Lcatins [80, 81] b. Impact f UK MV Netwrks (Maximum lad is 50 GW) [82] Figure 11 Electrical Lsses in Distributin Netwrk with DG Integratin Levels In regards t the impact f DG technlgy type (wind turbines, cmbinatin f wind turbines and CHP, PV, and CHP cnsidered independently) n lsses, as per study in [83], the wind turbines have the least psitive impact n lsses, because the injected energy is intermittent, presenting high time variability, and des nt match well with the feeder lad pattern, cntrary t ther technlgies, such as PV, where it typically fllws better daily lad variatins, and CHP, where the generatin utput is mre predictable and culd be adjusted t cincide with the lad demand. The OFGEM s 2007 study in [80] als suggests that small scale DG that wuld tend t perate during peak demand perids culd have very significant impact in reducing distributin netwrk lsses. Lw carbn DG technlgies, especially the wind, PV, etc., increasingly use cnverter-based technlgies (t meet grid cnnectin requirements, t maximise generatin utput via Maximum Pwer Pint Tracking (MPPT) functinality, etc.) are a surce f harmnics in the netwrk. Althugh, at lw levels f DG penetratin and utput it may lead t reductin in fundamental pwer lsses, the verall harmnic lsses with cnverter-based DG technlgies may rise cmpared t the netwrks withut r less f them; with increased share f cnverter-based DG in the netwrk, the harmnics lsses expected in the netwrk can be significant. In additin, increased levels f small scale single-phase DG-units can cause netwrk unbalances leading t increased lsses in the neutral cnductrs [84]. Furthermre, a 2011 IEEE paper n ptimal accmmdatin and smart peratin f DG fr lsses minimisatin in [85] cncludes that The multi-perid AC [alternating current] OPF-based [Optimal Pwer Flw] technique has demnstrated that ptimal accmmdatin cmbined with adequate pwer factr settings fr the DG units can harvest significant benefits in terms f lss reductin. This less technically cmplex slutin culd easily be implemented in mst distributin netwrks prvided that ptential cmmercial and regulatry barriers are alleviated thrugh the use f incentives. Further gains can be achieved by the use f Smart Grid-like cntrl schemes (crdinated vltage cntrl and adaptive pwer

33 Reprt: LEAN Reprt Page 33 f 107 factr cntrl), althugh the ecnmics f energy lsses alne might nt justify the required infrastructure. A study in [86] assessed the impact f DG dispersin levels and reactive pwer capacity in the netwrk n electrical lsses has cncluded that Minimum active pwer lsses are reached with high penetratin levels if DG sufficiently dispersed; and reactive pwer generatin capacity is enugh. Hwever, where there is a lw dispersin f DG in the netwrk, variable pwer factr peratinal capacity f the DGs can be utilised t reduce verall netwrk lsses Active Netwrk Management (ANM) It is generally accepted in the electrical industry tday that a full feature set f what ANM entails cntinues t evlve and mature with time [87-89]. Hwever, in general terms, accrding t [90], Active Netwrk Management (ANM) is cncerned with the real-time cntrl f energy prducing and cnsuming devices t maintain electricity netwrks within acceptable perating parameters. Until the end f last decade, i.e. 2010, the primary fcus f ANM in GB has been t allw fr accelerated grwth and integratin f renewable generatin int the netwrk, while maximising the utilisatin f cnnected electric netwrk and maintaining electrical circuit pwer flws and netwrk vltages within their respective peratinal limits. With renewed fcus n energy efficiency at bth UK natinal and EU level, and with intrductin f EU Energy Efficiency Directive 2012/27/EU and OFGEM s new DNO license t ensure that lsses n their netwrks are as lw as reasnably practicable, electrical netwrk lsses reductin pprtunity, as an additinal desired netwrk peratinal cnstraint, within ANM methdlgy is currently being explred [91, 92]. A study undertaken by Shn Assciates and Imperial Cllege Lndn in the UK fr Western Pwer Distributin in [6] assessed the benefits f fur active netwrk management techniques in term f enhancing the ability f the netwrk t accmmdate increased penetratin f wind generatin are [were] mdelled ; these techniques included generatin curtailment, Pwer Factr (PF) cmpensatin, areabased Online Line Tap Changer (OLTC) vltage cntrl, and in-line Vltage Regulatrs 11 (VR). Fr each technique r case study, the wind generatin capacity in the netwrk was increased frm 4 MW t 20 MW in steps f 2 MW steps, and the annual energy prduced in the netwrk was calculated. Althugh, the base case with applicatin f standard vltage limit wuld nly allw fr 6 MW f wind generatin capacity t be cnnected, the allwable capacity with ther studied ANM techniques in this study [6], as detailed in Figure 12a, was much better, especially with implementatin f area based OLTC and area based OLTC with in-line vltage regulatin ANM techniques; in Figure 12a, the lighter bars represent the net energy generated in the curse f ne year, while the darker bars represent the curtailed energy. 11 In-line vltage regulatrs are typically limited t applicatin in distributin system than in transmissin system [93], [94], [95].

34 Reprt: LEAN Reprt Page 34 f 107 Figure 12b shws crrespnding netwrk lsses fr varius cnsidered ANM techniques and their level f generatin accmmdatin in the netwrk. The study [6] cncludes that the applicatin f advanced active management techniques that wuld maximise the utilisatin f existing netwrks may increase lsses in the lcal netwrk very significantly. It further cncludes that, althugh, in ecnmic terms, the increase in lsses may be efficient when traded against the facilitatin f lw-carbn generatin cnnectins and avided netwrk reinfrcements, the netwrk lss increases [nnetheless] may be als described as a decrease in energy efficiency and as such, undesirable. a. Generatin accmmdatin with cnsidered ANM techniques: 0.98 PF, 0.95 PF, area based OLTC, and area based OLTC with in-line vltage regulatin b. Impact f ANM schemes n netwrk lsses fr varius level f penetratin f wind generatin Figure 12 Impact f Chice f ANM Technique n Generatin Accmmdatin and Lsses [6] A paper [96] presented at EWEA n cst f ANM schemes at distributin level, based n results presented in [97], suggests that the value f the ANM increases as the vlume f DG cnnectin t the system increases. A cmparisn f different ANM schemes highlights the imprtance f generatin curtailment (when used with vltage cntrl) t minimise lsses n the netwrk, and reduce investment csts. The exclusin f generatin curtailment wuld result in larger Static Var Cmpensatr (SVC) required fr vltage cntrl n the netwrk. T cntrast, the exclusin f SVC frm the ANM scheme reduces lsses and investment csts when cmpared with the use f bth curtailment and SVC Netwrk Reinfrcements ESB Netwrks, which has upgraded much f its verhead MV netwrk frm 10 kv t 20 kv ver the last 15 years (as detailed in [98]), reprted in [74] that The csts f 20kV cnversin were little mre than thse f rebuilding in 10kV, yet the vltage drp was halved, thermal capacity dubled and lsses reduced by 75%. Upgrade csts, were reprted in [99]: Frm '10 incl. the cst f the prgramme

35 Reprt: LEAN Reprt Page 35 f 107 fr renewing and upgrading existing netwrk plus cnstructin f new lines and transmissin /distributin statins is in excess f 2.5 billin. Cmments received frm Lenard Energy n ERGEG psitin paper n the treatment suggests that the Investments in new capacity culd in sme cases be justified by the reduced cst f lsses. The apprpriate tl fr such an investment decisin is Life Cycle Csting (LCC). It has been suggested that the ptimal average utilisatin rate f distributin netwrk cables shuld be as lw as 30% if the cst f lsses is taken int accunt. There is sme histry in GB f netwrk vltage upgrade wrk [[100, 101] almst the entire earlier 6.6 kv undergrund netwrk has been upgraded t 11 kv between the 1960 s and the present date. In many cases, it was fund t be ecnmically frtunate that the 6.6 kv cable was capable f reliable peratin at 11 kv. A netwrk vltage upgrade can prvide significant reductin in lsses; while its deplyment may be expensive, disruptive, and time cnsuming the benefits f an increase in capacity fr new lad and generatin cnnectins may suit the future lw-carbn custmer. Frm a utility s perspective n lsses reductin with mving t higher vltages f existing netwrks, as per Western Pwer Distributin s (electricity utility in GB) lsses strategy [102], An extraplatin frm the HV netwrk can be made nt the EHV and higher vltage netwrks. In general terms these netwrks are sized fr nrmal running and fault supprt and d nt perate in a radial fashin. The csts als utweigh the benefits in fllwing area: Uprating f EHV transfrmers (33kV and abve) Uprating f EHV cables (33kV and abve). SSEPD s wn netwrk lsses reductin study in [47] has als shwn that Althugh, the interventin ffers significant savings in netwrk lsses savings cmpared t any ther cnsidered interventin, and the upfrnt high investment utweigh any cumulative benefits ffered fr remainder f the assessment perid, with n expected ROI [Return On Investment] Other Opprtunities In additin t lsses reductin netwrk interventins r pprtunities discussed in the abve subsectins, i.e. frm Sectin and , there are als several ther lsses reductin methds as detailed in [36, 84, 103], such as lad phase balancing, harmnic current mitigatin, frequent live-line washing t reduce leakage currents, splitting f LV netwrks, etc. Hwever, applicatin f these methds is specific t netwrk tplgy, gegraphy, climatic cnditins, lad and generatin type, etc.

36 Reprt: LEAN Reprt Page 36 f Prjects This sectin prvides a brief cmmentary n varius technical lsses reductin prjects undertaken in GB and arund the wrld Distributin Netwrk Lsses Management Western Pwer Distributin, UK This is an IFI funded study, led by Western Pwer Distributin (WPD) and undertaken by Imperial Cllege and Shn Assciates, that has examined the pprtunities fr mving further twards lssinclusive netwrk peratin and design; key prject details, findings, and recmmendatins are summarised in [6]. The principal aim f the study was t prgress with the cncept f making distributin netwrks as energy efficient as is ecnmically pssible, raising awareness f the relevant issues and prpsing slutins which take mre accunt f lsses in netwrk design. The study, cmpleted in 2014, has accunted fr the fllwing aspects affecting netwrk technical lsses: Netwrk mdelling and analysis. Pwer factr and lsses. Phase imbalance. Impact f nn-diversified lading in lss calculatins. Impact f peak demand reductin n netwrk lsses. Vltage cntrl driven lad reductin. Impact f enhancing netwrk utilisatin n netwrk lsses. Principal cnclusins drawn in the study included the fllwing: Netwrk lsses shuld be included within netwrk design plicy and that netwrk energy efficiency shuld rank alngside safety and security f supply in the bjectives f verall netwrk management. There are several ecnmically viable interventins n existing netwrks. Tgether with new design plicies, these will enable a mve t higher netwrk energy efficiency. There is evidence f technical and practical slutins t harvest and use heat generated by electrical lss in rder t imprve verall energy efficiency f running the netwrk. Hwever, mre wrk is required if the develpments are t be deplyed in thse cases where there may be a match between heat demand and heat generatin frm the netwrk. Furthermre, the study in [6] anticipates that DNOs may develp a lng-term plan t discver new knwledge f netwrk lsses and t develp netwrk plicies, standards and netwrk designs fr future netwrks which may be demnstrably best practice in electrical pwer distributin, and prvides a list f

37 Reprt: LEAN Reprt Page 37 f recmmendatins fr effective management f technical lsses; the latter are reprduced here frm [6] fr cmpleteness f this dcument. 1. The netwrk mdelling and analysis tls used in the study are based n calibrated representative netwrk mdels data. Given the increasing imprtance f lsses, it wuld be apprpriate that DNOs establish the capability f mdelling and evaluating lss perfrmance f their present and future netwrks, under different future develpment scenaris. 2. DNOs t cnsider carrying ut mre systematic data gathering assciated with pwer factr t assess the materiality f the issue and t enhance the understanding f the csts and benefits f pwer factr crrectin at cnsumers premises. The business case fr pwer factr crrectin may then be develped. 3. Further wrk is required t assess the extent f the imbalance prblem and t test varius slutins, which will nt nly reduce lsses but deliver many ther benefits f a well-balanced netwrk. It may be apprpriate t develp plicies and wrking practices fr aviding excessive imbalance in future. 4. The inaccuracy f lss calculatin using half-hurly data at the edges f the LV netwrk shuld be recgnised when cnducting netwrk studies 5. As the benefits f peak demand reductin may be material an assessment f the pprtunities enabled by alternative smart grid techniques t achieve this shuld be carried ut. 6. As the benefits f active vltage cntrl in LV distributin netwrk may be significant, cmprehensive assessment f the pprtunities t further reduce netwrk lsses shuld be carried ut. 7. When cnsidering active netwrk management slutins and technlgies t facilitate lw-carbn cnnectins, the impact n lsses shuld be given full cnsideratin. 8. There is a clear case fr fundamentally reviewing cable and verhead line ratings t ensure that future lss csting has been included in the ecnmic rating calculatin. This culd be based n OFGEM s lss investment guidelines r n lss-inclusive netwrk design standards. 9. The transfrmer lss calculatins indicate that the benefits f investing in lw-lss transfrmers may be significant and this shuld be cnsidered further t establish r therwise the lw-lss transfrmer business case in line with UK energy and carbn plicy. 10. In future, lsses may drive early asset replacement when ecnmically efficient. If early replacement prgrammes are ecnmically justified and capable f being funded, apprpriate resurces wuld need t be made available t facilitate delivery f such prgrammes.

38 Reprt: LEAN Reprt Page 38 f Netwrk designers may cnsider the ptin f installing additinal distributin transfrmers t minimise LV netwrk reinfrcement cst and reduce lsses. 12. In the light f future develpments, particularly in relatin t the integratin f lw carbn demand and generatin technlgies, it may be apprpriate t recnsider lng-term distributin netwrk design. This may take a strategic view f future vltage levels and include cnsideratin f lsses in the decisin-making. 13. In rder t reduce lsses and prvide future flexibility within LV netwrks, LV tapering plicy may be re-examined. 14. A review f DNOs netwrk mdelling and analysis tls and capabilities may be required t supprt design engineers in applying new plicies and prcesses relating t lss-inclusive netwrk design. 15. There is pprtunity fr cnsiderable further learning in Eurpe and als frm Natinal Grid. It wuld be beneficial t share experiences f waste heat recvery installatins amng DNOs. 16. An Innvatin Prject, based upn learning frm this initial Study, may be initiated in rder t gather further insight int the technical and practical slutins which can be tested at mre sites. The Prject culd be scped t als tackle the regulatry and cmmercial market structural issues which will als need t be vercme t bring heat recvery and use int mainstream applicatin. 17. DNOs may maintain an awareness f the ptential fr heat recvery when planning the installatin f EHV transfrmers and seek t install mre systems where the recvered heat may be f cmmercial use. 18. Further wrk n heat strage may be integrated with future trials wrk n recvery f heat frm the distributin netwrk, as it may imprve the ecnmics f mre basic heat recvery systems. 19. DNOs shuld develp lss-inclusive netwrk design strategies, based n their specific data, in rder t ensure that the verall ecnmic netwrk peratin and design criteria are met. This shuld include netwrk mdelling capability fr answering what-if questins in rder t predict the impact f prpsed netwrk plices, prjects and netwrk demand frecasts n the verall reprted netwrk lsses. 20. DNOs, with supprt frm DECC and OFGEM, may determine the cmmn basis in relatin t lss mitigatin and lss-inclusive netwrk design and investment. 21. There is a need t establish the basis fr assumptins n future electricity csts and carbn prices that wuld be used in lss-inclusive netwrk investment that is cnsistent with the verall UK lw carbn plicy.

39 Reprt: LEAN Reprt Page 39 f Early in the RIIO-ED1 perid, DNOs may develp mre accurate means f measuring and reprting f distributin netwrk lsses. 23. The DECC/OFGEM cmparisn f reprted lsses shws a discrepancy which may cause a distrted view f GB DNO lsses, within industry, gvernment and internatinally. 24. DNOs may grasp pprtunities as they may arise t influence lss reprting in ther cuntries and as it is presented in internatinal studies. This is in rder t ensure that GB DNOs lss management perfrmance is presented accurately. 25. Industry, gvernment and regulatrs shuld cnsider develping apprpriate regulatry and cmmercial framewrks that wuld facilitate develpment f lss-generated heat schemes where ecnmically justified. 26. DNOs lss strategies may be stress tested t demnstrate that they can deliver an bjective f achieving an ecnmic level f lsses based upn avided lss valuatin, engineering csts and future netwrk demands Isle f Wight Netwrk Lsses Reductin Study ( ) SSEPD, UK SSEPD cmmissined S&C, in partnership with LIG and The University f Bath, t undertake an IFI funded desktp based detailed technical feasibility and cst benefit analysis f ptential netwrk interventins t reduce the Isle f Wight s 11kV distributin netwrk electrical lsses; the study was cmpleted in A cmprehensive set f netwrk interventins were cnsidered as listed belw and were investigated in detail; netwrk lss reductin perfrmance fr each interventin and assciated cstbenefits were benchmarked against the existing business as usual netwrk. Further details f this study can be fund in [47] and [59]. Netwrk autmatic recnfiguratin. Meshed netwrk peratin. Transfrmer autmatic switching. Incrprating energy strage. Cnservatin vltage reductin. Netwrk vltage upgrade. A detailed mdel f the island s 11kV netwrk frm 33/11kV primary transfrmers dwn t 11 kv/lv secndary transfrmers and a high level mdel f its 33kV and 132kV netwrks maintain system fault levels was develped in DIgSILENT PwerFactry based n detailed SEPD data. Annual half-hurly 11kV feeder data fr the year 2012, extracted frm SEPD s PI data histrian system, was mdelled and distributed in the netwrk using the feeder scaling tl. T reduce cmputatinal burden, the study

40 Reprt: LEAN Reprt Page 40 f 107 selected 8 representative days (weekday and weekend fr each f fur seasns) using relevant meterlgical degree-day data t grup data int seasns. Study results were then extraplated t represent the annual demand ver In assessing the technical perfrmance f the cnsidered cases, detailed time dependent simulatins were undertaken in DIgSILENT PwerFactry sftware t establish the Isle f Wight 11 kv netwrk electrical lsses, vltage prfile, equipment thermal lading, and shrt-circuit levels. A high-level expert pinin based assessment was als undertaken fr each case t qualitatively establish the netwrks vltage step-change, reliability and prtectin perfrmance and identify ptential related peratinal risks and cnstraints. The cst-benefit analysis fr each cnsidered interventin was undertaken using OFGEM s ROI methdlgy, capturing the fllwing cst-benefit metrics: capital investment, avidable DNO csts, nn-dno benefits, scietal benefits, net (and cumulative) benefits, etc. Principal lessns learnt frm this study include the fllwing: Study technical findings: Netwrk interventins with significant electrical lss and carbn savings frm the business as usual netwrk and thse indicating a psitive Return n Investment were identified as fllwing: transfrmer Aut Stp Start with Alternative Netwrk Tplgy, transfrmer Aut Stp Start acting alne, and cnservative vltage reductin (assuming a unity vltage lad dependency). The greatest electrical lsses reductin was fund t be achieved when the 11 kv netwrk is upgraded t 22 kv, but the verall cst wuld likely prhibit this apprach. It may be apprpriate, hwever, t cnsider a rural vltage upgrade, s as t avid majrity f csts due t undergrund cable replacement required as part f this interventin. It may als be apprpriate t cnsider if rural 11 kv verhead feeders may be cst effectively upgraded t 33 kv, especially where reinfrcement is necessary fr embedded generatin cnnectins. Additin f embedded generatin cnnecting at Isle f Wight 11 kv netwrk had the fllwing impact: Embedded generatin may increase ttal 11 kv netwrk electrical lsses; hwever, system wide electrical lsses (400 kv t LV) may reduce. The transfrmer Aut Stp-Start with Alternative Netwrk Tplgy interventin remained as the mst cst-effective slutin, with similar payback perids.

41 Reprt: LEAN Reprt Page 41 f 107 Cst-benefit assessment findings: Several interventins that indicate a psitive Return n Investment ver reasnable timescales were identified. In additin t their suitability fr practical demnstratins as netwrk lss reductin innvatins, n significant barriers t their deplyment were identified. Alternative Netwrk Tplgy (r meshed netwrk peratin) was fund t give the quickest return n investment, althugh the expected return n investment and sensitivity t higher netwrk lsses grwth rates is lw. Hwever, in cmparisn the Aut Stp-Start with Alternative Netwrk Tplgy interventin is expected t give a greater return n investment and imprved sensitivity t higher netwrk lsses grwth rates. Amng the cnsidered interventins, the transfrmer Aut Stp-Start acting alne, r its cmbinatin with Alternative Netwrk Tplgy case, is fund t be the ptimal interventin slutin (accunting fr bth netwrk technical and cst-benefit metrics ffered by these interventins) in reducing Isle f Wight s netwrk lsses Capacity t Custmers (C 2 C) Prject Electricity Nrth West Ltd., UK C 2 C, a 10 millin prject led by Electricity Nrth West Ltd. (ENW) in the UK, was funded via OFGEM s LCNF Tier-2 mechanism. The C 2 C prject had the fllwing listed brad aims, and was authrised t cmmence in January 2012 and is nw cmplete [104]. Release previusly untapped emergency netwrk capacity fr everyday use. Enable cnsumers t make savings by changing the way yu use electricity. Prevent huge infrastructure imprvement csts being passed n t custmers. Deliver vital benefits t the regin and t the whle f the UK. The technical aims [70], included: t test new technlgy, netwrk peratinal practices (i.e., perating with clsed distributin rings), and cmmercial demand respnse cntracts that will allw Electricity Nrth West t increase the ladings n a selectin f trial circuits representing apprximately 10% f its 6.6/11 kv netwrk withut resrting t cnventinal netwrk reinfrcement. In ther wrds, the prject will release inherent spare capacity in the 6.6/11 kv system in rder t accmmdate the future frecast increases in demand, whilst aviding (r deferring) the cst and envirnmental impacts that are assciated with traditinal netwrk reinfrcement. As part f the C 2 C prject, the study in [70] has analysed the level f electrical lsses in meshed distributin system was carried ut at the University f Strathclyde; the study mdelled the lsses assciated with 34 f the circuits that have been selected fr the C 2 C trial and results were later presented in a white paper. The study later cncluded that netwrk lsses are generally reduced, but the gains are marginal.

42 Reprt: LEAN Reprt Page 42 f Smart Urban LV Netwrk UK Pwer Netwrks, UK This is a 2.1 millin IFI and Tier-1funded UKPN led prject (in cllabratin with TE Cnnectivity Ltd.) that has develped a new slid-state switching technlgy fr use n LV distributin netwrks; the prject was cmmenced in July 2012 and was cmpleted in March Accrding, the prject s verview dcument in [105], the devices develped [as part f this prject] retrfit t existing LV plant, and the system prvides previusly unavailable remte switching, visibility and recnfiguratin f the LV netwrk. This prject is carrying ut a large-scale trial f the technlgy in tw areas f the Lndn LV netwrk. Secndary substatins have been equipped with CBs and link bxes with switches r demand mnitring devices. The ptential fr helping netwrk peratrs address the challenges faced with the transitin t a lw carbn ecnmy will be investigated. Key findings identified as part f the prject included the fllwing: Remte netwrk recnfiguratin and lad transfer has been demnstrated n UKPN s LV netwrk using prttype devices. Training has been delivered t cmmissining engineers and peratinal staff. A fully interactive Pwer n Fusin LV cntrl diagram has been develped fr the trial area. The LV remte cntrl and autmatin devices have been integrated with the LV Cntrl system, and perated remtely during testing. Key benefits expected frm implementatin technlgies studies and demnstrated as part f this prject, accrding t [106], include the fllwing: Existing LV plant t be utilised mre effectively. High lading issues t be addressed if transfer f lad is pssible. Imprvements in quality f supply perfrmance. Reductin in electrical netwrk lsses. LV netwrk reinfrcement t be better targeted r deferred Reactive Pwer Dispatch Using Distributed Generatin UK Pwer Netwrks, UK Accrding t UKPN s Innvatin Funding Incentive (IFI) r LCNF reprt in [106], The Pwer Netwrks Research Academy (PNRA) has been established thrugh a strategic partnership agreement between the Engineering and Physical Sciences Research Cuncil (EPSRC), electricity transmissin and distributin cmpanies, and related manufacturers and cnsultants. The Academy funds supprt PhD researchers in pwer-industry related prjects and helps maintain and imprve research and teaching capacity in pwer engineering subjects. As part f this PNRA partnership, the impact f reactive pwer

43 Reprt: LEAN Reprt Page 43 f 107 dispatch using DG n netwrk vltages and their effective management is currently being studied and investigated. Reference [106] als reprts that Prgress cntinues t indicate that bth pwer system lsses and vltage prfiles can be enhanced with the invlvement f DG, as perfrmed n the IEEE 30 bus system. As such, it is believed that the expected benefits f the prject can be realised Distributin System Lsses Reductin Mazn Electricity Cmpany, Oman A 2012 article in Wrld Academy f Science, Engineering and Technlgy, jurnal in [103] summarises a set f technical lsses reductin studies undertake n Oman s Mazn Electricity Cmpany (MZEC) distributin netwrk feeders; key excerpts frm this article, including findings, are presented belw. At MZEC, 29 substatins and 28 feeders were fund t be nn-cmpliant with the Distributin System Security Standard (DSSS). T address this, 33 prjects were prpsed. In the study, the largest part f MZEC s netwrk (Suth Batinah regin) was mdelled in ETAP sftware package; the mdel was extended t implement the prpsed prjects and their impact n netwrk technical lsses was investigated. The prpsed 33 prjects included: Installing parallel feeders fr verladed feeders accrding t the apprved security standard. Intrducing transfrmer(s) fr verladed substatin. Installing mre capacitr banks at the primary feeder which will help in bsting the vltage and imprving the pwer factr as well as reducing the reactive pwer lsses. Intrducing mre links between the feeders t facilitate lad sharing. Results reveal that there was a little decrease in the active pwer but the reactive pwer decreases cnsiderably. Using the lad factr f MZEC s netwrk, the average lsses were estimated fr the full year and then the cst f lsses in bth scenaris was calculated. Thereafter, the saving frm the lsses reductin was estimated fr the cming 4 years with the aim f studying the cst benefit f the prpsed prjects. Arund US$ 29 millin as net present value was estimated t be gained frm the prpsed prjects, which represents a payback perid f 17 years. Finally, the ecnmic analysis has revealed that the implementatin f the prpsed prjects in MZEC leads t an annual saving f abut US$ 5 millin Distributin System Lsses Reductin Electricité du Las, Las Electricite du Las (EDL), the state enterprise cmpany, is respnsible fr transmissin and distributin f electricity in La Peple's Demcratic Republic (La PDR). Accrding t the prject prpsal in [107] submitted t the Asian Develpment Bank in 1997, the Vientiane lad n the EDL s 22 kv netwrk was knwn t have lw pwer factr issues, which had cntributed t verall system lsses.

44 Reprt: LEAN Reprt Page 44 f 107 A desktp simulatin based study undertaken in [48] has assessed the level f technical lsses reductin achievable in EDL s Vientiane MV distributin netwrk in Las. This study cnsidered pwer factr crrectin by fixed/switched capacitrs r capacitrs placement and netwrk recnfiguratin r switching ptimizatin; the impact f these netwrk interventins n lsses reductin was cnsidered individually and then aggregated algebraically. Hwever, the impact f lsses reductin with cmbinatin f cnsidered interventins was nt investigated. Amng 9 substatins and 47 feeders assessed in this study, the ttal instantaneus technical lsses were fund t be 8.1 MW at peak lad. When the level f technical lsses reductin was cnsidered individually, the study fund that the capacitr placement may reduce technical lsses by 2.5 MW r 21.5 GWh with a payback perid f 0.7 years, while the netwrk ptimizatin has shwn t reduce by 1.4 MW r 12.1 GWh with a payback perid f 0.3 years. With cnsidered interventins cmbined, the study suggests a ttal reductin in technical lsses by 3.9 MW r 47.38% f the ttal instantaneus technical lsses figure in savings Cnclusins and Lessns Learnt With the grwing need fr lwer electrical lsses, driven by carbn reductin targets, rising electricity prices, and regulatry bligatins, traditinal DNO methds will have t be cmplimented by alternative methds that are able t deliver significant lss reductins while prviding return n investment within reasnable time scales. The lessns learnt frm this review wrk include the fllwing: T&D lsses estimatin is a difficult exercise t establish a reasnably accurate verall figure fr technical lsses in GB at bth DNO level and natinally, the cmparisn with lsses f ther cuntries is even mre prne t errr due t different bases fr assessment. Several factrs affect the validity f cmparisn f electrical lsses perfrmance; therefre, due care needs t applied when cmparing them acrss natinal r glbal electrical netwrk systems. Arund the wrld, n average, arund 9% f ttal electrical energy that enters the T&D system is lst befre it reaches end cnsumers. In GB, distributin electrical lsses represent abut 6% f the ttal energy supplied t cnsumers; these lsses represent an aggregate value f arund 1 billin a year. The transmissin and distributin electrical lsses in the UK accunt fr abut 1.5% f all greenhuse gas emissins. The breakdwn f technical lsses vary significantly with netwrk tplgy and peratin, netwrk vltages, asset management and prcurement plicy, level and types f netwrk lad and generatin, gegraphical area, climatic and seasnal cnditins, cnsumer use behaviur, lsses and breakdwn calculatin methdlgy, netwrk peratr/wner, etc.

45 Reprt: LEAN Reprt Page 45 f 107 The need fr reductin f technical lsses is in respnse t the grwing glbal cncern arund climate change, and grwing energy csts that utilities and custmers are frced t grapple with. Acrss the EU, including in the UK, there are currently energy efficiency targets t be achieved; in the UK, these are distributed acrss varius industrial sectrs, including the pwer sectr. In GB area, OFGEM requires DNOs t ensure that lsses n their netwrks are as lw as reasnably practicable, and t maintain and act in accrdance with their respective published lsses strategies. A brad set f netwrk lsses reductin methds and pprtunities were reviewed; lessns learnt and applicability f sme f these methds was evaluated using a sectin f SEPD s wn netwrk. SSEPD s wn studies have shwn that a dynamic tie pen-pint ptimisatin scheme, when slely applied t reduce netwrk lsses, may be expensive and may have limited return n investment. Hwever, studies elsewhere shw that when nn-dynamic penpint ptimisatin is emplyed, 1.4 MW (i.e.17% f ttal technical lsses at peak lad) f instantaneus technical lsses savings r 12.1 GWh f annual lsses savings can be achieved with a payback perid f 0.3 years. Althugh, amng studies and tests undertaken in the USA and ther places have shwn reductin in lsses using CVR implementatin up t 1% t 3%, practical studies t cnfirm the apprpriate CVR factrs fr SEPD/GB custmer and lad grups are required t quantify the benefits ffered by the CVR methd in GB. Depending n the day-in day-ut r 'critical peaks' based DSR implementatin, the reductins culd vary between frm 0% t 22% fr peak demand and frm 5% t 38% fr critical peaks respectively; hwever, there are several challenges assciated with the DSR implementatin, which are being researched and trialled acrss the wrld. SSEPD s wn studies have indicates that abut 9% reductin in verall 11 kv netwrk lsses (including 33/11 kv primary and secndary LV transfrmer lsses) culd be achieved thrugh the use f TASS scheme. Hwever, there is n evidence f similar schemes applied r trialled in GB r glbally. Althugh, the meshed netwrk peratin is implemented primarily fr the purpse f increase netwrk reliability and availability, reductin in lsses are achieved as a secndary benefit. SSEPD s wn studies have indicated that abut 1.7% reductin verall 11 kv netwrk lsses (including 33/11 kv primary and secndary LV transfrmer lsses) culd be achieved thrugh the use f netwrk meshed scheme, with a payback perid f abut 4 years.

46 Reprt: LEAN Reprt Page 46 f 107 EDL s netwrk MV lad in La was knwn t have pr factr. Studies undertaken n EDL s netwrk in Las have shwn that the ptimal placement f capacitrs can reduce instantaneus technical lsses by 2.5 MW (i.e. 30.4% f ttal technical lsses at peak lad) r 21.5 GWh with a payback perid f 0.7 years. SSEPD s wn netwrk lsses reductin study has shwn that battery based energy strage purely frm an electrical lsses reductin pint may nt be cst effective; hwever, this result may change with expected future reductins in per kwh cst and increase in battery density. Inclusin f DG in distributin netwrk can either imprve r wrsen electrical lsses; the level f electrical lsses als depend n several factrs, such as its prximity t lad, level f cnsumptin, DG technlgy, DG daily and seasnal generatin patterns, DG penetratin dispersin level in the netwrk, spillage f excess generatin t ther netwrk vltage levels, netwrk circuit cnductr sizes and selectin criteria, level f DG s cntributin and participatin in the external netwrk reactive pwer cmpensatin, etc. With renewed fcus n energy efficiency at bth UK natinal and EU level, electrical netwrk lsses reductin pprtunity, as an additinal desired netwrk peratinal cnstraint, within ANM methdlgy is currently being explred. Studies have shwn that the applicatin f advanced active management techniques that wuld maximise the utilisatin f existing netwrks may increase lsses in the lcal netwrk very significantly; hwever, the increase in lsses may be efficient when traded against the facilitatin f lw-carbn generatin cnnectins and avided netwrk reinfrcements. SSEPD s wn netwrk lsses reductin study has als shwn that while reinfrcement f existing netwrks ffers significant reductin in lsses, the upfrnt high capital csts utweigh any cumulative benefits. A number f netwrk lsses reductin prjects bth in GB and glbally were als reviewed and the lessns learnt frm these prjects are detailed belw. Netwrk lsses shuld be included within netwrk design plicy and that netwrk energy efficiency shuld rank alngside safety and security f supply in the bjectives f verall netwrk management. There are several ecnmically viable interventins n existing netwrks which, tgether with new design plicies, which will enable the mve t higher netwrk energy efficiency. There is evidence f technical and practical slutins t harvest and use waste heat generated by electrical lsses in rder t imprve verall energy efficiency f running the netwrk. Hwever, mre wrk is required if the develpments are t be deplyed in thse

47 Reprt: LEAN Reprt Page 47 f 107 cases where there may be a match between heat demand and heat generatin frm the netwrk. DNOs may develp a lng-term plan t discver new knwledge f netwrk lsses and t develp netwrk plicies, standards and netwrk designs fr future netwrks which may be demnstrably best practice in electrical pwer distributin. The TASS methd acting alne, r its cmbinatin with Alternative Netwrk Tplgy methd, was fund t be the ptimal interventin slutin (accunting fr bth netwrk technical and cst-benefit metrics ffered by these interventins) in reducing distributin lsses. The level f lsses reductin achieved using meshed netwrks is expected t be marginal. The level f lsses reductin achieved using LV netwrk recnfiguratin is currently under trials in the UK.

48 Reprt: LEAN Reprt Page 48 f TRANSFORMER AUTO STOP-START This sectin discusses the general principle f TASS Methd, prvides a review f assciated risks and mitigatin strategies based n existing literature, evaluates sme f the identified risks and mitigatin using simulatin based analysis, and summarises key bservatins fund during the prcess TASS Methd The TASS methd, which is used fr the purpse electrical lsses reductin in a substatin, is described in [1, 53] and was studied in [47, 53, 59]; the methd descriptin detailed in this sub-sectin belw was adpted frm [53]. Transfrmer electrical lsses cnsist f fixed lsses (als called n-lad r irn lsses) and variable lsses (als called lad r cpper lsses). Variable lsses in a transfrmer are prprtinal t the square f applied electrical lad. When lw r n electrical demand exists, fixed lsses can be significant cmpared t variable lsses. a. Substatin Lsses-Lad Curves fr Single and Tw Substatin Transfrmer b. TASS Substatin Implementatin Operated Separately Figure 13 TASS Methd Substatin Arrangement, Switching Lgic, and Lss Savings [53] Fr the purpse f security f supply in distributin netwrk based n ENA s ER P2/6 Security f Supply, mst primary substatins in GB emply a level f redundancy via a pair f transfrmers (usually f the same MVA rating) that are energised and at any given time laded up t a maximum f half their rated MVA capacity. A substatin with a single transfrmer has lw fixed lsses at lw substatin lads and has high variable lsses at high substatin lads, and cnversely, a substatin with tw transfrmers will have high fixed lsses at lw substatin lads and lw variable lsses at high substatin lads; this

49 Reprt: LEAN Reprt Page 49 f 107 behaviur, is illustrated in Figure 13a. The pint at which the lad-lsses curves fr these different perating regimes intersect is defined here as the Crssver-Pint. A high level hardware implementatin f TASS scheme at a 33/11 kv substatin is utlined in Figure 13b: tw breakers are required: ne n the HV side f the transfrmer selected fr de-energising (typically the ne with highest verall lsses amng the substatin pair), and anther between substatin transfrmers HV terminals. During nrmal peratin, the bus-sectin breaker and HV breaker n the transfrmer t be switched may be interlcked, such that at given time if a breaker amng the tw is pened then the ther is clsed, and vice-versa. During transitin f transfrmer energisatin state a shrt time delay may be applied between each breaker peratin where this is required t avid r amelirate any transients. As part f TASS, ne f a transfrmer pair is de-energised when the applied substatin electrical lad falls belw the Crssver-Pint value and vice-versa when the substatin s lad exceeds the Crssver- Pint (plus a dead band value t avid any hunting ). This may leads t transfrmer switching several times during the day depending n varying substatin lad; studies cnducted in [47, 53, 59] suggest, the number f substatin transfrmer switching peratins t a typical substatin lad culd be up t 3 times a day, averaged annually. Figure 14 Annual Substatin TASS Switching Operatins fr Varius Lad-DG Scenaris and Selected TASS Switching Hysteresis Bands (Slid bars fr ±2.5%, Checked fr ±5%, and Hrizntal fr ±7.5%) [53]

50 Reprt: LEAN Reprt Page 50 f Risks There are several risks assciated with implementing and successful peratin f TASS Methd; a review f sme f the key risks assciated with the TASS Methd is presented in subsectins belw Inrush Currents The transfrmer cre may becme saturated due t an abrupt change in the vltage applied t it. This may be caused by switching transients, ut-f-phase synchrnizatin f a generatr, external faults and fault clearance, and transfrmer s wn energisatin [108]. When saturated, a transfrmer absrbs a magnetizatin current, als knwn as inrush current, which can reach several times the nminal current f the transfrmer [108]. Transfrmer energisatin frm the netwrk by uncntrlled and randm clsing f a switching device prduces inrush currents that can have undesirable r unintended cnsequences; these inrush currents, accrding t [109], can cause adverse impacts n transfrmer itself (lss-f-life, mechanical damage t transfrmer winding) and pwer system peratin (reduced pwer quality, mis-peratin [r malperatin] f prtectin devices and temprary vervltages), and additinally, accrdingly t [108], can create unusual prblems such as pulsating lw frequency electrmagnetic trques in generatrs r large mtrs that are remte frm the energizing bus, causing mechanical vibratins, shifting f windings and slippage f shaft cuplings. Inrush currents and their severity are a functin f transfrmer design (e.g. a delta-star transfrmer may have higher inrush current during energisatin than a star-star cnnected transfrmer fr the same MVA rating [110], etc.), initial cnditins, and netwrk factrs [108, 109]. Transfrmer energisatin related inrush currents can als interact with the netwrk prtectin schemes. When the prtectin schemes are prly set/designed r may cnsist f devices that cannt discriminate between netwrk fault currents and transfrmer energistin related inrush currents, the prtectin schemes can cause nuisance tripping f netwrk circuits r equipment and therefre disruptin t custmers. This phenmenn has been extensively studied and tested [111], several mitigatin slutins are prpsed [112, 113], and mst mdern relays tday incrprate inrush discriminatin schemes against fault currents [ ] Harmnics Due t nn-linearity f the transfrmer s saturatin, the resultant inrush currents, in additin t fundamental frequency current, als cntain harmnic and DC ffset currents; this phenmenn is widely studied and cnfirmed in several field tests [108, 117]. The harmnic currents prduced during the transfrmer energisatin prcess can interact with the external netwrk impedances and cause vltage harmnics (and therefre vltage distrtin) in the cnnected netwrk; these vltage harmnics can then, in additin, prpagate t ther vltage levels (upstream r dwnstream), depending n crrespnding vltage level netwrk impedances (including resistances, capacitances, etc.), netwrk bundary

51 Reprt: LEAN Reprt Page 51 f 107 intercnnecting transfrmer winding ratis and impedances. Althugh, the energisatin prcess f a typical primary transfrmer is likely t be a mmentary event f up t tens f secnds duratin, in the presence f netwrk impedance resnances with significant magnitudes (caused due t matching f netwrk inductance and capacitance at resnant frequencies and lw netwrk resistance), significant vervltages can ccur, which may cause equipment insulatin failures, capacitr bank ver-current relay malperatin, nuisance tripping f vltage sensitive equipment, etc. [108, 118]. Factrs that can cntribute t a higher level f resnance vervltage are listed belw [119]: Higher rating f the transfrmer t be energised. Lwer value f surce fault level. Lnger circuit length. Smaller amunt f lad in the system. Higher than nrmal system perating vltage. Higher designed wrking flux density f the transfrmer (lwer knee pint). Psitin f the tap resulting in lwer turns rati Overvltages Interruptin f magnetising current f an un-laded transfrmer may lead t switching vervltages. CIGRÉ in [120] has published (in 1995) a guidance dcument n interrupting small inductive currents; this guidance dcument cvers interactins, especially the transient behaviur, between the un-laded inductive equipment (such as, reactrs, transfrmers, mtrs, etc.), series switch r CB, and external netwrk. The dcument states the peratinal experience shws that switching f unladed transfrmers nrmally causes n prblems irrespective f the switching device and that the published literature material indicates that the vervltage level is lw irrespective f the type f switch, transfrmer type and netwrk arrangements. In additin, the dcument als adds that fr medium vltage breakers smewhat higher vervltages are ccasinally recrded as expected but with average values generally belw 2.5 p.u [and in sme rare cases up t just belw 4 p.u]. Medium vltage switches will typically prduce lwer vervltages, perhaps n the rder f 1.5 p.u. r less. Althugh, the expected results will be dependent up n the type f switch emplyed, generally speaking, medium vltage switches d nt chp currents t the degree as circuit-breakers. And, even if sme chpping f current des ccur, the vervltages t be expected will be lw. Hwever, the impact f repetitive transfrmer switching (as expected with the implementatin f LEAN Methd), and resultant vervltages (even at 1.5 p.u t up t just belw 2.5 p.u), n the external netwrk and transfrmer s health and lifetime was nt discussed in this dcument.

52 Reprt: LEAN Reprt Page 52 f Sympathetic Interactins Sympathetic interactin can ccur when a transfrmer r shunt reactr is energized nt a system with lng transmissin lines in the presence f ther electrically clse and energized transfrmers r shunt reactrs [108]; this phenmenn can significantly change the duratin and the magnitude f the transient magnetizing currents in the transfrmers invlved during the energisatin prcess. Depending n the netwrk n HV s side f the transfrmer (t be energised) is cnnected and hw the netwrk n the LV side f transfrmer is t be energised, the sympathetic interactin phenmenn can be parallel r series; example circuits where these interactins are likely t ccur are shwn in Figure 15. a. Parallel Sympathetic Interactin b. Series Sympathetic Interactin Figure 15 Sympathetic Interactin Phenmenn [108] In the case f TASS Methd, where a primary transfrmer (unladed) will be switched n t an already energised parallel primary transfrmer, the parallel sympathetic interactins are mre likely t ccur. The substatin s LV side lad transfer between substatin primary transfrmers, where TASS Methd will be implemented, is expected t happen either befre r after transfrmer energisatin prcess, the likelihd f series sympathetic interactins may be limited; this is prvided if the lad transfer prcess itself will cause n abrupt and/r significant change in the substatin s LV side vltages. Frm a transfrmer prtectin pint f view, mst cnventinal prtectin schemes emply the secnd harmnic restraint apprach fr the purpse f discriminating transfrmer inrush currents against the transfrmer s internal fault currents [110, 121]. A study in [122] that has investigated the impact f sympathetic interactin in transfrmers cncluded that the sympathetic inrush current may nt have a sufficient amunt f the secnd harmnic in it t prevent the relay frm tripping Current Chpping Current chpping is the sudden reductin f the current in circuit breaker t zer at a time ther than the natural instant f current zer [123]. A transfrmer s residual flux [r remnant flux] is influenced by varius factrs, such as cre material characteristics, cre air-gaps, winding leakage capacitance and circuit breaker current chp characteristics [124], and additinally, n the vltage Pint-n-Wave (POW) switching angle.

53 Reprt: LEAN Reprt Page 53 f 107 When the transfrmer s winding capacitance value is very lw, the transfrmer behaves as a purely inductive circuit instead as a parallel LC circuit; a CB s ple n each phase is unable t chp an inductive current, thus the discnnectin is pstpned until the first current zer crssing [125]. In a parallel LC circuit, such as transfrmers with winding capacitances and CB s with stray capacitances, hwever, the current can be chpped since the parallel capacitance prvides an alternative circulatin path fr the current [125]; this results in remnant flux that is nt in phase with the rated flux. The magnetising current f transfrmers, accrding t [126], is ften smaller than the chpping current f the circuit breaker, the current will be interrupted prir t its natural zer crssing. As a cnsequence, the residual flux can reach any value between -1 p.u. and 1 p.u. Because n magnetising curve is able t exceed the maximum magnetising characteristic given by the prperties f the cre material, the residual flux margin will shrink t the range between the tw pints f the maximal residual flux [126]. In real substatins, hwever, the maximal accessible residual flux is further reduced t a value f apprximately 0.9 p.u. due t transients during de-energisatin [126]. As a cnsequence, current chpping may lead t uncertainty in the value f remnant flux in the transfrmer during its de-energisatin, and therefre, may apply sme level f uncertainty n the predictin n inrush currents during the transfrmer s energisatin prcess Health and Lifetime HFDE s reprt, attached t this reprt in Appendix D, details varius TASS Methd related transfrmer health and lifetime impact risks Mitigatin A wide variety f transfrmer inrush current and temprary vervltage mitigatin methds are available tday and the best practices were presented and discussed in CIGRÉ s and IEEE s guidance dcuments (in [108] and [127] respectively) n transfrmer energisatin in pwer systems; these are listed as fllwing: Inrush reductin methds: Cntrlling the switching times f the energizing CB. De-fluxing the transfrmer cre befre energisatin. Installing pre-insertin resistrs in series t the CB energizing the transfrmer r in the neutral winding fr star grunded transfrmers. Adjusting the n-lad tap befre energizing the transfrmer. Reducing the system vltage befre energising the transfrmer. Increasing the system fault level befre energising the transfrmer.

54 Reprt: LEAN Reprt Page 54 f 107 Energizing the transfrmer using air-break discnnect switches. Temprary vervltage reductin methds: Resistr-capacitr snubber circuit. Switching devices equipped with pre-insertin resistrs. De-tuning the parallel resnance in the path frm the netwrk t the transfrmer. Adding as much lad as pssible befre energizing the transfrmer. Selecting a lw impedance path fr energisatin f the transfrmer. HFDE s reprt, attached t this reprt in Appendix D, details varius TASS Methd related transfrmer health and lifetime impact risks and their mitigatin; these include, diagnstics and mnitring fr pwer transfrmers, electrical discharge activity, and managing transfrmer health and life Cntrlled Switching Fr existing n-site three-phase transfrmers, where the LEAN TASS methd may be applied, mdificatin f their cnstructinal features (such as saturatin characteristics) r the external netwrk cnditins fr the purpse f transfrmer inrush mitigatin may be difficult r impracticable t effect. Hwever, transfrmer energisatin related current inrush reductin r mitigatin using cntrlled switching has been extensively studied in [124, 126, 128] and was demnstrated using ABB s Switchsync T183 TM related field tests in [129]. Figure 16 - ABB s Switchsync T183TM Applicatin Schematic Fr N-Lad Transfrmer Inrush Current Mitigatin and Switching [129, 130]

55 Reprt: LEAN Reprt Page 55 f Cmplete Inrush Reductin r Eliminatin Strategy Transfrmers are generally energized by randm clsing f the circuit breaker cntacts, with the system vltage being applied n the transfrmer windings at randm instants, [129]. Reference [126] cncludes that If a transfrmer is energised at a randm instant, it is pssible that n transient inrush current will ccur; but mstly transient inrush currents will arise. This happens because transient inrush currents depend nt nly n the instant f energisatin, but als n the residual flux f the previus deenergisatin. a. Cre fluxes after Phase C is first switched ON b. Optimal instants fr clsing Phases A and B Figure 17 Transfrmer Inrush Mitigatin Strategy [126] The basic principle fr eliminating the magnetic ver flux, r the asymmetrical flux appearing in the transfrmer cre during its energisatin, is t ensure that the residual flux is equal t the presumable (r prspective) flux, [126]. Accrding t [126] again, a three-phase cre type transfrmer has inherent interactin amng the phase fluxes. In this type f transfrmer, after the first phase (r phases) energisatin new fluxes are established thrughut the pen circuit legs. These are called dynamic fluxes. T reduce transfrmer inrush currents, the ptimal transfrmer switching instances, accrding t [126], are when prspective fluxes and dynamic fluxes are equal r cincide. This is illustrated in Figure 17. The strategy detailed in Figure 17 will require accurate knwledge f transfrmer fluxes fllwing the deenergisatin f a transfrmer; these are typically calculated nline by cntinuusly integrating the transfrmer primary r HV winding terminal vltages. The remnant r residual transfrmer flux is the flux when the de-energised transfrmer terminal vltage scillatins have cmpletely decayed; hwever, the prspective fluxes calculatins (as lng as there is vltage at the supply side f the transfrmer energisatin CB) are cntinued in the backgrund fllwing the transfrmer de-energisatin. This allws fr calculatin f ptimal transfrmer switching instances f time/angles n the vltage wavefrms.

56 Reprt: LEAN Reprt Page 56 f Sensitivities and Tlerances Fr the purpse f transfrmer inrush mitigatin, ideally, each CB switch phase ple shuld clse at the exact calculated instance; hwever, due t uncertainty that exists within the mechanical switching mechanism f a CB, this is nt always achievable in practice. CB manufacturers typically specify a switching time and instance accuracy and tlerances fr their equipment; fr the purpse f transfrmer inrush mitigatin. Accrding t reference [126], a circuit breaker suitable fr cntrlled switching f this transfrmer must have at mst a clsing time-deviatin f 1.15 ms [r 20.7º POW] if n transient inrush currents shuld ccur. Therefre, a CB ple switching time errr tlerance f ±1 ms (r 18º POW) frm the calculated ptimal switching instance may be needed. In additin, there may als be vltage sensr measurement and/r flux calculatin errrs that might impact the verall inrush mitigatin algrithm s effectiveness in reducing inrush current t tlerable levels r eliminatin. Accrding t [126], the residual flux measurement device must have a minimum accuracy f 0.29 p.u. if an ideal circuit breaker is used. The allwable flux calculatin errr tlerances depend n the available headrm frm the calculated Remnant t the flux saturatin value Other Methds In additin t the described inrush current mitigatin strategy in Sectin 3.4.1, additinal strategies are als mentined in [131]; these are listed belw: Rapid Clsing Strategy: This strategy clses ne phase first and the remaining tw phases within a quarter cycle. It requires knwledge f the residual flux in all three phases, independent ple breaker cntrl, and a mdel f the transfrmers transient perfrmance (n studies were run t cmpare transient perfrmance f different transfrmer designs t determine errr frm assuming a standard mdel). Delayed Clsing Strategy: This strategy clses ne phase first and the remaining tw phases after 2 3 cycles. It requires knwledge f the residual flux in ne phase nly, independent ple breaker cntrl, but des nt require any transfrmer parametric data. Simultaneus Clsing Strategy: This strategy clses all three phases tgether at an ptimum pint fr the residual flux pattern. It des nt require independent ple breaker cntrl, but requires knwledge f the residual flux in all three phases and that the residual flux magnitudes in tw phases are high and fllw the mst traditinal residual flux pattern. Amng the abve listed three inrush current mitigatin r reductin strategies, the frmer tw methds will require transfrmer HV side CB with vltage POW and independent ple switching capability (if implemented, these tw cntrlled switching strategies will be part f LEAN prject Optin 3 as detailed in [1]), and the latter methd will require a CB with vltage POW and three-ple (simultaneus) thrw (if implemented, this cntrlled switching strategy will be part f LEAN prject Optin 2 as detailed in [1]).

57 Reprt: LEAN Reprt Page 57 f Analysis The analysis presented in this sectin is limited t an evaluatin f transfrmer energisatin impact n transfrmer electrical parameters such as terminal vltage, inrush currents, and internal fluxes and external netwrk. Other aspects related t transfrmer energisatin impact n its lng-term health, including diagnstics and mnitring, are detailed in HFDE s reprt, which is attached t this reprt in Appendix D. Accrding t the TASS Methd, as a minimum, there will be at least ne Energised Primary Transfrmer (EPT r un-switched primary transfrmer as detailed in Figure 13b) and ne Switched Primary Transfrmer (SPT, as detailed in Figure 13b); the frmer transfrmer, i.e. EPT, will remain energised thrughut the TASS methd, and the latter will be energised r de-energisatin depending n the substatin s varying secndary lad thrugh the day. The impact f SPT s energisatin and de-energisatin n the wider netwrk was investigated using a high level electrical mdel f a prspective substatin; this later mdel was then used t identify ptential risks that may ccur with the implementatin f the TASS Methd. The analysis presented here cnsidered the SPT s de-energisatin behaviur and relevant impact n remnant magnetisatin flux seen within the transfrmer cre t the fllwing aspects: Timing f the CB switching n the vltage AC wavefrm, i.e. the POW. Transfrmer surge capacitance f the SPT/EPT. Fllwing this, the analysis assessed the influence f the fllwing aspects related t SPT s energisatin: Remnant magnetisatin prir t SPT s energisatin. External netwrk fault levels and X/R ratis. Timing f the CB switching n the vltage AC wavefrm. The verall impact f the energisatin events n the external netwrk was assessed by mnitring the substatin s maximum bserved inrush currents and HV/LV side vltages. Principal findings frm these investigatins are detailed in this sectin, including presentatin f test system data, mdelling, assumptins, study methdlgy, test cases, and summary f results Data, Assumptins, and Mdelling Fr the set f investigatins, a simulatin test bed that cnsists f external netwrk mdel (capable f representing arbitrary netwrk fault levels), SPT s HV side energisatin and de-energisatin mechanism (incrprating either a simultaneus three-ple r independent single-ple switch), a typical 33/11 kv primary transfrmer as SPT (with saturatin characteristics) and unladed secndary winding was develped. The test bed was develped in Alternative Transient Prgram (ATP) Electr-Magnetic

58 Reprt: LEAN Reprt Page 58 f 107 Transient Prgram (EMTP) analysis sftware as illustrated in Figure 18a. Figure 18b illustrates the mdel accunting fr EPT and EPT s LV side lad. The fllwing mdelling assumptins were made in the analysis: Fr the purpse f investigatins presented here, it was assumed that the substatin, where TASS is implemented, will be a 33/11 kv primary substatin cmprising dual-redundant identical transfrmers (a SPT and an EPT) with parameters detailed in Table 2. T assess the impact n a wrst case transfrmer inrush current scenari, these transfrmers were assumed t cnsist f delta-star primary t secndary winding cnfiguratin. The analysis presented here nly accunts fr SPT switching aspect at the TASS Crssver- Pint, but des nt accunt fr aspect f SPT s LV side lad sharing frm (when substatin s LV side lad is abve the TASS Crssver-Pint) r transfer t (when substatin s LV side lad is belw the TASS Crssver-Pint) EPT. Fr the purpses f simplicity, the TASS Methd s hysteresis band applied t Crssver- Pint t avid hunting phenmenn was ignred. Based n the transfrmer data cnsidered fr bth SPT and EPT in Table 2, the Crssver-Pint n the EPT lad, where SPT is likely t be switched, was calculated as 56.6%; therefre, the secndary lad n the EPT was assumed t be 56.6% at 0.95 inductive pwer factr. The secndary lad n the SPT was assumed t be unladed (mdelled using a large resistr). The EPT lad was mdelled as a series RL lad; the impact f dwnstream secndary transfrmers n the EPT s perfrmance during SPT s switching peratin was neglected. The equivalent per phase-t-grund capacitance f the SPT/EPT was represented n its HV side, and was calculated (based n infrmatin detailed in [132]) and mdelled as 5.7 nf. The lading level n the EPT was assumed t be MVA at 0.95 inductive pwer factr; this is assumed t be the Crssver-Pint n the EPT lad, where SPT is likely t be switched. The 33 kv SPT s three-phase CB has a three-ple (simultaneus) thrw. The current chpping magnitude f the 33 kv SPT CB was assumed t be greater than the n-lad current f the SPT transfrmer fr all simulated cases. The study des nt cnsider the frequency dependency aspects f lsses within the transfrmer during energisatin events; instead, it represents lsses using typical 50 Hz values. The representatin f the external netwrk cnsists f equivalent fault level impedance cnsisting f resistive and inductive cmpnents and an ideal vltage surce. The equivalent resistance is assumed t be cnstant with frequency. The equivalent inductance is fixed fr all perating cases. Fr the purpse f high level investigatins presented here, it was assumed that the

59 Reprt: LEAN Reprt Page 59 f 107 vltage surce is free frm harmnics and fluctuatins and their impact n SPT and EPT during SPT switching aspect was nt studied. There is n general lading level assumed fr the external 33 kv netwrk. The nly lading present within the mdel is the lading f the EPT at 11 kv. Fr simplicity, the mdel des nt take int accunt phase-cupling effects between the phases f the mdel. This is a significant mdelling task and shuld be undertaken in mre detailed analysis cases. a. External Grid with Series CB and SPT b. External Grid with Series CB, SPT, EPT, and EPT s LV Side Lad Figure 18 TASS ATP Analysis Mdels

60 Reprt: LEAN Reprt Page 60 f 107 Table 1 Cnsidered Substatin s External Fault Levels N: Vl. Line t Line (kv) Fault Level Type Fault Level MVA ka X/R Remarks 1 33 Three-phase This was the default fault level Single-phase used in all studies in the reprt Three-phase Single-phase Three-phase Single-phase Three-phase Single-phase These fault levels were nly used as part f a special case t assess impact f external fault levels Table 2 Cnsidered SPT/EPT Transfrmer Parameters N: Parameter Prvided 1 Vectr grup Dyn11 2 Apparent pwer 15 MVA ONAN r 30 MVA ONAF 3 Vltage 33/11 kv 4 Impedance 25% 5 X/R rati 40 6 N-lad lsses 30 kw 7 Saturatin curve

61 Reprt: LEAN Reprt Page 61 f Methdlgy Investigatins presented here were undertaken in ATP EMTP sftware using the fllwing methdlgy: Step 1: The develped initial simulatin test bed mdel as detailed in Sectin and Figure 18a was adpted. Step 2: T assess the impact f POW de-energisatin f SPT, the SPT was initially energised and the transfrmer three-phase fluxes were allwed t settle and becme balanced; fllwing this, the series CB (a simultaneus three-ple thrw switch) n the HV side f SPT was signalled t pen (and therefre de-energising the SPT) at varius values f substatin s HV POW switching angles n Phase A (arbitrarily selected between the three phases) frm 0º t 360º in steps f 15º. Frm these studies, the vltage POW switching angle that will result in the lwest SPT remnant flux in particular phase (Phase A) was identified and studied. Step 3: T assess the impact f POW energisatin f SPT, the SPT was de-energised t retain a specific remnant flux value (maximum r minimum pssible value) n Phase A; the series CB (a simultaneus three-ple thrw switch) n the HV side f SPT was signalled t clse (and therefre energising the SPT) at varius values f substatin s HV POW switching angles n Phase A (arbitrarily selected between the three phases) frm 0º t 360º in steps f 15º. At each substatin vltage POW switching angle, the maximum remnant flux, inrush currents, and related vltage drp n all three phases f SPT were recrded and pltted. Frm these studies, the vltage POW switching angle that will result in least SPT inrush currents and vltage dip was identified and studied. Step 4: The adpted substatin mdel in Figure 18a was then mdified t include fr the EPT and the ETP lad; this is detailed in Figure 18b. Step 2 and 3 were then repeated n the new adpted mdel in Figure 18b. The fllwing additinal aspects were mnitred: At each substatin POW switching angle, the maximum remnant flux n all three phases f EPT were recrded and pltted. Step 5: In the adpted mdel in Figure 18b, the substatin s external fault level was varied frm 250 MVA t 1000 MVA, in steps f 250 MVA. The remnant flux and inrush current parameters were mnitred fr the best and wrst inrush current POW cases. Step 6: A sensitivity analysis was perfrmed t accunt fr a range f pssible SPT/EPT surge capacitance values that may impact the results. Step 7: The results btained frm the abve battery f studies were analysed, findings were nted and recmmendatins were drawn.

62 Reprt: LEAN Reprt Page 62 f Results and Discussin This sectin details principal results btained frm investigatins detailed in Sectin and prvides a brief discussin. Please nte that unless therwise mentined in the reprt, all vltages (instantaneus, RMS, trends, and vltage POW) in this sectin are line-t-grund vltages (e.g. Phase A refers t in the dcument as Phase A line-t-grund); the transfrmer fluxes, hwever, are line-t-line fluxes. During nrmal steady state cnditins, in the assumed circuits in Figure 18, the line-t-line vltages lead line-t-grund vltages by 30 º and line-t-line vltages lead line-t-line transfrmer fluxes by 90º Case A Impact f SPT s POW Switching n SPT s and Sub. s Perfrmance The results discussed in this sectin relate t investigatin undertaken using ATP mdel detailed in Figure 18a, which cnsiders an ideal case where substatin (supplied by external grid) has an unladed SPT and has n EPT. This is t investigate the effect f SPT s cntrlled POW switching n SPT s and substatin s perfrmance. Figure 19 initially shws (in Figure 19a and Figure 19b) the impact f SPT s 0º POW (n Phase A) deenergisatin n substatin s 33 kv vltages and SPT s internal fluxes, fllwed by presentatin (in Figure 19c and Figure 19d) f the impact f SPT s de-energisatin at 90º POW (n Phase A). It can be seen that SPT s 0º POW (n Phase A) de-energisatin leads t least amunt f remnant flux in Phase BC cmpared t the case with 90º POW de-energisatin in the same phase (i.e. Phase BC). The inrush currents drawn during the energisatin prcess and therefre their impact grid vltages are a functin f remnant flux prir t transfrmer s energisatin; this behaviur is shwn in Figure 20 and Figure 21. Fr the same POW transfrmer energisatin (e.g. 0º POW n Phase A in Figure 20 and Figure 21), but with different prir POW de-energisatin (e.g. 0º POW n Phase A in Figure 20 and 90º POW in Figure 21), the resultant substatin s maximum inrush current peak magnitudes are expected t be different (e.g. arund 0.97 ka inrush current in Figure 20 and 5 A in Figure 21). During cntrlled instances f POW transfrmer de-energisatin and energisatin, significant levels f transient vervltages were bserved; these can be seen n instantaneus vltage wavefrms in Figure 19 t Figure 21 as sharp spikes and marked using red circles. Further investigatin may be required t assess the level f impact that these transient vervltages may have n the health and lifetime f substatin transfrmers and ther cnnected equipment t the HV side f the energised transfrmer. Figure 22 initially presents (in Figure 22a) the expected three-phase maximum remnant flux peak trend plts varius instances f POW transfrmer de-energisatin (n Phase A POW); this is fllwed by presentatin f resultant three-phase maximum inrush current trend plts fr varius instances f POW transfrmer energisatin fr minimum (0º POW prir de-energisatin in Figure 22b) and maximum remnant flux (90º POW prir de-energisatin in Figure 22d). The expected substatin retained p.u. RMS

63 Reprt: LEAN Reprt Page 63 f 107 vltage fr inrush current trends in Figure 22b and Figure 22d are shwn in Figure 22c and Figure 22e respectively. The results in Figure 22 shw that the inrush current phenmenn during transfrmer energisatin prcess can be mitigated thrugh precisely cntrlled POW de-energisatin and energisatin prcess; these results als shw there may be a POW energisatin tlerance margin f apprximately ±15º (marked using red text and arrws in Figure 22b and Figure 22d) within which a minimum amunt f transfrmer inrush currents (and therefre maximum retained substatin vltages) culd be achieved. Hwever, further investigatin is needed t understand hw the POW de-energisatin switching errr fr remnant flux minimisatin may affect the allwable POW switching errr fr inrush mitigatin. These findings als cnfirm the general inrush current versus POW switching angle trends detailed in [133]. 40 [kv] Wb-t [s] 2.12 (f ile rm_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW De-energisatin n Phase A [s] 2.12 (f ile rm_0.pl4; x-v ar t) t:fluxab t:fluxbc t:fluxca b. SPT Fluxes at SPT s 0º POW De-energisatin n Phase A 40 [kv] Wb-t [s] 2.12 (f ile rm_90.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC [s] 2.12 (f ile rm_90.pl4; x-v ar t) t:fluxab t:fluxbc t:fluxca c. Substatin Vltages at SPT s 90º POW De-energisatin d. SPT Fluxes at SPT s 90º POW De-energisatin n n Phase A Phase A Figure 19 SPT Flux and Substatin Vltage Wavefrms at SPT s POW De-energisatin (Substatin with Unladed SPT Alne)

64 Reprt: LEAN Reprt Page 64 f [kv] Wb-t [s] 2.63 (f ile rm_0_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW Energisatin n Phase A [s] 2.63 (f ile rm_0_in_0.pl4; x-v ar t) t:fluxab t:fluxbc t:fluxca b. SPT Fluxes Vltages at SPT s 0º POW Energisatin n Phase A 15 [A] p.u [s] 2.63 (f ile rm_0_in_0.pl4; x-v ar t) c:igrida-x0009a c:igridb-x0009b c:igridc-x0009c [s] 2.63 (f ile rm_0_in_0.pl4; x-v ar t) t:vsarms t:vsbrms t:vscrms c. SPT Currents at SPT s 0º POW Energisatin n d. Substatin RMS Vltages at SPT s 0º POW Energisatin n Phase A Phase A Figure 20 SPT Flux and Substatin Currents and Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 0º POW n Phase A)

65 Reprt: LEAN Reprt Page 65 f [kv] Wb-t [s] 2.63 (f ile rm_90_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW Energisatin n Phase A [s] 2.63 (f ile rm_90_in_0.pl4; x-v ar t) t:fluxab t:fluxbc t:fluxca b. SPT Fluxes Vltages at SPT s 0º POW Energisatin n Phase A 800 [A] p.u [s] 2.63 (f ile rm_90_in_0.pl4; x-v ar t) c:igrida-x0009a c:igridb-x0009b c:igridc-x0009c [s] 2.63 (f ile rm_90_in_0.pl4; x-v ar t) t:vsarms t:vsbrms t:vscrms c. SPT Currents at SPT s 0º POW Energisatin n d. Substatin RMS Vltages at SPT s 0º POW Energisatin n Phase A Phase A Figure 21 SPT Flux and Current Wavefrms and Substatin Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 90º POW n Phase A)

66 Reprt: LEAN Reprt Page 66 f 107 a. SPT s Max. Remnant Fluxes with SPT s POW De-energisatin n Phase A POW Tlerance POW Tlerance b. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A c. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A POW Tlerance d. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A e. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A Figure 22 SPT s Remnant Flux, and Substatin s Inrush Current and Vltage Sag Trend Plts fr Multiple SPT s POW De-energisatin and Energisatin Events

67 Reprt: LEAN Reprt Page 67 f Case B Impact f SPT s POW Switching n SPT s, EPT s and Sub. s Perfrmance The results discussed in this sectin r Case B relate t investigatins undertaken using ATP mdel detailed in Figure 18b, which cnsiders a practical case f TASS Methd implemented at a primary substatin (supplied by external grid) with a minimum f tw transfrmers, perating as a dual-redundant pair; the investigatin presented in this sectin, cnsider TASS Methd peratin at the Crssver-Pint, where it cnsist f an unladed SPT and EPT supplying Crssver-Pint RL lad. This is t investigate the effect f SPT s cntrlled POW switching n SPT s, EPT s, and substatin s perfrmance, and in additin, capturing related sympathetic interactins between the SPT and EPT. Figure 23 initially shws (in Figure 23a and Figure 23b) the impact f SPT s 0º POW de-energisatin n substatin s 33 kv vltages and SPT s internal fluxes, fllwed by presentatin (in Figure 23c and Figure 23d) f the impact f SPT s de-energisatin at 90º POW. Similar t Case A, it can be seen that, with inclusin f EPT and its lad, SPT s 0º POW (n Phase A) de-energisatin leads t least amunt f remnant flux in Phase BC cmpared t the case with 90º POW de-energisatin in the same phase (i.e. Phase BC). The inrush currents drawn during the energisatin prcess impact the grid vltages. Bth parameters are a functin f remnant flux prir t transfrmer s energisatin; this behaviur was fund t be repeated with inclusin f EPT and its cnnected RL lad, as detailed in in Figure 24 and Figure 25. Fr the same POW transfrmer energisatin (e.g. 0º in Figure 24 and Figure 25), but with different prir POW deenergisatin (e.g. 0º POW in Figure 24 and 90º POW in Figure 25), the resultant substatin s maximum inrush current peak magnitudes, which are superimpsed n t substatin s lad (EPT s RL lad) current (arund 430 A), are expected t be different (e.g. arund 1.01 ka inrush current in Figure 24 and 430 A in Figure 25). In additin, the SPT s, EPT s and substatin s vltages and currents are impacted by the sympathetic interactins between the SPT and EPT; these are illustrated in Figure 26 and Figure 27, which are the same results as in Figure 24 and Figure 25 respectively, but their respective time axis scales were prlnged t illustrate the sympathetic interactin phenmenn. The magnitude f substatin s inrush current (with r withut sympathetic interactins between the SPT and EPT) is a functin f SPT s prir de-energisatin and energisatin POW angles. A SPT de-energisatin and energistin that will result in the wrst-case substatin inrush currents and vltage sags will result in greater sympathetic interactin and therefre higher than expected inrush currents (cntributed frm bth SPT due t energisatin and frm EPT due t change in terminal vltage). The impact f SPT s de-energisatin (fr the same de-energisatin vltage POW) n substatin vltages and SPT flux with inclusin f EPT and its lad (i.e. Case B) has similar effect as that f substatin with SPT alne (i.e. Case A). Hwever, the magnitude f transient vervltages with EPT and RL lad inclusin was fund t be reduced/damped during SPT s de-energisatin prcess; this can be bserved by cmparing substatin vltages in Figure 19a and Figure 19b against thse in Figure 23a and Figure 23b respectively. Hwever, further investigatin may be required t assess the level f impact that these

68 Reprt: LEAN Reprt Page 68 f 107 transient vervltages may have n the health and lifetime f substatin transfrmers and ther cnnected equipment t the HV side f the energised transfrmer. Figure 28 presents the expected three-phase maximum remnant flux peak trend plts varius instances f POW transfrmer de-energisatin; a trend result that was fund t be similar t substatin with SPT alne (i.e. in Figure 22a). Figure 29 presents resultant three-phase maximum inrush current trend plts fr varius instances f POW transfrmer energisatin fr minimum (0º POW prir de-energisatin in Figure 29a) and maximum remnant flux (90º POW prir de-energisatin in Figure 29d). The expected substatin and EPT s lad side retained p.u. RMS vltage trends fr inrush current trends in Figure 29a and Figure 29d are shwn in Figure 29b, Figure 29c, Figure 29e, and Figure 29f respectively. Cmparisn f substatin inrush current and vltage trends (results in Figure 22b and Figure 22d against thse in Figure 29a and Figure 29d, and in Figure 22c and Figure 22e against thse in Figure 29b and Figure 29e) and their shapes fr varius SPT s prir POW de-energisatin and energistin event fr Case A and Case B vary; this is due t bth intrductin f EPT s lad current (EPT and its cnnected RL lad) and mutual interactins (i.e. sympathetic interactins) between the SPT and EPT. Similar t Case A, where substatin with SPT is present alne, the results in Figure 29 shw that the inrush current phenmenn during SPT energisatin prcess in the presence f EPT and its cnnected RL lad can still be mitigated thrugh precisely cntrlled POW de-energisatin and energisatin prcess; these results als shw there may be a POW energisatin tlerance margin f apprximately ±15º (marked using red text in Figure 29a and Figure 22d) within which a minimum amunt f transfrmer inrush currents (and therefre maximum retained substatin vltages) can be achieved. Hwever, further investigatin is needed t understand hw the POW de-energisatin switching errr fr remnant flux minimisatin may affect the allwable POW switching errr fr inrush mitigatin.

69 Reprt: LEAN Reprt Page 69 f [kv] Wb-t [s] 5.12 (f ile rm_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW De-energisatin n Phase A [s] 5.12 (f ile rm_0.pl4; x-v ar t) t:afsab t:afsbc t:afsca b. SPT Fluxes at SPT s 0º POW De-energisatin n Phase A 40 [kv] Wb-t [s] 5.12 (f ile rm_90.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC c. Substatin Vltages at SPT s 90º POW De-energisatin n Phase A [s] 5.12 (f ile rm_90.pl4; x-v ar t) t:afsab t:afsbc t:afsca d. SPT Fluxes at SPT s 90º POW De-energisatin n Phase A Figure 23 SPT Flux and Substatin Vltage Wavefrms at SPT s POW De-energisatin (Substatin with Unladed SPT, EPT and its Cnnected RL Lad Included)

70 Reprt: LEAN Reprt Page 70 f [kv] Wb-t [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) t:afsab t:afsbc t:afsca a. Sub. Vltages at SPT s 0º POW Energisatin n Phase A b. SPT Fluxes Vl. at SPT s 0º POW Energisatin n Phase A 500 [A] p.u [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) t:vgrra t:vgrrb t:vgrrc c. Sub. Currents at SPT s 0º POW Energisatin n Phase A d. Sub. RMS Vltages at SPT s 0º POW Energisatin n Phase A 9000 [V] p.u [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) v :VLOADA v :VLOADB v :VLOADC [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc e. EPT LV Vltages at SPT s 0º POW Energisatin n Phase A f. EPT LV RMS Vl. at SPT s 0º POW Energisatin n Phase A Figure 24 SPT Flux, Substatin Currents and Vltage, and ETP LV Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 0º POW n Phase A)

71 Reprt: LEAN Reprt Page 71 f [kv] Wb-t [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) t:afsab t:afsbc t:afsca a. Sub. Vltages at SPT s 0º POW Energisatin n Phase A b. SPT Fluxes Vl. at SPT s 0º POW Energisatin n Phase A 1000 [A] p.u [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) t:vgrra t:vgrrb t:vgrrc c. Sub. Currents at SPT s 0º POW Energisatin n Phase A d. Sub. RMS Vltages at SPT s 0º POW Energisatin n Phase A 9000 [V] p.u [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) v :VLOADA v :VLOADB v :VLOADC [s] 5.63 (f ile rm_90_in_0.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc e. EPT LV Vltages at SPT s 0º POW Energisatin n Phase A f. EPT LV RMS Vl. at SPT s 0º POW Energisatin n Phase A Figure 25 SPT Flux, Substatin Currents and Vltage, and ETP LV Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 90º POW n Phase A)

72 Reprt: LEAN Reprt Page 72 f [kv] [A] [s] (f ile rm_0_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW Energisatin n Phase A [s] (f ile rm_0_in_0.pl4; x-v ar t) c:x0006a-ispta c:x0006b-isptb c:x0006c-isptc b. SPT Currents at SPT s 0º POW Energisatin n Phase A 500 [A] [A] [s] [s] (f ile rm_0_in_0.pl4; x-v ar t) c:x0047a-iepta c:x0047b-ieptb c:x0047c-ieptc (f ile rm_0_in_0.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc d. EPT Currents at SPT s 0º POW Energisatin n e. Substatin Currents at SPT s 0º POW De-energisatin n Phase A Phase A Figure 26 SPT-EPT Sympathetic Interactin Inrush and Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 0º POW n Phase A; same results as in Figure 24, but time axis scale was prlnged t 6 s)

73 Reprt: LEAN Reprt Page 73 f [kv] [A] [s] (f ile rm_90_in_0.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. Substatin Vltages at SPT s 0º POW Energisatin n Phase A [s] (f ile rm_90_in_0.pl4; x-v ar t) c:x0006a-ispta c:x0006b-isptb c:x0006c-isptc b. SPT Currents at SPT s 0º POW Energisatin n Phase A 500 [A] [A] [s] [s] (f ile rm_90_in_0.pl4; x-v ar t) c:x0047a-iepta c:x0047b-ieptb c:x0047c-ieptc (f ile rm_90_in_0.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc d. EPT Currents at SPT s 0º POW Energisatin n e. SPT Fluxes at SPT s 0º POW De-energisatin n Phase A Phase A Figure 27 SPT-EPT Sympathetic Interactin Inrush and Vltage Wavefrms at SPT s POW Energisatin (With Prir SPT De-energisatin at 90º POW n Phase A; same results as in Figure 25, but time axis scale was prlnged t 6 s) Figure 28 SPT s Flux Trend Plts fr Multiple SPT s POW De-Energisatin Events

74 Reprt: LEAN Reprt Page 74 f 107 POW Tlerance POW Tlerance a. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A b. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A c. EPT Max. LV Side Vltage Sag with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A POW Tlerance d. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A e. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A f. EPT Max. LV Side Vltage Sag with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A Figure 29 Substatin s (with SPT, EPT and EPT s LV Side Lad Included) Inrush Current and Vltage Sag Trend Plts fr Multiple SPT s POW Energisatin Events

75 Reprt: LEAN Reprt Page 75 f Case C Impact f Sub. s Fault Levels n POW Switching Inrush Current Mitigatin The ATP netwrk mdel detailed in Figure 18b was adpted. Substatin vltages and currents were btained fr the fllwing cnditins: Best and wrst case fr SPT s remnant flux (i.e. SPT s de-enerigsatin at 0º POW n Phase A and 90º respectively). Best case SPT s inrush current (i.e. SPT s enerigsatin at 0º POW n Phase A). A range f 33 kv fault levels (frm 250 MVA t 1000 MVA in steps f 250 MVA). The principal aim f studies presented in this sectin includes addressing the fllwing aspects: What is the impact n SPT s inrush currents and substatin s vltage and current perfrmance fr varius levels f fault currents if precise levels f POW based de-energisatin and energisatin is achievable and implemented t minimise SPT s remnant flux and inrush currents? What is the impact n SPT s inrush currents and substatin s vltage and current perfrmance fr varius levels f fault currents if precise level f POW based energisatin is achievable t minimise SPT s inrush currents, but precise level f POW based de-energisatin is nt achievable r implemented? Hw des the external fault level R/X rati impact the SPT s switching related substatin vltages? The plts fr substatin vltages and currents fr a case cnsidering the de-energisatin and energisatin f SPT ccurring at 0º POW (n Phase A) are shwn in Figure 30. The plts cnsider extremes f substatin fault levels (i.e. 250 MVA and 1000 MVA). The results shw that the variatin in external substatin fault level des nt diminish the POW cntrlled switching methd s ability t reduce remnant flux and inrush currents. Results in Figure 31 cnsider the same mdel cnditins as in fr Figure 30, but with SPT s 90º POW (n Phase A) prir t de-enerigsatin. These results (in Figure 31) shw higher substatin vervltages, higher inrush current and lwer vltage sags ccur with an increase in external fault levels. Figure 32 summarises the substatin vltage and current trends fr all cnsidered external fault level scenaris, which re-iterate the findings f results in Figure 30 and Figure 31. Finally, Figure 33 shws the impact f varius X/R ratis n substatin s transient vervltages; a scenari substatin SPT 0º POW (n Phase A) energisatin with prir 90º POW de-energisatin case with 250 MVA external fault level was used prduced these results; these results shw that reductin in X/R rati r increase in R/X rati f external system can reduce r damp transient vervltages seen during transfrmer de-energisatin prcess.

76 Reprt: LEAN Reprt Page 76 f [kv] [kv] [s] [s] 5.63 (f ile rm_0_in_0_250mv a.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC (f ile rm_0_in_0_1000mv a.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. 250 MVA Fault Level Substatin Vltages b MVA Fault Level Substatin Vltages 500 [A] [A] [s] [s] 5.63 (f ile rm_0_in_0_250mv a.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc (f ile rm_0_in_0_1000mv a.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc c. 250 MVA Fault Level Substatin Currents d MVA Fault Level Substatin Currents 1.10 p.u p.u [s] [s] 5.63 (f ile rm_0_in_0_250mv a.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc (f ile rm_0_in_0_1000mv a.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc e. 250 MVA Fault Level Substatin RMS Vltages f MVA Fault Level Substatin RMS Vltages Figure 30 Substatin Currents and Vltages (With Prir SPT De-energisatin at 0º POW n Phase A and Subsequent Energisatin at 0º POW n Phase A) fr Varius External Fault Levels

77 Reprt: LEAN Reprt Page 77 f [kv] [kv] [s] [s] 5.63 (f ile rm_90_in_0_250mv a.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC (f ile rm_90_in_0_1000mv a.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. 250 MVA Fault Level Substatin Vltages b MVA Fault Level Substatin Vltages 1000 [A] [A] [s] [s] 5.63 (f ile rm_90_in_0_250mv a.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc (f ile rm_90_in_0_1000mv a.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc c. 250 MVA Fault Level Substatin Currents d MVA Fault Level Substatin Currents 1.10 p.u p.u [s] [s] 5.63 (f ile rm_90_in_0_250mv a.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc (f ile rm_90_in_0_1000mv a.pl4; x-v ar t) t:vlrmsa t:vlrmsb t:vlrmsc e. 250 MVA Fault Level Substatin RMS Vltages f MVA Fault Level Substatin RMS Vltages Figure 31 Substatin Currents and Vltages (With Prir SPT De-energisatin at 90º POW n Phase A and Subsequent Energisatin at 0º POW n Phase A) fr Varius External Fault Levels

78 Reprt: LEAN Reprt Page 78 f 107 a. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A b. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A c. EPT Max. LV Side Vltage Sag with SPT s POW Energisatin n Min. Remnant Flux (r 0º POW Deenergisatin) n Phase A d. Substatin Max. HV Peak Inrush Currents with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A e. Substatin Max. HV Vltage Sag with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A f. EPT Max. LV Side Vltage Sag with SPT s POW Energisatin n Max. Remnant Flux (r 90º POW Deenergisatin) n Phase A Figure 32 Substatin s (with SPT, EPT and EPT s LV Side Lad Included) Inrush Current and Vltage Sag Trend Plts fr Varius Substatin s External Fault Levels

79 Reprt: LEAN Reprt Page 79 f [kv] [kv] [s] [s] 5.63 (f ile rm_90_in_0_250mv a.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC (f ile rm_90_in_0_250mv a_xr1.pl4; x-v ar t) v :VGRIDA v :VGRIDB v :VGRIDC a. X/R Rati is 5 Substatin Vltages b. X/R Rati is 1 Substatin Vltages 1000 [A] [A] [s] [s] 5.63 (f ile rm_90_in_0_250mv a.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc (f ile rm_90_in_0_250mv a_xr1.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc c. X/R Rati is 5 Substatin Currents d. X/R Rati is 1 Substatin Currents 9000 [V] [V] [s] [s] 5.63 (f ile rm_90_in_0_250mv a_xr5.pl4; x-v ar t) v :VLOADA v :VLOADB v :VLOADC (f ile rm_90_in_0_250mv a_xr1.pl4; x-v ar t) v :VLOADA v :VLOADB v :VLOADC e. X/R Rati is 5 EPT Lad Vltages f. X/R Rati is 1 EPT Lad Vltages Figure 33 Substatin Currents and Vltages (With Prir SPT De-energisatin at 90º POW n Phase A and Subsequent Energisatin at 0º POW n Phase A) fr 250 MVA External Fault Levels and Varius X/R Ratis

80 Reprt: LEAN Reprt Page 80 f Case D Impact f SPT s/ept s Effective HV s Side Capacitance n SPT s Remnant Flux A sensitivity analysis f the selected transfrmer s HV side effective line-t-grund capacitance, calculated as per [132] and used in all studies prir t this sectin, n SPT s remnant flux (90º POW de-energisatin n Phase A) was cnducted. Results frm these studies (given in Figure 34 n a lgarithmic capacitance scale) shw that the transfrmer s remnant flux may decrease with large increase in transfrmer and CB stray capacitance values fr a specific POW switching angle; the reductin in transfrmer s remnant flux is due t the scillatin f the circuit cmprising f the inductances and stray capacitances f the transfrmer and stray capacitances f the cnnected series CB [134]. Fr small variatin (±5 nf) n selected transfrmer s effective capacitance value f 5.7 nf, the SPT s remnant flux value fllwing its de-energisatin is nt expected t change significantly. Remnant Flux with 5.7 nf Transfrmer s Line-t-Grund Capacitance Value Figure 34 SPT s Remnant Fluxes fr Varius SPT/EPT Line-t-Grund Capacitance Values 3.6. Cnclusins and Lessns Learnt A review f TASS Methd and related technical risks frm the pint f transfrmer switching aspect was presented. The review has als cvered in detail the methdlgy fr cntrlled switching based transfrmer energisatin/de-energisatin methds, and its perfrmance and risks assciated with the methd were evaluated in detail. Based n the results presented in this sectin, the fllwing cnclusins can be drawn: It may be pssible t energise a primary substatin transfrmer withut adversely affecting the pwer quality within the substatin s 33 kv netwrk and it s cnnected 11 kv lad. It is likely that ple-independent cntrl and peratin f the SPT 33 kv circuit breaker will be required. The use f cntrlled POW switching appears t be a feasible strategy fr effectively limiting transfrmer energisatin related inrush currents.

81 Reprt: LEAN Reprt Page 81 f 107 Figure 35 shws the expected substatin HV side currents and vltages with uncntrlled (such as mechanism detailed in LEAN prject Optin 1 in [1]) and cntrlled (such as mechanisms detailed in LEAN prject Optin 2 in [1]) POW SPT switching n an already energised EPT supplying substatin lad in parallel; during the energisatin prcess, ne can expect high substatin mmentary inrush currents superimpsed n t substatin lad currents (as detailed in Figure 35a) with uncntrlled switching r substatin currents (as detailed in Figure 35c) clse t substatin lad currents (i.e. with negligible amunt f energising transfrmer s inrush currents superimpsed n t substatin lad currents) with cntrlled switching. Figure 35b and Figure 35d illustrate the impact n substatin RMS vltages as result f these transfrmer switching scenaris [A] p.u [s] 5.63 (f ile rm_0_in_180.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc a. Substatin Currents with Uncntrlled Switching (expected with LEAN Prject with Optin 1) [s] 5.63 (f ile rm_0_in_180.pl4; x-v ar t) t:vgrra t:vgrrb t:vgrrc b. Substatin RMS Vltages with Uncntrlled Switching (expected with LEAN Prject with Optin 1) 1500 [A] p.u [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) c:x0008a-igrida c:x0008b-igridb c:x0008c-igridc [s] 5.63 (f ile rm_0_in_0.pl4; x-v ar t) t:vgrra t:vgrrb t:vgrrc c. Substatin Currents with Cntrlled Switching (expected d. Substatin RMS Vltages with Cntrlled Switching (expected with LEAN Prject with Optin 2) with LEAN Prject with Optin 2) Figure 35 Substatin Currents and Vltages with Uncntrlled and Cntrlled Switching A cntrlled prir line-t-grund vltage POW de-energisatin at 0º fllwed by energisatin f the transfrmer at the same line-t-grund vltage POW angle and

82 Reprt: LEAN Reprt Page 82 f 107 phase is expected t draw an inrush current that is typically less than 10% f transfrmer nminal current; this result was als fund t be valid when substatin transfrmers sympathetic interactins were cnsidered. In additin, the impact f such POW transfrmer energisatin n substatin s HV and LV side RMS vltages is expected t be negligible. This result is based n the assumptin that the cnsidered three-phase three-ple series CB s chpping current is higher than the transfrmer s magnetizing current. If the CB s chpping current is fund t be lwer than the transfrmer s magnetizing current, then significant inrush currents may still be seen. Under these cnditins, a single-ple three-phase CB may be required t allw fr independent phase POW switching. With SPT energising in parallel with the EPT and its cnnected LV side lad, the currents seen by the external netwrk is expected t be SPT s inrush currents (including thse due t sympathetic interactins between SPT and EPT) superimpsed up n the EPT s HV side lad s current. In additin, there may be a POW energisatin tlerance margin f apprximately ±15º within which a minimum amunt f transfrmer inrush currents (and therefre maximum retained substatin vltages) culd be achieved. Hwever, further investigatin is needed t understand hw the POW de-energisatin switching errr fr remnant flux minimisatin may affect the allwable POW switching errr fr inrush mitigatin. The variatin in external substatin fault level des nt diminish the POW cntrlled switching methd s ability t reduce remnant flux and inrush currents; this was fund t be the case fr cnsidered fault levels frm 250 MVA t up t 1000 MVA. Hwever, external fault level des have an impact n transfrmer inrush currents, and it s wrth accunting fr when POW cntrlled switching methd f inrush current mitigatin is nt applied. With inrush currents reduced t less than transfrmer nminal current during its energisatin prcess, the impact f transfrmer switching n transfrmer s differential prtectin and netwrk prtectin schemes is expected t be minimal. The TASS risk analysis has shwn that a primary substatin transfrmer can be energised thrugh cntrlled switching withut causing significant inrush currents and any assciated issues; this relies n the fllwing assumptins: The distributin f the remnant flux f the energised transfrmer as seen frm the 33 kv side remains reasnably symmetrical (apprximately, 120º phase displacement). The current chpping magnitude f the circuit breaker is greater that the magnetising current f the switched primary substatin transfrmer at all times. In reality, hwever,

83 Reprt: LEAN Reprt Page 83 f 107 the current chpping magnitude is in part dependent n the circuit breaker design and the value f ttal system capacitance seen at the circuit breaker terminals. The current chpping magnitude is als subject t a large statistical variatin; hwever, in future studies the current chpping culd be accunted fr by cnsidered typical best and wrst case values and assessing their respective impact n transfrmer s remnant flux. The remnant flux within the transfrmer des nt decay by significant amunts between the de-energisatin and the energisatin f the switched transfrmer. Shuld a significant decay ccur, this decay is likely t lead t added asymmetry t the remnant flux distributin. Based n the simplified mdel f external netwrk used in the study, during cntrlled instances f pint-n-wave transfrmer de-energisatin and energisatin, significant levels f transient vervltages were bserved; hwever, the magnitudes f transient vervltages are a functin f several external factrs, such as netwrk resistances, lad, etc. A reductin in substatin external fault level s X/R rati (r increase in R/X rati) has shwn t reduce/damp the magnitude f transient vervltages; it is anticipate the latter result may als apply t the case when transfrmer energisatin netwrk lad in high with additinal resistance added t the netwrk. In practical situatins, switching vervltages may nt be a prblem due t the presence f netwrk lad and resistance in DNO netwrks; this shuld be verified via additinal studies r site r cntrlled labratry based transfrmer energisatin/deenergisatin tests. In additin, a sensitivity study assessing the effect f transfrmer stray capacitances (including any errr in calculatin f their magnitude). Prvided these are kept within tens f nf, they may have negligible impact n cntrlled switching methd s ability t minimise transfrmer remnant flux during the de-energisatin prcess; hwever, use f very large transfrmer stray capacitances values were fund t have significant impact n the transfrmer remnant flux values. These effects may als require calibratin by physical testing. Additinally, HFDE s reprt, attached t this reprt in Appendix D, prvides additinal cnclusins related t transfrmer health and lifetime impact risks and their mitigatin with implementatin f TASS Methd.

84 Reprt: LEAN Reprt Page 84 f RECOMMENDATIONS Recmmendatins drawn frm the undertaken TASS Methd risk and mitigatin evaluatin wrk include the fllwing: Based n cnsidered primary substatin transfrmer parameters and simulatin test cnditins, substatin peak current traces (with superimpsed transfrmer inrush peak current traces) detailed in Figure 36 can be expected theretically depending n if a substatin transfrmer was switched either using a uncntrlled POW mechanism (such as Optin 1 in LEAN prject) r a cntrlled POW mechanism (such as Optin 2 in LEAN prject); it is, hwever, recmmended that these inrush currents and their decay trends are validated (fr studied substatin transfrmers) based n parameters btained frm actual nsite transfrmers where TASS Methd may be implemented. High transfrmer peak inrush current superimpsed n t lad current due t uncntrlled substatin transfrmer switching Negligible transfrmer peak inrush current superimpsed n t lad current due t cntrlled transfrmer switching Figure 36 Substatin Peak Inrush Current Traces with Type f Transfrmer Switching Detailed technical risk evaluatin using electr-magnetic transient simulatin based analysis assessing the impact f the fllwing aspects is recmmended: Three-phase single- r three-ple circuit breaker transfrmer de-energisatin related chpping current impact n transfrmer s remnant flux and resultant inrush currents during its energisatin. Netwrk capacitance, and series circuit breaker and switched transfrmer s stray capacitances, n the vervltage transients that may ccur n the system during transfrmer energisatin. Opprtunities fr timed POW switching using existing and new HV and LV side breakers.

85 Reprt: LEAN Reprt Page 85 f 107 Transfrmer phase-cupling n inrush currents and cntrlled POW switching ability in reducing r eliminating inrush currents. Substatin LV side lad transfer switching impact, which will ccur prir t and after switched transfrmer switching actin, n substatin s vltages and transfrmers. Vltage harmnic distrtin n vltage POW angle detectin, and therefre impact n cntrlled switching mechanism ability in reducing transfrmer remnant flux and inrush current. External netwrk harmnic impedance resnances f significant impedance magnitudes (typically due t lw netwrk resistances) can significantly amplify netwrk vltages, even n small harmnic current injectins. Althugh cntrlled switching f transfrmer switching is expected t draw lw inrush current related harmnic currents frm the netwrk, their impact n harmnic vltages due t impedance resnances can be significant. While switching vervltages due t transfrmer switching are nt usually a prblem, it is a recmmendatin f this reprt t verify this with additinal studies r site r cntrlled labratry based transfrmer energisatin/de-energisatin tests. In the studies presented here, the impact f variatin in transfrmer s effective HV s side surge capacitance n transfrmer s remnant flux was investigated fr ne POW de-energisatin angle (i.e. 90º); it will be useful t understand such impact fr ther POW de-energisatin angles and further studies are recmmended. Frm the abve listed studies, a minimum set f equipment (accunting fr any mitigatin if fund necessary) and their respective specificatin fr TASS Methd implementatin shuld be identified. Fllwing this, it is recmmended that SEPD undertakes the fllwing set f tasks: A review f available ptins fr existing 33 kv and 11kV switchgear in SEPD s netwrk that may be able t prvide POW based cntrlled transfrmer switching. Explre characteristics f varius switchgear types (e.g. vacuum, air, il, etc.), their POW switching capabilities, and perfrmance (ple scatter, chpping current values, rated switching peratin numbers, etc.) that may be best suited and cst effective fr TASS Methd applicatin. Undertake a cntrlled site r labratry based testing f principal and critical cmpnents (i.e. equipment and peratinal philsphy) f TASS Methd. Results btained frm these tests can assist with calibrating a simulatin mdel f the same test system. HFDE s wn reprt (attached t this reprt in Appendix D) includes cnclusins and recmmendatins related t TASS methd impact n switched transfrmer s health and lifetime aspects which shuld als be cnsidered during the LEAN prject.

86 Reprt: LEAN Reprt Page 86 f 107 The LEAN business case may need t be re-evaluated t see if TASS Methd, with incrpratin f any risk mitigatin required, remains a viable and cst-effective slutin t reduce netwrk lsses and related carbn emissins and cst t cnsumers, while prviding a reasnable return n investment t the DNOs.

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90 Reprt: LEAN Reprt Page 90 f 107 [29] "Western Pwer Distributin RIIO-ED1 lsses reductin strategy," Ofgem, Lndn, UK, Open Letter, 30th Jul Available at: Accessed n: 13th May [30] Cnsultatin n the draft RIIO-ED1 Lsses Discretinary Reward Guidance Dcument [Web Page], Lndn, UK: Ofgem, Available at: Accessed n: 21st May [31] "Price Cntrls Explained," Ofgem, Lndn, UK, Fact Sheet N. 117, Mar Available at: Accessed n: 22nd May [32] Business Plan 2015 t 2023 [Web Page], Prtsmuth, UK: Scttish and Suthern Energy Pwer Distributin Limited, Available at: Accessed n: May [33] "Electricity Act 1989," Great Britain, Lndn, UK, Act N. ch. 29 pt. 1, Available at: Accessed n: 22nd May [34] "Helping Yu Save Energy and Mney," SSE, Reading, UK, Brchure N. SSE EN EFFICIENCY BROCH DEC 14, 19th Dec Available at: ncybklet.pdf. Accessed n: 22nd May [35] "Sustainability Reprt 2012/2013," UK Pwer Netwrks Hldings Limited, Lndn, UK, Reprt, Available at: Accessed n: 22nd May [36] N. P. Tbin and M. Lyns, "Practical Electricity Distributin System Lss Reductin," presented at the The energy efficiency challenge fr Eurpe: Prceedings f the 1993 Eurpean Cuncil fr an Energy Efficient Ecnmy (ECEEE) Summer Study, Rungstedgard, Denmark, 1st-5th Jun [37] "Revised RIIO-ED1 Business Plan - Annex 7: Lsses Strategy," UK Pwer Netwrks (Operatins) Limited, Lndn, UK, Business Plan, Mar Available at: nnexes/ukpn_revised_business_plan_whats_changed_&_why.pdf. Accessed n: 14th May [38] "Well Justified Business Plan ," Electricity Nrth West Limited, Warringtn, UK, Business Plan, Apr Available at: /re-submitted-well-justified-business-plan and-annexes.pdf. Accessed n: 22nd May [39] Our Business Plan [Web Page], Newcastle Upn Tyne, UK: Nrther Pwergrid, Available at: Accessed n: May [40] " Business Plan," Scttish Pwer Ltd., Glasgw, UK, Business Plan, Available at:

91 Reprt: LEAN Reprt Page 91 f dmar14.pdf. Accessed n: 22nd May [41] " RIIO-ED1 Business Plan," Western Pwer Distributin, Bristl, UK, Business Plan, Apr Available at: infrmatin/our-future-business-plan/wpd-rii-ed1-business-plan/wpd-riio-ed1-business- Plan.aspx. Accessed n: 22nd May [42] Business plan (2015 t 2023) [Web Page], Lndn, UK: UK Pwer Netwrks, Available at: nnexes/. [43] M. Clemence, R. Cccini, and A. Glatigny, "Hw Utility Electrical Distributin Netwrks can Save Energy in the Smart Grid Era," Schneider Electric SE, Rueil Malmaisn, France, White Paper N AR0, rev. 0, Available at: Accessed n: 1st Jun [44] W. M. Dahalan and H. Mkhlis, "Netwrk Recnfiguratin fr Lss Reductin with Distributed Generatins using PSO," presented at the 2012 IEEE Internatinal Cnference n Pwer and Energy (PECn), Kta Kinabalu, Malaysia, 2nd-5th Dec [45] "Autmatin t Optimise Netwrk Cnfiguratin in Real Time T Optimise DG Cntributin and Reduce Lsses," Institute fr Energy and Envirnment, University f Strathclyde, Glasgw, UK, Reprt, Cntract N. DG/CG/00096/00/00, URN N. 07/1653, Nv Available at: cg00096rep_v3.pdf. Accessed n: 20th May [46] M. Sedighizadeh, M. Dakhem, M. Sarvi, and H. Krdkheili, "Optimal Recnfiguratin and Capacitr Placement fr Pwer Lss Reductin f Distributin System using Imprved Binary Particle Swarm Optimizatin," Internatinal Jurnal f Energy and Envirnmental Engineering, Springer-Verlag GmbH, Berlin, Germany, vl. 5, iss. 1, pp. 1-11, [47] S. C. Vegunta, D. Hawkins, and M. Barlw, "Isle f Wight Netwrk Lsses Reductin Study Phase 1 Studies," S&C Electric Eurpe Ltd., Reading, UK, Reprt N RevC, 31st Oct [48] V. Phetlamphanh, S. Premrudeepreechacharn, and K. Ngamsanraj, "Technical Lsses Reductin f Electrical Distributin System in Vientiane Capital," in 2012 Internatinal Cnference n Renewable Energy Research and Applicatins (ICRERA), Nagasaki, Japan, 2012, pp [49] R. G. Pratt, M. C. W. Kintner-Meyer, P. Balducci, T. F. Sanquist, C. Gerkensmeyer, K. P. Schneider, et al., "The Smart Grid: An Estimatin f the Energy and CO2 Benefits," Pacific Nrthwest Natinal Labratry (PNNL), Washingtn, USA, Reprt N. PNNL-19112, Rev. 1, Jan Available at: _Revisin_1_Final.pdf. Accessed n: 20th May 2015.

92 Reprt: LEAN Reprt Page 92 f 107 [50] "Distributin Efficiency Initiative," Nrthwest Energy Efficiency Alliance, Prtland, USA, Reprt N. DEI Final Reprt 1207_ , Dec Available at: DEI_Reprt.pdf. Accessed n: 20th May [51] E. Diskin, T. Falln, G. O'Mahny, and C. Pwer, "Cnservatin Vltage Reductin and Vltage Optimisatin n Irish Distributin Netwrks," presented at the CIRED 2012 Wrkshp n Integratin f Renewables int the Distributin Grid, Lisbn, Prtugal, 29th-30th May [52] "Demand Side Respnse in the Nn-Dmestic Sectr," Element Energy Limited, Cambridge, UK, Reprt, Nv Available at: Accessed n: 28th May [53] S. C. Vegunta, D. Hawkins, S. A. Reid, C. Frank, and A. Steele, "Transfrmer Lss Reductin with Varying Substatin Lad-Generatin Prfiles," presented at the 23rd Internatinal Cnference and Exhibitin n Electricity Distributin (CIRED), Lyn, France, 15th-18th Jun [54] Demand Side Respnse [Web Page]: UK Pwer Netwrks, Available at: Accessed n: 28th May [55] G. Strbac, "Demand-Side Management: Benefits and Challenges," Energy Plicy, vl. 36, iss. 12, pp , Dec [56] "Demand Side Respnse in the Dmestic Sectr - A Literature Review f Majr Trials," Great Britain, Department f Energy & Climate Change (DECC), Lndn, UK, URN N. 12D/257, Aug Available at: demand-side-respnse-in-the-dmestic-sectr-a-lit.pdf. Accessed n: 27th May [57] "Equipment Energy Efficiency Prgram - Cnsultatin Regulatry Impact Statement - Review f Minimum Energy Perfrmance," Cmmnwealth f Australia, Bartn, Australia, Reprt, May Available at: rmers/ draft-cnsult-ris-transfrmers.pdf. Accessed n: 17th May [58] "Business Plan - Annex 1.4: Strategy f Technical Lssess," Nrthern Pwergrid Hldings Cmpany, Newcastle Upn Tyne, UK, Business Plan, Mar Available at: file. Accessed n: 14th May [59] S. C. Vegunta, D. Hawkins, and M. Barlw, "Isle f Wight Netwrk Lsses Reductin Study Phase 2 Studies," S&C Electric Eurpe Ltd., Reading, UK, Reprt N RevB, 31st Oct [60] R. F. Arritt, W. M. Grady, K. Frsten, D. Brks, and T. Shrt, "KCP&L Green Circuits Analysis," Electric Pwer Research Institute (EPRI), Inc., Pal Alt, USA, Reprt, 29th Apr Available at:

93 Reprt: LEAN Reprt Page 93 f Accessed n: 17th May [61] J. Triplett, S. Rinell, and J. Fte, "Evaluating Distributin System Lsses Using Data frm Deplyed AMI and GIS Systems," presented at the 2010 IEEE Rural Electric Pwer Cnference (REPC), Orland, USA 16th-19th May [62] W. F. Griesacker and J. L. Thierry, "Magnetic Cre Issues in Pwer Transfrmers and Their Diagnstics," presented at the 81st Internatinal Cnference f Dble Clients, Bstn, USA, 6th-11th Apr [63] "Pwer/Vac Prduct Family Applicatin Guide," GE Cnsumer and Industrial, Hustn, USA, Applicatin Guide N. GET-6600G, Aug Available at: G?TNR=Applicatin%20and%20Technical%7CGET-6600G%7Cgeneric. Accessed n: 21st May [64] S. Hesmndhalgh, W. Zarakas, and T. Brwn, "Appraches t Setting Electric Distributin Reliability Standards and Outcmes," The Brattle Grup Ltd., Lndn, UK, Reprt, Jan Available at: g_electric_distributin_reliability_standards_and_outcmes_hesmndhalgh_zarakas_brw n_jan_2012.pdf. Accessed n: 20th May [65] C. McCarthy, A. Jayantilal, M. Edmnds, and E. Bardman, "Smart Distributin thrugh Layered Intelligence fr Next Generatin Self-Healing Distributin Netwrks," presented at the 22nd Internatinal Cnference n Electricity Distributin (CIRED), Stckhlm, Sweden, 10th-13th Jun [66] J. Baker and M. Meisinger, "Experience with a Distributed-Intelligence, Self-Healing Slutin fr Medium-Vltage Feeders n the Isle f Wight," presented at the 2nd IEEE PES Internatinal Cnference and Exhibitin n Innvative Smart Grid Technlgies (ISGT), Manchester, UK, 5th-7th Dec [67] R. E. Gdin, T. S. Fahey, and A. Hansn, "Distributin Reliability using Reclsers and Sectinalisers," ABB, Inc., Lake Maryu, USA, White Paper N. 1VAL2601-WP, rev. A, Feb Available at: 0Reclsers%20and%20switches.pdf. Accessed n: 23rd May [68] F. Vybiralik and J. Tlusty, "Increasing f the Capacity f Lw-Vltage Netwrks with Higher Explitatin f their Cmpnents," presented at the 16th Internatinal Cnference and Exhibitin n Electricity Distributin (CIRED), Amsterdam, Netherlands, 18th-21st Jun [69] T. G nen, Electric Pwer Distributin Engineering, 3rd ed., Bca Ratn: Taylr & Francis, [70] S. Blair and C. Bth, "Analysis f Electrical Lsses in Meshed Distributin System Operatin," University f Strathclyde, Glasgw, UK, White Paper, Jun Available at:

94 Reprt: LEAN Reprt Page 94 f Accessed n: 23rd May [71] B. Brewin, J. Gdall, G. Williamsn, and K. Jacksn, "Assessment f Special Case fr SP Manweb Operating an Intercnnected Netwrk," Parsns Brinckerhff Ltd, Manchester, UK, Reprt N , Mar Available at: s_aj.pdf. Accessed n: 23rd May [72] J. Ware, "Pwer Factr Crrectin (pfc)," IEE Wiring Matters [Magazine], iss. 18, pp , Spring Available at: Accessed n: 20th May [73] J. Dixn, L. Mran, J. Rdriguez, and R. Dmke, "Reactive Pwer Cmpensatin Technlgies: State-f-the-Art Review," Prceedings f the IEEE, vl. 93, iss. 12, pp , [74] K. Niall and A. Walsh, "Maximising Benefits t Custmers frm Distributin Lsses Management An ESBN Perspective," presented at the 21st Internatinal Cnference n Electricity Distributin (CIRED), Frankfurt, Germany, 6th-9th Jun [75] "Electrical Energy Strage White Paper," Internatinal Electrtechnical Cmmissin (IEC), Geneva, Switzerland, White Paper N. IEC WP EES: (en), Dec Available at: Accessed n: 20th May [76] "Electricity Energy Strage Technlgy Optins A White Paper Primer n Applicatins, Csts, and Benefits," Electric Pwer Research Institute (EPRI), Inc., Pal Alt, USA, White Paper N , 23rd Dec Available at: Accessed n: 20th May [77] G. Strbac, M. Aunedi, D. Pudjiant, P. Djapic, F. Teng, A. Sturt, et al., "Strategic Assessment f the Rle and Value f Energy Strage Systems in the UK Lw Carbn Energy Future," Imperial Cllege Lndn, Lndn, UK, Reprt, Jun Available at: Accessed n: 20th May [78] A. Khtanzad, R. Afkhami-Rhani, T. L. Lu, A. Abaye, M. Davis, and D. J. Maratukulam, "ANNSTLF A Neural-Netwrk-Base Electric Lad Frecasting System," IEEE Transactins n Neural Netwrks, vl. 8, iss. 4, pp , [79] "Cst and Perfrmance f EV Batteries," Element Energy Limited, Cambridge, UK, Reprt, 21st Mar Available at: Accessed n: 28th May [80] G. Strbac, C. Ramsay, and D. Pudjiant, "Integratin f Distributed Generatin int the UK Pwer System - Summary Reprt," Ofgem, Lndn, UK, Reprt, Mar Available at:

95 Reprt: LEAN Reprt Page 95 f Accessed n: 14th May [81] D. M. Ca, D. Pudjiant, G. Strbac, A. Martikainen, S. Kärkkäinen, and J. Farin, "Csts and Benefits f DG Cnnectins t Grid System Studies n the UK and Finnish Systems," Energy research Centre f the Netherlands (ECN), Petten, Netherlands, Reprt, Dec Available at: Accessed n: 14th May [82] M. Scheepers, D. Bauknecht, J. Jansen, J. d. Jde, T. Gmez, D. Pudjiant, et al., "Regulatry Imprvements fr Effective Integratin f Distributed Generatin int Electricity Distributin Netwrks - Summary f the DG-GRID prject results," Energy Research Centre f the Netherlands (ECN), Petten, Netherland, Reprt N. ECN-E , Nv Available at: Accessed n: 14th May [83] V. H. M. Quezada, J. R. Abbad, and T. G. S. Rma n, "Assessment f Energy Distributin Lsses fr Increasing Penetratin f Distributed Generatin," IEEE Transactins n Pwer Systems, vl. 21, iss. 2, pp , [84] L. Degrte, B. Renders, B. Meersman, and L. Vandevelde, "Influence f Cnverter-based Distributed Generatrs n the Harmnic Line Lsses," presented at the 13th Internatinal Cnference n Harmnics and Quality f Pwer (ICHQP), Wllngng, Australia, 28th Sep [85] L. F. Ocha and G. P. Harrisn, "Minimizing Energy Lsses: Optimal Accmmdatin and Smart Operatin f Renewable Distributed Generatin," IEEE Transactins n Pwer Systems, vl. 26, iss. 1, pp Feb [86] F. M. Gnzález-Lngatt, "Impact f Distributed Generatin Over Pwer Lsses n Distributin System," presented at the 9th Internatinal Cnference n Electrical Pwer Quality and Utilizatin (EPQU), Barcelna, Spain, 9th Oct [87] D. Hllingwrth, I. Llyd, and R. Hetheringtn, "Custmer-Led Netwrk Revlutin Lessns Learned Reprt: Grand Unified Scheme," Nrthern Pwergrid (Nrtheast) Limited, Newcastle Upn Tyne, UK, Reprt N. CLNR-L167, rev. 1, 15th Dec Available at: Learned-Reprt-v1.pdf. Accessed n: 1st Jun [88] Y. Qiang, J. A. Barria, and T. C. Green, "Cmmunicatin Infrastructures fr Distributed Cntrl f Pwer Distributin Netwrks," IEEE Transactins n Industrial Infrmatics, vl. 7, iss. 2, pp , [89] "Active Management f Distributed Generatin: Making Sense f Market Trends an ELEXON Perspective," Elexn Limited, Lndn, UK, Reprt, Mar Available at: Generatin_March2015.pdf. Accessed n: 1st Jun

96 Reprt: LEAN Reprt Page 96 f 107 [90] R. Currie, B. O Neill, C. Fte, A. Gding, R. Ferris, and J. Duglas, "Cmmercial Arrangements t Facilitate Active Netwrk Management," presented at the 21st Internatinal Cnference n Electricity Distributin (CIRED), Frankfurt, Germany, 6th-9th Jun [91] R. L. Strry, M. Dlan, E. Davidsn, G. Ault, and I. Kckar, "A Case fr Lsses Minimisatin in Active Netwrk Management Systems," presented at the 21st Internatinal Cnference n Electricity Distributin (CIRED), Frankfurt, Germany, 6th Jun [92] L. McDnald, R. L. Strry, A. Kane, F. McNicl, G. Ault, I. Kckar, et al., "Minimisatin f Distributin Netwrk Real Pwer Lsses using a Smart Grid Active Netwrk Management System," in th Internatinal Universities Pwer Engineering Cnference (UPEC), 2010, pp [93] S. Thngkeaw and M. Bnthienthng, "Technique fr Vltage Cntrl in Distributin System," Internatinal Jurnal f Electrical, Cmputer, Electrnics and Cmmunicatin Engineering, vl. 7, p. 10, [94] "Distributin Vltage Regulatrs," Siemens Pwer Transmissin & Distributin, Inc., Richland, USA, Brchure N. E50001-U113-A510-X-US00, Available at: Accessed n: 2nd Jun [95] S. Griffith, "Vlt/VAR Technlgies Imprve Efficiency," NEMA electrindustry, vl. 18, iss. 11, p. 12, Nv Available at: Accessed n: 2nd Jun [96] L. Kane and G. Ault, "The Cst f Active Netwrk Management Schemes at Distributin Level," presented at the Eurpean Wind Energy Assciatin (EWEA) Annual Wind Energy Event 2013, Vienna, Austria, 4th Feb [97] D. Papadaskalpuls, E. Manitsas, P. Mancarella, and G. Strbac, "Optimizatin f Operating and Investment Csts f Active Management Deplyment in Distributin Netwrks," presented at the 2011 IEEE Trndheim PwerTech, Trndheim, Nrway, 19th-23rd Jun [98] N. McDnagh, W. Phang, and W. Bridgeman, "Testing f ESB s 20kV Faulted Phase Earthing System," presented at the Prtectin, Autmatin & Cntrl (PAC) Wrld Cnference, Dublin, Ireland, 21st-24th Jun [99] Our Infrastructure [Web Page], Dublin, Republic f Ireland: ESB Netwrks Ltd., Available at: Accessed n: 20th May [100] D. Hawkins, B. Currie, E. Davidsn, N. McNeill, and A. Gding, "Task 3.4: Review f Enablers, Slutins and Tp-Dwn Mdelling in TRANSFORM," Smarter Grid Slutins Ltd., Lndn, UK, Reprt N C, 13th Feb Available at: Accessed n: 20th May 2015.

97 Reprt: LEAN Reprt Page 97 f 107 [101] G. Stkes, Handbk f Electrical Installatin Practice, 4th ed., Oxfrd: Blackwell Science, [102] "Lsses Strategy," Western Pwer Distributin, Bristl, UK, Reprt, Jan Available at: Final.aspx. Accessed n: 7th May [103] Y. Al-Mahrqi, M. I.A., A. Al-Hinai, and A. Al-Badi, "Reductin f Pwer Lsses in Distributin Systems," Wrld Academy f Science, Engineering and Technlgy, vl. 6, iss. 3, pp , 21st Mar [104] Abut C2C [Web Page], Salfrd, UK: Electricity Nrth West Ltd., Available at: Accessed n: 24th May [105] "Smart Urban Lw Vltage Netwrk - Overview," UK Pwer Netwrks, Lndn, UK, Brchure, Available at: prjects/smart-urban-lw-vltage-netwrk/prject-dcuments/overview-smart-urban-lw- Vltage-Netwrk.pdf. Accessed n: 25th May [106] "IFI/LCNF Reprt April 2013 March 2014," UK Pwer Netwrks (Operatins) Limited., Lndn, UK, Reprt, 30th Jul Available at: pdf. Accessed n: 26th May [107] "La Peple s Demcratic Republic fr the Pwer Transmissin and Distributin Prject," Prpsal N. La 29273, Sep Available at: Accessed n: 12th Jun [108] "Transfrmer Energizatin in Pwer Systems: A Study Guide," Internatinal Cuncil n Large Electric Systems (CIGRÉ), Paris, France, Brchure N. 568, WG C4.307, Feb Available at: Accessed n: 26th May [109] J. Peng, H. Li, Z. Wang, F. Ghassemi, and P. Jarman, "Stchastic Assessment f Vltage Dips Caused by Transfrmer Energisatin," IET Generatin, Transmissin & Distributin, vl. 7, iss. 12, pp , Dec [110] R. S. Girgis and E. G. tenyenhuis, "Characteristics f Inrush Current f Present Designs f Pwer Transfrmers," in IEEE Pwer Engineering Sciety General Meeting, Tampa, USA, 2007, pp [111] R. Hunt, J. Schaefer, and B. Bentert, "Practical Experience in Setting Transfrmer Differential Inrush Restraint," in 61st Annual Cnference fr Prtective Relay Engineers, 2008, pp [112] Z. Wang, J. Ma, Y. Xu, and L. Ma, "A New Principle f Discriminatin Between Inrush Current and Internal Fault Current f Transfrmer Based n Self-crrectin Functin," in The 7th Internatinal Pwer Engineering Cnference (IPEC), 2005, pp

98 Reprt: LEAN Reprt Page 98 f 107 [113] M. Mathur and V. Barhate, "ANN based Technique fr Discriminatin between Magnetizing Inrush and Internal Fault Currents in Transfrmer," Internatinal Jurnal f Science and Research (IJSR), vl. 2, iss. 1, pp , Jan [114] B. Kasztenny and A. Kulidjian, "An Imprved Transfrmer Inrush Restraint Algrithm Increases Security While Maintaining Fault Respnse Perfrmance," presented at the 53rd Annual Cnference fr Prtective Relay Engineers, Cllege Statin, USA, 11th-13th Apr [115] "GR-200 Series: GRT 200 Transfrmer Prtectin IED," Kawasaki, Japan, Brchure N. GKP , rev. 1, Available at: Accessed n: 4th Jun [116] "MiCOM P40 Agile P64x: Transfrmer Prtectin IED - Technical Manual," Alstm Grid, Paris, France, Manual N. P64x-TM-EN-1, Available at: ftp://ftp.alstm.cm/alstm_manuals/p64x-tm-en-1.pdf. [117] H. S. Brnzead, P. B. Brgan, and R. Yacamini, "Harmnic Analysis f Transient Currents during Sympathetic Interactin," IEEE Transactins n Pwer Systems, vl. 11, iss. 4, pp , [118] J. A. Martinez-Velasc, Transient Analysis f Pwer Systems: Slutin Techniques, Tls, and Applicatins, 1 ed., Chichester, UK: Jhn Wiley & Sns, Ltd, [119] C. P. Cheng and S. Chen, "Simulatin f Resnance Over-Vltage During Energizatin f High Vltage Pwer Netwrk," Electric Pwer Systems Research, vl. 76, iss. 8, pp , 5// [120] "Interruptin f Small Inductive Currents," Internatinal Cuncil n Large Electric Systems (CIGRÉ), Paris, France, Brchure, Dec Available at: Accessed n: 3rd Jun [121] O. Ozgnenel and C. Akuner, "A Study n magnetizing Inrush Current f Different Cre Material," presented at the Internatinal Cnference n Electrical and Electrnics Engineering (ELECO), Bursa, Turkey, 7th-11th Dec [122] S. M., B. Albyaci, S. Öztürk, and H. B. Cetinkaya, "Case Study f Sympathetic Interactin Between Transfrmers Caused by Inrush Transients," presented at the Internatinal Cnference n Pwer Systems Transients (IPST), Mntreal, Canada, 19th-23rd Jun [123] C. H. Flurscheim, Pwer Circuit Breaker Thery and Design. Stevenage, UK: Peregrinus fr the Institutin f Electrical Engineers, [124] J. C. Oliveira, C. E. Tavares, R. Aplni, A. B. Vascncells, and H. S. Brnzead, "Transfrmer Inrush Mitigatin - Part I: Mdelling and Strategy fr Cntrlled Switching," presented at the Simpósi Brasileir de Sistemas Elétrics (SBSE), Campina Grande, Brazil, Jul [125] N. Chiesa, A. Avendan, H. K. Høidalen, B. A. Mrk, D. Ishchenk, and A. P. Kunze, "On the Ringdwn Transient f Transfrmers," presented at the Internatinal Cnference n Pwer Systems Transients (IPST), Lyn, France, 4th-7th Jun

99 Reprt: LEAN Reprt Page 99 f 107 [126] A. Ebner, "Transient Transfrmer Inrush Currents due t Clsing Time and Residual Flux Measurement Deviatins if Cntrlled Switching is Used," presented at the EEUG Meeting Eurpean EMTP-ATP Cnference, León, Spain, 24th-26th Sep [127] "IEEE Guide t Describe the Occurrence and Mitigatin f Switching Transients Induced by Transfrmers, Switching Device, and System Interactin," IEEE Std C , pp. 1-56, [128] J. H. Brunke and K. J. Frhlich, "Eliminatin f Transfrmer Inrush Currents by Cntrlled Switching Part II: Applicatin and Perfrmance Cnsideratins," IEEE Transactins n Pwer Delivery, vl. 16, iss. 2, pp , Apr [129] H. S. Brnzead, S. O. Pint, P. Jnssn, J. C. De Oliveira, and M. L. R. Chaves, "Transfrmer Cntrlled Switching t Eliminate Inrush Current - Part II: Field Tests n a 100MVA Three-phase Transfrmer," presented at the IEEE/PES Transmissin & Distributin Cnference (TDC) and Expsitin: Latin America, Caracas, Venezuela, 15th-18th Aug [130] "Cntrlled Switching Buyer s and Applicatin Guide," ABB AB, Ludvika, Sweden, Brchure N. 1HSM en, Aug Available at: trlled%20switching%20ed4.pdf. Accessed n: 19th May [131] J. H. Brunke and K. J. Frhlich, "Eliminatin f Transfrmer Inrush Currents by Cntrlled Switching Part I: Theretical cnsideratins," IEEE Transactins n Pwer Delivery, vl. 16, iss. 2, pp , [132] A. R. Hileman, Insulatin Crdinatin fr Pwer Systems. New Yrk: Marcel Dekker, [133] K. S. Smith, "Transfrmer Inrush Studies fr Wind farm Grid Cnnectins," presented at the Internatinal Cnference n Pwer Systems Transients (IPST), Mntreal, Canada, 19th-23rd Jun [134] J. A. Martinez-Velasc, Pwer System Transients: Parameter Determinatin. Bca Ratn, FL: CRC Press, [135] "Treatment f Lsses by Netwrk Operatrs ERGEG Psitin Paper fr Public Cnsultatin," Eurpean Regulatrs Grup fr Electricity and Gas (ERGEG), Brussels, Belgium, Reprt N. E08-ENM-04-03, 15th Jul Available at: NSULTATIONS/ELECTRICITY/Treatment%20f%20Lsses/CD/E08-ENM-04-03_Treatmentf-Lsses_PC_ pdf. Accessed n: 13th May 2015.

100 Reprt: LEAN Reprt Page 100 f GLOSSARY AC ANM ANNSTLF ATP C2C CB CHP CIGRÉ CIRED CO2 CVR DECC DER DG DIgSILENT DNO DSR DSSS DSTATCOM DUKES ED EDL EHV EMT ENA ENW EPRI Alternating Current Active Netwrk Management Artificial Neural Netwrk Shrt Term Lad Frecaster Alternative Transient Prgram Capacity t Custmers Prject Circuit Breaker Cmbined Heat and Pwer Internatinal Cuncil n Large Electric Systems Internatinal Cnference and Exhibitin n Electricity Distributin Carbn Dixide Cnservatin Vltage Reductin Department f Energy & Climate Change Distributed Energy Resurce Distributed Generatin DIgSILENT GmbH Distrubutin Netwrk Operatr Demand Side Respnse Distributin System Security Standard Distributin Static Synchrnus Cmpensatr Digest f United Kingdm Energy Statistics Electricity Distributin Electricite du Las Extra High Vltage Electr-Magnetic Transient Energy Netwrks Assciatin Electricity Nrth West Ltd. Electric Pwer Research Institute, Inc.

101 Reprt: LEAN Reprt Page 101 f 107 EPSRC EPT ER ERGEG ESB ETAP EU EV GB HFDE HV ICT IEA IEEE IFI La PDR LC LCC LCNF LDR LEAN LIG LV MPPT MV MZEC NDPL NPV OFGEM Engineering and Physical Sciences Research Cuncil Energised/un-switched Primary Transfrmer Engineering Recmmendatin Eurpean Regulatrs Grup fr Electricity and Gas ESB Netwrks Ltd. Electrical Transient Analyzer Prgram Eurpean Unin Electrical Vehicle Great Britain High Frequency Diagnstics & Engineering (HFDE) Ltd High Vltage Infrmatin and Cmmunicatin Technlgy Internatinal Energy Agency Institute f Electrical and Electrnics Engineers Innvatin Funding Incentive La Peple's Demcratic Republic Inductance and Capacitance Life Cycle Csting Lw Carbn Netwrk Fund Lsses Discretinary Reward Lw Energy Autmated Netwrks LIG Cnsultancy Services LLP Lw Vltage Maximum Pwer Pint Tracking Medium Vltage Mazn Electricity Cmpany Nrth Delhi Pwer Limited Net Present Value Office f Gas and Electricity Markets

102 Reprt: LEAN Reprt Page 102 f 107 OLTC OPF PF PNRA POW PV R/X RIIO RL ROI S&C SCADA SEPD SPT SSEPD Sub. SVC T&D TASS UK UKPN US USA USAID WPD X/R Online Line Tap Changer Optimal Pwer Flw Pwer Factr Pwer Netwrks Research Academy Pint-n-Wave Phtvltaic Resistance t Inductance Rati Revenue = Incentives + Innvatin + Outputs Resistance and Inductance Return On Investment S&C Electric Eurpe Ltd Supervisry Cntrl and Data Acquisitin Suthern Electric Pwer Distributin plc. Switched Primary Transfrmer Scttish and Suthern Energy Pwer Distributin Ltd. Substatin Where TASS Methd will be Implemented Static Var Cmpensatr Transmissin and Distributin Transfrmer Aut Stp-Start United Kingdm UK Pwer Netwrks United States f America United States f America United States Agency fr Internatinal Develpment Western Pwer Distributin Inductance t Resistance Rati

103 Reprt: LEAN Reprt Page 103 f 107 APPENDIX

104 Reprt: LEAN Reprt Page 104 f 107 A. EMBEDDING OF SOLUTION FOR LOSSES IN THE MARKET SYSTEM * Abve table taken frm ERGEG, Treatment f Lsses by Netwrk Operatrs ERGEG Psitin Paper, [135]

105 Reprt: LEAN Reprt Page 105 f 107 B. REGULATORY INCENTIVES TO REDUCE LOSSES * The abve table was taken frm ERGEG, Treatment f Lsses by Netwrk Operatrs ERGEG Psitin Paper, [135]

106 Reprt: LEAN Reprt Page 106 f 107 C. DSR TRIAL AVERAGE REDUCTION IN PEAK DEMAND SUMMARY Trial Cuntry Participant Numbers Average Reductin in Peak Demand Apprx. Peak t ff Peak Price Differential Califrnia State-wide Pricing Pilt ( ) USA % 200% CL&P Pilt (2009) USA % % PG&E's Trial ( ) USA 86,222 11% Varied Ireland Electricity Smart Metering Behaviur Trials ( ) Ontari Smart Price Pilt ( ) Ireland 2, % % Canada 124 0% 140% mypwer Trial ( ) USA % 187% Energy Demand Research Prject Trials ( ) UK 194 (EdF Energy), 1,352 (SSE) varied 165% Nrway EFFLOCOM Trial ( ) Nrway 237 Maximum 10% unknwn Nrthern Ireland Pwershift trial ( ) Nrthern Ireland 100 Small reductin 267% Integral Energy Trial ( ) Australia 241 unknwn unknwn Xcel Energy Trial USA 2,900 in the verall study 5.19% with central air cnditining, 10.63% withut unknwn Flrida Gulf Pwer Select Prgramme (2000 nwards) USA Unknwn fr the TU tariff, 2,300 fr the CPP tariff Unknwn fr the TU tariff, 22% fr CPP cnsumers during nncritical peak perids 266% fr the CPP rate n nn-critical days Idah DSR trial ( ) USA 85 0% 184% Missuri CPP trial ( ) USA 91 0% 349% PSE's TU trial ( ) USA 300,000 residential and small cmmercial 5% unknwn * The abve table infrmatin was taken frm DECC, Demand Side Respnse in the Dmestic Sectr - A Literature Review f Majr Trials, [56]

107 Reprt: LEAN Reprt Page 107 f 107 D. A REVIEW OF TASS IMPACT ON TRANSFORMER S HEALTH AND LIFETIME

108 High Frequency Diagnstics and Engineering Ltd c/ Suite 116, Baltic Chambers, 50 Wellingtn Street, Glasgw G2 6HJ e: t: w: Review f the effects f repeated transfrmer energisatins/de-energisatins and prlnged de-energisatin impact n transfrmer asset health and life Dcument Reference: S&C Electric B Revisin & date f issue: v1 4 th June 2015 Prepared by: Dr Martin Judd Technical Directr HFDE Ltd High Frequency Diagnstics and Engineering Limited Cmpany Number SC482878, registered in Sctland Registered address: 11 Smerset Place, Glasgw, G3 7JT VAT Reg. N

109 Executive Summary This reprt was cmmissined by S&C Electric Eurpe Ltd t investigate and summarise current knwledge and peratinal practice relevant t situatins in which pwer transfrmers are subjected t either (i) repeated energisatins/de-energisatins, r (ii) prlnged perids f de-energisatin. Presented in the frm f a literature review, the reprt is grunded in engineering practice thrugh its cllabrative preparatin by HFDE Ltd (Dr Martin Judd, Technical Directr, having many years f pwer transfrmer research experience at the University f Strathclyde) and Plaris Diagnstics & Engineering Ltd (Mr Ian Hunter, Technical Directr, having managed peratinal pwer transfrmer fleets at transmissin and distributin vltages fr mre than 15 years). This study has been carried ut in cntext f the LEAN* prject, which aims t reduce lsses (and thereby carbn emissins) in the electricity distributin netwrk by practively managing transfrmer utilisatin, switching units ut f circuit when they are nt necessary t meet demand. The reprt reviews the ptential effects f repeated transfrmer energisatins/de-energisatins and prlnged deenergisatin impact n transfrmer asset health and life that are particularly relevant t 33/11 kv primary transfrmers. Methds that have been prpsed r are in use fr mitigating inrush currents and their adverse cnsequences are described and discussed. T validate the prpsed new transfrmer perating regime within the LEAN prject, a representative selectin f transfrmers will be selected fr a trial phase, during which they will experience mre frequent energisatin/de-energisatin than has previusly been the case. Diagnstic measurements and mnitring techniques that can be used n-site t quantify any effects f additinal transfrmer switching and the cnsequential altered lading patterns that will ccur during the trial phase are described. Key issues are identified, particularly in relatin t aspects f transfrmer health and degradatin mechanisms that shuld be bserved with particular care during the initial trial phase in rder t maximise cnfidence with regard t subsequent rll-ut f the scheme. The reprt als serves t identify key dcuments that are relevant t planning f the practical trial phase f the transfrmer utilisatin cmpnent f the LEAN prject in rder t ensure that current understanding f assciated technical issues is taken int accunt. * Lw Energy Autmated Netwrks, a prject prpsed by SSE Pwer Distributin fr the Secnd Tier Lw Carbn Netwrks Fund see 1

110 Disclaimer Infrmatin prvided in this reprt has been referenced as far as pssible t ensure that it accrds with the latest scientific understanding and engineering practiced in the field. While every effrt has been made t ensure that the material is accurate, High Frequency Diagnstics and Engineering Ltd accepts n liability fr any cnsequential lsses in relatin t the cntent f this reprt, hwever incurred. A number f cmmercial prducts are referred t r depicted in this reprt fr illustratin purpses. This des nt imply endrsement by the authr, his cmpany r cntributrs t this reprt f any particular prducts r manufacturers. 2

111 Table f Cntents 1. Energising Issues fr Pwer Transfrmers 5 page 1.1 Intrductin Ageing and failure f transfrmers Magnetic inrush current Cnsequences f energising transients Techniques fr mitigating transfrmer inrush currents Pint-n-wave (PW) switching f the energising breaker Pre-insertin resistrs in series with the energising breaker Adjusting the OLTC setting befre energising the transfrmer Reducing system vltage befre energising the transfrmer Energising the transfrmer using the discnnect switch De-fluxing the transfrmer cre befre energising References Diagnstics and Mnitring fr Pwer Transfrmers Intrductin Disslved gas analysis (DGA) and interpretatin On-site transfrmer tests Pwer factr/tan delta and capacitance Plarizatin index Dielectric spectrscpy Frequency dmain spectrscpy (FDS) Sweep frequency respnse analysis (SFRA) Furanic cmpunds and paper cnditin Enabling transfrmer mnitring References Electrical Discharge Activity Partial Discharges (PD) Transfrmer PD levels in service Techniques fr n-site PD investigatins References 34 3

112 4. Managing Transfrmer Health and Life Cnsumptin f paper life Thermal cycling and intermittent lading Transfrmer Health Index Site selectin fr the trial phase f LEAN Testing f transfrmers in the LEAN trial Maintenance f de-energised transfrmers References Cnclusins 47 4

113 1. Energising Issues fr Pwer Transfrmers 1.1 Intrductin A prject entitled LEAN (Lw Energy Autmated Netwrks) prpsed by Scttish and Suthern Energy Pwer Distributin (SEPD) has been established t demnstrate nvel methds f reducing electrical lsses n the distributin netwrk. One f the prpsed appraches invlves de-energising 33/11kV transfrmers during perids when they are nt required t meet demand. The prject aims t deliver significant netwrk lss savings withut affecting custmers quality f supply r cmprmising lngterm asset health f the transfrmers themselves. SSEPD s studies have established that significant reductins in lsses culd be achieved thrugh the prpsed transfrmer switching scheme. Hwever, implementing this strategy will invlve a significant mid-life change in perating regime fr the transfrmers included in the scheme. If nt prperly managed, switching f transfrmers (particularly energising) has the ptential t degrade their electrical and mechanical integrity t a certain extent, s a change in practice that invlves increasing the frequency f switching by tw r three rders f magnitude needs t be planned and managed in an infrmed manner. When transfrmers are prcured fr applicatins that can invlve aggressive lad cycles (such as tractin r furnace units r generatr step-up transfrmers fr pumped strage), they wuld usually be purchased at a premium t a specificatin requiring suitably rbust cnstructin and resilient electrical insulatin. Hwever, in the cntext f this prject, the transfrmers shuld be regarded as being f standard cnstructin. Any cnsideratin f upgrading transfrmers in anything but exceptinal cases wuld undermine the aim f the LEAN prject, bth in terms f cst and the cnsumptin f additinal raw materials tgether with the carbn emissins that manufacturing entails. Transfrmer energisatin is a regular peratin in certain parts f the pwer netwrk fr example, at a pumped strage generatin site several energisatins may ccur each day. Althugh transfrmer energisatin is a far less frequent ccurrence in 33/11kV distributin netwrks, it is an peratin which transfrmers and the netwrk shuld be expected t handle withut adverse cnsequences. Hwever, the ptential impact f switching peratins n transfrmers themselves is becming increasingly pertinent due t changes in the use and cnfiguratin f pwer netwrks [1]. These changes include accmmdating distributed and renewable energy surces and integratin f a whle suite f nvel technlgies that can be gruped under the smart grid banner. Changing demands n lng-established transfrmers, such as the ptential prliferatin f electric vehicles, may als lead t changes in hw they age [2] and cnsequently respnd t being switched in and ut f circuit. The primary cnsideratins frm a transfrmer health perspective when changing t an perating regime with cmparatively frequent transfrmer switching are the extent t which the fllwing issues pse a risk: 1. Ptential physical damage resulting frm the magnetising inrush currents that ccur during energisatin. These currents can cause large magnetic frces that exert a significant transient mechanical shck n the cre and active part, which may distrt the internal structure and slacken the winding clamping frces ver time. Furthermre, there is ptential t cause damage t slid insulatin thrugh either direct mechanical damage r secndary effects such as partial discharges r arcing driven by the transient vervltages that can ccur during energisatin. 2. Increased rate f aging f the paper insulatin as a result f mre frequent thermal cycling f the transfrmer. When a transfrmer is perating at a stable temperature well belw that at which significant paper ageing wuld ccur at the ht spt (60C being a gd illustrative example f a cmfrtably warm temperature), the stable thermal equilibrium may be mre cnducive t an extended life than frequent thermal cycling between ambient and elevated temperatures because the latter invlves migratin f misture between the paper and the il. 5

114 These issues are the central themes f this reprt, in which varius relevant aspects are discussed with reference t published literature, standards and technical recmmendatins. This reprt seeks t highlight references that culd be f particular value during the planning and trial phases f the LEAN prject. Hwever, as it is neither permissible nr practical t duplicate large sectins f the surce material here, cpies f the riginal dcuments may need t be btained. A prprtin these (particularly the relevant IEC standards) are likely t already be in the pssessin f a DNO such as SSEPD. Otherwise, IEC and IEEE standards can be purchased n-line at the relevant pages, respectively. In additin, crprate r individual members f CIGRE shuld be in pssessin f lgin details that give access t and free dwnlad f all CIGRE brchures via the link at: Finally, the authr f this reprt wuld be happy t assist with surce any dcuments that are in the public dmain (subject t the usual restrictins f cpyright & wnership). 1.2 Ageing and failure f transfrmers Fig. 1.1 shws a representative active part f a primary distributin transfrmer. In a well-designed and mechanically sund pwer transfrmer, the mst significant factrs gverning its health and lifetime are the level f misture in the il and its thermal perating regime. The extent t which misture cnsumes the available lifetime f the pwer transfrmer s insulatin is intimately linked t its lading and the ambient temperature, bth in an abslute sense and as a result f significant variatins in its lading prfile. This is because thermal cycling causes migratin f misture ut f the cellulse-based insulatin (paper, pressbard) when the transfrmer is running ht and its re-absrptin by the insulatin when the transfrmer returns t ambient temperature (issues discussed in Sectins 4.1 & 4.2 f this reprt). clamping blcks clumn f pressbard spacers paper insulated windings inter-phase pressbard barriers steel frame Fig. 1.1 Internal view f a 5 MVA, 33/11 kv primary transfrmer. 6

115 There are a number f mechanisms by which this mvement f water cntent can degrade the transfrmer ver time. Firstly, chemical reactins with water mlecules, prmted by heat, break dwn the cellulse mlecule chains and weaken the paper structurally. This lss f mechanical integrity f the cellulse insulatin weakens the transfrmer in tw ways: 1. In an electrical sense, due t a reductin in the dielectric strength f the insulatin (its ability t resist inisatin under the influence f a strng electric field). Severe paper damage (charring) can liberate carbn and therefre intrduce an element f electrical cnductivity (see Figs. 1.2 and 1.3). The detectin f thermal faults and partial discharges thrugh disslved gas analysis (DGA) is cvered in Sectin 2.2 f this reprt. 2. In a mechanical sense, reductins in the vlume and rigidity f paper and pressbard reduces their resilience t mechanical shck and slacken the clamping f the windings, making them mre prne t failure when subjected t high levels f transient magnetic frce such as may ccur when the transfrmer experiences a thrugh fault r lengthy inrush current at energisatin (see examples in Figs. 1.4 and 1.5). Lss f paper cnditin is discussed in Sectin 2.4 f this reprt. (a) (b) Fig. 1.2 (a) View f windings with paper insulatin and pressbard spacers in gd cnditin. (b) Paper insulatin that has undergne thermal damage, becming blackened and brittle. Fig. 1.3 Paper insulatin taken frm the winding shwing five stages f thermal degradatin frm reasnable cnditin (LHS) thrugh t ttal degradatin (RHS). 7

116 disldged clamping plates clamping blt clamping plate winding Fig. 1.4 Failure f winding clamping assembly caused by an 11kV cable thrugh-fault exacerbated by lss f clamping pressure attributed t shrinkage f the cellulse-based insulatin. Clamping blck disldged at bttm f winding Fig. 1.5 Mechanical damage due t magnetic frces caused by a thrugh fault, mst likely due t repeated incidences f such events causing slackening f the clamping arrangement. A cre-type 33/11kV transfrmer cre will cmprise f 3 limbs and a tp and bttm yke, cnstructed frm layer f laminated steel. The yke and limb jints are usually mitred and interleaved t minimise reluctance in the magnetic circuit, which tends t be cncentrated at these jints. The limbs and ykes can either be clamped tgether by the use f: Cre blts the cre blts are inserted thrugh pre-drilled hles in the yke r limb and are blted at each end, t prvide a cmpressive frce, hlding the laminatins tgether. Each individual blt is lcated within an insulating sleeve that is necessary t maintain the electrical insulatin between the cre laminatins (t avid additinal lsses and lcalised heating). Bands A banded cre will cmprise f each limb and yke being held tgether by bands, which wrap arund each part f the cre and are trqued t prvide the necessary cmpressive frce t hld the laminatins tgether. The advantage f a banded assembly is that the cre des nt need t be drilled and there are n degradatin mechanisms f the type described belw that is assciated with cre blts. 8

117 Like any ther insulatin, cre blt insulatin will degrade and age ver time due t misture and temperature effects. These are exacerbated by the fact that, due t its lcatin, the cre blt insulatin des nt necessarily benefit frm the main tank cling methds. As example f this mde f degradatin can be seen in Fig. 1.6, which shws blts remved frm a transfrmer at end f life, with heating and cracking evident n the cre blt insulating sleeves. In a situatin where the cre blt insulatin has severely degraded, the cre blt will be less mechanically secure and prne t mvement r lsening ff, which may lead, in the wrst case, t a yke displacement. This will lead t a lss f cmpressin between the individual laminatins and the cre jints, which will increase the reluctance f the magnetic circuit, increasing the cre lss and generating mre heat which will in effect accelerate the ageing f the active part. Fig. 1.6 Examples f cre blts having degraded insulating sleeves. In the remainder f Sectin 1, this reprt reviews the rigins f the electrical and mechanical stresses that transfrmers experience during energisatin and utlines the peratinal techniques that can be used t mitigate these effects. Sectin 2 cvers diagnstic measurements and mnitring techniques that are relevant t assessments f the health f transfrmer subjected t repeated energisatin/deenergisatin r prlnged perids f de-energisatin. Sectin 3 fcuses particularly n the issue f internal electrical discharges and their diagnsis in pwer transfrmers. Finally, Sectin 4 summarises the principles f managing transfrmer health and life under the prpsed nn-standard energisatin regimes. 1.3 Magnetic inrush current Transfrmer energisatin at a nn-ptimal pint n the vltage cycle can create large cre flux asymmetries and peratin at magnetic field strengths that exceed the saturatin levels f the transfrmer s magnetic circuit [1]. Randm switching f transfrmers ften causes large inrush currents that can reduce residual life and may als cause inapprpriate tripping f prtectin relays and/r reductins in pwer quality. Inrush currents tend t be rich in harmnic cntent and can als include a significant DC cmpnent that nly decays ver a perid f many secnds. Inrush current arises as a cnsequence f the permanent remnant magnetisatin that is trapped in the transfrmer cre at the instant f de-energisatin. That is, the magnetic flux level in the cre at the pint f pening the breakers persists until the transfrmer is re-energised. 9

118 current rati (peak inrush : peak nrmal) Suppse the transfrmer is designed t perate nrmally with a peak cre flux variatin within the range Φ nrmal. If the design f the magnetic circuit is efficient, there will nt be a large margin between the nrmal peak flux and the cre saturatin flux. In the wrst case, if the transfrmer was de-energised at the instant when the remnant flux happened t be Φ residual = Φ nrmal and re-energised at a pint n the vltage cycle that wuld crrespnd t the phase psitin where the magnetic flux wuld have previusly been -Φ nrmal, linear peratin f the circuit wuld require the peak flux t reach a value f 2 Φ nrmal + Φ residual, r 3 times the nrmal peak cre flux [1], driving the cre well int a nn-linear regin that results in the drawing f a large inrush current. Fig. 1.7 illustrates sme typical inrush current wavefrms that culd ccur in a 3-phase transfrmer. Fig. 1.8 and Table 1.1 give an idea f the magnitude and time cnstant respectively f the inrush currents as a functin f transfrmer pwer rating in MVA. Fig. 1.7 Typical inrush current wavefrms in a 3-phase transfrmer, illustrating the severe nn-linearity that ccurs [2] transfrmer rating ( MVA ) Fig. 1.8 Range f magnetising inrush currents relative t rated peak current as a functin f transfrmer pwer rating [3]. 10

119 Table 1.1 Time cnstants f inrush current relative t transfrmer pwer rating [3]. Nminal pwer (MVA) Time cnstant (s) > Disturbances caused by the switching f transfrmers in and ut f circuit have the ptential t cause a number f adverse effects, including degrading the transfrmer s electrical insulatin system. Transient vervltages may cause internal electrical discharging, initiating partial discharge surces and leading t physical damage t the insulatin and lss f dielectric strength t resist cmparable events r system transients thereafter. Such effects will cause damage t accumulate if they ccur regularly as a result f frequent transfrmer switching, with the affected areas becming mre prne t discharge activity thrugh the creatin f electrically cnducting tracks n insulatr surfaces. Abnrmal magnetic frces, driven by large inrush currents with harmnic cntent and DC ffset cause bth mechanical shck and vibratins that put the unit s mechanical integrity t test. This particularly affects the ends f the windings, where the magnetic field and physical structure are less unifrm. Axial and radial frces can in severe cases lead t distrtin f the windings r even cause sme f the turns t escape frm the clamping arrangement. Furthermre, mechanical mvement can cause r exacerbate insulatin damage, reducing dielectric strength and leading t weakness that may be mre prne t the electrical frms f damage utlined abve. 1.4 Cnsequences f energising transients Of particular relevance t this study is the 2014 reprt f CIGRE Wrking Grup C4.307, Transfrmer energizatin in pwer systems [4]. This technical brchure sets ut t prvide guidance n transfrmer energisatin issues and practice. While its central theme is system transients and vltage-related prblems and the tls required t analyse and mdel these effects, it includes a cmprehensive descriptin f the inrush current phenmenn in its chapter 2. A useful review f mitigatin techniques and best practice is prvided in its chapter 4, fcusing particularly n a prven and effective technique based n cntrlling the clsing times f the energising circuit breaker. The switching f any system that cntains reactive impedances will generate transient currents and vltages. In many situatins, these transient are safely damped within the pwer netwrk. Hwever, in certain situatins, the transient currents and vltages generated during transfrmer energisatin can lead t pwer quality issues in the supply netwrk and, in extreme cases, prduce transfrmer insulatin degradatin r surge arrester failure. Adverse system effects can include: 1. RMS-vltage drp caused by high transient currents, 2. temprary vervltages generated when a parallel resnance within the supply netwrk is excited by energisatin currents, and 3. inapprpriate tripping f prtectin when the inrush current is interpreted as fault current. In severe cases, transfrmer insulatin damage may ccur if either a slw-frnt vervltage r a temprary vervltage is generated during the energisatin. The ccurrence f slw-frnt vervltages depends n the capacitive and inductive characteristics f bth the transfrmer and the neighburing supply netwrk t which it is cnnected. The ccurrence f temprary vervltages depends n the existence f lw-frequency parallel resnances in the supply netwrk, which may be excited by the transfrmer inrush current with its rich harmnic cntent. The latter may be f particular interest during 11

120 netwrk restratin fllwing a system-wide blackut. This is mainly because during such times the netwrk tends t be weak, i.e. characterized by a relatively lw shrt circuit level and hence higher system inductance with the pssibility f resnance at a much lwer frequency accmpanied by ineffective damping f vervltages due t light lading cnditins. In existing pwer netwrks, where equipment may be appraching r exceeding its intended lifespan, ne f the main cnsideratins is t prlng the life f transfrmers due t their capital replacement cst. This implies a need fr prper maintenance t cunter ageing, which may include an apprpriate level f cnditin mnitring. Hwever, in additin, it may be advisable t adpt new technlgies that can significantly mitigate inrush currents, in rder t reduce mechanical and electrical stresses n the transfrmers. 1.5 Techniques fr mitigating transfrmer inrush currents Nrmal, uncntrlled (i.e., randm phase) energising f a transfrmer prduces inrush currents that are similarly uncntrlled. While experience suggests that the prbability f damage is lw n a statistical basis, the risk rises with increasing ccurrence f transfrmer switching. In additin t the pwer quality issues and their ptential t damage transfrmer insulatin and mechanical integrity, transfrmer inrush currents have been knwn t create unusual prblems such as pulsating lw frequency electrmagnetic trques in generatrs r large mtrs that are remte frm the energising bus, causing mechanical vibratins, shifting f windings and slippage f shaft cuplings [4]. Methds listed in [4] that have been prpsed r are being used t mitigate inrush currents are listed belw, each f which will be utlined in subsequent sub-sectins: 1. Cntrlling the switching times f the energising circuit breaker 2. Installing pre-insertin resistrs in series with the energising breaker 3. Adjusting the n-lad tap befre energising the transfrmer 4. Reducing the system vltage befre energising the transfrmer 5. Energising the transfrmer using the discnnect switch instead f the breaker 6. De-fluxing the transfrmer cre befre energising With regard t mitigating temprary vervltages in the pwer netwrk, a number f impedance-related techniques are listed in [4], which are essentially cncerned with altering the netwrk impedance t reduce ptential resnances that culd be excited by the rich harmnic cmpnents f the inrush current. A related apprach is t ensure that any transient phenmena are subjected t as much damping as pssible by maximising lading f the transfrmer befre energising. Hwever, the extent t which this culd be achieved in the cntext f the LEAN prject may be restricted by the fact that the transfrmer being energised will be taking n nly a share f the lad that is already being fed frm anther surce (which may r may nt be a sister transfrmer in the same substatin). In relatin t the cntrl f vervltages, it shuld be nted that surge arresters (installed t prtect equipment frm lightning strikes and switching surges) have a limited ability t absrb energy during the lnger temprary vervltages that can be caused by energisatin. This is because their maximum cntinuus perating vltage must be safely abve the maximum steady state perating vltage (including during cntingencies) at the busbar. This means that they will nrmally be ineffective fr temprary vervltages up t abut 1.8 p.u. and, in the case f mre severe harmnic vervltages, failure f the surge arrester due t thermal runaway becmes pssible [4]. Hence, surge arresters are generally nt cnsidered apprpriate fr cntrlling harmnic vervltages excited by transfrmer energisatin. 12

121 1.5.1 Pint-n-wave (PW) switching f the energising breaker Methds t cntrl and reduce transfrmer energisatin inrush currents began t be develped in earnest in the early 2000s [5, 6] and a cmprehensive verview f these methds is cntained in the 2004 reprt f CIGRE Wrking Grup A3.07, Cntrlled switching f HVAC circuit breakers guidance fr further applicatins including unladed transfrmer switching,... [1]. This reprt was ne f a set f three Technical Brchures [1, 7, 8] prduced by this wrking grup dealing cmprehensively with many aspects f cntrlled switching f HVAC breakers (amunting t 146 pages in ttal). Cntrlled switching f breakers t minimise stress placed n cmpnents f the electrical netwrk was intrduced fr applicatins such as switching capacitr banks and shunt reactrs. Fr example, the ptimal clsing instant fr shunt capacitrs is at the instant when the surce vltage is equal t the vltage n the capacitr. In an analgus manner, recent develpments have made transfrmer switching taking int accunt residual flux a realistic prpsitin [1]. The detailed descriptin f this prcedure given in [1] can be summarised with reference t Fig At each f the instants labelled P-A r P-B (either side f the vltage zer-crssing), the prspective flux matches the residual cre flux and results in a flux that scillates with a zer bias level and remains within the limits f nrmal peratin. In effect, the transfrmer picks up frm where it left ff. Hwever, the wrst case scenari is represented by the switching instant P-W n the psitive zer-crssing f the vltage wavefrm. In this case, the flux must cntinue t increase ver the whle f the half-cycle fr which the vltage is psitive, resulting in a peak flux f 2.8 p.u. As the cre saturates, the benefits f high cre permeability are lst and many mre amps/weber are required t sustain the flux demanded by the vltage, which is the cause f the inrush current. Fig. 1.9 Simulated energisatin f the first phase f a 3-phase transfrmer with a residual flux f 0.8 p.u. [1]. See the main text fr the assciated narrative. This illustratin raises the prspect that inrush current can be reduced r eliminated subject t the fllwing requirements f the cntrl system: 1. It must be pssible t accurately measure / predict and stre the residual flux level at the instant the transfrmer was de-energised. 2. The energising circuit breaker must be cntrlled t perate at a precise phase psitin n the vltage cycle (pint-n-wave) s that the prspective flux expected at the instant f vltage applicatin matches the previusly stred value f residual flux. 13

122 These capabilities are nw embdied in cmmercially available PW switching packages that include calculatin f residual flux based n measurements frm the previus de-energisatin f the transfrmer [1]. Hwever, the cmmissining f any PW system fr transfrmer switching invlves a number f tests, including sme n-line uncntrlled/randm clsings in rder that the cntrller can determine the parameters necessary fr its subsequent peratin. Sme f these calibratin tests will prduce a high inrush current and the wner/peratr must therefre be prepared t accept the assciated risks. Nevertheless, case studies have shwn that inrush current fr ptimal clsing can be reduced t abut 1% f the wrst case value, virtually eliminating the inrush effect [4] Pre-insertin resistrs in series with the energising breaker This methd invlves inserting a resistance in series with the energising breaker and bypassing it nce the inrush transient has decayed. Its effectiveness is largely influenced by the time f inserting the resistance and the time at which it is bypassed. If the pre-insertin resistr is nt prperly bypassed after the switching peratin, the breaker culd be damaged. This ptin des nt appear t be relevant within the scpe f the LEAN prject, based n the fllwing brief cverage it receives in [4]: Pre-insertin resistrs require relatively large resistrs t be installed in series t the main circuit breaker interrupters and an effective reductin f inrush current is achieved nly by an ptimal chice f the resistance value and the pre-insertin time. Hwever, the additin f switching devices equipped with pre-insertin resistrs in series with the energizing breaker is prbably nt practical because such devices wuld have t be specially rdered frm the manufacturer at a high cst. Circuit breakers equipped with pre-insertin resistrs are als n lnger available ff-the-shelf fr vltages less than 500 kv since mdern EHV breakers are designed fr use with POW clsing. [4] This view is cnfirmed in [9], which indicates that this slutin was nly emplyed until better alternatives were intrduced. The resistrs had t be included in the circuit breaker design, ptentially necessitating additinal maintenance, reducing the inrush currents, but unlike mdern techniques, withut addressing their rt cause Adjusting the OLTC setting befre energising the transfrmer The effectiveness f this technique has been demnstrated particularly in cases f energising generatr step-up transfrmers in high vltage netwrks [10, 11]. Pre-setting f the OLTC (n-lad tap changer) t its maximum psitin can mderate inrush current t a certain extent by ensuring that mre turns are in-circuit at the instant f energisatin. In a distributin netwrk cntext, this apprach suppses that the LV side breaker is pen during energisatin and that, nce the energisatin transient has died away, the tap psitin wuld be adjusted back dwn t match the utput vltage with LV netwrk requirements. This apprach remves the pssibility f maximising lad n the transfrmer at the time f energising, which is cnsidered t ffer a means f damping dwn ptentially damaging ver-vltages during the energising inrush. Once the transfrmers are cnnected in parallel and sharing the lad, the extent t which their tap settings can differ will be restricted t ne r tw tap psitins at mst by the need t limit circulating currents between the tw transfrmers. Hence, t implement the OLTC adjustment scheme, it must be pssible t verride any restrictins n tap psitin differences during the energisatin f the islated transfrmer in the pair. An investigatin int the level f inrush current reductin that can be achieved (fr a single transfrmer) by adpting this apprach is reprted in [12]. The authrs state that in a cmparisn between tap psitins crrespnding t 1.1 p.u. and 0.9 p.u., the peaks f the inrush current were reduced by nearly 50%. Clearly, this is cmparing rather extreme variatins in tap psitin, rather than a mre realistic cmparisn between 1.1 p.u. and 1.0 p.u. Hence the achievable reductins in inrush current are prbably mre in the regin f %. 14

123 Additinal cautin is prmpted in [12] by the cmments relating t energising transfrmers in parallel (emphasis in italics has been added), which might be characterised using apprpriate mdelling packages: The magnitude f the inrush current in a transfrmer being energised thrugh windings with Lad Tap Changer (LTC) can be significantly reduced if the tap is cnveniently psitined t allw a much higher number f turns t be excited. This fact shuld be used by electric utilities as an peratinal prcedure as it will reduce the disturbances caused by transfrmer inrush transients. This paper presents an analysis f this prcedure, supprted by theretical and practical simulatin results. It is als shwn the imprtance f cnsidering the saturatin characteristic f the transfrmers already cnnected t the system as the sympathetic interactin between transfrmers (transferred saturatin) may ccur, affecting the magnitude and duratin f the inrush current. The simulatins were perfrmed using the Electrmagnetic Transient Prgram [12]. Sme detailed analysis and mdelling f transfrmer energisatin transients can be fund in a recent PhD thesis by a candidate at the University f Manchester [13], which reviews and implements mdelling techniques t study inter-transfrmer interactins and wider system effects, including pre-adjustment f the tap psitins. The use f OLTC setting cntrl t mitigate inrush currents wuld be implemented using electrnic vltage cntrl relays, which are a well-established technlgy (e.g., [14]) Reducing system vltage befre energising the transfrmer This apprach has parallels with the OLTC setting cntrl, in that reductin f the feeder vltage wuld be used t reduce inrush current during energisatin instead f increasing the number f turns n the primary side. In fact, this apprach might be implemented in cmbinatin with adjustments t OLTC setting but the extent f any inrush mitigatin will remain subject t the same verall limit with regard t maintaining vltage levels abve the statutry minimum. A literature search has nt identified any reference t this practice within distributin systems (it is usually mentined in cnnectin with generatr step-up units). It is highly unlikely that a DNO wuld be willing t significantly reduce the 33kV system vltage t reduce transfrmer inrush current. An alternative ptin that culd be investigated at the 33kV level wuld be t determine whether there are any ptential advantages in reducing the fault level by splitting the 33kV busbar at the 132/33kV grid site as a means f limiting energisatin inrush current effects. Evaluating this apprach wuld require sme site-specific mdelling bth t cnfirm that there is a benefit t the transfrmer and t ensure that there are n wider adverse effects in terms f pwer quality delivered t custmers Energising the transfrmer using the discnnect switch This unusual apprach seems unlikely t be applicable within the LEAN prject, based n the fllwing descriptin: Energizing the transfrmer using discnnect switches instead f a breaker is a technique that has been used by sme utilities t reduce the inrush currents during transfrmer energizatin. One Canadian utility used this methd fr many years t energize HVDC cnverter transfrmers with, apparently, n negative cnsequences. A discnnect switch, being a relatively slw device, will result in pre-striking r flash ver clse t the vltage peak. Hwever, with the arc in air, there culd be multiple extinctins and re-striking f the current. High frequency transients culd be impsed n the transfrmer windings and this culd stress the transfrmer insulatin. Als, the discnnect switch cntact has t be rated fr this duty, t prevent the cntact frm becming welded clsed [4]. 15

124 1.5.6 De-fluxing the transfrmer cre befre energising This methd cntributes t the reductin f inrush currents by ensuring that the wrst-case misalignment f remnant flux and pint-n-wave f energising cannt ccur. In describing a means t perfrm this peratin n-site, authrs frm OMICRON have published a useful review [15], which is summarised here. Of the three methds available fr demagnetizing magnetic materials (vibratin, heating t the Curie temperature, and electrical methds), nly electrical methds can be used n a transfrmer. Under factry cnditins, manufacturers can apply nminal vltage at nminal frequency and by gradually reducing the vltage, prgressively demagnetise the cre. Fr n-site demagnetisatin using prtable equipment, it is usually nly pssible t apply lwer vltages than the peratinal level and t use nly a single phase surce. The high-vltage winding is mre suitable fr use in demagnetizatin since the greater number f turns will generate a magnetic flux mre efficiently. Experiments have shwn that the middle limb is the mst suitable fr demagnetisatin using a single-phase surce [15]. The demagnetisatin system described in [15] begins with a calibratin f several parameters relating t the magnetic circuit, based n preliminary measurements and a prcess f iteratin whereby the cre is demagnetised t belw 1% f its maximum value. The primary purpse f this type f equipment is t de-flux prir t taking certain fingerprint measurements that can be affected by residual magnetisatin (such as SFRA). Hwever, they can als be used t de-flux a transfrmer befre re-energising after a series f ff-line tests, which will help t mitigate inrush effects at the next time f re-energising. Overall, this apprach des nt appear t be a practical ptin within the cntext f the LEAN prject and its ptential rll-ut because it wuld require an autmated de-fluxing system t be develped and installed n-site t perfrm this peratin every time the transfrmer was de-energised. Hwever, an awareness f this manual de-fluxing technique is useful as it might need t be perfrmed during the ffline fingerprint tests f transfrmers that are prpsed in Sectin 4.5 f this reprt. 16

125 1.6 References [1] CIGRE WG A3.07, Technical Brchure 263, Cntrlled switching f HVAC circuit-breakers - guidance fr further applicatins including unladed transfrmer switching, lad and fault interruptin and circuit-breaker uprating, CIGRE (Paris), Dec [2] P. Grahn, J. Rsenlind, P. Hilber, K. Alvehag, and L. Sder, A methd fr evaluating the impact f electric vehicle charging n transfrmer htspt temperature, 2 nd IEEE PES Internatinal Cnference and Exhibitin n Innvative Smart Grid Technlgies (ISGT Eurpe), Manchester, pp. 1-8, Dec [3] CIGRE WG B5.05, Technical Brchure 463, Mdern Techniques fr prtecting, cntrlling and mnitring pwer transfrmers, CIGRE (Paris), June 2011 [4] CIGRE WG C4.307, Technical Brchure 568, Transfrmer Energizatin in Pwer Systems: A Study Guide, CIGRE (Paris), Feb * [5] J.H. Brunke and K.J. Frhlich, Eliminatin f transfrmer inrush currents by cntrlled switching. I. Theretical cnsideratins, IEEE Trans. Pwer Delivery, Vl. 16, N. 2, pp , April 2001 [6] J.H. Brunke and K.J. Frhlich, Eliminatin f transfrmer inrush currents by cntrlled switching. II. Applicatin and perfrmance cnsideratins, IEEE Trans. Pwer Delivery, Vl. 16, N. 2, pp , April 2001 [7] CIGRE WG A3.07, Technical Brchure 262, Cntrlled switching f HVAC circuit breakers, CIGRE (Paris), Dec [8] CIGRE WG A3.07, Technical Brchure 264, Cntrlled switching f HVAC circuit-breakers - Planning, specificatin and testing f cntrlled switching systems, CIGRE (Paris), December 2004 [9] A. Ebner, Transient Transfrmer Inrush Currents due t Clsing Time- and Residual Flux Measurement-Deviatins if Cntrlled Switching is used, EEUG Meeting, Eurpean EMTP-ATP Cnference (León, Spain), Sept [10] J. Pntt, J. Rdriguez, J.S. Martin and R. Aguilera, Mitigatin f sympathetic interactin between pwer transfrmers fed by lng verhead lines caused by inrush transient currents, Prc. IEEE Industry Applicatins Cnference (New Orleans, USA), pp , 2007 [11] A. Prs and N. Brwne, Mdelling the energisatin f a generatr step-up transfrmer frm the high vltage netwrk, Prc. Australasian Universities Pwer Engineering Cnference (AUPEC, Sydney, Australia), pp. 1-5, Dec [12] H.S. Brnzead and J.C. de Oliveira, The influence f tap psitin n the magnitude f transfrmer inrush current, Internatinal Cnference n Pwer System Transients (IPST), Budapest, 1999 [13] J.S. Peng, Assessment f Transfrmer Energisatin Transients and Their Impacts n Pwer Systems, PhD Thesis, Schl f Electrical and Electrnic Engineering, The University f Manchester, 2013 [14] Example f vltage cntrl equipment fr use with OLTC transfrmers can be seen at (accessed 26 May 2015) [15] M. Pütter, M. Rädler and B. Unterer, Reliable demagnetizatin f transfrmer cres, OMICRON electrnics GmbH, available frm (accessed 29 May 2015) * Key reference within the cntext f the LEAN prject 17

126 2. Diagnstics and Mnitring fr Pwer Transfrmers 2.1 Intrductin In an peratinal envirnment that invlves switching transfrmers mre frequently than they have experienced cnventinally, it wuld be advisable t implement a surveillance prgramme t mnitr the cnditin f the transfrmers during a trial phase. Data relating t plant health shuld be trended ver time until a sufficient histrical recrd has been built up, allwing the extent f any measureable effects n transfrmer health t be assessed with cnfidence. Bth nline and ffline testing have a rle t play during the trial phase, as fllws: The mst imprtant and cst-effective safeguard fr nline mnitring wuld be t implement a rbust system f il sampling fr disslved gas analysis (DGA). DGA can detect thermal and misture issues, and PD that might arise as a secndary effect f vervltage r mechanical damage. DGA is typically carried ut n an annual basis fr primary transfrmers perating in a cnventinal manner. A reasnable apprach t trials invlving mre frequent switching wuld be t reduce the il sampling interval by a factr prprtinal t the increase in switching frequency. As a cst effective means f primary surveillance fr assessing whether r nt the energisatin/de-energisatin is having an impact n transfrmer health, trending f these DGA results will sn prvide evidence that wuld allw the il sampling interval t be safely increased, mst likely t smething n the scale f mnths rather than weeks. DGA results shuld be mnitred and evaluated by an experienced engineer, wh will be able t interpret underlying r deterirating cnditins that cmputer-generated il diagnstics reprts, as issued by chemistry labratries, fail t identify. DGA interpretatin is discussed in Sectin 2.2. Mechanical damage t the cre and/r winding may ccur withut initially leading t the generatin f any diagnstic gases if the electrical insulatin is nt seriusly cmprmised. The nly ptins fr detecting changes in the internal structure are ff-line tests, with SFRA (sweep frequency respnse analysis) representing the mst useful diagnstic tl. There are a number f ther standard ffline tests that can be perfrmed t assess cnditin, which are best implemented thrugh a fingerprinting apprach, where present measurements are cmpared with previus nes t identify significant changes in cnditin r deviatins frm acceptable limits. Tests f this nature are listed in Sectin 2.3. Since the cnditin f paper insulatin is ne f the key factrs gverning transfrmer life in the ideal scenari f graceful deteriratin, Sectin 2.4 summarises the mechanisms f cellulse degradatin and the ptential fr identifying this prcess thrugh the detectin f furanic cmpunds in the il. Mnitring f health may need t be increased as a particular transfrmer ages. Sectin 2.5 reviews the recmmendatins f CIGRE Wrking Grup A2.27, which relate t the incrpratin f facilities t enable apprpriate mnitring transfrmers accrding t their criticality within the pwer netwrk. Mre detailed infrmatin n varius transfrmer mnitring techniques can be fund in the 2011 technical brchure prepared by CIGRE Wrking Grup B5.05, Mdern techniques fr prtecting, cntrlling and mnitring pwer transfrmers [1]. 2.2 Disslved gas analysis (DGA) and interpretatin DGA is in itself a vast field f investigatin (and cnsiderable debate) in which varius sets f interpretatin methds have evlved. The standard IEC 60599, Mineral il-impregnated electrical equipment in service Guide t the interpretatin f disslved and free gases analysis [2] and IEEE Standard C Guide fr the Interpretatin f Gases Generated in Oil-Immersed Transfrmers [3] are the principle dcuments pertaining t disslved gas analysis fr mnitring f defect cnditins and 18

127 develping faults, prviding a cded list f faults detectable by DGA. Hwever, this is cmplemented by a number f ther publicatin wrthy f mentin here, including: IEC Oil-filled electrical equipment - Sampling f gases and analysis f free and disslved gases - Guidance [4], which specifies the gd practice necessary t ensure that il samples are taken in a manner that will give reliable results, free frm cntaminatin. IEC Mineral insulating ils in electrical equipment supervisin and maintenance guidance [5] deals with the maintenance measurands f il such as clur & appearance, breakdwn vltage, water cntent, acidity, dielectric dissipatin factr (DDF), resistivity, inhibitr cntent, sediment & sludge, interfacial tensin (IFT), particle cunt and flash pint. These quantities are all related t the verall quality f the insulatin system, whereas DGA is mre related t specific defects/prblems f a mre lcalised nature. CIGRE Technical Brchure 296, Recent develpments in DGA interpretatin [6] discusses sme debate ver anmalies in typical gas cncentratin values and rates f increase bserved in service and reprted in IEC in relatin t transfrmers. IEC standard [2] sets ut clear diagnsis methds fr DGA, but there is smetimes cnfusin due t disslved gas threshlds described as typical values being interpreted as threshlds, belw which n actin is required. This is nt the case there is n substitute fr experience when examining the DGA histry f a transfrmer in rder t diagnse a defect and determine what, if any, remedial actin is required. Many articles have been published dealing with DGA practice and interpretatin. The 2014 review in [7] prvides a gd intrductin t this field. One widely used methd fr interpreting DGA data is t plt the cncentratins f varius diagnstic gases n Duval s triangles [8] (f which there are nw a prliferatin f types fr different circumstances and types f equipment). A typical example f the diagnstic regins in a Duval triangle is reprduced in Fig Fig. 2.1 Crdinates and fault znes in the Duval triangle methd [7]. 19

128 2.3 On-site transfrmer tests The list f tests described in this sectin represents a varied tlbx (with sme diagnstic verlap acrss methds) capable f identifying mst cmmn transfrmer defects and degradatin prcesses, particularly when histrical recrds f previus spt test r fingerprint measurements n the same unit (r sister units) are available. These tests are described because they are amng thse recmmended in Sectin 4.5 f this reprt as being relevant t the surveillance f transfrmers invlved in the LEAN trials. Nte that many f these measurements will be affected by any netwrk cmpnents that remain cnnected t the unit while it is being tested. Hence it is necessary t discnnect busbars and/r cables (by remving links in the cable end bx) t ensure that readings reflect the transfrmer s prperties rather than thse f peripheral equipment Pwer factr/tan delta and capacitance These are typically 10 kv AC tests, which can be perfrmed using a Dble M4100 Insulatin Analyzer, fr example. In a duble wund, three phase transfrmer it wuld be typical t measure the pwer factr and capacitances f the HV winding t earth (CH), LV windings t earth (CL) and the HV t LV (CHL). The test results are cmpsite measurements representing the cmplete insulatin system (i.e., paper insulatin and il), s it is nt pssible t determine whether a prblem exists n an individual phase. The measurements are affected by ambient values f temperature and relative humidity, which must be recrded t facilitate cmparisn with future measurements that may take place under different cnditins. This test is mst beneficial when individual units are fingerprint tested at initial cmmissining and tested peridically during service life, typically every 5 years in nrmal service, where any insulatin deteriratin wuld be detected. During the trial phase f the LEAN prject, an interval f several mnths may be mre apprpriate t track any ptential changes in transfrmer cnditin Plarisatin index This is a DC test, typically carried ut at 5 kv, fr example using a Megger MIT525 insulatin resistance tester. The plarisatin index (PI) is the rati f the reading at 10 minutes t that at 1 minute, which prvides a measure f the insulatin cnditin. The PI can be trended r cmpared t reference data that prvides an indicatin f the cnditin f the winding. In additin t taking the simple rati, the readings may be pltted in increments ver the test perid, which gives a useful characteristic in additin t the simple rati. Lwer values f PI tend t indicate a prblem with the insulatin system, typically that the insulatin has becme carbnised r has absrbed cntaminatin. A pr reading wuld indicate that additinal testing is necessary Dielectric spectrscpy A typical instrument fr perfrming this measurement is the ALFF Engineering PDC Analyser, where PDC refers t the measurement f plarizatin/deplarizatin currents, typically at 1 r 2 kv. A first winding is cnnected t a vltage surce while a secnd winding is cnnected t a sensitive ammeter. One measurement nrmally cnsists f a set f tw current curves: the plarisatin current curve when applying the vltage and the de-plarisatin curve when switching back t zer vlts. The measurements can ptentially be f several hurs duratin. By a prcess f iterative curve-fitting it is pssible t establish the il cnductivity and the misture cntent f the pressbard Frequency dmain spectrscpy (FDS) Applied vltages d nt usually exceed 200 V during these tests, which can be perfrmed using an insulatin diagnstic analyser such as the Megger IDAX350. This instrument generates diagnstic data using true AC dielectric frequency respnse methds fr reliable test results in high interference envirnments. The resulting test spectrum allws autmated measurement and analysis f misture 20

129 cntent, tan delta/pwer factr and il cnductivity, which can be temperature crrected fr cmparisn with data recrded under different ambient cnditins Sweep frequency respnse analysis (SFRA) In equivalent circuit terms, transfrmer windings cnsist f distributed inductive and capacitive impedances that frm an electrical netwrk with precise (but cmplex) frequency-dependent characteristics. Since the inductances and capacitances assciated with the winding are gverned by the dimensins and psitins f cnductrs as well as the electrical prperties f the assciated insulatin materials, physical changes in gemetry r variatins in material prperties can be detected by the way in which they influence the frequency respnse f the unit, which can be measured externally. This is the basis f the technique knwn as sweep frequency respnse analysis (SFRA), which has been increasingly used t fingerprint transfrmer windings. Applicatins include detecting mechanical defrmatins f the windings caused by shrt-circuit faults, damage t the cre structure as a results f jlts experienced during transprtatin r installatin and slackening clamping frces due t ageing f slid insulatin. These types f mechanical fault are ften hard t detect using ther diagnstic methds. Damage that can be identified by SFRA is nt limited t that assciated with gemetrical changes it can als detect changes in the magnetic circuit f the cre and electrical faults in the windings. Knwn failure mdes that can be detected thrugh SFRA measurements [9] include: Radial defrmatins (hp buckling) Axial defrmatins and displacements Overall bulk and lcalised winding mvements Shrt-circuited turns in windings Lse windings Cre defects Cntact resistance Flating shield SFRA is particularly sensitive t the first fur defect mdes in the list abve, while its ability t detect the ther kinds f defect is dependent n their severity and the type f transfrmer. Since the different types f defect influence the SFRA respnse in different ways, their identificatin smetimes requires a number f cmplementary measurements f different transfer functins. SFRA measures the transfer functin between test terminals and is typically presented as a gain/frequency plt ver the range 20 Hz t 2 MHz. Cnnectins are made externally t the transfrmer terminals s the measurements can be cnveniently perfrmed n-site. While the test is straightfrward in principle, it is essential t bserve the required standards and fllw crrect prcedures t ensure repeatability. The test is sensitive t cnnectins and set-up and requires an awareness f the ptential effects f nise n the measurement instrument. Transfer functins f transfrmers can ften include minima f -90dB r belw, s a wide dynamic range is required and earthing practices are critical. These and ther practical issues such as the influence f residual magnetisatin are cvered in CIGRE Technical Brchure 342, Mechanical cnditin assessment f transfrmer windings using frequency respnse analysis [10]. Open and shrt circuit cnditins can be detected as well as inter-turn cnditins. On a typical duble wund transfrmer, a minimum f 9 individual measurements wuld be taken, frm which it is pssible t detect and diagnse a defect n an individual winding r phase withut the need fr a reference fingerprint. Hwever, the real benefit is gained when individual transfrmers are fingerprinted during cmmissining and then peridically thrughut the life f the transfrmer. The test interval might be every 5 r 10 years under cnventinal perating cnditins, but additinal tests are especially recmmended fllwing a Buchhlz relay trip, clse-in thrugh fault r lightning strike. Transfrmers with aggressive lad cycles, such as tractin, furnace r rectifier units wuld als benefit frm a shrter SFRA testing interval t verify cntinuing integrity f the active part. 21

130 A gd verview f the principles and practice f SFRA can be fund in [11], frm which the examples shwn in Figs. 2.2 and 2.3 are included by way f illustratin. Since the value f SFRA results will be lst if crrect prcedures are nt fllwed, they shuld be carried ut by an experienced peratr if they are t frm part f the fingerprinting prcess fr transfrmers in the LEAN prject. Fig. 2.2 Open-circuit impedance measurement f three identical single phase units that frm a three phase bank. These units had cnsecutive serial numbers. Nte the similarity f the frequency respnse up t at least several MHz [11]. Fig. 2.3 Example f shrt-circuit impedance respnse in the MHz range fr a 10 MVA transfrmer, cmparing a gd reference winding with ne that has experienced significant axial defrmatin [11]. 22

131 2.4 Furanic cmpunds and paper cnditin The material in this sectin firstly summarises hw paper insulatin degrades and lses its strength in a manner that can reduce clamping pressure n the winding (thrugh lss f bulk and rigidity) and cmprmise its ability t withstand transient vervltages. A methd f measuring paper degradatin based n the detectin f furans in the il is then intrduced. Fig. 2.4(a) shws the chemical cmpsitin f a few segments in the cellulse chain, in which the degree f plymerisatin (DP) is equal t the average number f cellulse mnmer units in the chain length. Fr Kraft paper f the quality expected in new pwer transfrmers, DP values in excess f 1000 are expected. Over time, the DP value reduces thrugh schisms between segments due t chemical prcesses, leaving shrtened cellulse chains (with lwer DP values), as illustrated in Fig. 2.4(b). The difference in appearance between new and severely degraded paper is shwn in Fig While this example is at the extreme end f the scale, it serves t illustrate hw chemical degradatin f the cellulse chains leads t a lss f mechanical integrity and resilience. H CH 2 OH C CH 2 OH (a)... C C C C C C... OH C H O H H C OH O H H C OH C OH H H C CH 2 OH O O H C OH C H O H H C OH H CH 2 OH C O O (b)... C C C C C C... OH C H H H C OH H H C OH C CH 2 OH Fig. 2.4 (a) Chemical cmpsitin f the cellulse chain. The DP value is equal t the number f segments in the chain. (b) Splitting f the chain int tw shrter chains as a result f chemical reactin with a water mlecule (images curtesy f TJ H2b). OH C H H O OH H CH 2 OH C OH C H O H H C OH (a) (b) Fig. 2.5 (a) Cnductr newly insulated with il impregnated Kraft paper. (b) Paper insulatin that has been severely degraded as a cnsequence f chemical and thermal actin. Nte the fluffy appearance that results frm the shrtening f cellulse mlecule chains (images curtesy f TJ H2b). 23

132 As the cellulse insulatin ages, its plymer chain breaks dwn. When splitting f the chain liberates a glucse mnmer, it underges further chemical reactin and becmes ne f several furanic cmpunds [12]. These are partially sluble in il and their measurement can be used (with cautin) t give an estimate f the degree f plymerisatin (DP). A widely accepted relatinship between the level f 2-furaldehyde in the il and DP value was develped by Chendng [13], wh derived the fllwing equatin based n field data and labratry studies: DP = 1.51 lg 10(2FAL) where 2FAL is the cncentratin (in ppm, by weight) f 2-furaldehyde in the il. While this equatin was derived frm an extensive study f many units, it shuld be nted that these were predminantly lcated in Eurpe. Cnsequently this expressin is mst applicable t transfrmers that d nt emply thermally-upgraded paper. Mdified equatins have been prpsed fr transfrmers with thermallyupgraded paper [14], where it is cnsidered that the chemicals used t upgrade the paper als have a tendency t break dwn furanic cmpunds. Fig. 2.6 shws Chendng s theretical relatinship between the measured values f 2FAL and the expected DP value f the paper insulatin. Hwever, cnsiderable care is needed in the interpretatin f 2-furaldehyde levels due t issues f thermal equilibrium this tpic is discussed further in Sectin 4 f this reprt DP value furfura l cncentra tin (ppm by we ight) Fig. 2.6 Predictins f DP value as a functin f 2FAL ppm accrding t the Chendng equatin. Testing fr furans is particularly recmmended if a transfrmer is knwn t have been verheated, r presents high levels f carbn mnxide and carbn dixide in the DGA. If the transfrmer is lder than 25 years, sme wuld recmmend furan testing n a peridic basis [15, 16]. This tpic is revisited in Sectin 4.1 f this reprt, which discusses the lss f transfrmer life caused by breakdwn f cellulse, and in Sectin 4.2, which deals with practical limitatins f furan-based analysis when transfrmers underg thermal cycling and describes hw carbn xides shuld als be taken int accunt in the assessment f transfrmer ageing. 2.5 Enabling transfrmer mnitring In 2008, CIGRE Wrking Grup A2.27 published its Technical Brchure, Recmmendatins fr cnditin mnitring and cnditin assessment facilities fr transfrmers [17], the cntent f which is summarised in the fllwing statement: Transfrmer n-line cnditin mnitring has been the subject f significant research and develpment ver the past few years and it is becming increasingly imprtant that mnitring can be easily and safely applied t a transfrmer at whatever pint in its lifetime. The brchure cnsiders the benefits f pssible standardisatin f the interface between mnitring systems 24

133 and the transfrmer with the aim t prvide a guide t the specificatin f a transfrmer intended t be "cnditin mnitring ready." [17]. The dcument sets ut the interfaces that shuld be included t facilitate the installatin f sensrs, rather than the details f the sensrs themselves (recgnising that there are many existing and emerging sensrs available that culd be useful fr n-line mnitring). Hwever, the extent f any sensr installatin facilities/interfaces shuld be selected accrding t the size, age, cnditin and imprtance f the transfrmer with respect t the pwer netwrk. Table 2.1 summarises the recmmendatins fr different levels f mnitring prvisin apprpriate t different categries f transfrmers, which are defined as fllws [17]: Level 1: Level 2: Level 3: Minimum set f sensrs required t prvide basic infrmatin abut transfrmer peratin and the minimum facilities fr further mnitring shuld a fault develp. Sensrs and facilities that can prvide a gd level f infrmatin n transfrmer cnditin, as wuld be apprpriate fr large and imprtant transfrmers. In general, where it is recmmended that a sensr is fitted then it will be mnitred using the substatin SCADA system even if a bespke mnitring system is nt used. Sensrs and facilities f value fr cnditin mnitring, including thse fr specialised and critical applicatins. Including all f these sensrs and facilities n a single transfrmer is unlikely t be ecnmically justified. Where it is recmmended t fit a sensr at Level 3 that is nly a facility at Level 2, this indicates that it is recmmended the sensr shuld frm part f a mnitring system if this is prvided frm new. In additin t regarding the levels f mnitring as increasing with criticality f the unit based n a cmbinatin f its capital value and hw strategically it is lcated within a supply netwrk, it may be wrth escalating units smewhat up this scale n the basis f age and any unfavurable histry with similar units elsewhere n the netwrk. Table 2.1 Summary f the recmmended cnditin mnitring facilities (excluding cler, bushings and tap changer). The 3 Levels relate t the size and imprtance f the transfrmer, as defined abve. Fit means a recmmendatin that the sensr shuld be installed and mnitred frm new. Facility suggests that prvisin shuld be t allw the sensr t be fitted later, shuld it be required. Sensr Level 1 Level 2 Level 3 Tp Oil Temperature fit fit fit Bttm il temperature facility fit Gas-in-Oil Cntent (single utput) facility facility fit Misture in Oil facility fit Oil Level in Cnservatr alarm fit fit Oil Level in Cnservatr indicatin fit fit Multiple gas mnitr facility facility Partial discharge sensr facility facility DC Neutral current facility Magnetic circuit facility facility In additin t the gas-in-il (i.e., DGA) facility, which relates t PD, the ther types f PD sensr fr which an installatin facility is recmmended in Table 2.1 culd include: Electrical PD sensrs (cnventinal r new designs) that make use f bushing tap fittings. UHF PD sensrs, either windw munted r f the prbe type. Fr the prbe type, fitting f additinal DN50 il valves in apprpriate psitins is suggested. Acustic PD sensrs, which, as well as being temprarily munted externally n the tank, might als be intrduced thrugh an il valve prbe type mechanism fr imprved sensitivity. 25

134 2.6 References [1] CIGRE WG B5.05, Technical Brchure 463, Mdern Techniques fr prtecting, cntrlling and mnitring pwer transfrmers, CIGRE (Paris), June 2011 [2] IEC 60599:1999+AMD1:2007 CSV Cnslidated versin, Mineral il-impregnated electrical equipment in service - Guide t the interpretatin f disslved and free gases analysis, Ed. 2.1, May 2007 [3] IEEE Standard C , Guide fr the Interpretatin f Gases Generated in Oil-Immersed Transfrmers, 2008 [4] IEC 60567:2011, Oil-filled electrical equipment - Sampling f gases and analysis f free and disslved gases Guidance, Ed. 4, Oct [5] IEC 60422:2013, Mineral insulating ils in electrical equipment - Supervisin and maintenance guidance, Ed. 4, Jan [6] CIGRE JTF D1.01/A2.11, Technical Brchure 296, Recent develpments in DGA interpretatin, CIGRE (Paris), June 2006 [7] N.A. Bakar, A. Abu-Siada and S. Islam, A review f disslved gas analysis measurement and interpretatin techniques, IEEE Electrical Insulatin Magazine, Vl. 30, N. 3, pp , May/June 2014 [8] M. Duval, A review f faults detectable by gas-in-il analysis in transfrmers, IEEE Electrical Insulatin Magazine, Vl. 18, N. 3, pp. 8-17, May/June 2002 [9] IEEE Standard C , Guide fr the Applicatin and Interpretatin f Frequency Respnse Analysis fr Oil-Immersed Transfrmers, 2012 [10] CIGRE WG A2.26, Technical Brchure 342, Mechanical-Cnditin Assessment f Transfrmer Windings using Frequency Respnse Analysis (FRA), CIGRE (Paris), Apr [11] N Abeywickrama, Detectin f transfrmer faults using Frequency Respnse Analysis with case studies, Chapter 12 in Electrmagnetic transients in transfrmer and rtating machine windings, C Q Su (ed), IGI Glbal, pp , 2013 [12] T.A. Prvst, H.P. Gasser, R. Wicks, B. Glenn and R. Marek, Estimatin f Insulatin life based n a dual temperature ageing mdel, Weidmann ACTI Inc. Fifth Annual Technical Cnference, (Albuquerque NM), Nv [13] I. Chendng, Mnitring paper insulatin ageing by measuring furfural cntents in il, Prc. 7 th Int. Symp. n High Vltage Engineering (Dresden), pp , Aug [14] R.D. Stebbins, D.S. Myers and A.B. Shklnik, Furanic cmpunds in dielectric liquid samples: Review and update f diagnstic interpretatin and estimate f insulatin ageing, Prc. 7 th Int. Cnf. n Prperties and Applicatins f Dielectric Materials, Vl. 3, pp , June 2003 [15] ABB Service Handbk fr Transfrmers, 2 nd Ed., ABB Management Service Ltd (Zurich, Switzerland), 2007 [16] M. Wang and K.D. Srivastava, Review f cnditin assessment f pwer transfrmers in service, IEEE Electrical Insulatin Magazine, Vl. 18, N. 6, pp , Nv/Dec 2002 [17] CIGRE WG A2.27, Technical Brchure 343, Recmmendatins fr cnditin mnitring and cnditin assessment facilities fr transfrmers, CIGRE (Paris), April

135 3. Electrical Discharge Activity 3.1 Partial Discharges (PD) A partial discharge (PD) is a lcalised current pulse caused by inisatin f the insulatin in a regin where the electric field stress exceeds the dielectric strength f the insulating medium. This may be due t an abnrmal structural stress-raiser (sharp prtrusin r edge n a cnductr, metallic inclusin) r a lcalised weakening f the insulatin (vid in slid insulatin, air bubble in il, carbnised r wet pressbard). The term partial discharge arises because the gap between the system cnductrs is nly partially bridged by the inised discharge path. This temprary inisatin may be cnfined t a very small regin (< 1 mm at a sharp metallic prtrusin) r can be dispersed ver quite large areas (many tens f cm in surface tracking n damp pressbard). PD self-extinguishes when the electric field falls belw the level required t sustain further inisatin, but the cyclic plarity reversal f the field in HVAC systems leads t patterns f repetitive re-ignitin. When PD invlves the inisatin f a strng dielectric medium, such as mineral il, the initial breakdwn f the PD pulse ccurs n a very shrt timescale, typically having sub-nansecnd risetimes. Detectable effects f PD arise frm the fllwing phenmena: The shrt risetime f each PD pulse creates a transient electrmagnetic disturbance within the transfrmer tank that can be detected particularly well as a radi signal in the UHF (ultra-high frequency) band. The physical displacement f charge that ccurs during the PD is balanced by a current pulse that is detectable in the external supply circuit using apprpriate frequency discriminatin. This frms the basis f the cnventinal electrical technique fr PD measurement (such as ccurs during transfrmer acceptance tests), defined in terms f an apparent charge in pc by IEC [1]. PD is accmpanied by a rapid expansin f gas within the inised channel, generating an acustic pressure wave, which can be detected at the tank walls using acustic (ultrasnic) sensrs if the signal has sufficient energy. PD als results in the emissin f light frm excited mlecules, and (mst imprtantly fr transfrmers) the creatin f chemical breakdwn prducts. In principle, any f these electrical, physical and chemical cnsequences f PD culd be used t detect its presence. The ABB Handbk Testing f Pwer Transfrmers [2] cntains a wealth f infrmatin abut PD surces in transfrmers, including their physics and their phase-reslved patterns f ccurrence with respect t the applied vltage sinusid (see example in Fig. 3.1). The ABB Handbk als gives a thrugh descriptin f varius electrical and acustic methds fr lcalising PD activity, althugh these are mainly applicable nly under factry test cnditins. 27

136 Fig. 3.1 Representatin f PD pulses (in this case due t bubbles and surface discharge) as a clur intensity plt using an advanced digital PD measurement system [2]. Partial discharges always cause damage t insulatin systems thrugh thermal and chemical mechanisms, althugh their presence des nt always mean that interventin is required. Fr example, capacitive sparking at a lse cre blt will prbably prduce quite a strng DGA signature, but it may be pssible t live with such a defect if it is nly degrading the il but nt the paper (il culd be reprcessed during a shrt utage, if necessary). Hwever, a smaller PD surce lcated n the winding will be burning the paper insulatin, reducing its dielectric strength and creating undesirable areas f cnductivity. Even if this is nt sufficient in itself t cause failure, it may make the unit mre vulnerable t internal flashver in the event f a lighting impulse r a transient vervltage caused by switching. Even PD that takes place purely in the il, withut initially invlving paper, may lead t the prductin f gas bubbles that can rise and becme trapped against anther part f the active circuit. The much lwer dielectric strength f gas cmpared t il, cmbined with its lwer dielectric cnstant, makes such trapped bubbles prne t PD activity that may initiate paper damage in what is therwise a healthy regin f the transfrmer insulatin system. A secnd way in which benign PD can induce physical damage is thrugh the prductin f ins in the il, which tend t becme trapped n insulating surfaces. This prcess alters the electric field distributin intended by the designer and may lead t verstressing f the insulatin at certain pints n the pwer frequency vltage wavefrm. Such phenmena can be initiating factrs fr electrical tracking alng pressbard surfaces, in which lcally trapped charges prduced by the PD exacerbate the electric field and lead t grwth f the tracking in a manner that reduces the safety clearance fr electrical flashver. Persistent electrical discharge activity can als prduce sludge which can then migrate as a result f il flw r gravity (see Fig. 3.2), leading t cntaminatin f ther parts f the insulatin system with this material, which may be partially cnducting due t its carbn cntent that arises frm breakdwn f the hydrcarbn-based insulatin. 28

137 Stain mark frm sludge that flwed thrugh the tube abve site f PD insulating pressbard tubes Fig. 3.2 Internal view f a transfrmer during frensic investigatin f PD activity shwing secndary cntaminatin due t the generatin f sludge. 3.2 Transfrmer PD levels in service In the Duval triangle f Fig. 2.1, what is termed PD ccupies nly a very small regin at the tip f the triangle. Hwever, areas D1 (lw energy discharges), D2 (high energy discharges) and DT (discharges in cmbinatin with thermal issues) als invlve electrical discharge activity that is ccurring n a larger and mre energetic scale. While activity in these regins might mre strictly be defined as ther electrical discharge cnditins, it is usually regarded as an extensin f the PD scale, with D1 and D2 effectively being medium and large PD surces. Small discharges that are strictly in the PD area f the triangle are likely t prve very challenging t lcate when a transfrmer is in service. Hwever, discharges in classes D1, D2 and DT are particularly amenable t the n-site investigatin methds utlined in Sectin 3.3. Taken tgether, PD D1, D2 and DT cver mst f the area f Duval s triangle, which is the reasn fr devting a full chapter f this reprt t a discussin f electrical discharge activity. Infrmatin abut PD levels fr transfrmers in service is scarce, since the cmmnly applied cnditin mnitring technique (DGA) des nt quantify PD in an electrical sense. Prbably the best reference is still CIGRE Brchure 227, Life Management Techniques fr Pwer Transfrmers [3] published in Making reference t papers frm the bibligraphy f this Brchure, the fllwing bservatins have been made n PD levels: Pr impregnatin caused discharges f abut 1,000-2,000 pc. Large (3-5 mm in diameter) air/gas bubbles in il resulted in discharges ranging in magnitude frm 1,000 t 10,000 pc. (this statement is attributed t R T Harrld, The Influence f Partial Discharges and Related Phenmena n the Operatin f Oil Insulatin Systems at Very High Electrical Stresses, IEEE Trans. Electrical Insulatin, Vl. EI -11, N. 1, 1976) Mechanism f PD actin and classificatin f PD fr defect-free and defective insulatin: Defect free pc Nrmal deteriratin <500 pc Questinable pc Defective cnditin pc Faulty (Irreversible) >2500 pc Critical >100,000-1,000,000 pc 29

138 (these levels are attributed t V Sklv et al, On-site PD Measurements n Pwer Transfrmers, Prc. 67 th Annual Internatinal Cnference f Dble Clients, Sec. 8-10, 2000) In general, PD level ver 2500 pc (in paper) and ver 10,000 pc (in il) may be cnsidered as a destructive inizatin in a lng-term actin. (this statement is attributed t S Yakv et al, Crna in Pwer Transfrmers, CIGRE Reprt 12-06, 1968) At the time f writing, CIGRE Wrking Grup D1.29 is wrking n the final draft f its Technical Brchure entitled Partial Discharges in Transfrmers, which includes within its scpe the fllwing: 1. Survey f the available detectin systems (i.e. bandwidth and frequency range) and influence f detectin sensitivity. This includes all methds that can respnd t the individual PD pulse. Fr example, acustic, ultra-high frequency (UHF) and bushing tap measurements are included, but nt disslved gas analysis, which is an imprtant but separate tpic. Fr each technique, practical issues fr deplyment are described, i.e., number f sensrs required, methds f sensr attachment/installatin, etc. 2. Survey f the mre cmmn types f defect that are evidenced by PD t: identify which cmpnents f the transfrmer are mst imprtant fr PD detectin, grup defects accrding t the actins that are mst likely t be taken if they are discvered (e.g., cntinue in service, repair, replace), and infrm the develpment f realistic mdel PD surces that can be used t benchmark different PD detectin techniques. 3. Cllate infrmatin n time-reslved and frequency-reslved signatures frm typical defects and prvide examples frm field measurements f defects, their signatures and characteristics. The interpretatin f cnventinal phase-reslved PD measurements is als cnsidered, since the phase f the lcal electric field at a specific pint in the tank can differ frm the phases at the bushings. 4. Obtain sme cnsensus abut the criteria fr evaluatin f severity f a detected defect (lcatin, defect recgnitin, signal level). Fr example, hw PD threshlds might be defined fr in-service transfrmers. This dcument [4] prmises t be the mst cmprehensive review f the subject fr many years. 3.3 Techniques fr n-site PD investigatins When DGA results indicate the presence f internal discharging that gives cause fr cncern, a prcess f investigatin shuld be initiated t determine what, if any, remedial actin shuld be taken. One apprach t this task is illustrated in Fig While this example is based n the use f ultra-high frequency (UHF) PD sensrs nce internal PD has been cnfirmed, there are ther ptins that can be adpted depending n the facilities available fr installing/attaching PD sensrs. The purpse f the initial site assessment is t establish whether there is evidence f PD activity internal t the transfrmer that can crrbrate the DGA interpretatin. Such measurements can be carried ut with a hand-held RFI (radi frequency interference) detectrs such as the Dble PDS-100 shwn in Fig In additin t antenna-type sensrs, systems that use high frequency current transfrmers (HFCTs) r transient earth vltage (TEV) sensrs can be used. These are illustrated in Fig When the level f PD activity inside the transfrmer tank is regarded as unacceptable fr lng-term peratin, the pssibility f repair must be cnsidered. As this is a cstly and invasive prcess, it is imprtant that the surce f PD be lcalised as accurately as pssible while the transfrmer is still in service. This is particularly true because the physical signs f PD may nt be evident even if the 30

139 transfrmer is de-tanked (fr example, if PD was taking place underneath a thick wrapping f paper insulatin). Methds t lcate PD usually invlve deplying a number f sensrs arund the tank and using differences in time f arrival f PD pulses t triangulate their pint f rigin. Fig. 3.6 shws acustic sensrs that have been magnetically attached t the transfrmer tank fr this purpse. Fig. 3.3 Suggested investigatin prcess fr pwer transfrmers with suspected PD r ther internal electrical discharge activity. In this example, UHF sensrs are prpsed, but alternative ultrasnic r electrical sensrs may be used accrding t the available ptins fr sensr munting/installatin. (a) (b) Fig. 3.4 (a) RFI surveys using hand-held equipment with a small antenna can help t cnfirm the rigins f PD activity in a substatin envirnment. (b) If the ptin exists t intrduce a UHF prbe-type PD sensr thrugh a transfrmer il valve, as shwn here, additinal certainty can be gained. 31

140 (a) (b) Fig. 3.5 Methds that can be used t cnfirm the presence f PD in a transfrmer withut an utage include (a) high frequency current transfrmer (HFCT) devices, and (b) transient earth vltage (TEV) sensrs. ultrasnic sensr fr PD detectin accelermeter (lw frequency) t detect mechanical issues signal acquisitin hardware Fig. 3.6 Acustic sensrs fr detecting prblems internal t the tank while the transfrmer is in service. Nte that these sensrs can be cnveniently attached using pwerful magnets. Using either a set f acustic (usually ultrasnic) PD sensrs, UHF PD sensrs, r a cmbinatin f bth types, time f flight (r time-difference f arrival) methds can be used t triangulate the surce f PD activity. The main reasn fr perfrming this test wuld be t assess whether the PD was in a lcatin where it might give rise t imminent failure f the unit, r whether at the ther extreme, the PD activity might be tlerated indefinitely. Knwledge f the PD lcatin can als help t infrm the decisin as t whether an n-site repair might be necessary / pssible. Fig. 3.7 shws a set f UHF signals used t successfully lcate a PD surce in a distributin transfrmer, the internals f which are shwn in Fig Further details f this apprach can be fund in [5] and the principles f acustic-based lcatin techniques are very similar. 32

141 Fig. 3.7 A set f fur UHF signals captured simultaneusly frm a PD f 200 pc during a 1.3 U verptential test n the transfrmer represented in Fig. 3.8 belw [5]. Fig. 3.8 Lcatin f a PD surce at the tp f phase A winding using UHF time f flight methds, as described in [5]. 33

142 3.4 References [1] IEC 60270:2000, High Vltage Test Techniques - Partial Discharge Measurements, Ed. 3, Dec [2] A. Carlsn, J. Fuhr, G. Schemel and F. Wegscheider, Partial Discharge Measurements, Ch. 9 in Testing f Pwer Transfrmers - Rutine tests, Type tests and Special tests, ABB AG, Zurich, 1 st Editin, 2003 [3] CIGRE WG A2.18, Technical Brchure 227, Life Management Techniques fr Pwer Transfrmers, CIGRE (Paris), June 2003 [4] CIGRE D1.29, Technical Brchure TBD, Partial Discharges in Transfrmers, in preparatin, 2015 [5] M D Judd, Partial discharge detectin and lcatin in transfrmers using UHF techniques, Ch. 13 in Electrmagnetic transients in transfrmer and rtating machine windings, C Q Su (ed), IGI Glbal, pp ,

143 4. Managing Transfrmer Health and Life 4.1 Cnsumptin f paper life As mentined at the utset, fr a well-maintained transfrmer in which misture levels are prperly managed, the mst significant issue gverning life (excluding external catastrphic influences) will be its thermal perating regime. Transfrmer il des nt degrade significantly belw abut 140C, but it is generally accepted that paper insulatin degrades with rapidly increasing severity if its temperature rises abve abut 90C [1]. IEC Pwer Transfrmers: Temperature Rise [2] and ther standards set ut limits fr permissible temperature rise that are dictated by cnsideratins f achieving a service life f at least 30 years. A cmmnly adpted limit fr ht spt temperature in this regard is 98C. A nte f cautin is necessary at this pint, in that it is widely recgnised that levels f misture, acidity and xygen cntent f the il are all factrs that will influence the rate f ageing at high temperature and these in turn are dependent n the integrity f the transfrmer and its breathing system and maintenance [1]. Further details cncerning thermal issues can be fund in the relevant standards such as [2] and IEC Pwer transfrmers: Lading guide fr il-immersed pwer transfrmers [3]. The three agents f cellulse ageing are water (hydrlysis), xygen (xidatin) and heat (pyrlysis) [4]: In hydrlysis, a single water mlecule reacts with the xygen bridge between adjacent mnmers in the cellulse chain t frm tw OH grups, each attached t the mnmers that nw frm the ends f tw separate chains. This prcess des nt generate any gases r residues. In transfrmers where xygen is present, O 2 reacts with carbn atms in the glucse rings t frm aldehydes and acidic grups. In sme cases the damaged glucse ring breaks dwn with the liberatin f gases including CO, CO 2 and H 2, which frm indicatrs in the DGA. In particular, CO and CO 2 frmatin increases with temperature and with the xygen cntent f il and the misture cntent f paper [5]. Pyrlysis refers t thermal damage that degrades cellulse withut the requirement fr water r xygen, leading t the frmatin f charred residues and anhydrides that may cntribute t sludge frmatin. Bth xidatin and pyrlysis prduce water, which may cntribute t, and be partly cnsumed by, further hydrlysis. An illustratin f the penalties f perating transfrmers utside the nrmal thermal perating regime (typically a ht spt temperature f 98C with an ambient temperature f 20C) is given in [7]. During times when the ht spt temperature is abve that crrespnding t nrmal ageing, insulatin life is being used up faster than the rate crrespnding t nrmal life expectancy. If this is nt t reduce the verall life expectancy, there must be balancing perids during which the insulatin life is being used up less rapidly than nrmal. Using an expressin derived frm the Arrhenius law f chemical reactins, an expressin fr the rate f ageing, r, can be derived [6] in a frm that is widely accepted (althugh different investigatrs take different views n the criteria fr end f life and the exact rate cefficient fr ageing): r = Me pθ where θ is the temperature in C, p is the cefficient f temperature variatin and M is a cnstant that depends primarily n the level f misture and availability f xygen. The cefficient p can be regarded as cnstant ver the temperature range C and is generally accepted t have a value such that the rate f ageing dubles with every 6C increase in temperature fr mst f the slid insulating materials used in transfrmer cnstructin. Using this value f p fr the purpse f illustratin and using 98C as the reference ht spt temperature fr nrmal ageing, the relative ageing rate R at any ther temperature θ h in the range C wuld be: R = 2 (θ h 98) 6 35

144 This equatin predicts that 24 hurs f nrmal ageing life wuld be cnsumed within 1 hur at a ht spt temperature f abut 125C. Table 4.1 lists the predictins f this equatin in terms f relative rate f life cnsumptin ver a range f ht spt temperatures. Table 4.1 Predictins f relative ageing rates as a functin f ht spt temperature relative t nrmal ageing rated fr a ht spt temperature f 98C. ht spt temperature, θ h relative rate f life cnsumptin, R 86C C C C C C C C Thermal cycling and intermittent lading Building n the discussin f furan measurements and cnditin f insulating paper, a significant analysis f experience with UK transfrmers was reprted in [7], which reviews the main areas f uncertainty invlved in estimating the residual life f transfrmer insulatin frm a practical user s pint f view. The authrs agree with the prpsal that a DP value f 200 is a reasnable criterin fr end f life but nte that published labratry derived crrelatins f furan v. DP appear t significantly verestimate furan levels seen in real transfrmers fr a number f reasns, and therefre will lead t serius underestimates f insulatin ageing [7]. They attribute this discrepancy t a need t take int accunt temperature dependence f the measured furan cncentratins and the effects f il prcessing. This review makes it clear that it is nt pssible t evaluate residual life n the basis f simple threshlds n furan levels it requires a cnsiderable knwledge f the assets themselves. On the ther hand, an interesting suggestin within this paper, based n evidence frm large transmissin transfrmers that have been scrapped in the UK is that it culd even be argued that fr practical purpses it is nly necessary t identify and assess units with pr cling r thermal faults ageing fr ther units nt being a significant issue [7]. Overall, it must be cnsidered that furans are smewhat unstable measurands and diagnses f paper cnditin shuld nt be cnducted n the basis f a snapsht furan measurement alne. A further interesting cncluding bservatin made in this paper is as fllws: Anther indicatr f paper ageing is said t be prvided by carbn mnxide and dixide levels in disslved gas analysis results. Traditinally the CO 2 : CO rati is suppsed t be the key indicatr f nrmal r abnrmal ageing, but UK experience with mainly free breathing transfrmers is that CO 2 levels shw randm variatins, presumably being affected by ambient cnditins, and are therefre nt reliable indicatrs. Hwever, CO cncentratins are very cnsistent ver time and between transfrmers, nrmal levels fr UK transmissin transfrmers being abut 250 ppm, and fr generatr transfrmers smewhat higher, perhaps up t 600 ppm, while levels arund 1,000 ppm wuld be f cncern. UK experience is that transfrmers that exhibit higher than average furan levels als exhibit higher than average CO levels. Therefre CO appears t be a useful cmplementary indicatr f paper ageing, which wuld be useful when furan results are missing r in dubt. Nte als that CO levels are less affected by il prcessing, returning t previus levels within abut 6 mnths [7]. 36

145 CIGRE Technical Brchure 323, Ageing f cellulse in mineral-il insulated transfrmers [8], makes similar bservatins, alluding t cntradictry evidence that has led t a mre reluctant attitude twards accepting simple relatins between furan cncentratins and winding ageing. The fllwing cmments are als made with regard t CO and CO 2: Water and carbn xides are the main by-prducts frm cellulse degradatin. Hwever, ther degradatin prcesses prduce these gases and they may cme frm the utside atmsphere. Since the cellulse material is the main prducer f carbn xides these gases can be used fr a rugh assessment f the cellulse ageing. This assessment is based n gas levels, prductin rates and n ratis. Recently, CIGRE JTF D1-01/A2-11 made an extensive summary f typical abslute values and gas increase rates [8]. Cnstant lading regimes are usually less nerus fr the transfrmer, as mechanical stability is established at cnstant temperature and there is a stable equilibrium f misture distributin between the il and paper. Variable lading regimes, such as thse illustrated in Fig. 4.1, will have different effects n transfrmer ageing. Generatr step-up transfrmers r industrial furnace transfrmers are prbably the best examples fr which a very large n/ff swing in terms f lading may be experienced. When the lading is high and pwer dissipatin in the transfrmer increases, water migrates frm paper t il. When the lad is remved and cling ccurs, water migrates back int the paper. In the mst extreme cases, when a transfrmer is n full lad fr a lng perid f prlnged heating, misture migratin t the il will be at a maximum. If the transfrmer is suddenly de-energised and cls rapidly, cndensatin f discrete water drplets in the il may ccur. Being denser than il, these drplets tend t cllect at the bttm f the tank as free water. Ptential prblems include crrsin, but additinally, if water drplets becme mbile in circulating il, they may cme t rest n insulatin surfaces and lead t misture damage r the initiatin f PD. Lading 25 % duty 120 % lad 50 % duty 60 % lad 100 % duty 30 % lad 120% 60% 30% 0% 0 24 hurs 48 hurs Time Fig. 4.1 Different lading regimes with the same average level will result in different rates f cnsumptin f the paper insulatin life. A discussin f thermal cnsideratins fr insulatin life in generatr step-up units (GSUs) where thermal lading is mre predminant is given in [9]. A mre practical apprach that has been prpsed fr evaluating the ageing f distributin transfrmers n the basis f daily lad prfiles is given in [10]. Further infrmatin n transfrmer lading practice can be fund in the standard IEC [3]. Prper management f misture in transfrmers is clearly ne f the main keys t lngevity. Misture levels can be measured r mnitred using prbes such as thse manufactured by Vaisala. Their website [11] lists a range f misture sensrs that are suitable fr installatin n pwer transfrmers. An 37

146 interesting new prduct sn t be released is the MHT410 misture, hydrgen and temperature transmitter fr nline transfrmer cnditin mnitring. The Vaisala MHT410 is claimed t measure directly frm representative transfrmer il, giving bth reliable hydrgen trend as well as fast misture data. Misture readings btained frm the transfrmer will t an extent depend n the psitining f the prbe insertin pint, regardless f any temperature cmpensatin applied t the misture reading itself. Therefre, if misture is an issue it may be wrth emplying mlecular sieve technlgy t keep the transfrmer il dry. These systems wuld nrmally be equipped with relative humidity sensrs n the incming and utging pipes. A typical justificatin fr and descriptin f such a system, taken frm the Transec website [12] is as fllws: Excessive misture will saturate the insulatin and increase its cnductivity. At higher temperatures vapur r free misture can develp increasing the risk f partial discharge. TRANSEC is an n-line mlecular sieve, designed t cntinuusly remve water frm the insulating il f a pwer transfrmer. As the ppm level in the il is reduced, water frm the slid insulatin (where ver 95% f the water will accumulate) will migrate frm the insulatin int the il t redress the natural equilibrium. This prcess nt nly reduces ageing, but will imprve the dielectric strength f the insulatin, and increase reliability. Fig. 4.2 shws an installed Siemens drying system that als emplys mlecular sieve technlgy. In additin, a number f cmmercial nline DGA systems are available. These shuld be preferably f the 8 gas type, but they are nrmally nly used n transfrmers at transmissin vltage levels. Fig. 4.2 Open frame versin f Siemens SITRAM DRY uses mlecular sieve technlgy t maintain lw levels f il misture. 38