DISCRIMINATION BETWEEN INRUSH AND FAULT CURRENTS IN AN UNLOADED THREE PHASE POWER TRANSFORMER BASED ON PRE-FLUXING AND HARMONIC ANALYSIS TECHNIQUES

Transcription

1 DISCRIMINATION BETWEEN INRUSH AND FAULT CURRENTS IN AN UNLOADED THREE PHASE POWER TRANSFORMER BASED ON PRE-FLUXING AND HARMONIC ANALYSIS TECHNIQUES by SHANTANU KUMAR (Achieving inernaional excellence) This hesis is presened for he degree of Maser of Engineering Science (Research) of The Universiy of Wesern Ausralia Energy Sysem Cenre The School of Elecrical, Elecronics, and Compuer Engineering 03

2 Shananu Kumar 5, Braemoun Road, Darch, Perh, WA 6065 Ausralia @sudenuwaeduau Mobile : The 3s of July, 03 The Head of he School, School of Elecrical, Elecronics and Compuer Engineering The Universiy of Wesern Ausralia Nedlands, WA,6009 Ausralia Dear Sir, I wish o submi his hesis iled : "DISCRIMINATION BETWEEN INRUSH AND FAULT CURRENTS IN AN UNLOADED THREE PHASE POWER TRANSFORMER BASED ON PRE-FLUXING AND HARMONIC ANALYSIS TECHNIQUES" as par of he requiremen for he degree of Maser of Engineering Science Yours sincerely, (SHANTANU KUMAR) i

3 ACKNOWLEDGEMENTS I express my sincere graiude o my supervisors, Associae Professor Vicor Sreeram and Associae Professor Tam Nguyen for giving me an opporuniy o underake his research Due o heir able guidance, consan suppor and invaluable encouragemen hroughou my Maser's candidaure, I could complee he hesis in spie of my full ime employmen I specially hank Associae Professor Vicor Sreeram and DrSushama Rajaram Wagh for he pains aken by hem o proof read he conens and answer any echnical queries beyond heir official duy hours on his hesis My boundless graiude o my parens who passed away during he course of my maser's sudy They coninuously inspired me o pursue academic excellence and succeed in life I also hank my wife, Naasha and daugher, Radhika who sacrificed heir precious ime and curbed heir insinc o underake holidays, during my candidaure I also ake his opporuniy o hank all my brohers, sisers and siblings in India and oher counries who gave me moivaion and encouragemen o work owards my objecive I ake his opporuniy o express my graiude o DrVan Liem Nguyen for imparing his knowledge in power sysem and giving necessary correcions he simulaion and injecing houghs on new areas o be probed ino his research I also hank my friend Mr Pariosh Tripahy who helped me in he presenaion of his hesis and colleagues a Wesern Power, Mr Don Wijayasinghe, MrKerry Williams and Mr Dilan Amarasinghe who suppored me in my academic pursui Finally, I would like o hank Dr Sao Juniper and oher saffs in UWA graduae research school office, who suppored my candidaure during my difficul period and were exremely flexible in allowing me o complee he hesis in spie of having a challenging personal circumsance ii

4 ABSTRACT This hesis is devoed o he applicaion of suiable echniques o miigae inrush phenomenon and discriminae inrush from fauls in a hree phase unloaded power ransformer Furher, his hesis applies appropriae echnique o esimae and eliminae DC componen embedded in he inrush and faul wave during he power ransformer energizaion The echnique used for miigaing inrush applying pre-fluxing is based on seing he power ransformers residual flux o a known polariy afer he ransformer has been de-energized and conrolling he incomer circui breaker closing ime The device used in he pre-fluxing echnique is simple o use and easy o consruc and doesn' require prior flux knowledge of he ransformer core The key driver of his device is a pre-fluxing device which can operae a a lower volage level as compared o he overall volage of he ransformer Using his model a specific flux paern is esablished in he power ransformer, prior o is energizaion in an unloaded condiion In he second par of he pre-fluxing echnique applicaion, he circui breaker associaed wih he power ransformer is energized a a posiive or negaive polariy Wih he applicaion of his echnique, inrush is grealy reduced and is very close o prospecive flux during ransformer energizaion The moivaion o apply pre-fluxing echnique is validaed using sofware wherein he inrush is reduced considerably wih respec o he normal curren as opposed o 0 imes inrush wihou pre-fluxing device Having esablished he pre-fluxing echnique mehod of miigaing inrush in he fourh chaper, a mehod o esimae and eliminae unwaned DC componen embedded wihin inrush and faul is aken up in he fifh chaper, based on harmonic analysis The pracical problems associaed wih DC componen on he proecion sysem has been elaboraed in his hesis including, he fundamenal concep and iniial appearance of DC componen Due o presence of he DC componen he proecion sysem response is slow and usually delays he discriminaion abiliy of he relay One of he significan conribuions in his hesis is he esimaion and iii

5 eliminaion of DC componen appearing as a noise and idenifying inrush from faul condiion based on second harmonic raio o fundamenal, of an unloaded power ransformer In order o eliminae he DC componen, a compensaing signal is generaed based on Taylor series expansion and Leas Square Mehod (LCM) for he inrush and faul curren As he second harmonic raio (SHR) is a dominan feaure in he inrush, discriminaion has been carried ou by comparing i wih a prese value Deails on SHR are found in he review chapers of his hesis The algorihm developed o discriminae inrush from faul curren based on harmonic analysis is validaed using MATLAB Alhough wo case sudies of discriminaion and miigaion of inrush and faul currens has been modelled and simulaed on a sep-up power ransformer bu his concep of pre-fluxing and DC componen esimaion and suppression could be exended o oher vecor groups of hree phase power ransformers in he nework which is a fuure scope of work for his auhor The noable conribuions in his hesis has been he applicaion of he pre-fluxing echnique, miigaion of DC componen and applicaion of SHR o discriminae inrush from he faul The research can be exended o ulra high volage raed power ransformer paricularly operaing a 800kV and 00kV as he resuls obained for inrush by swiching on a 0kV power ransformer canno be exrapolaed o ha on an 800 kv ransformer which is ransmiing power a an ulra high volage level iv

6 Table of Conens ACKNOWLEDGEMENTS ii ABSTRACT iii Table of Figures Lis of Tables 3 Chaper Inroducion 4 BACKGROUND AND SCOPE OF THE RESEARCH 4 OBJECTIVES 6 3 OUTLINE OF THE THESIS 7 4 CONTRIBUTIONS OF THE THESIS 8 Chaper Power Transformer Proecion and is issues 0 INTRODUCTION 0 EQUIVALENT CIRCUIT AND PHASOR DIAGRAMS OF CT 3 Transformaion Errors 9 Curren Error (Raio Error) 0 3 Phase Error 4 Composie Error 3 TYPICAL CHARACTERSTICS OF AN IDEAL CT 3 4 CT BURDEN EXPLAINED 4 5 CT CHARACTERSTICS AND ITS EFFECT ON PROTECTION RELAY 5 6 DIFFERENTIAL PROTECTION 6 7 FUNCTION OF INTERPOSING CT (ICT) 8 8 CT SELECTION AND APPLICATION OF STANDARDS 8 8 Accuracy Class 30 9 FACTORS DETRMINING CT RATIO AND SELECTION 30 0 NEW TRENDS IN CT MANUFACTURE 3 v

7 TRANSIENTS AND ITS EFFECT ON PROTECTION SYSTEMS 3 EFFECT OF HARMONICS ON POWER TRANSFORMER 34 3 PRINCIPLE OF DIGITAL RELAYS 36 4 POWER TRANSFORMER PROTECTION 43 5 SHORT CIRCUIT CURRENTS AND ITS EFFECT ON POWER TRANSFORMER 44 6 EFFECTS OF SHORT CIRCUIT ON POWER SYSTEM AND OTHER MECHANICAL APPARATUS 45 7 CONCLUSION 46 Chaper 3 Algorihms for he Proecion of a Power Transformer 47 3 INTRODUCTION 47 3 OVERVIEW OF THE UNIT PROTECTION SCHEME APPLICATION OF ALGORITHMS IN DIGITAL DIFFERENTIAL RELAYS 5 33 Finie duraion impulse response filer mehod (FIR) Fourier analysis mehod Flux based algorihm Leas Square Mehod REVIEW OF DIGITAL DIFFERENTIAL PROTECTION ALGORITHM COMMON METHODS FOR DETRMINING INRUSH AND FAULTS Harmonic resrain mehod Waveform based resrain mehod Flux resrain mehod 7 36 ANALYTICAL EXPRESSION FOR INRUSH CURRENT 7 37 CONCLUSION 75 Chaper 4 Miigaion of Inrush Curren in a Three phase Power Transformer using Pre-Fluxing Technique 76 4 INTRODUCTION 76 vi

8 4 NATURE OF INRUSH TRANSIENTS PRE-FLUXING TECHNIQUE MITIGATION OF INRUSH CURRENT IN TRANSFORMERS USING PRE- FLUXING 8 44 Sep-: Pre-fluxing device 8 44 Sep II: Conrolled Swiching 8 45 MODELLING OF TRANSFORMER FOR INRUSH CURRENT STUDY SIMULATION RESULTS Inrush curren in power ransformer wihou using pre-fluxing device Harmonic analysis wihou filers Inrush curren in ransformer using pre-fluxing Inrush curren using pre-fluxing in ransformer wih filer 9 47 CONCLUSION 94 Chaper 5 Eliminaion of DC Componen and Discriminaion of Inrush and Faul based on Harmonic Analysis mehod 95 5 INTRODUCTION 95 5 BACKGROUND OF DC COMPONENT DISCRIMINATION OF INRUSH FROM SHORT CIRCUIT AND ELIMINATION OF DC COMPONENT MODELLING OF THE INRUSH CURRENT Esimaion of DC componen from inrush curren Esimaion of DC componen from faul curren Discriminaion beween inrush and faul curren SIMULATION RESULTS Case I : Discriminaion beween inrush and faul curren Case II: Three phase power ransformer under faul condiion 56 CONCLUSION Chaper 6 Conclusions and Fuure Work 3 vii

9 6 CONCLUSIONS 3 6 FUTURE WORK 5 Bibliography 8 Appendix A: Deerminaion of DC and AC Componens during ransiens of an unloaded Transformer Appendix:B Publicaions 6 viii

10 Table of Figures Figure Curren ransformer and impedance burden [8] 4 Figure Equivalen circui referred o primary [8] 5 Figure 3 Equivalen circui as refrerred o Secondary [8] 6 Figure 4 Phasor diagram of a CT referred o he secondary [8] 7 Figure 5 Phasor diagram of CT having inducive burden [8] 8 Figure 6 CT open circui exciaion characerisics [8] 3 Figure 7 Burden conneced o a secondary circui of a CT [8] 5 Figure 8 Biased differenial relay wih inerposing CT [8] 7 Figure 9 Incorrec swiching ime leads o inrush [] 34 Figure 0 No inrush occurrence due o correc swiching ime [] 34 Figure Block diagram of a ypical digial relay [5] 40 Figure Surge proecion circui [5] 4 Figure 3 Characerisics of an (a) ideal filer response (b) pracical filer response of a low pass filer [5] 4 Figure 4 Time course of AC volage [6] 45 Figure 3 Basic uni proecion scheme of a ransformer [7] 49 Figure 3 Three phase dela-sar ransformer wih bias se on he differenial relay [7]5 Figure 33 Typical dual slope bias characerisics of a differenial relay [7] 5 Figure 34 Impulse responses of FIR filers [5] 56 Figure 3 5 Magniude of he frequency response of filers [5] 57 Figure 36 Two winding Transformer [5] 6 Figure 37 Transformer magneising curve [5] 6 Figure 38 Faul and non faul regions in dψ/di- i plane [5] 63 Figure 39 Leas square curve fiing mehod [5] 64 Figure 30 Block diagram for deermining he Second Harmonic Raio (SHR) [5] 67 Figure 3 Transformer Equivalen Circui 7 Figure 3 Simplified wo slope sauraion curve 7 Figure 4 Pre-fluxing device 8 Figure 4 Connecion of pre-fluxing device in a hree phase power ransformer 8 Figure 43 Three - phase simulaneous conrolled swiching wih phase volage 83 Figure 44 MATLAB model o deermine inrush curren in an unloaded ransformer 84 Figure 45 Inrush curren and fluxes in individual phase and collecively 87 Figure 46 Magniude of harmonics wihou filers 88

11 Figure 4 7 (a), (b) and (c) Inrush curren in Phase A,B and C, 9 Figure 4 8 (a), (b), (c) Harmonics in differen phases wih pre-fluxing 93 Figure 5 Saus of DC componen during closing of an inducive circui 98 Figure 5 Single line diagram of he nework simulaed 0 Figure 53 MATLAB model for harmonic analysis of inrush and faul curren 07 Figure 54 Magniude of inrush curren a zero degree energizaion 08 Figure 5 5 Harmonic conens presen in phase A inrush curren 09 Figure 56 DC componen in inrush curren 09 Figure 5 7 Digial filer used o compensae DC componen 0 Figure 58 Signal for compensaion of DC componen 0 Figure 5 9 Inrush and faul curren Figure A DC Componen characersic 5 Figure A AC componen characerisics 5

12 Lis of Tables Table CT raing plae 9 Table Ampliude of harmonics presen during inrush [9] 35 Table 4 : Summarizing he comparaive resuls for harmonic analysis for hree phases wihou applying pre-fluxing 89 Table 4 Comparison of inrush curren before and afer miigaion 9 Table 43 Harmonics in differen phases of an unloaded ransformer afer applying filers and pre-fluxing device 93 Table 5 RMS value of inrush currens for various swiching insans (wih and wihou faul condiions) of one phase A 3

13 Chaper Inroducion BACKGROUND AND SCOPE OF THE RESEARCH Rapid growh in primary plan asses and he complexiy involved in he power sysem nework has lead he operaors and asse owners o focus on proecing hese vial equipmens Power sysem operaors are emphasising on he reliabiliy of proecion design Sysem engineers are sriving owards improving he proecion, by invening novel engineering echniques o keep power sysem healhy Power sysem is severely esed for reliabiliy in he adven of a faul which could be due o he applicaion of nonlinear devices, swiching operaions, large inerconneced grids and harmonics in he sysem Researchers are consanly upgrading exising mehods and inroducing novel ones Wih a compeiive marke environmen in which power sysems grid currenly operaes, he need for reliable and robus proecion of he cosly asses like power ransformers, ransmission lines, underground high volage cable feeders, circui breakers, capacior banks, reacor banks ec couldn' be overemphasised Wih he applicaion of compuer in he power sysems, exensive design and research is being carried ou o proec sysem asses prevening damage due o shor circui or oher fauls Sysem engineers are consanly working owards providing consumers wih a reliable power flow which is of high significance o he indusry and uiliy nework operaors Sysem engineers ensure ha he nework remains seady and power flow remains smooh in he even of spurious posiive spikes like inrush hereby 4

14 prevening inadveren relay operaion Furher, proecion engineers by applying discriminaive proecion, isolae fauly equipmens ou of nework speedily in mili seconds while mainaining coninuing power flow in he healhy secion of he nework In a ypical case, he consequences of a fauly sysem could lead o one of he following:- May cause generaors o go ou of synchronizaion in a generaing saion, hereby risking sysem wide insabiliy Risk of causing damage o men and maerial in he saion (ie arc flash or blow ou) 3 Propagaion of fire due o faul in a safe plan In order o preven fauls affecing he nework, sysem engineers are consanly designing new relays wih smarer logics Today digial relays wih smar algorihm are easy o use and occupy less space in he conrol room These digial relays enhance sysem sabiliy agains fauls and quickly isolae fauly secion Failure of he sysem or nework could occur due o :- Incorrec seing of he proecion relay Mechanical failure of he equipmen 3 Inrinsic design or componen failure 4 Mal-operaion of proecion 5 Incorrec insallaion 6 Elecrical inerference 7 Mechanical vibraions Given he cos associaed wih he primary plan asses, i is of vial ineres o he asse owner o ensure he equipmens in he nework are adequaely proeced from abnormal ransiens o mainain a seady sae supply I is a difficul proposiion o weigh he cos associaed in employing iers of proecion agains he cos associaed in having basic proecion compromising he sysem reliabiliy and o spell ou wha is he opimum amoun o be invesed in order o proec hese asses In a disribuion sysem, speed of faul clearance is relaively less imporan in comparison o a ransmission sysem 5

15 Transmission sysem faul clearance is of much imporance, as i forms backbone of he nework and ouages have a significan impac on he number of he indusrial and zone cusomers affeced Among he elecrical proecions available, he mos popular one which reliably proecs power ransformer is differenial proecion Power ransformer need proecion agains nuisance ripping and faul occurrence during energizaion a he ime of is swiching In his hesis, wo mehods have been proposed o reliably miigae inrush a saring and discriminae inrush from fauls during iniial swiching operaion of an unloaded ransformer OBJECTIVES The main objecive of his hesis is o miigae inrush and idenify inrush from fauls in an unloaded hree phase power ransformer locaed in a generaing subsaion The idea of proecion of he unloaded power ransformer is based on pre-fluxing and second harmonic raio algorihm This hesis has been broadly divided ino six pars wih firs par saing he moivaion o carry ou he research, second par reviewing differenial proecion scheme and is associaed componens eg curren ransformers, circui breaker swiching ec The hesis also gives an overview of inrush and shor circui generaion and is effec on he nework Furher, i also gives an overview of he principles of modern digial relays from differenial relaying perspecive and he reason for inclinaion owards microprocessor based relays Third par overviews he equaions associaed wih digial differenial relay proecion Fourh par relaes o he pre-fluxing mehod [] and applies a echnique o miigae inrush on a sofware model Fifh par discusses esimaion and eliminaion of DC componen and discriminaion of inrush from shor circui [] Sixh chapers conclude he hesis and emphasises he fuure work ha could be underaken based on he given echniques on ulra high volage power ransformer Summarizing, he objecive of he hesis which is divided ino he following:- (a) Power ransformer proecion and issues: This par discusses proecion schemes associaed wih emphasis on differenial proecion, curren ransformer errors and 6

16 applicaion o he proecion, inrush and shor circui phenomenon and fundamenals of digial differenial proecion using block diagram and he algorihm applied (b) Pre-fluxing and harmonic analysis echniques: mehodology of pre-fluxing and harmonic analysis echniques employed in power ransformer proecion including oulining of algorihmic models and daa analysis inerpreing he simulaion carried ou using MATLAB This par discusses he eliminaion of DC componen and recommends second harmonic echnique (SHR) o compare and idenify inrush and faul curren in an unloaded power ransformer (c) Recommendaion: Pre-fluxing and harmonic analysis mehodology resuls simulaed in MATLAB are analysed The generaed resuls are inerpreed for sep up ransformers o show he benefis of he echniques (d) Fuure work: Wih he use of compuer and applicaion of digial based relays in he modern power sysem, he reliabiliy, speed and discriminaion capabiliy of he relay has been significanly enhanced However, here are number of challenges which sill lies unresolved due o parallel operaion of power ransformers, mal operaion of he relays due o ap change, differen vecor groups of ransformers conneced o he bus in a generaing and ransmission subsaions, ec Fuure work shall moivae researchers o produce novel echniques which are no only reliable and accurae bu quick o ac in isolaing hese asses during inrush and faul condiion, saving he operaor cosly ouages and financial burden due o asse replacemen [3] 3 OUTLINE OF THE THESIS This hesis is organised in six main chapers Begining wih he background and scope of he research, he firs chaper presens he objecive and moivaion leading o he need for research on he improved mehod of proecion during iniial energizaion of an unloaded ransformer Chaper revisis exising power ransformer proecion schemes and discusses radiional elecromechanical and solid sae relays wih respec o modern digial differenial relays Furher, i discusses he inrush and shor circuis occurrences in he 7

17 sysem and he impac of ransiens on he proecion sysem I also addresses he issues of DC componen during inrush and shor circui and examines he impac of DC componen on he proecion sysem Chaper 3 presens modelling of power ransformer proecion schemes including various mahemaical models which goes ino he relay logic I also reviews some of he popular algorihmic echniques used in he relay logic wih special emphasis on leas square mehod (LSM) and second harmonic raio (SHR) echnique Chaper 4 elaboraes on he need for pre-fluxing echnique, sauraion of curren ransformer, conrolled swiching of he circui breakers A compuer based model is simulaed on MATLAB applying pre-fluxing echnique and he resuls obained reflec significan miigaion of inrush by insering flux ino he power ransformer [4] Chaper 5 oulines inrush and shor circui phenomenon I also discusses he DC Componen issues and proposes a mehod of is eliminaion A case sudy is underaken of an unloaded sep up ransformer wherein simulaion during inrush and faul is carried ou on MATLAB Comparing he resuls abulaed using SHR mehod which is based on LCM echnique, idenificaion beween inrush and faul currens is carried ou This logic is incorporaed ino relay logic for proecion purpose Chaper 6 deals wih conclusions and fuure work on he mehods exhibied based on pre-fluxing and harmonic analysis echniques and gives a scope for he fuure research o be underaken on power ransformer proecion in he generaing and ransmission subsaions 4 CONTRIBUTIONS OF THE THESIS The hesis has made wo original conribuions as described in he following: (a) Pre-fluxing echnique used in reducing inrush during energizaion of an unloaded hree phase power ransformer In his echnique, a device having residual flux of known polariy is insered ino an unloaded ransformer This device is simple in consrucion and operaes a a lower volage Furher, his echnique does no 8

18 require prior knowledge of he ransformer flux Deailed discussion can be found in chaper 4 (b) Esimaion and eliminaion of DC componen and harmonic analysis of an unloaded hree phase ransformer based on harmonic echnique In his mehod, idenificaion of inrush and faul curren is carried ou by comparing he raios of second harmonic o fundamenal ie second harmonic raio (SHR) which is compared o a pre-se value and hus idenifies inrush from fauls, which could be used in he relay and proecion schemes The deailed discussion and simulaion on a sep up power ransformer can be found in chaper 5 9

19 Chaper Power Transformer Proecion and is issues INTRODUCTION Power ransformer is inegral primary plan equipmen wihin a modern power sysem nework and plays an imporan role in ransforming volages from one level o anoher for ransmission and disribuion purposes However, due o shor circui, poor workmanship, conducion of elecric discharges in he suspended paricles of an insulaing oil, poor welding or joining echniques employed, displacemen of winding due o sress occurrence, failure of he insulaion ec are few of he causes in is failure I has been noed ha he earh shields placed beween primary and secondary windings leads o sress a he edges causing failures in he long run [5] Sudy indicaes ha 70%- 80% failure occur because of elecromagneic forces produced due shor circui sresses on he power ransformer windings The oher reasons of failures of power ransformer are due o susained load, emperaure rise due o poor cooling mehods employed and failure of proecion relay o isolae he power ransformer quickly in he even of an exernal or inernal shor circui Hence, i is of grea imporance o he asse owners o proec hese cosly apparaus wihin he subsaion environmen wih suiable and reliable proecion echniques 0

20 Researchers are consanly upgrading he proecion sysem and employing new echniques o check exernal and inernal fauls affecing power ransformer from damages Damage may occur due o fauls aribued o exernal facors like overvolage, overload, power sysem fauls, excessive flux densiy or due o inernal causes like winding failure, iner urn failure, phase o phase fauls, ank and accessory fauls, core fauls ec [3] Transformers raed below MVA are usually proeced wih fuses and simple mechanical proecion schemes due o economical consideraions However large power ransformers serving criical loads, need advanced proecion schemes comprising of phase failure, earh faul, over curren, differenial proecion in he primary and secondary side For proecion agains lighning srikes and swiching surges, design engineers employ meal oxide surge arresors on he primary side, using rod gaps ec Oher echniques such as conrolled swiching of upsream circui breakers using poinon-wave (POW) reduces he chances of power ransformer exernal fauls and are a useful pracical mehod employed while energising ransformer a no load Power ransformer mechanical fauls are usually proeced using pressure relief devices, Buchholz's relay, oil emperaure and winding emperaure sensors However, proecion of power ransformer agains elecrical fauls require sensiive proecion relays which akes ino consideraion over curren, earh faul and differenial curren arising due o sysem unbalance, unequal loading, ransiens during swiching ec In his hesis, differenial proecion has been discussed employing suiable algorihm o discriminae inrush from shor circui Till dae, differenial scheme is he mos popular mehod of proecion employed by he uiliies and indusries for he proecion of all criical ransformers in he nework In a differenial scheme, unequal or differenial curren arising due o phase and ground fauls wihin he zone of proecion, helps in discriminaing exernal and inernal fauls affecing he ransformer Addiionally, oher fauls such as swiching-in volage rise, swiching-in curren rush, shor circui curren rush which have a derimenal impac on ransformer winding, is also aken care by differenial relay The inrush phenomenon occurring during ransformer swiching a no-load lass several seconds and can cause caasrophic damage o he power ransformer winding Proecion engineers employ

21 robus differenial schemes o damp his high magniude inrush and se he relay operaing parameers o resrain during an inrush and rip during a faul condiion I is observed a he ime of energizaion of a power ransformer on no-load ha, he iniial curren waveform is rich in harmonics and consiss of fundamenal, second and fifh harmonics For he maer of proecion, hese lower order harmonics are employed in he relay algorihms o discriminae inrush from shor circui while higher order of harmonics beyond fifh order are negleced due o is non exisence Proecion specialiss have found magniude of second harmonic curren o be one of he main indicaors of he presence of inrush curren which ofen resuls in mal-operaion of he proecion relay These harmonic curren signals, when fed o a differenial proecion module equipped wih complex algorihms, discriminaes ransien inrush from fauls [6] Oher reasons of inrush occurrences could be due o ransformer core sauraion or curren ransformer (CT) sauraion, largely due o magneic core maerials used and flux produced CTs a primary and secondary end of a ransformer conver high magniude curren signal ino smaller secondary curren for proecion purpose a a cerain raio depending on primary o secondary urns In a normal condiion, curren enering he ransformer and exiing a he secondary remains seady However, during an abnormal condiion like inrush or faul in one or more phases of he ransformer circui, his balance condiion is disurbed resuling in CT secondary curren o sharply rise This necessiaes relay o eiher resrain during an inrush or rip he upsream circui breaker in case of a faul and hus avoiding damage o he ransformer Usually, all differenial relays are fied wih a bias feaures This bias enables he relay o resrain for few seconds a he ime of iniial energizaion o preven mal-operaion In order o build a bias feaure ino a differenial scheme, relay designers ake ino accoun phase shif beween differen phase currens, ap changing in he ransformer winding and CT urns raio ec Differenial relay is heavily dependen on he secondary inpu of CT's locaed on primary and secondary of ransformer Improper inpu due o errors in he CT raio could severely limi he proecion scheme CT sauraion is an imporan phenomenon and has been discussed in he conex of differenial proecion scheme in his chaper All CTs including modern opical based CTs have magneic cores The magniude of curren required o magneise he core and mainain is accuracy wih minimum loss during he ransformer operaion is a

22 challenging ask, as he CT sauraion could lead o he mal-operaion of differenial relay In he nex few secions of his chaper, CT errors and is effec on differenial relaying has been elaboraed Correc selecion of CT raios and working wihin he specified errors, is of imporance o he operaors and proecion engineers which oherwise could lead o unnecessary ripping of he ransformer during is energizaion EQUIVALENT CIRCUIT AND PHASOR DIAGRAMS OF CT Appearance of ransiens a he ime of ransformer energizaion is a common phenomenon and i no unusual o find CT wih errors resuling ou of hese disurbances CT errors could seriously compromise he proecion or cause spurious operaion, fed via he secondary leads o he differenial relay Hence, he locaion of CTs on eiher side of he ransformer has a significan relevance from differenial proecion scheme perspecive and canno be ignored [7] Fig indicae a wo winding CT where he curren in he secondary side of a CT I s core produces a magneic field which couner balances he primary curren I p [8] Almos all CTs works wih he principle ha magneic field of primary and secondary curren balance ou each oher and here is a direc relaionship wih respec o he raio of primary o secondary urns which is almos close bu no idenical The magneic core of CT produces wo opposie fluxes in he primary and secondary winding, which oppose each oher These opposing fluxes balance ou each oher and mainain a relaionship beween primary N p o secondary N s urns I mus be noed, CT secondary should never be kep open as he secondary curren I s wih a normally conneced burden Z b will circulae he curren in a loop However, when he secondary erminal is accidenally lef open wih CT energized will encouner cable and source impedance Furher, his volage appearing a he open circui end of he secondary erminal, will acquire a disproporionae magniude which could be up o he ens of kilovols This is a risky siuaion from safey poin of view for he operaor and insulaion of he componen 3

23 I P I s Primary Winding N P Ψ p Ψ s N S Secondary Winding Z b Magneic Core Secondary Circui Figure Curren ransformer and impedance burden [8] where, - Polariy marking N p - Number of primary windings I P - Curren primary windings Ψ p - Flux in primary N S - Number of secondary windings I s - Curren in secondary windings Ψ s - Flux in secondary Z b - Secondary burden Fig exhibis an equivalen circui of a simplified wo winding CT model which replicaes an acual CT of Fig Curren flowing ino he primary I P circui and ou of he secondary I s are denoed by marking dos on he winding N P and N s respecively I mp represens he primary curren which ses up flux in he ransformer core and cause hyseresis and eddy curren losses in he CT Represening he CT shown in Fig wih an equivalen circui in Fig, Leakage resisance and reacance on he primary side is denoed by R p and X p while on secondary side are given by R s and X s respecively Magneising curren fluxes are 4

24 produced due o I mp on he primary side and he common flux, which links boh hese fluxes, is called muual flux This muual flux is responsible for elecromagneic coupling beween he wo windings Losses in he CT ie magneic and hyseresis are caused due o I mp I p I s R p jx p R s jx s I ep V p I cp R cp X mp I mp Z b V s N p N s Figure Equivalen circui referred o primary [8] where: R p Primary winding resisance X p Primary winding leakage reacance R cp Resisance of core losses X mp Magneising reacance R s Secondary winding resisance X s Secondary winding leakage reacance Z b Secondary burden impedance - Polariy marking Fig 3 shows he equivalen circui model referred o he CT secondary The value of R s and X s represens he oal of acual resisance and leakage reacance on he secondary side of he CT I also represens a primary when referred o he secondary as he primary side resisance (R) and reacance (X) being very low, can be negleced On he secondary side in Fig 3, a core loss componen (R c ) and magneisaion reacance (X m ) branch of he CT are shown hrough which core losses of CT dissipaed This 5

25 equivalen circui model of he CT replicaes an acual CT in he high volage ransmission nework Sudy of equivalen circui referred o secondary of he CT is of ineres because secondary curren in he CT is usually no grealy affeced by he change in burden raings and i helps o gain knowledge of he CT errors such as raio and phase angle errors which can be deermined by knowing he CT magneizaion characerisics I p I ps I s V i I e R jx I c I m R c jx mp Z b VS N p N s Primary Secondary Figure 3 Equivalen circui as refrerred o Secondary [8] where, R c - Resisance of core loss X m - Magneizaion reacance R - Toal winding resisance referred o secondary Z b - Secondary burden impedance X - Leakage reacance referred o secondary I ps - Primary curren referred o secondary The relaionship beween primary and secondary curren in a CT is bes described by a phasor diagram given in Fig 4 Taking he secondary curren I s as he reference, V s leads I s a angle Ø s where he value of his angle depends on burden Z b Secondary resisance R is in phase wih I s and volage drop across X is shown o be perpendicular 6

26 o I s in he phasor diagram Furher, he inernal CT secondary volage V i is he phasor sum of volage drop across shun branch combinaion of R c and X m and V s as shown in he equivalen circui diagram in Fig 3 Assuming he CT reacance is less han he resisance, he volage drop RI s is normally greaer han he inducive circui XI s which is refleced in he phasor diagram in Fig 4 The exciaion curren I e is he vecor sum of I c and I m which represens core loss and magneisaion componen of he CT The inernal CT volage drop V i and I c are in phase while V i lags I m by 90 Phasor diagram in Fig 4 is consruced referring o he secondary side of he equivalen circui shown in Fig 3, wih he horizonal axis curren I s as he reference and secondary volage V s subending angle Ø s wih I s Volage drop referred o secondary is denoed by RI s and XI s The phasor sum of I e and I m gives rise o I ps, which is he oal curren referred o secondary ransformed from primary I c is he componen which causes core loss and is in phase wih V i and perpendicular o I m Vecor summaion of magneising curren I m and core loss componen I c gives rise o he exciaion curren I e The relaionship beween I p and I ps is sricly based on N p, urn raio on primary and N s, urns raio on secondary V i V s XI s RI s Φ s I c I s I m I e I s Figure 4 Phasor diagram of a CT referred o he secondary [8] 7

27 Fig 5 shows he phasor diagram of a CT which exhibis he relaionship beween primary and secondary curren when here is an inducive burden Phasor I c and I m gives he magniude and phase error in he CT and is direcly depends on he phase displacemen beween secondary curren I s and secondary volage V s [8] I ps which represens he oal curren of primary ransformed o secondary and is he phasor sum of I s and I e, where I e represens he exciaion curren and is vecor sum of I m and I c In he vecor diagram of Fig 5, he componen I c is in phase wih V i and is responsible for he core loss I p and I s have are linked by he primary urns raio N p and secondary urns raio N s and is given by () V i X I s V s R I s I c I e I ps ø s I m I s Figure 5 Phasor diagram of CT having inducive burden [8] Fig 5 shows he phasor diagram of a CT which exhibis he relaionship beween primary and secondary curren when here is an inducive burden Phasor I c and I m gives he magniude and phase error of he CT and is direcly proporional o he phase displacemen beween I s and V s [8] Assuming a large inducive burden in he secondary side of CT in Fig, i is useful o sudy he effecs of secondary burden on he ransformaion errors in Fig 5 In his 8

28 phasor diagram I ps is almos in phase wih I s Volage drop RI s being greaer han XI s in magniude in he phasor diagram Fig 5 and I ps being almos in phase wih I s, i is observed ha phase angle errors reduces as he power facor of he burden becomes low Alernaively, higher power facor and higher inducive burden (wih less X/R raio in he secondary circui) causes more phase angle errors CT errors vary direcly wih he burden and operae beer a low power facor giving rise o lesser phase angle errors To reduce magniude errors in he CT, urns compensaion is inroduced as an alernaive and is accomplished by reducing few urns in he secondary The relaionship of number of urns on primary side wih respec o secondary is given by () N I N I () where, N p - Number of urns in he primary side of CT I p - Curren in he primary side of CT N s - Number of urns referred o secondary side of CT I ps - Toal curren ransformed from primary o secondary of a CT p p s ps Errors in ransformaion occur when here is a mis-mach in urns raio wih he curren raio and i is called ransformaion errors However, in an ideal siuaion CT primary side curren should replicae secondary side wihou any errors bu in a pracical case wih CT's on eiher side of he ransformer, his is no possible and many oher errors occur eg raio, phase, composie, ransformaion errors ec All hese errors cause maloperaions of he proecion relay and sauraion of he CT cores Therefore, i is of ineres o revisi various errors affecing he CT's which has been oulined in secion o 4 Transformaion Errors In a CT, when he curren is ransformed from primary o secondary, i mus preserve all is curren characerisics of he primary and reflec i on he secondary side for meering and proecion bu usually ha is no he case However, in a pracical se up, hese errors occur due o burden and phase angle displacemen beween secondary volage and secondary curren of he CT and largely depend upon he raing of he burden 9

29 Higher impedance of he burden cause greaer ransformaion errors due o magniude and phase shif beween secondary volage and curren Transformaion errors are direcly relaed o he power facor (PF) Lower he power-facor (ie he more inducive he burden) beer is he CT operaion wih respec o ransformaion errors The mehod o resolve he ransformaion errors caused due o magniude and phase shif is by reducing he number of urns in he secondary side known as urns compensaion Curren Error (Raio Error) CT is said o be having curren error when here is a difference beween I p and I s as he primary curren I p is no a replica on he secondary side I s, when muliplied by he urns raio This is caused by he core exciaion curren and as given in () ' '' K I I n p p Curren error % = 00 ' I p () where, I' s RMS value of he supply frequency componen I' p RMS value of he supply frequency componen of he primary curren K n Raed ransformaion raio of he CT K n I' s - Turns raio Expressing curren ransformaion raio in anoher form [8] ( Is' Ip' / Kn) Curren error % = 00 Ip' Kn (3) where, I p' / Kn in (3) is denoed as nominal secondary curren Usually he curren error is deermined o be negaive as he value of I' p is greaer han K I ' n s in () However, in an ideal lossless CT wih zero magneising curren requiremens, he available curren oupu is less han he ideal case which is normally no he siuaion Curren or raio error is imporan from relaying poin of view during 0

30 shor circui condiion and he percenage increase in he error is direcly proporional o he increase in primary curren 3 Phase Error Secondary curren can lead or lag he primary curren depending upon he power facor Mos CTs have volage leading he acual curren by a small amoun leading o phase angle error Phase error represens values indicaing he phasor posiion of secondary wih respec o primary When I s leads I p, phase error is saed o be posiive and when i is behind i raed negaive and he mehod o compensae his phase error is by reducing one or wo secondary urns [7] Assuming leakage reacance of he burden o be negligible, he maximum error ha could occur is limied o +70% when he burden is very low The range of values in a pracical siuaion for he measured errors varies beween 0 o 6 degrees for CT Phase angle errors a a low power facor below 7 or less and has a significan impac on proecion sysem and measuring insrumens conneced in he nework when errors are below 7 power facor (PF) Manufacurers of CT's are improving windings maerials and fine uning o obain zero phase angle error by improving he CT accuracy a lower PFs 4 Composie Error The exciaion componen I e as seen in he equivalen circui Fig 3 produces harmonics and increases as CT progresses owards sauraion This exciaion curren causes waveform disorion and inroduces composie errors in he CT The error occurs when here is a difference of an insananeous value of primary I p and secondary curren I s inegraed over one complee cycle of operaion a supply frequency as shown in (4) and (5) [8] When expressed as he RMS value, composie error is expressed as T 00 {Kn is ()- i p ()} d ' T (4) I p 0

31 Alernaive expression for composie error is given by (5) Where, T 00 {is ()- is ()/K n} d ' I / K T (5) p ε - Composie error n 0 T - Time period over one complee cycle over he supply frequency is - Insananeous secondary curren ip - Insananeous primary curren If harmonics during power ransformer energizaion are considered, composie errors assume a posiive value bu if harmonics in he sysem are discouned, he expression of composie error becomes (6) I e (6) I p 00 K n Where, I e - supply frequency exciaion curren phasor I s and I P - supply frequency phasor Equaion (6) has errors resuling due o phase angle and magniude of I e and I p in seady sae operaions aking he harmonics ino consideraion Hence, i is called composie errors As menioned earlier, ideal CT's are expeced o be free from errors bu ha no being he case, he minimum expecaion of a measuring or proecion CT funcionaliy, is o keep he errors wihin specified limis and perform is funcion wihin is characerisics as enumeraed in secion 3 In order o keep CT errors in check, manufacurers use he following maerials and echniques: Consruc high permeabiliy and low hyseresis maerials Keep raed burden close o he acual burden Reduce flux pah and increasing cross secional area of CT core Minimizing he joins in he core Decreasing he secondary inernal impedance

32 3 TYPICAL CHARACTERSTICS OF AN IDEAL CT During CT energizaion, wih primary kep open and if volage is measured across secondary, a characerisic curve resuls and his ypical exciaion characerisics is given in Fig 6 This characerisic is divided ino four regions namely: From origin o ankle poin From ankle poin o knee 3 Knee poin region 4 Sauraion region Fig (6) shows ypical CT exciaion characerisics in which a non linear response in he graphical form is shown Where, V k -Knee poin open circui volage I k -Knee poin exciaion currens V k V k Open-Circui Volage 0% increase in Open-circui Volage 50% increase in exciaion curren I k 5 I k Figure 6 CT open circui exciaion characerisics [8] V k ie knee poin volage of he CT is said o have aained, when 0% increase in he open circui volage gives rise o 50% increase in he exciaion curren Any furher 3

33 increase in he open circui volage of CT would no increase he exciaion curren a he secondary and CT is said o be driven owards sauraion This sauraion exiss as long as primary ransien curren is no reduced An ideal proecion CT is expeced o perform above ankle and below knee poin region while a measuring ype CT is expeced o perform below ankle poin Beyond he knee poin region occurs he sauraion of CT In a shor circui condiion, CT sauraion is a common phenomenon and has an impac on he proecion relaying operaions by is slow response, reduced oupu ec CT is deermined o have enered a sauraion zone when he curren signals on he secondary side is found o be disored and has a grea impac on he proecion sysem paricularly in a differenial scheme for he proecion of a power ransformer In differenial relay, shor circui could drive CT o saurae fas causing disurbance in balance, speed, operaion of he relay To keep he CT in balance sae and minimise he effec of sauraion, CT burden conneced o he secondary erminal should be chosen accuraely which plays an imporan role in relay resrain and operaion 4 CT BURDEN EXPLAINED The load side of he CT consiss of secondary cables, relays or he meers which has resisance and reacance The volage and curren across hese componens consiue as burden and is exhibied in Fig 7 which shows a ypical burden on he secondary side of a single core CT circui which are in he form of coils or elecronics of a relay Designers deermine he CT secondary cable parameers and relay reacance while sizing he CTs Deerminaion of CT burden is an imporan aspec while configuring proecion seings [8] 4

34 CT Connecing Leads Proecion Relay ~ I p R s!~ r V i I' V s R L I s R R --;~ V R -'-- Bay in Subsaion r:- R L Swichyard r:- Relay Room Figure 7 Burden conneced o a secondary circui of a CT [8] Where, R s - Secondary winding resisance R L - Lead resisance for one conducor from swichyard o he proecion relay uni R R - Relay Resisance I s - Secondary curren V R - Volage drop across he relay winding In Fig 7 he resisance of he lead wire is considered while reacance of he wire is negleced because he secondary cable from swichyard o relay conrol room is no oo long Burden is direcly relaed o he secondary curren and assumes an imporan role during CT curren ransformaion raio CT mus always be procured based on burdens conneced on he CT secondary ie relays, lead cables If he burden calculaed is larger han CT raing, CT size will become large and uneconomical 5 CT CHARACTERSTICS AND ITS EFFECT ON PROTECTION RELAY When he knee-poin on he CT characerisics is exceeded, sauraion akes place and resuls in waveform disorion This has a pronounced effec on relay and may cause mal-operaion of he relay paricularly in he differenial proecion wherein wo or more CTs are required o balance each oher ou in normal condiion However during a faul 5

35 his equilibrium is disurbed resuling in an ou of balance and insabiliy in he proecion [8] Furher, due o CT sauraion he balance beween primary and secondary CT is grealy disurbed resuling in mal-operaion of differenial relay However, i is observed ha CT sauraion has less effec on over curren and disance proecion Hence i is of ineres o he auhor o analyze he differenial proecion scheme wih respec o CT sauraion and is effec on i 6 DIFFERENTIAL PROTECTION Differenial proecion is he mos comprehensive uni proecion ha isolaes he power ransformer from exernal and inernal fauls In his mehod of proecion he inpu signals a he primary end of he ransformer is compared wih he secondary side The CT raio a he primary and secondary side of he ransformer have a linked relaionship which is disurbed in a faul condiion as he magniude of he faul curren in he CT secondary shoos up during a faul resuling in a large difference in phase and magniude of he primary and secondary curren If hese curren increases beyond a pre-deermined value, he relay will rip he circui breaker associaed wih he ransformer a he primary side prevening permanen damage However, when he ransformer is energized wihou load, here is a sudden rush of he curren o saurae he core This phenomenon is ermed as inrush and could las for several seconds characerised by high magniude and rich in harmonic As his is a ransien phenomenon which decays slowly, i is expeced of he differenial proecion o resrain iself during an inrush condiion By incorporaing a biased feaure in he differenial scheme, he relay resrains iself during an inrush and hereby enhancing he sabiliy of he power ransformer during iniial energizaion CT sauraion has a grea effec on he differenial proecion in erms of disorion, loss of accuracy, secondary curren zero shif and loss of secondary peaks In order o eliminae hese issues, CT in a bias differenial scheme is insalled as shown in Fig 9 This diagram exhibis a differenial proecion wherein for an exernal faul, he faul curren flows eiher from lef o righ or from righ o lef side of he plan depending 6

36 upon he faul locaion However, during an inernal faul, he flow of he curren is in opposie direcion o he normal curren flow Main Transformer Proeced r'-'-'-'-'-'-'-'-'-' 600/ kv 3kV 00/ 30 MVA CT A ---;~~--~~~ ~+'------F ~ B c Inerposing CT Raio / Biased Differenial Proecion Figure 8 Biased differenial relay wih inerposing CT [8] Sum of I p /K p + I s /K s for hrough fauls while i is he difference of quaniy ie I p /K p - I s /K s for inernal fauls Where; I p - Primary curren I s - Secondary curren K p and K s are ransformaion raio referred o primary and secondary side of he power ransformer Biased differenial proecion relay operaes on he principle ha when operaing curren exceeds he resraining curren, he relay rips Furher, biased differenial relay provides sabiliy during an exernal faul while ripping during an inernal faul However, i is observed ha during a hrough faul, sabiliy of he biased differenial proecion is much lower compared o an unbiased differenial proecion and hence here is a necessiy of inroducing inerposing CT (ICT) in he relay scheme, paricularly for elecromechanical ype of relays Also, ICT help he biased differenial relay o measure he raed curren, when he full load curren flows in he proeced circui By 7

37 inroducing ICT i is possible o correc his error Deails of differenial proecion has been elaboraed furher in chaper 3, secion 3 7 FUNCTION OF INTERPOSING CT (ICT) The funcion of ICT is o mainain high hrough faul sabiliy in he differenial relay and are conneced beween oupu of he secondary side of a CT and differenial relay ICT oppose he phase shif of proecion signal due o winding connecions of he ransformer proeced, which oherwise would been he funcion of he main CT A ypical connecion of he inerposing CT is given in Fig 8 I is observed ha he primary side of he ICT is sar conneced while he secondary side of he ICT is dela conneced because of he need o block he zero-sequence curren arising ou of dela connecion of he ransformer primary Wih he advancemen in echnology, numeric differenial proecion schemes hese days has replaced ICTs as i can couner zerosequence currens by incorporaing suiable algorihms in he sofware of he relay ICT exhibied in Fig 8 may be arranged in he same panel as he differenial relay This elecrical proximiy by being locaed in he same panel grealy enhances ICT's oupu and ensures speed of operaion of differenial relay during inernal faul condiions of a ransformer CT sizing and selecion as applicable o differen vecor group of ransformers are of imporance as a wrong selecion or use of a non sandard size could compromise he proecion significanly Inernaional elecroechnical commission (IEC) and Ausralian sandards (AS) specify he sandard burden, raing and percenage errors of he various CTs o be manufacured for measuremen and proecion relaying 8 CT SELECTION AND APPLICATION OF STANDARDS As menioned in previous secion CTs play an imporan role during measuremen and proecion of he power ransformer Correc CT sizing is of imporance o engineers and designers as improper CT conneced o he primary and secondary side of power 8

38 ransformer causes mal-operaion of he relay Hence, sandards associaions around he world have laid sric guidelines while selecing CTs CTs supplied by he manufacurer has been sandardised by various counries for proecion, measuremen, accuracy and errors ec As per Ausralian Sandard (AS), he sandard associaed wih CT selecion and sizing is AS which guides he user o specify he key elemen of CTs such as:- Definiion for CT errors, accuracy, performance, raings, service condiions and erminal markings CT ess : Type ess, rouine ess and special ess 3 Classificaion of CTs : Class P, PL, PS ec 4 Applicaion of CTs 5 Summary of service condiion ie ambien emperaure, local amospheric and climaic condiion IEC defines CTs on General applicaion, raing, applicaion, ess Definiion, accuracy, ess for accuracy, name plae raing 3 Measuremen of CTs 4 Proecion of CTs As per AS 6044 designaion of a ypical Class P, CT may be read as [8] Table CT raing plae Iem in Designaion Example of Enries AS P 00 F 0 Meaning of differen iems in Table and CT specificaion is given by:- Iem : Ausralian sandard o which CT complies Iem : Year of publicaion of he sandard 9

39 Iem 3: Raed composie error in percen a he accuracy limi curren Iem 4: CT class Iem 5: Raed secondary reference volage Iem 6: Raed accuracy limiing facor Iem 7: Value of he accuracy limi facor For CTs o be idenified for proecion or measuremen i is imporan o know he knee- poin volage and accuracy class parameers 8 Accuracy Class In elecromagneic CTs, he proecion scheme suffers from phase angle and raio errors Hence, i is imporan o limi hese errors by applying sandards o define he lower and upper limi of he CT accuracy wihin specific olerances for differen applicaions [9] Following symbols are usually marked while specifying he accuracy class of a CT: Class P : CT is for general purpose applicaion where ransien response is no so imporan (Example: for slow speed over curren and earh faul relay) Class PL : Applicable o relay requiring good ransien response (Example: High speed over curren, disance and differenial) Class PS : Anyhing ha does no fall wihin P and PS and is differen from he above applicaion 9 FACTORS DETRMINING CT RATIO AND SELECTION Raio of he CT is affeced by:- Coninuous primary curren Nominaed coninuous secondary curren Specific applicaion in proecion The mos commonly used secondary raings are 5A, A, A 30

40 Coninuous primary curren The raio of CT = Coninuous secondarycurren (7) Following facors are considered while specifying he relevan CT selecion:- CTs are seleced based on he mos conservaive hrough faul condiion CT sauraion is aken ino accoun while deermining proecion response o inernal fauls Typical burdens conneced o CTs are given below : lead (phase and neural ogeher) over curren relay on 5A ap earh-leakage relay on A ap insananeous Relay Ω 055 Ω 78 Ω 00 Ω 0 NEW TRENDS IN CT MANUFACTURE There has been new rend in he manufacure of he CT in erms of maerials used and design One such innovaion is opical CTs Opical CTs [7] are finding applicaion in he indusries and uiliies hese days due o is compacness and small foo prin The insulaor srucure is considerably reduced due o use of composie ligh weigh maerials Reducion of weigh causes savings in civil foundaions and srucural suppors for he oudoor CTs insalled in a air insulaed high volage subsaion Use of fibre opic cables on he secondary side also has economic benefi o he cuomer Furher, he elecromagneic inerference and convenional secondary wiring is considerably minimized using hese insrumen ransformers as he sensor and opical fibre could carry ou he ask of a copper cable wih higher efficiency and lesser human effor for he insallaion In recen years hybrid echnology is gaining populariy Hybrid opical CT's combine he design of a convenional CT in conjuncion wih passive opical sensing medium The sensors used in a opical CTs are less immune o sauraion and elecromagneic inerference and hence provide greaer proecion feaures o differenial proecion 3

41 The use of opical CTs is expeced o bring echnical changes in he differenial proecion However, fuure works need o be expanded on hese novel insrumens o fully undersand is behaviour during ransiens TRANSIENTS AND ITS EFFECT ON PROTECTION SYSTEMS When a power ransformer is energized, here is an abrup change in is saus due o sudden rush of curren in sauraing he core winding before decaying in few cycles o sabilize and his ransien inrush phenomenon could be influenced due o one or combinaion of reasons like [0]:- Removal of a shor circui Sympaheic inrush due o ransformers in parallel 3 Synchronizaion of muliple ransformers in a generaing saion The ransien phenomenon causes CTs o be driven ino sauraion causing differenial relay o mal-funcion as he relay ends o misinerpre his ransien phenomenon as a shor circui hereby commanding he relay o rip Inrush due o ransiens lass few seconds and is magniude is ypically 5-7 imes he raed curren of a power ransformer In he las decade, proecion engineers had relied on blocking resisors o check his unwaned relay operaion due o inrush by pre-insering resisors in he circui and by performing harmonic blocking ec Few oher mehods of miigaing ransiens are:- Removal of residual flux Adjusmen of phase angle volage 3 Inserion of resisance, PWM inverers ec I is observed ha o miigae inrush of a single phase ransformer easier han a hree phase power ransformer, because i is no easy o remove he magneic flux in a hree phase power ransformer Also, in a hree phase sysem differen phase angles do no swich-on a he same ime and i is usually due o he iming of he hree phase circui 3

42 breaker which doesn' closes and opens a he same ime There is always a lead or lag of he closing mechanism by few mili seconds of he hree poles in a circui breaker Large inrush curren reflecs a paern similar o ha which occurs during a shor circui and his abnormal elecromagneic force sresses he winding of he ransformer If he sress becomes a regular occurrence, i could affec he enire power ransformer winding Eliminaion of inrush is of grea ineres o he proecion engineer and can be achieved by proper conrol of swiching angle of he circui breaker a he primary side of he ransformer and by eliminaion of residual magneisaion [] Based on above discussion, i is essenial from power sysem prospecive o formulae an algorihm which is accurae and idenifies shor circui from inrush aking ino consideraion of he following:- Inducance of he power ransformer core Impedance of he power ransformer including he HV cable conneced Vecor configuraion of he power ransformer ie Sar-Sar, Sar-Dela ec Fig 9 exhibis a ypical inrush phenomenon due o incorrec swiching angle of he circui breaker [4, ] In his diagram is shown an inrush curren magniude is dependen on residual flux and he angle of energizaion of circui breaker is no carried during AC curren passage hrough poin zero resuling in an inrush Fig 0 shows no inrush occurrence, as he circui breaker has been closed when he curren passes hrough zero insance which is usually an ideal case [3] The waveforms shown in Fig 9 and Fig 0 are sysem volages in solid lines Flux of he ransformer is shown in Y axis and Time in X axis The combinaion of prospecive flux and residual flux in he ransformer during energizaion gives rise o wors peak as shown in Fig 0 The magniude of inrush diminishes gradually [4] afer shor ime Beside inrush phenomenon caused by swiching of circui breaker, harmonics also play a role owards he unwaned operaion of proecion relay described in he subsequen secion 33

43 I I \ I Flux Flux Symmery 0 Time Residual \ \ ---+!,L---\-T-7 Flux Flux Volage! Time Curren T \ Figure 9 Incorrec swiching ime leads o inrush [] Flux I I \ I Flux Symmery 0 Time \ ~4:;zC--VT-:7 \ Flux Volage Time Curren i \ Figure 0 No inrush occurrence due o correc swiching ime [] EFFECT OF HARMONICS ON POWER TRANSFORMER In early 960's applicaion of elecronics in indusries grew rapidly leading o use of non-linear devices which produced harmonics Harmonics are impure or disored sinusoidal waves caused due o inroducion of recifiers, diodes, converers ec ermed 34

44 as non-linear devices Embedded wihin harmonic waveforms is inrush curren which carry fundamenal, second, hird and higher order of harmonics bu i is he second harmonic which is mos pronounced Hence, second harmonic deecion is deemed as he one of he preferred mehod in idenifying inrush and discriminaing i from faul currens Percenage of second harmonic o fundamenal is shown in Table Modern digial relays are equipped wih reliable algorihms incorporaed ino he sofware which enables differenial relay o resrain during an inrush, while operaing on a fauly condiion o rip Table abulaes he resuls of he ampliude percenage of differen harmonic componens presen in he curren waveform during iniial energizaion of a ypical hree phase power ransformer [9] I is inferred from he given able ha i is of high imporance o invesigae second and hird harmonics by virue of is large percenage presence in he inpu signal o he ransformer Table Ampliude of harmonics presen during inrush [9] Harmonic componen in Ampliude as % of magneising curren fundamenal Second 630 Third 68 Fourh 5 Fifh 4 Sixh 37 Sevenh 4 When a ransformer is energised, he inducance being high compared o resisance in he circui, large inrush of curren occurs which 5 o 7 imes he raed ransformer normal curren This inrush lass several cycle afer swiching insance and could be due o one or muliple causes as enumeraed below:- Size of he ransformer Type of he magneic maerial of he ransformer core Presence of residual flux in he ransformer core 35

45 Swiching angle of energizaion are differen for each phases Configuraion of ransformer ie line volages appearing in dela conneced ransformer winding is differen o ha of sar conneced one Sauraion occurs only in some of he limbs of he ransformer The effecs of harmonics on power ransformers are:- Increase in copper loss Increase in resisance o he curren flow and hereby giving rise o emperaure rise and creaing hospos in he power ransformers Increased iron loss Reducion in ransformer life Malfuncioning of proecion relays Decreased power facor For he proecion schemes o be reliable and robus, i needs o be equipped wih suiable algorihms o face inrush and shor circui Hence, he nex chaper in his hesis overviews some of imporan algorihms and is principles applicable o differenial relays based on digial echnology as convenional elecromagneic echnology is fading ou 3 PRINCIPLE OF DIGITAL RELAYS Wih he arrival of microprocessor and digial echnology, proecion of power ransformer has undergone remendous changes in he recen ime Proecion of ransformer has been improved o an exen ha engineers and sysem operaors expec i o be fas acing & sensiive o discriminae inrush from faul condiions in mili seconds wihou unduly causing nework insabiliy which oherwise could cause [5]:- Loss of synchronizaion in generaing saion Mechanical sress, fire and explosion in generaors or ransformer 3 Damage o oher healhy plan wihin subsaion and power nework 4 Injury o echnical saffs wihin he generaing subsaion 36

46 Researchers have analyzed ha power sysem nework is usually hrown ou of sep due o mal-operaion of proecion sysem by 30% and due o inernal fauls of power ransformer by 50% Proecion of large power ransformers which is one of he coslies elecrical asse in he nework, remains ill dae, a challenging problem for he relay designers as hey have o consider complex problems like miigaing magneizing inrush during energizaion of power ransformer under no-load besides conrolling over exciaion due o over-volage, ap changing, CT sauraion, ground fauls, inernal and exernal fauls ec In he recen years, proecion relay engineers have manufacured relays which are compac and ligh in weigh based on microprocessor and digial echnology These proecion relays are found o be more reliable and sensiive han is previous ones ie numerical and mechanical operaed relays due o is fas acing algorihm and compac dimensions Algorihms used in digial proecion sysem play a criical role in he relay operaions I is expeced o have beer performance in wihsanding mal-operaions in power ransformer during magneizing inrush and sauraion of CT's during energizaion These modern digial relays are equipped wih complex algorihms which analyses iniial curren signals including DC offse, harmonics and overexcied condiions I resrains from mal-operaions during inrush while ripping he relay a faul condiions Fig gives he basic model of a curren digial differenial relay [7] Usually curren imbalance akes place during an inrush condiion which predominanly carries a second harmonics componen of he iniial signal and is dominan during circui breaker swiching By applying suiable algorihms in he relay logic, i is possible o design a robus and sensiive digial differenial relay Some of he popular algorihms used in he proecion of he power ransformer by he researchers [5] are lised below :- Leas squares mehod Finie impulse response filers Walsh funcions Harr funcions Recangular funcions Kalman filering echnique 37

47 The problems associaed wih hese algorihms are, eiher hese are pracically no applicable in he proecion of various vecor groups ransformer vecor groups (ie Sar-Dela, Dela-Dela, Dela-Sar, parallel operaion of Transformer ec) or he accuracy and response o isolae during a faul are found o be inconsisen across all kinds of bus configuraion ie single bus, double bus, breaker and half, ring ype ec In order o miigae such issues, his hesis relies on he combinaion of algorihmic and MATLAB simulink models o analyses volage and curren daa, he deails of which are explained in chaper 4 and 5 wih he pre-fluxing echnique and harmonic analysis mehod as improved echniques respecively In he following secion, an insigh ino he design of a curren digial differenial relay using a funcional block diagram has been described by exhibiing a hardware model in Fig New digial proecion of power ransformer so designed exhibis, dependabiliy (no missing operaions), securiy (no spurious ripping) and fas operaions (shor faul clearing ime) Digial relay archiecure is divided ino hree pars [5] as shown in Fig :- Signal condiioning subsysem Conversion subsysem 3 Digial processing relay subsysem Signal condiioning subsysem has ransducers in he form of CTs and VTs CT ransducers scale down he primary curren o much lower value which could be measured, analyzed and applied o he filers before subjecing he signals o proecion relay sysem Similarly VT ransducers reduce high volages o 0 vol alernaing curren, which forms inpu signal o he digial relays Sauraion of he CT is a maer of concern bu i is adequaely compensaed using suiable algorihm and by considering physical parameers of cables and power ransformer impedances ino he digial proecion relay logic These digial relays compare he analogue signals agains many parameers such as swiching of he circui breaker, analogue inpus, iming ec Digial relays have scope for up gradaion ie in case of any change of algorihms hese microprocessors wihin he digial relay sysem can accep he new algorihm wihou 38

48 any change of is par or physical componen Wih digial pors available on he relay, i is user friendly in communicaing daa o cenral conrol room via supervisory conrol daa acquisiion sysem (SCADA) Digial relay acs as a source of informaion as i can accuraely record deails of he fauls or inrush occurrence wih accurae ime samping Digial relays are proeced from swiching and lighning surges Surge proecion devices locaed in he relay circui which are in he form of capaciors and zener diodes These are conneced o CTs and VTs via capaciors and zener diodes which proec he elecronic circuis agains swiching and lighning surges Careful screening echniques during he insallaion of surge arresors, filers ou large ampliude of surges by passing i hrough filer circui and furher ransmiing hese signals o he processing hardware 39

49 DIGITAL RELAY COMPONENT LAYOUT r , ransducer SIGNAL CONDITIONIN G SUBSYTEM surge proecive circui LP filer LP = Low pass A/D =analogue o digial D/A = digial o analogue CPU = cenral processor uni D/I = daa inpu D/O = daa inpu analogue muliplexer r I I L _ ~ r , I I I I I I I I sample hold circui A/D converer digial muliplexer _ ----r D/O D/I memory CPU I I I I I I I -~ D/A I _ J I I I I I I I I -l I DIGITAL PROCESSING RELAY SYSTEM Trip Signal(s) Remoe locaion daa Figure Block diagram of a ypical digial relay [5] 40

50 Surges in power sysem appear due o insananeous swiching of lighning srikes and i is characerized by sudden rise and slow decay in he waveform paern In Fig is shown a surge proecion circui where unwaned high frequency componen of he curren and volage signals originaing from CTs and VTs and are screened ou using a low pass filer which is nohing bu a combinaion of capacior and isolaing ransformer Depending upon he digial relays daa requiremen, he sampling is carried ou as per Fig 3 (a) and (b) Wih low pass filer, he rise ime, overshoo and seling ime is exhibied for a given oupu signal wih clariy Furher, analogue signal componen from main CT is aenuaed o avoid errors in subsequen digial processing Analogue low pass filers perform "ani-aliasing" funcion as shown in Fig 3 (a) which has an ideal low pass filer characerisics Surge Capacior Isolaing ransformer a b To Filers From main CT and VT c d Figure Surge proecion circui [5] 4

51 Figure 3 (b) indicae a pracical low pass filer which has ransiion beween pass and sop bands and which in pracice is difficul o achieve Undersanding he dynamics of he low pass filer and seady sae characerisics is imporan as i gives insigh ino digial relay a feaure wih is dynamic response like :- Rise ime Overshoo Seling ime Oupu Volage / Inpu Volage (a) f c f / (b) f c f Figure 3 Characerisics of an (a) ideal filer response (b) pracical filer response of a low pass filer [5] 4

52 The signals originaing as an ou of A/D converer as seen from he block diagram in Fig is nex sen o oupu channels which is more like a roary swich which obains a single inpu signal and convers many signals o digial processing sysems In he final sage of processing of he signal i eners ino a digial relay subsysem which comprises of hardware and sofware modules like cenral processing unis (CPU) memory, daa inpu (I/O) ec The sofware of his digial sub sysem is grealy influenced by he algorihm used, he sampling frequency and harmonics presen in he signal which is usually eliminaed by applying suiable filers 4 POWER TRANSFORMER PROTECTION Tradiionally ripping of power ransformer was prevened by swiching off power ransformer proecion relay during iniial energizaion condiion [] Wih he adven of modern echnology, resraining of he differenial relay has been carried ou using harmonic analysis, wave form analysis, fuzzy logic mehod ec Resraining he relay by insering resisors and swiching off he power ransformer is no longer deemed reliable for he inrush proecion of large power ransformers which could seriously compromise he safey of he asse High volage (HV) hree phase power ransformers wih a raing of 0 MVA and above normally have online ap changers During an exernal faul, he raio of primary o secondary could be hugely affeced variaion of he ap changers urns raio Furher, mismach beween primary and secondary side of he ransformer could occur due o sauraion or CT raio error One mehod of overcoming such malfuncion is o resrain he relay using harmonic rerained mehod In his mehod, he second harmonic being he larges harmonic componen is deeced in he inrush and compared o a fundamenal componen hereby resraining he proecion o operae 43

53 5 SHORT CIRCUIT CURRENTS AND ITS EFFECT ON POWER TRANSFORMER Reliable operaion, mainenance and economical power sysem nework requires a safe and a robus sysem which can wihsand, discriminae and clear fauls quickly in he form of shor circui In order o achieve his, proper planning in he design, consrucion and commissioning of he elecrical apparaus are required [6] Shor circui may occur due o lighning srikes on phase conducors of overhead lines or damages o cables due o inernal fauls ie aging of insulaion Design engineers ry o miigae shor circui by inroducing swichgears and fuses ha isolae he circui in he even of shor circui wihin mili-seconds Shor circui curren no only pose a risk o elecrical apparaus in which i occur, bu also forms hazard o he general public who may be unaware of he inducion effec of he shor circui ha may induce impressible volage in he neighbouring meallic pipeline, communicaion and power circui I also simulaes oscillaion in generaors which can have cascading effec on banks of generaors in operaion and lead o insabiliy in he nework ha could furher escalae o a sysem blackou Figure 4 exhibis a ypical ime course characerisics of a curren wave paern u and i are he RMS value of he volage and curren and ω is he angular frequency ф u and ф i are he phase angle of volage and curren The ime course has been shown on a real axis on he righ hand side of he Fig 6 wih an emphasis on following four parameers which has been discussed in chaper 3 Toal ime duraion Peak shor circui curren 3 RMS value 4 Shor circui breaking curren 44

54 IT ' HI) li (r); i(r)," u () o, ---T / f\\ (; U - "''>" ', - HI, I \ :',, I " "I - ' \' \ I I I \ / -, " ' / J, ~, - ",I Figure 4 Time course of AC volage [6] 6 EFFECTS OF SHORT CIRCUIT ON POWER SYSTEM AND OTHER MECHANICAL APPARATUS There are several damaging effecs of shor circui currens on he power sysem apparaus if no miigaed insanly a he incepion and could lead o hermal and insulaion failures of he ransformer Three phase and double phase shor circuis wihou earh connecion can cause highes mechanical force and he duraion of his shor circui effec depends on he ime unil i lass Also, shor circui has a mechanical effec on rigid and flexible conducors creaing a dynamic force and bending sress on he conducor ubes Paricularly in air insulaed subsaions, his could cause disaser as a large secion of HV oudoor swichyard comprises of ubular and flexible bus conducors IEC gives he mechanical effecs of he shor circui curren and gives pracical calculaion mehods for various shor circui currens Shor circui effec also leads o inerference problems in mechanical apparaus in he viciniy of overhead lines and cables Inerference of shor circui causes inducive, ohmic and capaciive coupling of he shor circuied pah and he circui affeced 45

55 Permissible values of volages induced in mechanical pipelines are governed by sandards in place by various counries Germany allows ouch volages o be 00V o 500V for faul duraion of 5s Brazil and Ausralia allow a maximum permissible 700V and 500V respecively As an example, if he mos severe resricion is applied on a 50Kg body weigh wih faul duraion being 50ms, he maximum ouch volage shall no be greaer han 350V 7 CONCLUSION This chaper gives he background ransformer proecion including discussions on sizing and selecion of CTs I gives an overview of radiional differenial proecion and use of ICTs in a differenial scheme wih explanaion of various CT errors and is limis I discusses he use of opical CTs and is effec on differenial relay Furher, i explains he phenomenon of harmonics, inrush and shor circui in he sysem and mehods of miigaion I discusses he issues relaed o harmonics and ransiens and how i reduces he life of ransformer and also causes mal-operaion of he relay I gives various mechanical and elecrical proecion commonly used like, pressure relief valve, oil, winding emperaure sensors, surge arresors, conrolling he swiching angle ec which has been employed currenly for he ransformer proecion While here are proecions schemes like over curren, resriced earh faul available and currenly being used, bu his chaper focuses on large power ransformer in a generaing saion sepping he volage using differenial relay as he primary and back up proecion This chaper oulines few imporan algorihms used in digial proecion sysems while giving a broad overview of he basic digial proecion relay block diagram used in a digial proecion scheme Furher, he chaper discusses he cause and effec of inrush and shor circui on power ransformers, subsaion primary plans and a recommendaion for is miigaion, which is one of he key ineres area of his hesis and has been deal in chaper 4 and 5 wih novel echniques 46

56 Chaper 3 Algorihms for he Proecion of a Power Transformer 3 INTRODUCTION In chaper wo, he background of ransformer proecion has been discussed and he imporance of discriminaion beween inrush and shor circui has been emphasised Pracically, his is achieved by resraining he relay, prevening CT sauraion and incorporaing a reliable algorihm ino he relay logic ec Modern digial relays usually have microprocessor in is hardware equipped wih algorihms which performs complex ieraions and commands circui breaker o rip in milliseconds These algorihms respond o inrush and faul condiions faser han he elecro mechanical relays and are finding is usefulness in he modern power sysem proecion in ligh of is weigh, compacness and real esae occupied in he conrol room cubicles In he previous chaper, i has been emphasised regarding he effecs of harmonics on he ransformer leading o degradaion of winding insulaion and CT sauraion Furher, inrush componen, which is a derivaive of he harmonics deerioraes he power qualiy significanly Togeher wih decaying DC componen and higher order of harmonics, inrush causes heaing, noise, shor life of he ransformer 47

57 Presen chaper enumeraes few mahemaical models used in curren based relay proecion I highlighs he limiaions of a radiional differenial mehod and suggess improved echniques o miigae DC componen and harmonics arising ou of inrush and shor circui Miigaion of inrush is of imporance which oherwise would make relay vulnerable o ap changing, poin-of-wave swiching, fluxes in he magneising core ec Basic uni proecion of a power ransformer along wih reliable algorihms, discussed in his chaper, gives an insigh in o inrush conrol and faul isolaion 3 OVERVIEW OF THE UNIT PROTECTION SCHEME Over curren scheme employed o proec ransformer do no always mee he requiremen due o unsaisfacory grading and reliabiliy in proecing he ransformer in a complex power sysem nework This is overcome by uni proecion wherein a secion of he power sysem nework is proeced A uni proecion which is also known as differenial proecion is a reliable form of proecion in comparison o proecion employing fuses for over curren The fundamenal principle on which uni proecion works is by comparing proecion signals derived from he inpu and oupu side of he primary plan CT and feeding hem o he relay During a healhy operaional condiion, curren signals on primary and secondary side of he ransformer are in balanced sae However, during a faul wihin he proeced zone, he relay rips he circui breaker in he upsream Thereby, isolaing he ransformer from an imminen danger of damage o is windings Such a differenial proecion forms a uni proecion wherein he boundaries of proecion are defined Fig 3 shows a uni proecion scheme wherein wo CTs, on each side of he ransformer are shown feeding signals o a high impedance differenial relay (I d >) and defining he boundary of he proecion zone [7] These CTs are conneced on o he primary and secondary side of he power ransformer forming a circulaing curren loop sysem The phase and raio errors of hese CTs are compensaed by connecing sardela CT's on a dela-sar conneced power ransformer is shown in Fig 3 On a normal operaing condiion, curren flowing ino he primary side of he ransformer and he secondary curren on he load side are in a balanced sae However 48

58 during a faul, curren imbalance occurs beween primary and secondary side of he CTs which feeds signal o he relay (I d ) causing he relay o operae Primary side CT Δ Y Secondary side CT Dela-Sar power ransformer I d > High impedance relay Figure 3 Basic uni proecion scheme of a ransformer [7] A cerain degree of biasness is provided in he uni proecion o preven spurious ripping as a resul of hrough faul curren due o ap changing operaion in he ransformer In addiions o he problems encounered due o ap changing operaion, biasness is also required in he uni proecion scheme due o he following reasons [7] :- Phase shif across he ransformer windings Effecs of earhing and winding arrangemens Deecion of unbalance signals Effecs of inrush during energizaion However, he major concern of a proecion engineer has been o calculae accuraely he relay bias seing due o ap changing operaions, paricularly for he large power ransformer wherein, he on load ap changer (OLTC) aemps o mainain a regulaed oupu in he even of an unseady source volage This unseady volage frequenly occurs due o volage flucuaion on he source side of he ransformer OLTC is usually locaed on he primary side of a ransformer in order o give a regulaed oupu, as i easier o handle smaller curren on he high volage side An off nominal ap posiion is 49

59 deemed o be an inernal faul and in which case uni proecion rips he upsream circui breaker bu in a pracical siuaion, he proecion relay should mainain sabiliy agains an off nominal ap posiion Sysem engineers achieve his by resoring o a relay seing wih minimum bias adjusmen which is greaer han he sum of he maximum ap of ransformer and possible CT errors Fig 3 shows ha CT on eiher side of he primary plan ie ransformer, play a major role in defining he uni proecion However, correc operaion of he relay wihin he uni proecion scheme depends on he following [3] :- Phase correcion - primary and secondary of he volages of he ransformers measured by he differenial relay are in phase irrespecive of vecor relaionship Raio correcion - mismach due o raio of primary o secondary ransformaion during ap-changing operaion could resul in ripping In elecromechanical relay insallaion days, i was carried ou by using inerposing CTs bu hese days all digial differenial relays are equipped wih suiable algorihm o deal wih his siuaion 3 Zero-phase sequence curren filer - applying his filer, i is easy o discriminae exernal earh faul from a in zone earh faul 4 Magneizing inrush sensiiviy during energizaion In Fig 3 is shown a digial uni proecion scheme of a dela-sar ransformer, wherein he phase compensaion raio correcion of CTs and filering of zero sequence currens are achieved by incorporaing suiable algorihms ino he differenial relay sofware Use of he sofware eliminaes he requiremen of ICT making i compac Mos of he power ransformer elecrical fauls can be aken care by differenial relay using uni proecion scheme such as [3] :- Primary winding phase faul Primary winding phase earh faul Secondary winding phase-phase faul Secondary winding phase-earh faul Iner-urn faul Core faul 50

60 R ~------' Dela Sar Y B Primary CT Sep up power ransformer Secondary CT Id> Id> Id> Differenial relay wih bias se Figure 3 Three phase dela-sar ransformer wih bias se on he differenial relay [7] Fig 33 shows wo secion dual slope bias characerisics The firs secion has a slope which represens ransformer magneising curren and prevens spurious ripping due o inrush In he second secion, 30% slope prevens mal-operaion of he relay due o off nominal seing Third slope is kep a 70% or higher o ensure ha he relay operaes only on heavy hrough fauls Applying hese bias seings on he differenial relay, i is possible o operae or resrain during an inernal faul or inrush Diff Curren (Id) 4 Operae 3 70% slope Seing Range (0-05 I d ) Resrain Effecive bias (x In) 30% slope 8 9 Figure 33 Typical dual slope bias characerisics of a differenial relay [7] 5

61 I is observed ha radiional proecion schemes are no so reliable during a combinaion of inrush and faul curren a he ime of ransformer energizaion The radiional elecromechanical scheme doesn' guaranee resrain in an over exciaion siuaion One such radiional scheme developed by Sharp and Glassburn [7] used harmonic blocking insead of resraining However heir mehod suffered from cerain limiaion as i was unable o deec low harmonic conen presen in he operaing curren which lead o furher research and developmen [8] Tradiional relays behave correcly when he CT's replicae he primary curren in he correc raio on he secondary side bu such ideal siuaion don' occur normally due o CT mismach, sauraion and errors In addiion o he problems of CTs, radiional differenial relays suffers from inaccuracy in esimaion of inrush and over exciaion, inabiliy o deec low harmonics during over exciaion, blocking of relay even while higher order harmonics are presen and aking full one cycle o analyse he inrush curren Reliable algorihms when applied on o a microprocessor digial relay give a robus proecion 33 APPLICATION OF ALGORITHMS IN DIGITAL DIFFERENTIAL RELAYS In order o miigae he disadvanages explained in 3 in erms of speed, reliabiliy, flexibiliy, cos/benefi consideraion, operaional performance and accuracy, microprocessor based digial differenial relay processors using algorihms described in secion 34 considerably improves he performance over radiional proecion relay [5] These algorihms are consanly being improved by researchers o increase he efficiency A basic digial differenial relay block diagram has been explained wih hardware and sofware componens involved in he secion in Fig 3 These algorihms have many advanages over convenional relay such as economy, reliabiliy, compacness in he relay size and improved performance ec Some of he commonly used algorihms used in he digial differenial relays are [5]:- Finie duraion impulse response filer mehod Fourier series mehod 3 Flux resrained curren differenial mehod 4 Leas Square Mehod (LCM) 5

62 33 Finie duraion impulse response filer mehod (FIR) In his digial signal processing echnique, magniude of fundamenal and second harmonics are esimaed using four filers, wo each for fundamenal and second harmonic shown in (3) o (3) Finie filers used in his echnique play an imporan role in he esimaion of he second harmonic raio Using his echnique he magniude of second harmonic raio (SHR) o fundamenal is calculaed and if he raio is deermined o be greaer han he hreshold value i is assumed o be inrush condiion [5] The four filers used in hese algorihms represen he harmonics for a period of one cycle (T) The sysem frequency responses are given in he form of sine and cosine funcion from (3) o (3) o he four filers The inpu currens are in he form of samples wih values ranging from + o - 0 T / S ( ) (3) T / T C ( ) 0 T /4, 3T /4 T T /4 3T /4 (3) The impulse response of fundamenal harmonic is given by (3) and (3) while second harmonic componens are represened by (33) and (34) 0 T /4, T / 3T /4 S ( ) (33) T /4 T /,3T /4 T C ( ) 0 T /8, 3T /8 5T /8,7T /8 T /8 3T /8,5T /8 7T /8 T (34) The sysem frequency response of he above four filers is given in sine and cosine form from (35) o (36) F s T 53 jt / ( ) e cos j (35)

63 54 4 sin cos / ) ( T T T j e j c F (36) 4 sin cos / ) ( T T T j e j F s (37) 8 sin 8 sin sin / ) ( T T T T j e j F c (38) Where, T = π /ω is for period when sysem frequency f 0 F s and F s are he frequency response using cosine filers of fundamenal componen F c and F c are he frequency response of cosine and sine pars of second harmonics Equaion (3) o (34) are furher used o exrac he curren inpus of he fundamenal and he second harmonic by compuing he impulse response of he ransformer curren and evaluaing a =T These oupus from four filer are summed up for a period of one cycle as shown from (39) o (3) Mahemaically, four equaions (35) o (38) are expressed wih N samples per cycle of curren i() and is chosen as muliple of eigh / / () S N k N k i k i (39) 4 / 4 / 3 / 4 / () C N k N k i N k i N k i k i (30) 4 / 4 / 3 / 4 / () S N k N k i N k i N k i k i (3)

64 N / 8 C () k Where, i k i k N / 8 i k N / 4 i k 3N / 8 i k 5N i k N N - Number of sample cycles in muliples of eigh / 8 / i k 3N - Time beween successive samples ie = π / N ω 0 and i k = i ( k ) is he k h sample a any ime = k / 4 i k 7N / 8 (3) S (), S (), C () and C () - impulse responses of he four filers used ie wo for fundamenals and wo for second harmonics During a faul wihin he ransformer, here is a high value of fundamenal componen and a low value in he second harmonic componen Alernaively, for an inrush condiion, he second harmonic componen's magniude is higher and faul is characerized by smaller magniude wih respec o he hreshold Typical values for inrush lies wihin he range: 0 ε 046 for X/R =5 0 ε 0093 for X/R =0 0 ε 046 for X/R=0 Where, X/R is he sysem reacance resisance raio Typical values of ε = 5 Any value greaer han 5 is regarded as inrush and shall render he relay inoperaive Fig 34 exhibis a ypical impulse response of he fundamenal and second harmonic filers whose oupu responses have a value ranging from - o + depending upon he responses of four filers o he ransformer inpu curren The simpliciy of digial FIR algorihm is bes appreciaed when he muliplicaion process required for convoluion is a simple sign change from + o - as shown in Fig34 55

65 S () 0 T/4 T/ 3T/4 T - a C () 0 T/4 T/ T 3T/4 S () b T/4 T/ 3T/4 T C () c 0 T/4 T/ 3T/4 T - d Figure 34 Impulse responses of FIR filers [5] In Fig 35 he ampliude of he raio ω/ω 0 is ploed (X axis) agains frequency response (Y-axis) which is obained by solving (3) o (34) for one period (T) and he ypical frequency responses of fundamenal and second harmonic are exhibied for he four filers in he Fig 35 56

66 F s (ω) a 6 ω / ω 0 Fc (ω) ω / ω 0 b F s (ω) ω / ω 0 c F c (ω) ω / ω 0 d Figure 3 5 Magniude of he frequency response of filers [5] The main advanages of his algorihm is finie impulse response (FIR) filers : Require no feedback Inherenly sable Can be designed o be linear phase (used in phase sensiive applicaions) 57

67 However, FIT suffers from he following disadvanages: Filer requiring more ime in compuaion Low frequency harmonics may no ge filered 33 Fourier analysis mehod This mehod is based on he assumpion ha he faul generaes a waveform wihin cerain period of ime say ( 0) o (T+ 0 ) conaining fundamenal, second and fifh harmonic waveforms Expanding a periodic funcion f() and applying Fourier series afer digially exracing he harmonics [9] from equaion (30) where, a0 f() n a cos nω b n sin n nω n 0 0 (33) f () - Periodic funcion which varies wih ime a, 0 0 a, n b - Coefficiens of he periodic funcion n n - n h order of angular frequency - Angular fundamenal frequency = πf 0 0 Coefficiens can be deermined from (34), (35) and (36) a T 0 f ( ) d T T T a n 0 T T b n 0 (34) f ( )cos n d (35) f ( )sin n d (36) The main advanages in he Fourier analysis mehod is ha i makes no assumpions of he fauled waveform having boh volage and curren signals and can be migraed o he frequency domain Using he daa obained from volage and curren he impedance is calculaed of he faul 58

68 Fourier series suffers from he following limiaions: Boundary condiions need o be defined and disconinuiy of he waveform periodiciy may arise if no defined clearly Odd and even harmonics are difficul o exrac Specificaion of odd and even harmonics around he boundary condiions is difficul o predic a imes 333 Flux based algorihm In his mehod resrain funcion is obained by flux-curren relaion of power ransformer and requires less compuaion han Fourier analysis If he flux is esimaed correcly hen over exciaion and magneising inrush could be deal easily Fig 35 gives a simple wo winding ransformer where primary is linked o he secondary by flux ψ Assuming he resisance of he winding o be negligible he relaionship esablishing beween primary volage and he muual flux linkage ψ is given by dip( ) d ( ) Lp vp( ) (37) d d Applying rapezoidal rule afer rearranging and inegraing (34) Where, [ v ( ) v ( )] L [ i ( ) i ( )] ( ) ( ) (38) p p p p p v - Primary applied volage on he power ransformer p i p - Primary applied curren on he power ransformer ψ - Muual flux linkage of on he power ransformer Expressing (35) as he k h sample of volage and curren waveform and calculaing he muual flux linkage of a ransformer 59

69 k k ( vp, k vp, k) Lp( ip, k ip, k ) (39) Where, L p - Leakage inducance of he primary winding i - kh samples of primary curren p, k ip, k - k h samples of primary volage Equaion 39 is normally used o calculae he muual flux linkage ψ of he ransformer A ime k, he differenial curren of a power ransformer is given by (37) i d, k i i (30) p, k s, k Where, i s, k - k h sample of he secondary curren i d, k - Represens he differenial curren in he ransformer Equaion 30 represens he differenial curren of a ransformer which is equal o magneisaion curren of a ransformer If i d, k and are ploed, i will be observed ha k he resuling curve aligns wih he open circui magneisaion of he ransformer Using his flux resrained mehod, he firs sep is o deec he fauls wihin he ransformer a every sampling inerval In second phase, a check is carried ou on he locaion of he poins i d, k and If he sampling poins lie ouside he open circui magneising curve, k hen a rip signal is issued wih an assumpion ha an inernal faul wihin he ransformer has occurred In Fig 36, he relaionship beween curren and flux is shown for a wo winding ransformer The accuracy of his mehod depends on he correc esimaion of magneisaion flux, which would discriminae inrush from faul condiion, depending on is value k 60

70 ψ I p() I s() Vp () v s () Figure 36 Two winding Transformer [5] When he residual flux is close o zero, he above echnique works fine, bu pracically he case is differen as i and characerisics varies as shown in Figure 37 (b) when d k compued in equaion (30) This is because esimaed value is subjec o an error o he residual flux linkages value Equaion (38) shows he way o miigae he problem by using flux resrain mehod which is deermined by he slope d k di i i k k k V i p, k p, k d raher han using flux ψ iself di V i p, k p, k L p (3) where, L - Leakage inducance p V p, k - k h sample of he primary volage Figure 37 designaes a region conaining faul and non-faul zones wihin which he inrush curren alernaes in he dψ/di - i d plane Fig 37 (a) shows he faul wihin a sauraed par, while 37 (b) operaes in an unsauraed par During power ransformer inernal faul, curren samples dψ/di remains coninuously in region However during inrush, hey oscillae beween he wo regions This 6

71 phenomenon is generally used o creae an index of resrain k whenever i falls in region Index is decreased whenever sample pair eners region ψ No faul Faul Faul i d No Faul (a) ψ I d (b) Figure 37 Transformer magneising curve [5] (a) Faul and non faul region (b) effec of remanen flux Fig 37 (b) exhibis he ypical characerisics of an i d and ψ plane wih he effec of remanen flux I is observed pracically residual core flux doesn' work close o zero 6

72 d Fig 38 shows a plane which shows he faul and non-faul region In he non-faul d region, flux is locaed in an unsauraed par ie region However, during inrush condiion, he flux oscillaes beween region and region,, dψ/d Region : no faul region Region : Faul region l Figure 38 Faul and non faul regions in dψ/di- i plane [5] In Fig 38 a rip condiion resuls when he pair d i, k k is in region, where d regarded as he resrain index The value of his index increases in region, while i decreases in region I is noed ha, he value k r never reaches he hreshold for all non faul condiions Threshold value acually depends upon he sampling rae and hence k r mus be deermined experimenally k is r The above algorihm has an advanage when he residual flux circulaes in he ransformer core close o zero Pracically his difficul and idenificaion of fauled region is difficul o deermine 334 Leas Square Mehod In his mehod, fundamenal and higher order harmonic conens are exraced from curren signals by daa sampling and ploing a curve as shown in Figure 39 The plo is 63

73 esablished by exracing volage and curren waveform coordinaes Leas square curve fiing mehod (LSC) differeniaes inrush from inernal fauls by comparing curren based signals wih a se hreshold value [5, 0] In Fig 3 (X,y ), (X,y ),(X n,y n ) are a se of N coordinaes where X represens curren and i is an independen variable aken a he i h measuremen, y represens volage variable and i is a dependen variable Using many X, y coordinaes, a curve is ploed and he funcion of y is derived using Taylor series expansion in equaion (3) Unknown polynomials are solved using marix for equaion (33) Y Leas square polynomial curve (u) X X X X X X X X X,y X X X,y X X X X X X X X Verical displacemen from Leas square polynomial curve x Figure 39 Leas square curve fiing mehod [5] Fundamenal componen of curren is expressed in phasor form equaion (39) and is given as [0] where, ( ) 5 i I e sin ( )] [ m 0 m 0 m m I (3) τ - Time consan of any decaying DC componen m - Harmonic order I - Harmonic curren samples (up o 5 h order) m - Angular frequency 0 I 0 - Decaying DC curren 64

74 65 Assuming ha he inrush curren consiss of fundamenal o fifh harmonic only and rewriing (3) o solve for ataylor series expansion expression (33) ) cos( sin ) sin( cos ) ( m m m m m m m I m I e I I i (33) The unknowns in (33) are 0 I, e I 0, m m I cos, m m I sin (where, m=,,5) which can be wrien in marix form ) ( ) ( ) ( cos sin cos cos 5 sin 5 cos 5 sin 5, cos sin cos 5 sin 5, cos sin N N N n i i i I I I e I I (34) Rewriing marix operaion in (34), in general form, x a x a x a x a a (35) 0 I x a / 0 I x a k,,5 cos sin n n a n a k k n n,,5 cos sin 5 n I x I x n n n n n n (36) To solve for unknown elemens of x n (n=,5), m equaions can be consruced from N curren samples in a marix form having samples as shown in (33) in simplified form

75 66 N m Nm N N N N i i i i x x x X a a a a a a a a a A (37) Marix (37) is solved by furher simplificaion A X = i (Nx) (x) (N x) Or X = (B) * (i) (38) where, B = {A T A) - (39) and A is pseudo inverse of A and A T is he ranspose of marix A Solving for real and imaginary pars of he fundamenal and second harmonic, N n n i n b x I 3 ) (3, cos (330) N n n i n b x I 8 ) (8, sin (33) N n n i n b x I 4 ) (4, cos (33) N n n i n b x I 3 ) (9, sin (333) where, k x is he k h elemen of vecor X and b (k, n) is he k h row and nh column of marix B

76 Ampliude of fundamenal and he second harmonic is calculaed by I n ( I n cos ) ( I sin ) where n, (334) n n n For 5 h harmonic, above echnique may also be applied in a similar manner bu i has been negleced for higher order as i does no carry significan inrush componen Discriminaion is obained when we compare second harmonic componen I wih fundamenal componen I in 334 Deerminaion of magniude of he curren is done by 334 and calculaed by Second Harmonic Raio (SHR) I SHR (335) I If he SHR is greaer han a se value hen inrush is assumed oherwise, if i is less is regarded as inernal faul Fig 30 gives he four filers which represen he digial filers and which has he real par (x 3 and x 4 ) and imaginary pars ( x 8 and x 9 ) I shows he graphical oupu of he filers where unwaned harmonics are filered ou B(4,n)in X 4 x 4 x 9 I B(9,n)in X 9 B(3,n)in X 3 SHR B(8,n)in x 8 x x 3 8 I In Figure 30 Block diagram for deermining he Second Harmonic Raio (SHR) [5] 67

77 In he LCM deerminaion of sampling frequency and filraion of noise using digial filraion is an imporan aspec Sampling frequency mus be greaer han wo imes he highes sysem frequency and i is no realisic o have lower sampling frequency below 600Hz and incorrec curve may occur if aken Sampling frequency mus be aken more han 0 samples o ge a correc resul beween 600Hz o 300Hz Higher order harmonics ie 5h, 6h, 7h,9h harmonics don' have major impac in he calculaion using LCM echnique and can be eliminaed 34 REVIEW OF DIGITAL DIFFERENTIAL PROTECTION ALGORITHM In view of he benefis which digial algorihms provide over convenional relaying, i is of ineres o review he bes mehod o be used in he differenial proecion However no single algorihm is chosen as he bes one because applicaion of he algorihm depends on he ransformer connecion, ap changing mechanism ie dela-sar, sarsar, on-load and off load ap changing ec[] Rockefeller, in 969 was he firs o propose he differenial proecion algorihm for ransformer by wave shape analysis wih he simulaion carried ou on a sampling rae of 000Hz and having response ime of 0 mili seconds (ms) Larson, Schweizer and Flechsig proposed Finie impulse response filers (FIR) for inrush deecion having a sampling rae of 480 Hz wih a response ime of 6 ms using Forran language He simulaed he es on a single phase ransformer Thorp and Phadke used discree Fourier ransformer (DFT) model for simulaion in 98 They successfully simulaed in he differenially relay for a sampling window of 70Hz in a full cycle and obained he relay response ime wihin 6 ms Degens is credied wih he LCM mehod in 98 using leas square curve fiing mehod o proec he ransformer wih a 600Hz sampling window on a full cycle having a response ime of he relay being less han 30ms, bu his es was carried ou on a single phase ransformer in he laboraory on a smaller raed ransformer Thorp and Phadke in 983 are credied wih esablishing flux resrain mehod wih a sampling window of 70Hz and relay response ime being ms The specific requiremen of an algorihm depends on he exciaion curren and harmonics produced during iniial energizaion of he ransformer For a harmonic based 68

78 algorihm, he requiremen is ransformer inpu and oupu currens while flux and volage based resrained mehod he requiremen is more complex having no only he requiremen of ransformer inpu and oupu volage and curren bu also he scaling and isolaion signals, low pass filers and A/D filers Deails of he common mehods for deermining inrush and fauls in a hree phase ransformer are elaboraed in he nex secion 35 COMMON METHODS FOR DETRMINING INRUSH AND FAULTS When an unloaded ransformer is energized i draws a non symmerical magneising curren known as inrush which spuriously rips proecion relay [4] The main cause of inrush is ransformers core sauraion Inrush if no checked, could cause ransformer winding o be sressed and ho spos o occur and ulimaely leading o insulaion failure Hence i is very imporan o miigae he inrush righ a is onse a he ime of swiching Tradiionally, ripping of ransformer was prevened by swiching off he ransformer proecion during energizaion Wih he advancemen in numerical and digial relaying echnology, resraining during inrush and ripping on faul has undergone changes In oday's world, complex algorihms using harmonic analysis, wave form analysis, fuzzy logic mehod ec are being used o resrain he digial relay These algorihms are incorporaed ino he relay, cerain boundary condiions o rip Resraining he relay by insering resisors and swiching off he ransformer are no longer considered reliable o conrol spurious ripping during inrush proecion of a large ransformer Subsecion 34 o 333 describes mehods commonly used for proecion agains inrush and operaion during fauls [3] 35 Harmonic resrain mehod Volage waveform when disored in is sinusoidal form is regarded as harmonics Harmonics generae inrush curren which causes heaing, sress on ransformer insulaion ulimaely leading o ransformer failure As his causes nuisance ripping of relay, few mehods have been developed o resrain a he ime of inrush and harmonic resrain is one of hem There could be muliple reasons for inrush occurrence during he ransformer energizaion beside ransformer core sauraion Inrush currens are 69

79 ypically characerised by high harmonics due o several facors like presence of nonlinear devices in he circui, swiching he ransformer a a non zero swiching angle ec Presence of second order harmonics is one of he significan characerisics of inrush and he proecion relays are designed o resrain during inrush condiion However, he same proecion sysem shall ac due o he presence of lower order harmonics ie fundamenal harmonics which could be laced wih faul signals Some of he mehods developed by he manufacurers are per phase mehod, cross blocking mehod, percenage average blocking mehod and summing-ype harmonic sharing mehod [] However, he mos popular mehod used by relay manufacurers are second harmonic resrain mehod wherein he SHR is compared o a se value The relay logic resrains if SHR is greaer han he se value and rips if SHR is below he se value However harmonic resrain mehod suffers from few shorcomings as given below: Accurae esimaion of second harmonic is difficul Amoun of higher harmonics may drop below 0% and second harmonics could be well below 7% and inaccurae esimaion chances are high 3 Second harmonics raio could be well below safe 0% due o ransiens 4 Volage drop across nework reacance due o harmonics 35 Waveform based resrain mehod As described previously, inrush currens have high peak waves and slow decays This high magniude peaks and is decaying par are analysed for deecing and resraining he relays by waveform based resrain mehods The wo fundamenal wave form based mehods of resrains are given in wo approaches [3]: Approach : Pays aenion o he periods of low and fla values in he inrush curren known as dwell ime Magneising inrush is ruled ou as i does no show up every cycle lasing aleas for /4 h Cycle in which he shape of he waveform is boh fla and close o zero Weaknesses of his mehod are: Inernal faul vs magneisaion akes one full cycle which is regarded slow CT sauraion may cause false ripping 70

80 During CT sauraion, secondary currens may show periods of low and fla values which may cause relay o miss an operaion Approach : I has is peaks displaced by half cycles and does no have wo consecuives peaks of he same polariy Weaknesses of his mehod are: No easy o deec peak values Timing beween wo successive peaks may be checked Difficuly in checking he peaking imings beween wo consecuive peaks A any given period shall have all hree phases inrush in uni-polar waveform and likely o fail he operaion of he relay However approach is preferred over approach due o is robusness in oleraing CT sauraion 353 Flux resrain mehod In his approach he inernal fauls are discriminaed from over exciaion and inrush based on he flux in he ransformer core Flux recognizaion is one of he preferred mehods and a echnique o miigae inrush using his mehod has been deal in chaper 4 of his hesis as he pre-fluxing mehod of miigaing inrush In his echnique sauraion of he ransformer core is used o deec curren imbalance and he relay commands he circui breaker o resrain due o presence of high flux 36 ANALYTICAL EXPRESSION FOR INRUSH CURRENT Transien performance is bes undersood by modelling he equivalen circui of a wo winding ransformer and applying he basic equaions Consider he equivalen circui of power ransformer given in Fig 3 having dual slope sauraion characerisics shown in Fig 3 [3] 7

81 i s r p lp lsp r sp Vp L m i Vs Primary Secondary Rn v n Figure 3 Transformer Equivalen Circui [3] λ L S Lm I S I m Figure 3 Simplified wo slope sauraion curve [3] where, r - Primary resisance p l - Leakage reacance p L (i) - Non linear inducance of iron core m r sp - Primary resisance referred o secondary l - Primary side leakage reacance p v - Primary ground erminal volages p v - Secondary ground erminal volages p 7

82 During ransformer energizaion he equaion governing he sauraed ransformer core is as given in (336) d r R i( i di p n p d d v ( ) ) (336) p Using he relaionship beween he flux linkages wih magneising curren and rewriing he equaion (336) v p di r R i( ) i di L p n p core d d ( ) (337) The soluion o (337) is found by inroducing non linear inducor as a linear inducor in unsauraed L m and sauraed L s modes of operaion During ransformer energizaion iniial flux λ 0 shall go below sauraion level λ s unil he cores are sauraed There is no effec of hysersis on s ie sauraion ime s s v d 0 sin s m (338) 0 s 0 ( ) cos [ ( )] 0 Where, - Nominal peak flux linkages n - Iniial flux 0 s - Sauraion flux ω - Angular frequency v m - Nominal peak supply volage n (339) Upon reaching sauraion he curren equaion of he ransformer and is parameers shall be wrien as: i / ( ) A e B sin where s (340) i / ( ) ( i A ) e B sin s where s (34) 73

83 74 Where, A, A - Magniude of decaying DC offse of fundamenal and second harmonic τ - Decaying DC ime consan B, B - Ampliude of he fundamenal an second harmonic, - Phase angle of fundamenal and second harmonic - Angular frequency To deermine he parameers A, B A, B, θ, θ,is,τ, τ followings expressions are used )]} ( [ ) {( p m n p m l L R r v B (34) )]} ( [ ) {( p S n p m l L R r v B (343) sin B A (344) ) sin ( s B A (345) ] ) ( [ an n p p m R r l l (346) ] ) ( [ an n p p s R r l l (347) s s s i i / (348) n p p m R r l L (349) n p p s R r l L (350) Equaion 340 and 34 are simplified furher o deermine he peak inrush per phase and is given by equaion / ] [ n n peak R R (349) Simplifying and rearranging (349)

84 / R n (350) R peak n Expressing he equaion for inrush in general form I peak ( s)/ peak ( R ) A e B n (35) 37 CONCLUSION Digial proecion is gaining ground over convenional relays by employing algorihms described in his chaper and due o is compacness and reliabiliy in proecing he primary plan asses wihin he uiliy nework Wih he applicaion of hese algorihm digial differenial proecion has become more economical, flexible and has improved he performance considerably over radiional relays I is observed in able ha inrush curren conains high percenage of second harmonic componen Hence, i is of grea ineres o engineers and relay manufacurers o uilise SHR o discriminae inrush from fauls Modern relay uses he some of he algorihms described in his chaper like fundamenal and second harmonics o discriminae inrush from shor circui curren Four commonly used algorihms on harmonic deerminaion, flux evaluaion and leas square curve fiing mehod has been described in his chaper such as FIR, Fourier analysis, Flux resrain mehod and LCM These algorihms are used in digial proecion wih due consideraion given o he signal processing for relaying purpose Removal of DC componen has been a challenging ask and usually akes complex calculaions o remove he DC offse in a real ime applicaion Esimaion of DC decaying componen using simple and numerically efficien mehod has been proposed in his chaper based on simplified equaion and is eliminaion of DC componens is given in chaper 4 Presence of DC componens ends o make he relay operaion slow and cause large errors A real ime soluion and miigaion of inrush and faul curren has been proposed in furher chapers using Fourier filers in a MATLAB model The performance of wo of hese algorihms for he proecion of an unloaded hree phase power ransformer has been explained wih a case sudy, in chaper 4 and 5 75

85 Chaper 4 Miigaion of Inrush Curren in a Three phase Power Transformer using Pre-Fluxing Technique 4 INTRODUCTION A he ime of ransformer energizaion under no-load, a high curren is drawn due o is core sauraion known as ransien inrush curren This ransien inrush could rise up o seven o eigh imes he nominal full load curren of he ransformer and his phenomenon may las for ens of seconds Furher, hese ransiens could produce mechanical sress on he power ransformer assembly and could lead o mal-funcion of proecion sysem Inrush curren ofen affecs he power sysem qualiy and may disrup he operaion of sensiive elecrical loads conneced o he sysem The mehod of miigaion of ransien currens has become an imporan concern o he proecion specialiss, as i conains rich harmonics laced wih DC componens [4] Decaying DC componens are produced due o inrush and fauls which inroduces errors up o 5% in he proecion sysem Convenionally, he mehod o couner hese ransiens was o de-sensiise he relay or increase he size of he fuses or 76

86 conrol he swiching angle of he circui breaker or perform a poin on wave swiching, bu i is observed ha hese mehods require he knowledge of residual flux of he ransformer prior o energizaion and ime consan τ of he sysem This is a challenging ask for he relay manufacurers as he values of hese parameers depends on he sysem configuraion, circui breaker swiching angle and faul locaion Few oher known mehods of ransien miigaion include, placing capaciors a he secondary side of he ransformer, using disribued line on he secondary side of a low pass filer However, hese echniques canno guaranee he desired oucome due o differen vecor groups of ransformers and ap changers operaion As he securiy and sabiliy of ransformers is of grea ineres o he sysem operaions, in which hree-phase ransformer is a key componen, i is imporan o proec he primary plan reliably The large ransien curren of ransformer due o flux sauraion in he core, which is called inrush curren, ofen causes he mal-funcion of he proecive relaying sysem This ransien curren affecs ouage ime of ransformer as he engineers have o examine closely he ransformer and he proecive sysem, o check for fauls The large ransien curren causes serious elecromagneic sress impac and shorens he life of ransformer I is very imporan o solve he effecs of inrush curren [5] Unconrolled energizaion of ransformer produces high inrush currens, which can reduce he ransformer life and can also lead o he unexpeced operaion of proecive relays and power qualiy reducion This curren depends upon various operaing condiions, such as he magniude of he volage, he swiching-on angle, he residual flux, he hyseresis-characerisics of he core, he resisance in he primary circui, and many oher parameers which has been described in [6] There are hree negaive side-effecs of inrush currens Firs, he proecive devices for overloads and inernal fauls may falsely operae and disconnec he ransformer from he power sysem spuriously In order o preven his occurrence, number of mehods has been incorporaed in he relay logic o disinguish beween fauls and inrush currens which could reduce hese undesirable rips Second, he windings are exposed o mechanical sresses ha can damage he ransformer and hird, power-qualiy problems may arise like high resonan harmonic over volages and volage sags [7] 77

87 In he recen years, many digial differenial proecive sysems for ransformer have been developed Differen echniques based on complex circuis or microcompuers have been proposed o disinguish inrush curren from faul curren However, he proeced ransformer mus sill wihsand large elecromagneic sress impac caused by he inrush curren wihou unduly causing he mal-operaion Furher, ransformer being a sensiive componen and being exposed o power sysem ransiens, mus have a robus and fas acing proecion sysem o preven any damage o is core as a consequence of harmonics I is noed ha non-sinusoidal harmonics are generaed from differen nonlinear sources wihin he power sysem These harmonics flow hrough ransformers and have a derimenal effec on he equipmen in he nework The main facors affecing he magneizing inrush curren are poin on-wave volage a he insan of energizaion, magniude and polariy of he remnan flux Addiionally, oal resisance of he primary winding, power source inducance, air-core inducance, he geomery of ransformer core and he maximum flux carrying capabiliy of he core maerial are also affeced by inrush curren [7, 8] In ligh of his discussion, i is of ineres o miigae inrush while isolaing he hree phase ransformer from furher damage due o faul Hence, his chaper proposes a echnique o miigae inrush curren in a hree phase ransformers by injecing predefined value of DC flux in he primary of he ransformer applying a process known as pre-fluxing Using his pre-fluxing echnique, he ransformer is energized by a convenional conrolled swiching A sample case is considered in which a hree phase ransformer is conneced o a supply source, a MATLAB simulaion model is designed and developed by considering his case and verifying he effeciveness of he proposed pre-fluxing mehod The resuls derived are validaed using his model for is efficien inrush curren conrol This chaper proposes his novel echnique o miigae inrush curren of hree phase power ransformer called pre-fluxing in which pre-fixed value of DC flux is injeced in primary of ransformer before energizaion 4 NATURE OF INRUSH TRANSIENTS The sauraion of he magneic core of a ransformer is he main cause of an inrush curren ransien The sauraion of he core occurs due o an abrup change in he 78

88 sysem volage which could be caused due o swiching ransiens, ou-of-phase synchronizaion of a generaor, exernal fauls and fauls resoraion The energizaion of a ransformer yields mos severe case of inrush curren and he flux in he core could reach a maximum heoreical value of wo o hree imes he raed value of peak flux Inrush during ransformer energizaion causes operaional problems o he power sysem I affecs he ransmission lines being energized afer an ouage or line being loaded suddenly Inrush characerisics are usually unidirecional which rises sharply and decays slowly The maximum rise of inrush curren occurs afer he firs half cycle The magniude and duraion depends mainly on four facors [9] : Poin on he volage wave a he ime of ransformer energizaion Impedance of he circui 3 Residual flux linkage 4 Non-linear magneic sauraion of ransformer core The firs wo facors, depends on he vecor group and connecion of he power ransformer while he second wo facors, depends on he flux in he magneic circui which again is dependen on he maerial and seel used in he ransformer core Deails of magneic circuis are difficul o obain, as manufacurers do no submi deailed composiion of he core maerials used Addiionally, here is no direc evidence ha he energizaion of a ransformer can cause an immediae failure due o high inrush currens However, insulaion failures in power ransformers which are frequenly energized under no load condiion suppor he suspicion ha inrush currens have a disasrous effec A more ypical problem caused by he energizaion of ransformers is due o harmonics ineracion wih oher sysem componens ha develops over-volages and resonan phenomenon The sudy of he energizaion of a ransformer insalled in an indusrial faciliy carried ou o address problems due o harmonics, over-volages and resonances In few inernaional conferences [30], he auhors have discussed how he harmonic disorions caused by swiching on of lighly loaded or unloaded ransformers may be amplified during a power sysem resoraion process, creaing high harmonic over-volages During he process of energizaion of large MVA ransformers in EHV subsaions a he end of long ransmission line is observed o cause significan emporary disurbances when harmonic resonances are reached This phenomenon paricularly occurs, when here are ransformers which are already conneced o he bus 79

89 and he disurbances caused by he energizaion of one more ransformer have greaer duraion and inensiy In [3], i is discussed how ransformer inrush curren can excie resonance frequencies in iner-conneced grid sysem 43 PRE-FLUXING TECHNIQUE In he pas few years, conrolled swiching has been deemed as a popular echnique o miigae inrush curren However, he key aspec of his mehod is o gain he knowledge of residual flux in he ransformer Earlier, several echniques had been suggesed o obain residual flux a he insance of ransformer swich off, bu i is a lugubrious and ime consuming process In order o make a simplisic model having minimum applicaion of residual flux, his chaper proposes a new echnique o se he iniial fluxes of ransformer o a desired value, which is known as pre-fluxing [] The innovaion behind he pre-fluxing inrush curren reducion sraegy lies in he prefluxing device iself The pre-fluxing device (capacior) is charged o a user specified volage and hen discharged ino he ransformer during which he circui breaker swich is closed For he pre-fluxing device, i is necessary no only o se he residual flux of a ransformer as high as possible o minimize he inrush curren, bu also doing i efficienly The pre-fluxing reducion sraegy is a wo par process Firs, he ransformer residual flux is se as close as possible o is maximum achievable residual flux when he ransformer is de-energized The second par of process conrols he circui breakers (CBs) o energize he ransformer There are hree conrolled sraegies for he conrol of circui breaker swiching, firs is rapid closing, second is delayed closing and hird is simulaneous closing [4] In he rapid closing mehod, CB closes one phase firs and he remaining wo phases wihin a quarer cycle Knowledge of he residual flux prerequisie and required for all hree-phases, independen pole breaker ype conrol, and a model of he ransformers ransien performance In a delayed closing mehod, firs pole closes insanly and he remaining wo phases afer 3 cycles However, his needs knowledge of he residual flux in one phase only of he independen pole breaker conrol, bu does no require any ransformer parameers In simulaneous closing mehod, all hree phases closes ogeher a an opimum poin of he residual flux paern I does no require independen pole breaker conrol, bu requires knowledge of he residual flux in all hree phases and he residual flux 80

90 magniudes in wo phases which are high and follow he mos radiional residual flux paern [7, 8] These closing imes of he circui breaker (CB) are chosen as a par of an inrush curren reducion sraegy for he hree phase ransformer ha enable he use of he hree pole CBs The pre-fluxing device shown in Fig4 is equipped wih a capacior, a diode, and a swich A charging circui no given in his Fig 4 provides he iniial volage across he capacior The device is used across he primary winding of he ransformer bu no when he ransformer is conneced o he nework The reason for using high volage winding is o reduce he magneizing curren on his winding However, i is o be noed ha his pre-fluxing device is used only when he ransformer is isolaed sae and operaed a a very low volage The advanage lies in using relaively inexpensive isolaor swiches which can connec he pre-fluxing device o he ransformer Figure 4 Pre-fluxing device The pre-fluxing device is sized o operae around he ransformer s magneizing curren level, so he capacior, diode and swich can be sized for a fracion of he ransformer raed curren [] 44 MITIGATION OF INRUSH CURRENT IN TRANSFORMERS USING PRE- FLUXING This secion describes miigaion of inrush curren in wo seps The inrush curren is miigaed using pre-fluxing device as shown in Fig4 8

91 Figure 4 Connecion of pre-fluxing device in a hree phase power ransformer 44 Sep-: Pre-fluxing device The pre-fluxing device is conneced o he primary winding of an unloaded hree phase ransformer The device should be conneced o he hree phase ransformer only during is isolaion sae, because i can back feed DC flux o he core before energizaion of he ransformer During he ransformer energizaion, i is observed ha he pre-fluxing device will be bypassed using an independen swich a he same insan[3] Furher, his pre-fluxing device ses a known residual flux in he primary of ransformer Applying poin on wave swiching [], he ransformer will be energized according o he residual flux The capacior will be charged o he maximum value of he ransformer volage Prefluxing device injecs DC flux ill he ransformer is energized The simulaion of his model has been designed and developed using MATLAB and filers have been applied o limi he harmonics 44 Sep II: Conrolled Swiching The conrolled swiching of he hree phase ransformer has been used o miigae inrush curren afer applying he pre-fluxing device The swiching operaion is conrolled by circui breakers Three circui breakers (CB) are conneced on each phase which is normally open When CBs are open, ransformer is isolaed from he 8

92 sysem and a ha insan, he pre-fluxing device is allowed o ge conneced o he ransformer via swich [] As he CB closes and power is given o he ransformer, he pre-fluxing device will be disconneced hrough isolaor Disconnecion of prefluxing device occurs insananeously as soon as he ransformer is energized The conrolled swiching is applied afer he pre-fluxing I has hree sraegies, described in Secion III In his chaper, simulaneous closing sraegy has been used for conrolled swiching [4, 33] Fig43 shows he simulaneous closing in hree phase ransformer, which uses he basic concep of conrolled swiching A hree-phase AC volage consising of hree volage waves U, V, and W is shown in Fig 43a Figure 43 Three - phase simulaneous conrolled swiching wih phase volage 83

93 The magniudes of he residual magneic fluxes, which depend on he phase angle a which he circui breaker opened, are ploed in Fig 43b In his example, he maximum residual magneic flux is in magneic phase U The sum of he residual magneic fluxes in he hree iron cores in a hree-phase ransformer wih dela windings is zero Consequenly, when he residual magneic flux is a maximum in a cerain core, he residual magneic fluxes in he oher wo cores have he reverse polariy and smaller absolue values The magneic flux magniudes ha would be induced by he volage waves, as funcions of he phase angle a which he volages are applied is ploed in Fig 43b 45 MODELLING OF TRANSFORMER FOR INRUSH CURRENT STUDY A 3-phase, unloaded, sep up power ransformer, having a raing of 50 MVA, 5 kv/400 kv, sar-sar, 50 Hz, is conneced o a 3-phase 5 kv source is shown in Fig44 The core magneizing resisance and inducance is 450Ω and 435 H of hree phase ransformer respecively Using he reference model formed by [] where he magneic modelling of core is carried ou and ransformed ino an elecric model, which is developed in MATLAB for simulaion sudy Figure 44 MATLAB model o deermine inrush curren in an unloaded ransformer The ransformer is energized wih appropriae iniial flux and sauraed core in each phase o ge he value of inrush curren When he ransformer is energized, he flux of 84

94 all hree-phase increases unil i reaches is maximum value Afer aaining he maximum value, his flux will become sauraed and draw more curren from he source, which will be 7 o 8 imes greaer han raed curren The main reason of sauraion of flux is residual flux is cerain amoun of flux which remains in he ransformer core a he ime of swiching off of he ransformer Residual flux depends on he raing of ransformer and de-energizaion insan I will have differen values for differen raings of ransformer [3] 46 SIMULATION RESULTS 46 Inrush curren in power ransformer wihou using pre-fluxing device The resuls of he model explained in Secion 43, he inrush curren in each phase is deermined wihou pr-fluxing device Fig 44a,44b,44c shows he inrush curren in phase A, B and C and Fig 45(d) show he fluxes in all hree phases The value of curren in phase A, Fig 45 (a) is 650 A and reaches a seady sae condiion in 8000 ms (8 s) (a) Inrush curren in phase A 85

95 (b) Inrush curren in phase B (c) Inrush curren in phase C 86

96 Flux (wb) 500 Flux in hree phase wihou Pre-flux device Time (s) (d) Fluxes in all hree phases Figure 45 Inrush curren and fluxes in individual phase and collecively In Fig 45 (b) he value of his inrush curren is found o be 670 A and i reaches in seady sae condiion in 7500 ms (75 s) in phase B which is less compare o phase A curren Fig 45(c) shows he inrush curren in phase C where he value of he inrush curren is 630 A and i goes in o seady sae condiion in 7400 ms (74s) The inrush curren in phase C is lowes curren compared o oher wo phase currens Fig 45(d) describes he variaion of flux in each phase The maximum flux in phase A is 300 Wb, in phase B is 00 Wb and in phase C is 00 Wb 46 Harmonic analysis wihou filers In secion 4 of his chaper, i is seen ha inrush curren is a harmonic rich curren and hence he resuls suppor ha he oal harmonic disorion (THD) in each phase are much high Harmonic analysis of inrush currens wih residual flux in phases A, B and C is shown in Fig 46 Now in Fig 46 (a) THD is 70% The DC componen in his phase is 58% and second harmonics is 6 % In Fig 46 (b) THD indicaes o be 066% and he DC componen in his phase is 5% which is slighly less in comparison o phase A and second harmonic is greaer han phase A which is 78% Fig 46(c) shows harmonics in phase C Highes THD generaed in his phase which is 075% The DC componen in his phase is 50% which is lesser compared o phase A and phase B Second harmonic is 79% which is highes among all hree phases 87

97 (a) Harmonics in phase A (b) Harmonics in phase B (c) Harmonics in phase C Figure 46 Magniude of harmonics wihou filers 88

98 Summarizing he resuls in a abular forma: Table 4 : Summarizing he comparaive resuls for harmonic analysis for hree phases wihou applying pre-fluxing Parameers Phase A Phase B Phase C THD (%) DC Componen (%) Second Harmonic (%) Inrush curren in ransformer using pre-fluxing Now, using a pre-fluxing device by connecing i o a hree phase power ransformer having he parameers given in secion 45 and MATLAB model in Fig 44 and injecing some amoun of DC flux in he primary side by removing i a he insan when a power ransformer is energized resuls in a wave paern as shown in Fig47 wih inrush curren in phase A, B and C Fig 48 shows fluxes of all hree phases Fig 47(a) shows he inrush curren in phase A in power ransformer currens using pre-fluxing echnique, he magniude of curren is 9A On comparing wih Fig 45(a), he inrush curren is miigaed from 600A o 9A 89

99 (a) Miigaed curren in phase A (b) Miigaed curren in phase B 90

100 (c) Miigaed curren in phase C (d) Fluxes in all hree phases Figure 47 (a), (b) and (c) Inrush curren in Phase A, B and C, (d) Fluxes in all hree phases 9

101 Fig47 (b) shows he inrush curren in phase B of he power ransformer using prefluxing The magniude of curren is 7A There is momenary shoo up o 50A a 00ms bu laer i decays o 7A for phase B and same for phase C Comparing wih Fig45 (b), he inrush curren is miigaed from 650A o 7A Fig 47 (c) showed he inrush curren in phase C in power ransformer using pre-fluxing The magniude of curren in his phase is reduced o 6A from 630A Fig 48 shows he fluxes in each phase The maximum flux in phase A is 00Wb, in phase B is 000Wb and in phase C is 990Wb Summarizing he above in a abular form in Table 4 and abulaing:- Table 4 Comparison of inrush curren before and afer miigaion Phases Inrush curren before miigaion (A) Inrush curren afer miigaion (A) A 65 9 B 65 7 C Inrush curren using pre-fluxing in ransformer wih filer I is observed ha when filer is conneced o he 3 phase power ransformer, he percenage of DC componen and THD will decrease drasically in magniude which is shown in Fig48 Afer connecing filers, THD in phase A is 05% The and DC componen is 4 % in Fig 48(a) In phase B, THD is 498 % and DC componen in his phase is % In phase C, THD is 538 % and DC componen in his phase is 6% 9