Fazeli, Meghdad (2011) Wind generator-energy storage control schemes for autonomous grid. PhD thesis, University of Nottingham.

Transcription

1 Fazeli, Meghdad (2011) Wind geneato-enegy stoage contol schemes fo autonomous gid. PhD thesis, Univesity of Nottingham. Access fom the Univesity of Nottingham epositoy: Copyight and euse: The Nottingham epints sevice makes this wok by eseaches of the Univesity of Nottingham available open access unde the following conditions. This aticle is made available unde the Univesity of Nottingham End Use licence and may be eused accoding to the conditions of the licence. Fo moe details see: Fo moe infomation, please contact

2 WIND GENERATOR-ENERGY STORAGE CONTROL SCHEMES FOR AUTONOMOUS GRID By Meghdad Fazeli Thesis submitted to the Univesity of Nottingham Fo the degee of Docto of Philosophy Decembe 2010

3 ABSTRACT Conventionally the powe netwok opeatos wee obliged to buy all the wind enegy geneated by wind fams. Howeve, as the penetation of wind enegy (o geneally any othe sot of enewable souce) in a powe system is inceased, the ability of othe geneatos to balance the demand becomes limited. This will necessitate the contol of wind tubines in ode to geneate a given demand powe athe than extacting the maximum wind powe. This contol appoach is temed Powe Demand Contol in this thesis. In contast to Powe Demand Contol, Powe Smoothing Contol utilizes enegy stoage systems in ode to absob high fequency wind fluctuations, hence, deliveing a smoothe vesion of wind powe into the gid/load. The dawback of the Powe Smoothing appoach is that the aveage powe into the gid/load is still detemined by the available wind powe athe than the system opeato. The Powe Demand Contol appoach, which has eceived little attention in liteatues, is the main focus of this thesis. This eseach poposes contol schemes with and without extenal enegy stoage fo the Powe Demand Contol stategy. This thesis studies diffeent possible methods of applying Powe Demand Contol, in paticula the doop contol method. It is shown that a doop-contolled wind fam does not need a cental Supevisoy wind Fam Contol unit to detemine the powe demanded fom each DFIG. Moeove, a doop-contolled wind fam has the advantage of contolling the local gid voltage and fequency. This means that no extenal voltage and fequency souce is equied which makes a doopcontolled wind fam a moe suitable option fo integation of wind enegy at distibution level. The classical doop contol is modified in ode to make the DFIGs shae the demand powe not only accoding to thei atings but also to thei associated available wind powe. The applications of the contol paadigm ae discussed, including: integation into micogids, AC gids and HVDC connection feedes. This wok mainly concentates on micogid applications. i

4 An Enegy Management System is poposed in ode to keep the enegy level of the enegy stoage (o the DFIG s shaft speed) within its limits using an Auxiliay Geneato and a Dispatchable Load. Diffeent possible system configuations ae intoduced and thei advantages and dawbacks ae discussed. It is illustated though simulation that the poposed contol scheme can inheently ide-though a gid fault with no need fo communication. Futhemoe, it is shown that the contol scheme can opeate if the wind speed dops to zeo. The simulations ae caied out using the PSCAD/EMTDC softwae. ii

5 ACKNOWLEDGEMENTS Fistly, I would like to expess my sinceest gatitude to Pofesso Geg Ashe whose wide age of expetise helped me to make this eseach a success. His invaluable advices and citicisms have been vey constuctive thoughout my PhD ove the last fou yeas. My special thanks ae also extended to D. Chistian Klumpne, D. Sehiy Bozhko and my othe colleagues in PEMC goup fo thei numeous helps and advices duing the eseach. I would also like to thank Alstom Gid paticulaly D. Liangzhong Yao, fo his financial suppot, hospitality and technical advices. Last but not least, I would like to expess my deepest appeciation to my family, especially my paents, fo thei neve-ending love, suppots and wods of comfot. iii

6 LIST OF SYMBOLS R s, R L s, L, L o l s,l p J e,, s i d, i q, i sd, i sq, i ms i gcd, i gcq v d, v q, v sd, v sq, s, θ s T e, T m m n N S C V dc R β P DFIG, P HVDC Q DFIG,Q HVDC, Q FILTER I o R C L C E o ρ β Stato and oto esistance Stato, oto and mutual inductance Stato and oto leakage inductances Pole pai numbe Combined tubine and geneato inetia Supply, oto and slip angula fequency Machine oto and stato cuents in dq axis Machine magnetising cuent Font-end convete cuent in dq axis Machine oto and stato voltages in dq axis Machine stato and oto flux Stato flux angle Electical and mechanical toque Active powe-fequency doop gain Reactive powe-voltage doop gain Gea atio Machine slip Damping facto DFIG DC-link capacitance DFIG DC-link voltage Pitch angle maximum slew ate Machine and HVDC link active powe Machine, HVDC link and AC filte eactive powe HVDC ectifie fiing angle HVDC ectifie DC-link cuent HVDC DC-link cable esistance HVDC DC-link cable inductance Invete output voltage in the DC-link Ai density Pitch angle iv

7 A C p V w λ dq _ pu pu gen P es E es P ext P ave Roto blade aea Powe coefficient Wind speed Tip speed atio stationay axis dq otating axis Vecto vaiables Refeence vaiables Pe Unit (based on the total wind fam) Pe Unit (based on the associated DFIG) Enegy stoage powe Enegy level of enegy stoage Extactable wind powe (i.e. the powe tansmitted to the shaft with pitch angle=0) Aveage of the extactable wind powe v

8 Glossay of tems DFIG: Doubly Fed Induction Geneato SCIG: Squiel Cage Induction Geneato GB: Geabox DSFO: Diect Stato Flux Oientation ISFO: Indiect Stato Flux Oientation LCC: Line Commutated Convete VSC: Voltage Souce Convete MPT: Maximum Powe Tacking CTM: Constant Toque Mode CPM: Constant Powe Mode DG: Distibuted Geneation T&D: Tansmission and Distibution PSC: Powe Smoothing Contol PDC: Powe Demand Contol AG: Auxiliay Geneato DL: Dispatchable Load EMS: Enegy Management System PFC: Powe Flow Contolle vi

9 Table of contents Abstact...i Acknowledgments...iii List of symbols..iv Glossay of tems..vi Table of contents...vii 1. INTRODUCTION AND TECHNICAL BACKGROUND ENVIRONMENTAL IMPACT TO ENERGY USE WIND ENERGY AND ENERGY STORAGE Review of cuent ES technologies DISTRIBUTED GENERATION Micogids CONTRIBUTION OF THIS THESIS Layout of the thesis CONTROL OF DOUBLY-FED INDUCTION GENERATOR AND WIND TURBINE GENERATOR DRIVE TECHNOLOGIES FOR VARIABLE SPEED WIND TURBINES CONTROL OF DOUBLY-FED INDUCTION GENERATOR Mathematical deivation of DFIG model in two-phase otating fame Contol of DFIG using Diect Stato Flux Oientation Vecto contol scheme of oto-side PWM voltage souce convete fo gid-connected DFIG Vecto contol scheme of gid-side PWM voltage souce convete fo gid-connected DFIG Contol of DFIG using Indiect Stato Flux Oientation Vecto contol scheme of oto-side PWM voltage souce convete Vecto contol scheme of gid-side PWM voltage souce convete WIND TURBINE GENERATOR CONTROL Wind tubine model Contol of wind tubine geneato unde Maximum Powe Tacking Contol of wind tubine geneato unde Constant Powe Mode Contol of wind tubine geneato unde Constant Toque Mode vii

10 2.4 REPRESENTATION OF WIND PROFILE IN PSCAD CONTROL OF WIND TURBINES-ES SYSTEM WITH EXTERNAL VOLTAGE AND FREQUENCY SOURCE STRATEGIES OF CONTROLLING ES DSFO-CONTROLLED DFIGS-ES SYSTEM WITH DIRECT AC GRID INTERFACE Powe Smoothing Contol fo wind tubine-es connected diectly to the AC gid Powe Demand Contol fo wind tubine-es connected diectly to AC gid DSFO-CONTROLLED DFIGS-ES SYSTEM WITH AC/DC/AC INTERFACE Intoduction of HVDC technologies DSFO-contolled DFIGs-ES system connected to VSC-HVDC Powe Smoothing Contol fo wind tubine-es connected to a VSC-HVDC link Powe Demand Contol fo wind tubine-es connected to VSC-HVDC link DSFO-contolled DFIGs-ES system connected to LCC-HVDC link Powe Smoothing Contol fo wind tubines-es connected to LCC-HVDC link Powe Demand Contol fo wind tubines-es connected to LCC-HVDC link DISCUSSIONS AND CONCLUSIONS LOCAL GRID VOLTAGE AND FREQUENCY CONTROL USING ISFO- CONTROLLED DFIGS INTRODUCTION Intoduction to technical equiements fo wind fams CONVENTIONAL METHODS FOR SUPPORTING GRID VOLTAGE AND FREQUENCY STATCOM-less solutions fo LCC-HVDC connected wind fams to contol wind fam gid WIND FARMS AND MICROGRIDS ISFO-contolled wind fam and micogids VOLTAGE AND FREQUENCY CONTROL USING CLASSICAL DROOP CHARACTERISTICS Intoduction and applications Voltage and fequency doops Simulation of a doop-contolled wind fam connected to a LCC-HVDC link DISCUSSIONS AND CONCLUSIONS DROOP-CONTROLLED WIND FARM DELIVERING A CONSTANT DEMAND POWER WITHOUT AN EXTERNAL ES INTRODUCTION Constant Powe Mode (CPM) contol CONSTANT DEMAND POWER DELIVERY USING PITCH CONTROL Calculation of the efeence pitch angle viii

11 5.2.2 Pitch angle contolle Simulation esults of constant demand powe delivey using pitch angle contol CONSTANT DEMAND POWER DELIVERY USING EXTERNAL ENERGY SOURCE Enegy Management System (EMS) Simulation esults of EMS fo constant demand powe delivey Vaiable doop gain contol Simulation esults of the vaiable doop gain method SIMPLIFIED ISFO-CONTROLLED DFIG MODEL Explaining the simplified model Simulation esults of fou doop-contolled DFIGs using the simplified model DISCUSSIONS AND CONCLUSIONS WIND TURBINE-ES SYSTEM DELIVERING A CONSTANT DEMAND POWER WITHOUT AN AUXILIARY GENERATOR INTRODUCTION ELECTRICAL TORQUE CONTROL BY REGULATING ES POWER Toque contol loop design Pefomance of the electical toque contol loop MATHEMATICAL DERIVATION OF THE SIZE OF ES FOR A GIVEN WIND PROFILE Intoduction Mathematical deivation of ES ating Stability study Simulation esults fo ES enegy and powe ating DISCUSSIONS AND CONCLUSIONS DROOP-CONTROLLED WIND FARM DELIVERING A CONSTANT DEMAND POWER WITH EXTERNAL ES AND AUXILIARY GENERATOR INTRODUCTION ENERGY MANAGEMENT SYSTEM FOR ES Pitch angle contol ES ACTUATED BY SHAFT SPEED Simulation esults fo ES actuated by shaft speed ES POWER REGULATING THE DFIG ELECTRICAL TORQUE Simulation esults fo ES contolling the DFIG electical toque TORQUE CONTROLLING-ES SYSTEM USING SIMPLIFIED DFIG MODEL Simulation esults of fou simplified doop-contolled DFIGs equipped with distibuted T e -contolling ES systems DISCUSSIONS AND CONCLUSIONS ix

12 8. STUDYING DIFFERENT SYSTEM STRUCTURES AND OPERATIONAL SCENARIOS INTRODUCTION ZERO WIND SPEED AND FAULT RIDE-THROUGH SCENARIOS Zeo wind speed ide-though Simulation esults of zeo wind speed ide-though Fault ide-though to a balanced gid fault Simulation esults of fault ide-though on local gid SYSTEM STUDIES WHEN ES AND DL ARE DISTRIBUTED Simulation esults of distibuted ES and DL SYSTEM STUDIES WHEN ES AND DL ARE BOTH AGGREGATED Simulation esults of aggegated ES and DL DISCUSSIONS AND CONCLUSIONS CONCLUSIONS SUMMARY OF THE THESIS FUTURE WORK REFERENCES PUBLICATIONS Appendix A.204 Appendix B.213 Appendix C.215 Appendix D. 219 Appendix E.220 Appendix F.223 x

13 1 Intoduction and technical backgound 1. Intoduction and technical backgound 1.1 Envionmental impact to enegy use The Kyoto Potocol is an intenational ageement, negotiated in Decembe The objective of the Kyoto Potocol is the stabilization of geenhouse gas (CO 2, NO x, SO x, etc) in the atmosphee at a level that would pevent dangeous anthopogenic intefeence with the climate system [1]. Thity six counties ae equied to educe thei geenhouse gas emissions below the level specified fo each of them in the teaty [2]. Some industialized nations have committed to making substantial eductions in thei geenhouse emissions by Ove one hunded counties have atified the potocol, but have no obligation beyond monitoing and epoting thei geenhouse emissions. Theefoe, many counties ae setting tagets to incease the amount of the electical enegy poduced by enewable enegy souces. The United Kingdom is a signatoy to the Kyoto Potocol. The enegy policy of the United Kingdom fully endoses goals fo cabon dioxide emissions eduction and is committed to popotionate eduction in national emissions. To achieve this, the Govenment has set a taget that 15% of the geneated electical enegy will be poduced fom enewable souces by The main enewable souces of enegy ae wind, sola, geothemal and tidal. Amongst theses enewable souces, wind enegy geneation is eceiving much inteest all ove the wold. The ated capacity of installed wind powe eached nealy 46000MW woldwide duing 2004 and is expected to each MW by 2012 [3]. This wok is concentated on the wind enegy geneation. 1.2 Wind enegy and enegy stoage Due to the envionmental and economical easons the penetations of wind enegy in powe systems is apidly inceasing woldwide. It is pedicted [4] that by 2020 up to 12% of the wold s electicity will be supplied fom wind powe. The 1

14 1 Intoduction and technical backgound impacts of wind enegy on powe gids is bette undestood at pesent compaed to a decade ago, but its integation into powe gids continues to be a topic that eceives a consideable amount of inteest in the intenational community [5]. The effects of wind enegy on powe systems have been investigated to some extent: spinning eseve equiements, effects on powe quality, eactive powe demands and voltage contol [6-8]. As a esult of these studies, many utilities have evised thei existing gid codes to include specific functionalities which must be satisfied by wind geneatos [9]. Technically this equies modifications to taditional wind fam designs in the fom of added equipment, implementation of moden wind tubine technologies, sophisticated pediction and contol stategies, o a combination of the above [7]. Fo example, conventionally, the powe netwok opeatos wee obliged to buy all the wind enegy poduced by wind fams. Howeve, this is subject to change as the wind enegy penetation inceases and ecently the Danish powe opeato intoduced a fine fo the wind fams poducing enegy moe than the demanded value. The pimay poblems associated with the wind enegy ae due to the natue of the souce itself, which is both time vaying and difficult to pedict [10]. Consequently, the output powe of wind fams is time vaying and unpedictable as well. Since the enegy to the gid must equal that of the total demand, a stict powe balance must be upheld. The wind powe vaiations in powe systems with low level of wind enegy penetations ae moe o less toleable. Howeve, in cases whee wind enegy eaches a high level of penetation, the effect of these vaiations become moe evident as the ability of othe geneatos to balance the load equiements becomes limited. The situation is deteioated in cases whee the wind geneatos ae connected to a weak system o a distibuted feede. In these cases the oscillating wind powes ae eflected in voltage and fequency fluctuations at the point of connection, which can esult in undesiable seconday effects. Although the majoity of the wind fams ae connected to the tansmission systems, it is noted that in a consideable numbe of these cases they ae connected to a weak system since wind enegy souces ae often fa fom the main gid and cental geneatos. At the distibution level, the need to supply the local load is not 2

15 1 Intoduction and technical backgound always equied. Howeve, in cases whee island opeation is equied (i.e. micogids); the entie local load needs to be supplied by the distibuted geneatos including the wind tubines. The fluctuating powe esults in a numbe of poblems such as voltage flicke, balancing the demand, and instability, especially in the cases of emote o islanded powe systems [10-12]. Although combining the geneato with powe electonic intefaces can enable the eactive powe-voltage contol, the eal powe contol emains an issue which equies futhe attention. It is noted that spatial distibution of many tubines acoss the fam does in fact educe the oscillation of the total wind fam output powe as an aveaging effect is poduced acoss all the wind tubines due to the phase shift in thei associated wind speeds. Howeve, in cases whee the numbe of wind tubines is small o when the capacity of the wind fam is significant compaed with the othe geneatos, the need fo powe management and impovement in powe quality still exist. Futhemoe, the poduced wind powe is still unpedictable which will equie an auxiliay souce and enegy management scheme in ode to maintain the powe demanded by the load. Enegy Stoage (ES) systems have emeged as a potential solution to ovecome the intemittency and the shot tem vaying natue associated with wind enegy geneation [13]. Integation of the ES system into wind enegy geneation can benefit the powe system in diffeent aspects such as [3, 14, 15]: Smoothing the wind powe fluctuations though absobing its highe fequencies [16]. Contolling active powe balance [5, 17]. Poviding spinning eseve in ode to suppot the local gid fequency contol [18, 19]. This uses the concept of active powe-fequency contol in which as powe inceases the fequency dops. Theefoe, the ES capacity can be used to absobs o inject enegy in ode to povide fequency contol. 3

16 1 Intoduction and technical backgound Suppoting the low voltage ide-though capability by seving as a powe sink duing low system voltages [14]. Recent gid codes equie the wind geneato to stay connected duing voltage depessions. In such occasions, in which the wind powe has no place to go, the ES can seve as a powe sink to absob the wind enegy. This application equies ES technologies with lage enegy capacity especially in the case of lage wind fams. ES systems can be classified into shot-tem o long-tem. The shot-tem ES systems ae usually used to smooth out wind powe fluctuations while the longtem ES systems ae contolled to level the imbalance between the demand and available wind enegy. In a shot-tem ES system the nomal appoach has been fo powe smoothing in which the ES absobs the highe wind fequency fluctuations. This is called Powe Smoothing Contol (PSC) [5, 16, 17]. This thesis will addess shot-tem (o medium-tem) ES, not by PSC, but as an aid to meeting the equied use powe demand in conjunctions with othe auxiliay enegy management hadwae. This will be called Powe Demand Contol (PDC). This wok is mainly concentated on the PDC stategy. The wind tubine oto inetia can also be used as an ES mechanism [20-22]. Howeve, due to the obvious limitation esulting fom the ovespeed ating of the geneato and the fact that the wind tubine exhibits unstable behaviou fo low shaft speed; the use of the tubine inetia as the only ES mechanism is not vey beneficial. Obviously using an extenal ES can enhance the benefits of the ES fo the powe system. This thesis will popose contol stuctues both with and without an extenal ES Review of cuent ES technologies A numbe of diffeent ES technologies cuently exist. In all of them a powe electonic inteface is needed in ode to be popely integated with the gid. In selecting the type of the stoage device fo a given need, both the powe ating and enegy ating of the device must be consideed. Moeove, the chaging and dischaging chaacteistics and efficiency ae impotant factos in choosing the 4

17 1 Intoduction and technical backgound type of the ES device. While the natue of the stoage device will influence the powe electonic inteface stuctue, the limitation of the system and the enegy capacity of the ES impose the need fo a management scheme in ode to coodinate the flow of enegy to and fom the device. The most common ES technologies can be summaized in tems of shot-tem, medium-tem and longtem ES [14, 23] as is shown in Table 1.1: Time scale Shot-tem <10s Medium-tem 10s-60mins Long-tem 1-24hos Capacitos Batteies Hydo-pumped Stoage Supecapacitos Flywheel Hydogen geneation types Flywheel Hydo-pumped Regeneative fuel cell SMES Hydogen geneation Compessed ai Table 1.1. Compaing diffeent ES technologies in tems of shot-, medium- and long-tem [14] The shot-tem ES systems ange fom 100W to 500kW, the medium-tem ES systems ange up to 1000kW and the long-tem ES systems can be ated up to 20MW [23]. It is noted that this wok concentates mainly on shot-tem to medium-tem ES systems. Howeve, it will not deal with a cetain type of ES technology. It is intended to study the possible contol scheme fo the ES systems and possible locations fo them. Theefoe, thoughout this thesis, the ES system is simulated by an ideal DC-voltage souce connected to the gid though an AC/DC convete. The diffeent ES technologies ae biefly eviewed as follows: Batteies Batteies come in many diffeent types and ae pehaps the most vesatile than any of the stoage devices as they offe desiable stoage chaacteistics fo wide anges of applications and ae geneally cheape in most cases. Rechageable batteies such as valve-egulated Lead-acid o nickel-cadmium ae the most popula due to thei availability and eliability [24]. 5

18 1 Intoduction and technical backgound Supecapacitos Simila to batteies, supecapacitos [5, 16] ae based on an electochemical system and ae voltage based device which ae usually intefaced using a DC/DC choppe. Supecapacitos ae able to manage simila enegy densities as the batteies but with longe lifetime and lowe maintenance. Howeve, they ae only available fo vey low voltage (about 3V) [24]. Supeconducting Magnetic Enegy Stoage (SMES) In SMES [10, 12, 25], the enegy is stoed in fom of a dc cuent ciculating in a lage inducto. The esistance of a supeconducto is zeo so the cuent can flow without eduction in its magnitude. The vaiable cuent though the supeconducting coil is conveted to a voltage, which can be connected to an invete. SMESs ae well-suited fo fast exchange of lage amount of powe. Howeve, thei long-tem stoage capacity is limited and they ae elatively expensive [24]. Flywheel Flywheel ES [19, 26, 27] systems stoe enegy mechanically in the fom of kinetic enegy by spinning a mass about an axis. The electical enegy input keeps the flywheel oto spinning until called upon to elease the stoed enegy though a geneato [24]. Hydogen geneation In hydogen geneation ES [11, 28-30] systems, as its name suggests, the exta wind (o sunshine) enegy is used to geneate hydogen (e.g. by electolysing wate) which late can be used in fuel cells to geneate electical enegy when thee is lack of enegy. The dawback of this system is thei slow esponse to fast powe tansient due to the slow intenal electochemical and themodynamic chaacteistic of fuel cells. This poblem can be solved by using supecapacitos in ode to impove the dynamic esponse of the system [24]. A fuel cell woks like a battey but does not need echaging. It will poduce electicity as long as hydogen is supplied. A fuel cell consists of electodes-an anode and a cathodesandwiched aound an electolyte. Hydogen is fed to the anode, and oxygen is fed to the cathode. Activated by a catalyst, hydogen atoms sepaate into potons and 6

19 1 Intoduction and technical backgound electons, which take diffeent paths to the cathode. The electons go though an extenal cicuit, ceating a flow of electicity. The potons migate though the electolyte to the cathode, whee they eunite with oxygen and the electons to poduce wate. The Regeneative (o evesible) fuel cell poduces electicity fom hydogen and oxygen, but can also be evesed and poweed with electicity to poduce hydogen and oxygen. Hydo-pumped In Hydo-pumped stoage systems the enegy stoed by pumping wate up to a lage esevoi and is eleased though a wate tubine connected to a geneato wheneve equied. This system is usually used in peak shaving but is well-suited fo poviding balance sevices as well. Howeve, it usually equies suitable geological location [15]. Compessed ai Compessed ai [31] enegy stoage system uses an intemediay mechanicalhydaulic convesion also called the liquid-piston pinciple. These systems ae aising inteest as they do not poduce any waste. They also can be integated with a cogeneation system, due to the themal pocesses associated with the compession and expansion of gas. Thei efficiency can be optimized by combining them with othe stoage system [24]. As mentioned befoe, this wok will not concentate on a paticula ES technology and will simulate the ES by an ideal DC-voltage souce intefaced though an AC/DC convete. The enegy capacity and the powe ating of the ES system ae povided in pu. Howeve an example of flywheel ES is consideed in Appendix D in ode to povide a pespective of the physical size of the equied ES. 1.3 Distibuted geneation The apid gowth in the electical enegy demand puts the tansmission system unde geate stess evey yea esulting in a system opeation close to its edge i.e. geate possibility fo stability poblems than any time in the past [32]. The basic solution to this poblem is to constuct moe tansmission lines, which is vey 7

20 1 Intoduction and technical backgound difficult especially with this inceasingly apid gowth in demand. The othe solution is to use the existing system in a moe effective way. Moeove, it is wellknown that the etail customes ae equiing much highe powe quality than eve befoe due to the incease of digital systems and sophisticated contol [33, 34]. On the othe hand, facing global envionmental poblems makes the incease in enewable enegy penetation inevitable which in tun makes the highe powe quality equiements even hade to meet [35, 36]. Theefoe the thee majo difficulties fo futue Tansmission and Distibution (T&D) systems can be summaized as follows: Poviding fo the apid gowth in demand and enhancing the obustness of system with minimum incease in tansmission lines. Coping with the incease in penetation of enewable enegy such as wind and photovoltaic systems. Impoving the local eliability to ensue the powe quality demanded by customes. These difficulties necessitate the e-thinking o even e-deigning of T&D systems in ode to find a moe effective way to use them. Decentalisation of geneation and stoage systems, which is called Distibuted Geneation (DG), has emeged as a pomising solution fo the difficulties mentioned above [33, 37, 38]. DG is a vaiety of small powe geneatos and stoage facilities which ae located as close as possible to uses. The use of DG enables customes to have some degee of enegy independence, inceases the eliability of sevice, impoves the efficiency of enegy, and finally inceases the ability of system to exploit moe enewable enegy. Futhemoe, DG benefits the electic utility by educing congestion on gid, deceasing the need fo new geneation and tansmission capacity, and offeing sevices such as local fequency and voltage contol [36, 39, 40]. As the penetation of enewable enegy inceases, the intemittent natue of enewable enegy becomes a geate poblem equiing the cental geneation to povide the back-up enegy. This inceases both the stability poblems (simila to 8

21 1 Intoduction and technical backgound those found in intemittent load such as ac funaces) and also gid losses [34, 36]. Distibuted geneation and stoage can povide the equied back-up enegy with minimum loss and stability poblem. Renewable enegy geneation must be supplemented with dispatchable esouces such as stoage and local geneation in ode to balance the geneated enegy with demand [33, 35] Micogids Micogids have eceived inceasing attention as a means of integating DG into the electicity netwok. The micogid is an integated enegy delivey system that consists of inteconnected DG units and contollable loads which may be opeated autonomously and can opeate in paallel with, o isolated fom, the main powe gid [34, 36]. Conventionally the T&D systems wee not designed to accommodate geneation and stoage at distibution level. Theefoe the main challenge is how these DG units can be integated as a micogid to fom units that ae contollable and well-behaved at gid level. If the micogid is connected to the main gid, it will appea as a load to the main gid when the DG units cannot meet the local load. Howeve, when the DG output exceeds the local demand, the micogid appeas as a geneato to the main gid. Hence, the main challenge is when the micogid is islanded fom the main gid since the DG units must meet the local demands. Obviously, the situation deteioates as the enewable enegy penetation into the micogid inceases. Advanced powe electonics and contol technologies have made it possible to integate a ange of distibuted enegy geneation and stoage with existing electical powe systems [34, 36, 41]. Figue 1.1 shows an example of micogid consisting of a ange of DG units which ae integated into a thee-level hieachy though powe electonics. Using such a contol stuctue, the micogid can be egaded as a self-contolled entity within the powe system [34, 36]. The Static Switch (SS) is used to connect the micogid to the main gid wheneve equied. Micogids ae custome-fiendly, as they ae designed to meet thei local needs fo electicity and heat. They can also benefit customes though poviding uninteuptible powe, enhancing local eliability, educing tansmission loss and 9

22 1 Intoduction and technical backgound suppoting local gid voltage and fequency. In ode to achieve this functionality each component of the new distibution system must eact to local infomation such as voltage and fequency to coectly change its opeation point [32, 33, 35, 42]. The othe impotant featue is to make sue that thee is no component like a maste contolle o a cental stoage unit which is citical fo opeation of the micogid i.e. the micogid can continue opeation with the loss of any component. SS Gid AC Load DC Load DC Load AC Load AC Load AC/ DC DC/ DC DC/ DC AC/ DC DC/ DC DC/ DC AC/DC/AC Micotubine PV Battey Wind Fuel Cell Supecap Diesel geneato Flywheel Figue 1.1. Example of hieachical micogid with both AC and DC links [36] One conventional and yet obust way to achieve local contol without fastcentalised communication is to contol active and eactive powe flow to and fom each component by utilizing fequency and voltage doops [33-35]. This is petty much simila to how cuent T&D systems ae contolled. Howeve the biggest obstacle is the enewable enegy and the apid incease in thei penetation. Wind geneatos ae taditionally integated to the gid by powe electonic convetes that equie a voltage souce to povide voltage oientation fo the contol of the eal and eactive powe flow. It implies that they can neithe be easily integated within micogids (especially in island mode) no connected to weak gids since they cannot alone contol the local gid voltage and fequency. 10

23 1 Intoduction and technical backgound This becomes moe cucial in cases whee the wind powe supplies a significant pat of the load in the micogid. This thesis is intended to addess this poblem. 1.4 Contibution of this thesis This wok augments an aay of wind geneatos with fequency and voltage doop chaacteistics in ode to shae an active and eactive load. Although the poposed contol stuctue is mainly concentated on micogid applications, the method is quite applicable fo diect ac gid connection as well as connection via HVDC links. The active powe-fequency doop will be adjusted in ode to make the wind geneatos shae the load not only accoding to thei atings, (which is the case fo classical doop contol), but also accoding to the available wind powe. The method will be validated fo both with and without auxiliay ES units. This thesis also poposes a novel ES contol method in which the output powe of the wind geneato(s)-es system is totally smooth and equal to the powe demanded by the load. An Enegy Management System (EMS) fo both with and without extenal ES is poposed and illustated though PSCAD simulations. This thesis will also investigate the diffeent possible places fo the ES and othe components. The gid fault and zeo wind powe ide-though scenaios ae also studied in this wok Layout of the thesis The thesis is stuctued as follows: Chapte2 eviews geneato dive technologies fo vaiable speed wind tubines. The Doubly Fed Induction Geneato (DFIG) will be chosen in this wok. Diffeent contol methods fo DFIG and wind tubine ae studied. The Mathematical model of a DFIG is developed and the vecto contol scheme fo both gid-side and oto-side convetes of the DFIG ae explained. Two diffeent contol methods fo DFIGs ae identified: Diect Stato Flux Oientation (DSFO) and Indiect Stato Flux Oientation (ISFO). The DSFO-contolled DFIGs, which 11

24 1 Intoduction and technical backgound ae also called gid-connected DFIGs [43], ae field oientated off the gid voltage and appea as cuent souces in a powe system. Howeve, the ISFO-contolled DFIGs, which ae also known as standalone stuctue [44], opeate like voltage souces in a powe system and hence can be equipped with doop chaacteistics. Since in an ISFO-contolled DFIG the toque-component of the oto cuent is detemined by the load, thee is lack of the diect toque contol which is the main dawback of this contol method. This poblem will be addessed in the following chaptes. The PSCAD\EMTDC wind tubine model, which is used thoughout this thesis, is also biefly explained and diffeent wind tubine contol modes ae discussed. Finally wind chaacteistics and the PSCAD wind model ae explained. Chapte 3 consides DSFO-contolled DFIGs with extenal ES system(s) and identifies two contol stategies: Powe Smoothing Contol (PSC) and Powe Demand Contol (PDC). The application of the two stategies in a DSFOcontolled wind fam connected to both AC gid and HVDC link will be discussed. The diffeent wind fam gid voltage and fequency contol stuctues in each case will also be illustated. Though PSCAD simulation it will be illustated that in a PSC, the powe into the gid is a smoothed vesion of the wind powe, and the PSC stategy may not be appopiate fo a powe system with a high penetation of wind enegy. It will be also shown that in the PDC stuctues fo a DSFOcontolled DFIGs-ES system, communication between the system opeato and the wind fam is necessay in ode to detemine the efeence powe and pitch angle fo each wind tubine. This is in addition to the enegy management communication which is usually needed to maintain the enegy level of the ES within its limits. It will be discussed that the DSFO-contolled wind fams seem not to be the best choice fo integating with micogids especially if the wind fam supplies significant pat of the load. Chapte 4 eviews the diffeent functionalities that may be equied fom a wind fam including the active powe-fequency and the eactive powe-voltage contol. The chapte also explains the existing methods fo suppoting the local gid 12

25 1 Intoduction and technical backgound voltage and fequency and agues that an extenal voltage and fequency souce is still equied. Howeve, an ISFO-contolled wind fam augmented with doop chaacteistics has the potential to fully contol the local (wind fam) gid voltage and fequency. This chapte explains the diffeent applications of a doopcontolled wind fam including AC gid connection, HVDC connection, and integation within a micogid. This chapte will equip an aay of ISFO-contolled DFIGs with the classical fequency and voltage doops and compaes the active and eactive powe shaing using doops with that of without doops. It will be shown though PSCAD simulation that a doop-contolled wind fam is inheently able to ide-though loss of the gid with no need fo communication. Chapte 5 consides a micogid including a doop-contolled wind fam with no extenal ES. In this scenaio the DFIGs contol the local gid voltage and fequency and shae the local load. The wind tubine moment of inetia opeates as a shot-tem ES such that the shaft speed can be consideed as an indicato fo the excess o shotfall of enegy. In ode to keep the shaft speed within its limits, an Enegy Management System (EMS) is equied. The EMS consists of an Auxiliay Geneato (AG) and a contollable o Dispatchable Load (DL). These ae explained with thei integated opeation with tubine pitch contol. This chapte also adjusts the gains of the fequency-active powe doops in ode to make the DFIGs shae the load accoding to the available wind powe. This is called vaiable doop method. Although the poposed contol scheme woks with the standad doop, it will be shown that the vaiable doop can significantly educe the enegy needed fom AGs. Chapte 6 consides a wind geneato-es system deliveing a constant powe demanded by the load while no AG is available. Not having an AG is obviously impactical due to the limited capacity of ES systems. Howeve, such a scenaio is included hee since it is a case study fo a DFIG unde a non-maximum Powe Tacking (MPT) contol which is moe appopiate fo this scenaio. The chapte consists of two main pats. The fist pat poposes and designs a geneato 13

26 1 Intoduction and technical backgound electical toque contol scheme though egulating the ES powe. The same contol stuctue will be used in the following chaptes to contol the toque of the geneato. The second pat of this chapte attempts to deive a mathematical expession fo the size of the equied ES fo a given wind pofile. The mathematical esults ae quite close to the simulation ones in case of a sinusoidal base wind speed pofile. Although this not the case fo a eal wind speed pofile, a simila appoach might be adopted in futue eseach to obtain satisfactoy esults in case of eal wind speed as well. The second pat coves wok which was discontinued, but it is included hee fo completeness and it may also have achival value. Chapte 7 consides a micogid consisting of vaiable doop-contolled DFIGs, ES, AG and DL while the local gid voltage and fequency ae fully contolled by the DFIGs. Two methods fo contolling the ES system will be investigated. The fist method exploits the tubine inetia as an ES mechanism. This method is simila to the Chapte 5 while the ES appeas as a buffe between the tubine inetia and the AG and DL. In the second appoach the ES powe is egulated in ode to contol the electical toque of the DFIG. The advantages and the disadvantages of the two appoaches ae discussed. A pitch angle contol scheme will also be poposed which is applicable fo both ES contol methods. An EMS will also be explained which uses the AG and DL in ode to keep the enegy level of the ES within its limits. Although vaiable doop method is used in this chapte, the poposed contol schemes ae also applicable with the standad doop at the expense of moe enegy demanded fom the AG. Chapte 8 consides the same micogid as Chapte 7 (i.e. an aay of vaiable doop-contolled DFIGs, ES, AG and DL) and studies the diffeent system configuations and fault ide-though scenaios. It will be shown that the doopcontolled wind fam is inheently able to ide-though a fault on the local gid with no need fo communication. This chapte also explains the ide-though scenaio in case of zeo wind speed situation. In Chapte 7 the ES units ae 14

27 1 Intoduction and technical backgound distibuted amongst individual DFIGs while the DL is aggegated on the local gid. This chapte will illustate that it is also possible to aggegate the ES on to the local gid and/o to distibute the DL amongst the individual DFIGs. It should be emphasized that the poposed contol schemes in this thesis ae quite applicable fo small micogid as well as lage wind fams. Thoughout this thesis, all the esults will be given in pu in ode to make it easie to follow. Howeve, this PhD was initially focused upon lage offshoe wind fams connected to HVDC links. Theefoe, the paametes ae given in pu based on 1000MVA. In cases with two DFIGs, the ating of the DFIG1 is 0.66pu and that of the DFIG2 is 0.34pu. In such cases, the powe and the enegy level of the distibuted elements (ES and/o DL) will be given based on the atings of thei associated DFIG which is denoted as pu gen thoughout this thesis. The DFIGs paametes, which ae given in Appendix B, ae oiginally fom the pevious woks caied out in Nottingham Univesity [4, 48]. It is emphasized that since all esults in this thesis will be displayed in pu (o pu gen ), the ating of the DFIGs ae not impotant. Two eal wind speed pofiles will be used thoughout this thesis. The two wind speed pofiles ae sampled at each second, howeve, the methods of thei measuements ae unknown. 15

28 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine 2. Contol of Doubly-Fed Induction Geneato and Wind Tubine In this chapte, geneato dive technologies fo vaiable speed wind tubines ae eviewed and then diffeent contol methods fo Doubly Fed Induction Geneato (DFIG) and wind tubine ae studied. Mathematical models of a gid-connected DFIG and a standalone DFIG ae intoduced. PSCAD\EMTDC wind tubine model, which is used thoughout this thesis, is biefly explained and diffeent wind tubine contol modes ae discussed. Finally wind chaacteistics and PSCAD wind model ae shotly explained. 2.1 Geneato dive technologies fo vaiable speed wind tubines Thee ae geneally two types of wind tubines: fixed-speed and vaiable speed. In fixed-speed wind tubine a squiel cage induction geneato (SGIG) is used to convet the mechanical enegy into electical enegy. In this case almost always capacito banks ae needed to compensate the eactive powe dawn by the SCIG. Due to the almost constant shaft speed (less than 1% vaiation), the wind powe fluctuations ae conveted into mechanical and theefoe electical powe fluctuations. This causes voltage vaiations especially in case of a weak gid, which is known as flicke [45, 46]. Integation of powe electonics with wind tubines enabled shaft speed contol [43, 44] which offes advantages such as: Moe wind enegy captue by optimum shaft speed opeation of wind tubine. Less dive tain stess and less powe fluctuations to gid since some wind fluctuations ae stoed in shaft inetia athe than diectly conducted to dive tain [4, 47]. 16

29 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Reduction in noise at low otational speed and impoved fault ide-though ability [48]. The otational speed of a wind tubine is vey low and must be adjusted to the electical fequency. This can be done by using eithe a gea box o a geneato with high numbe of poles. Thee ae mainly thee types of vaiable speed wind tubine. The fist one, which is known as Doubly-Fed Induction Geneato (DFIG), uses a wound oto induction geneato with a patial scale powe electonic convete (ated at appoximately 30% of nominal geneato powe) connected to the oto cicuit while the stato cicuit is connected to gid. In this type, which is shown in Figue 2.1.a, the speed ange typically compises synchonous speed - 40% to +30% depends on the size of the convete [45]. Simila to the fixed-speed type, it needs a geabox (GB). Its main dawbacks ae the needs of potection to meet ide-though egulations and highe maintenance due to slip ings. New technologies enable decoupling between geneato and gid using full scale convete system. This allows a wide ange of geneato contol, the possibility of elimination of geabox in cetain cases, and bette contol of enegy flow to the gid [15]. Figue 2.1.b shows a SCIG connected to the gid though a full scale powe electonic convete. Unlike DFIG it does not need potection to meet idethough egulation but this is achieved at the pice of much bigge convete. Howeve it still needs a geabox. The thid type of vaiable speed wind tubines, shown in Figue 2.1.c, is called diect-dive wind tubine as it does not need a geabox. In this type a low speed multipole synchonous geneato with the same otational speed as the wind tubine is used to convet mechanical enegy to electical enegy. The geneato can have eithe a wound oto o a oto with pemanent magnets [45]. The stato cicuit is connected to the gid though a full scale convete. This type is the most expensive one since it needs a full scale convete connected to a multipole geneato. 17

30 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine GB ~ (a) GB ~ (b) ~ Figue 2.1 Diffeent types of vaiable speed wind tubines (a) DFIG with wound oto induction geneato and geabox (b) squiel cage induction geneato with geabox and full scale convete (c) diect dive wind tubine with multipole synchonous geneato and no geabox Despite the fact that DFIGs need a geabox, they ae still the most popula option in the maket since they ae much cheape and yet offeing geat advantages such as decoupled contol of active and eactive powe and easonable shaft speed vaiations. Theefoe DFIGs ae used thoughout this thesis. (c) 2.2 Contol of Doubly-Fed Induction Geneato The DFIG is a wound oto induction geneato with its oto connected to the gid though powe electonic convetes which in this study ae two back-to-back voltage-fed PWM convetes. As shown in Figue 2.1 the stato cicuit is diectly connected to gid (fo sake of simplicity the geabox will not be shown fom now on). The ability to supply/subtact powe to/fom the oto makes it possible to opeate the DFIG at sub-o supe-synchonous speed. 18

31 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Stato P s, Q s Roto P, Q Roto- side convete Gid- side convete Pgc, Qgc DSP/PC Figue 2.2. DFIG with powe flow The use of DFIG as a geneato is consideed in many papes and theses [43, 44, 48-51]. The DFIG is suitable fo both gid connected and also standalone (o isolated load) stuctues. The gid connected DFIG is field oientated off the gid voltage i.e. the stato flux is detemined by the gid voltage. This stuctue which is also known as Diect Stato Flux Oientation (DSFO) has eceived moe attention [43, 48, 49]. In the standalone stuctue [44], the stato voltage is not detemined by gid voltage but is set though egulating the oto excitation cuent. The standalone stuctue uses a field oientated contol known as Indiect Stato Flux Oientation (ISFO). The designs of both DSFO and ISFO have been explained in detail in liteatues [43, 44, 48-50]. Howeve the next subsections biefly eview thei contol design Mathematical deivation of DFIG model in two-phase otating fame Each phase of the thee-phase stato windings sets up a magnetic field otating sinusoidally (in time domain) aound the cicumfeence of ai gap. The stato field 19

32 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine can be visualized as a set of noth and south poles otating aound the cicumfeence of stato. The same magnetic distibution can be obtained fom an equivalent two-phase system called αβ. The 3-phase abc and the 2-phase αβ systems ae tansfomable to one anothe by [43]: F F F F T F abc whee F stands fo vaiables in αβ fame, F stands fo vaiables in abc fame αβ 0 abc while F 0 0 and: T Consideing the equivalent 2-phase stato windings, which ae fixed in space on the stato, the voltage acoss each winding is witten down using Kichhoff s law: v s i s R s d s dt (2.1) Consideing the equivalent 2-phase oto windings, which ae otating at oto speed, the voltage acoss each winding can be witten as: v i R (2.2) whee the diection of flux vecto and is the diection of peak flux linkage in space. d dt s Equations (2.1) and (2.2) ae usually tansfomed to the stationay αβ fame, which is fixed to the stato. They can also be tansfomed to a otating dq fame which is fixed to the otating magnetic field poduced by the stato. In this case the dq 20

33 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine fame otates at the electical angula velocity ω e. The advantage of the otating dq fame is that the time vaying paametes of the 3-phase system become constant when efeed to the otating dq fame. This advantage is independent of the choice of efeence fame [52]. Some assumptions ae needed to be made in ode to develop the dq fame: The stato and oto windings ae symmetic and sinusoidally distibuted. The ai gap eluctance is constant. Satuation of magnetizing and mutual inductances ae neglected. Tansfomation fom (2.1) and (2.2) into dq otating fame can be done by substituting the vaiables in (2.1) by x s e j t e x s and vaiables in (2.2) by x jst e x [43, 49] (Whee x stand fo i, v and ). The electical angula velocity of the otating fame is ω e and is aligned on the stato flux; ω is the oto fequency and ωs is called slip fequency. The equations in otating dq fame ae: v v s i i whee whee s R R s L i s s L i L s l s o d j s e s dt d j s dt s s L o L L 0 i i s e (2.3) (2.4) (2.5) (2.6) (2.7) L l L 0 (2.8) whee L,, and L ae stato, oto and mutual inductances, l, l ae stato and s L 0 oto leakage inductances. s 21

34 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Combining (2.5) and (2.6) with (2.3) and (2.4) and then sepaating into eal and imaginay pats: v v v v sd sq d q i i i i sd sq d q R R R R s s d sd e dt d sq e dt d q s dt d q s dt (2.9) (2.10) (2.11) (2.12) Selecting i d, i q, i sd and i sq as state vaiables, the DFIG model can be descibed as: di dt sd di dt sq di dt d di dt d R L R L R L R L s s s s sq 2 LsL Lo whee. L L s s s s L L L L o o i i sd sq i i sd i s i sq sd L L L L o o i i sq sd q d R L L L R L sq sd s s L L o o i i R L R L d q i i L L d q s s L L s s o o i L L L 2 o L L L i 2 o q d i i v L v L q d sd s sq s Lov L L s s sd Lov L L Lo L L sq s s Lo L L v v v L d v L q d q (2.13) Assuming that the magnitude of the αβ vecto is equal to the ms phase quantity (i.e. x whee x stands fo i, v and ), the electical toque would be [44, x ms 49]: T e p Lo 3 i s 2 L s whee p is the numbe of machine pole pai. (2.14) 22

35 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Contol of DFIG using Diect Stato Flux Oientation This section explains the vecto contol scheme fo a gid-connected DFIG which is also known as Diect Stato Flux Oientation. In this method the stato flux is detemined by gid voltage Vecto contol scheme of oto-side PWM voltage souce convete fo gid-connected DFIG The objective of the contol of the oto-side convete is to obtain a decoupled contol between the stato active and eactive powe. This can be achieved by choosing a synchonously otating dq fame with the d-axis oiented along the stato flux vecto position. Once the oientation is coectly done, an independent contol of toque and flux is achieved i.e. the toque is contolled by toque poducing cuenti q. The stato flux is detemined by stato voltage as the stato windings ae diectly connected to the main voltage souce [43, 49, 50]. Howeve fom (2.5) the magnetising cuent fo can be supplied by eithe i o i i.e. fom stato supply o fom the oto convete. The stato flux angle θ s can be calculated by measuing stato cuent and voltage: s s s s s v v -1 tan s s R i s s R i s s s s dt dt (2.15) A Phase Lock Loop (PLL) is used to deive the position of the stato voltage. Neglecting stato esisto, the stato flux vecto lags the stato voltage vecto by almost 90º due to the integation effect in (2.15). Aligning the d-axis of the efeence fame along the stato flux vecto position gives sq 0 and sd s L 0 ims, whee i ms is machine magnetizing cuent. Equations (2.5) and (2.6) can also be descibed in dq fame: 23

36 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine sd sq L i s L i s sd sq L i o L i o d q L i o ms 0 i i sq sd L L Loi s o i q ms L i L s o d (2.16) d q L i o L i o sd sq L i q (2.17) Since stato windings ae diectly connected to main gid, the stato flux can be consideed constant, thus v v sd sq i i sd sq R R And oto equations: v v d q i d R s s L i d d dt d dt d dt sd L i sq d L 2 o ms d s 0. Assuming negligible stato esistance: dt 2 2 Ls L Lo Lo whee and Lmm. Fom (2.20) and (2.21) one can define: L L L v d v Theefoe the tansfe functions between i v q d ' d i q v v s s R d q i v d dt s q L i s s (2.18) (2.19) (2.20) (2.21) (2.22) (2.23) and oto cuents become: which means the oto cuent can be egulated by v using a cuent contolle whee L i and L i L i ae compensation tems fo v and v espectively. L L L s e e L i s L sq s s q d sd q L i L i q ' q s s s mm ms 1 L s R d 2 o 2 o i 0 d q i i d q d R e R s q L i sd d L L di s di L L 2 o s d dt q dt i ms L s s L L i s v q L i L q mm ms L s 2 o i d L i s d mm ms d 24

37 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Figue 2.3 illustates the vecto contol scheme of oto-side convete fo a gid connected DFIG. L s L i s i mm ms q L i d DC- Link Q s + - PI i d + - PI - V d 2/3 PWM P s + - PI i q + - PI V q i i q i d 3/2 - ω DFIG + Stato Flux Angle Calculation i s V s P s Q s Stato Powe Calculation Figue 2.3. Roto-side PWM voltage souce convete fo gid-connected DFIG As can be seen fom Figue 2.3, the stato active and eactive powes ae contolled by egulating i d and i q espectively. Figue 2.4 shows the schematic diagam fo the cascaded powe and cuent contol loops of the oto-side 2 L Ls L Lo 3L convete whee ( ) and k 0 vs. R L L 2L s s 25

38 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Inne Cuent Loop p s, Qs - + k ip s a s ip i dq + - k i s ai s ' V dq 1 / R s 1 i dq ' k p s, Q s PI Contolle PI Contolle Plant Plant Figue 2.4. Cascaded powe and cuent contolles fo oto-side convete The deivation of contol plants and design of PI contolles fo the powe and cuent loops ae explained in the liteatue [43, 48, 49] using diffeent methods. In Appendix A, Pole-placement and Chaacteistic Equation methods ae used to design the PI contolle fo a 1000MVA DFIG (with paametes given in Appendix B) using Mathcad softwae. Appendix A can be used to get the PI contolle s popotional and integal gains fo a DFIG of any ating by changing the appopiate paametes. In this thesis Appendix A has been used to design the contolles fo DFIGs with 1000MVA, 660MVA and 340MVA atings. These ae used thoughout the thesis Vecto contol scheme of gid-side PWM voltage souce convete fo gid-connected DFIG The objective of the gid-side convete is to keep the DC-link voltage constant egadless of magnitude and diection of oto powe. The efeence fame used fo the vecto contol is oiented along the stato (gid) voltage vecto position. This enables the independent contol of the active and eactive powe flowing between gid and the gid side convete. The PWM convete is cuent egulated with the d-axis cuent used to egulate the DC-link voltage and the q-axis cuent used to contol the eactive powe [43, 49]. 26

39 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine i dc i dcg + L f R f i dc i ga V ga V dc C i gb V gb i gc V gc - VCa VCb VCc Figue 2.5. Schematic diagam of gid-side convete and line inductances and esistances Figue 2.5 shows the schematic diagam of the gid-side PWM convete whee L f, R f, V g and V c ae the connecting tansfome inductance, esistance, gid voltage and convete voltage espectively. The thee-phase equation elating the convete voltage and gid voltage in given in (2.24) uses Kichhoff s law: V Cabc V gabc R f i gabc L f di gabc dt (2.24) The 3-phase equation (2.24) can be tansfeed into a otating dq fame with a fequency of ω e using the same technique explained above: V V Cd Cq With the d-axis of the otating fame oientated on the gid voltage vecto, and V V V gd gq gd V g R R f f i i gd gq L L f f di di gd dt gq dt L e e L f f i i gq gd (2.25) (2.26). Using the same scaling facto explained in 2.2.1, the active and V gq 0 eactive powe flow is: P Q g g V gdigd Vgqigq Vgdigd 3 V gqigd Vgdigq Vgdigq 2 2 The angula position of the gid voltage can be calculated as[43, 49]: (2.27) (2.28) 27

40 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine v dt tan e 1 V V g g (2.29) Active and eactive powe flow between gid-side convete and gid ae contolled by egulating i gd and i gq espectively. It can be defined: V ' gd V Cd V gd L e f i gq (2.30) V ' gq V Cq V gq L e f i gd V Cq L e f i gd (2.31) Theefoe the tansfe function between cuents and voltages is i V gd ' gd s s i V gq ' gq s s 1 L s R f f DC- Link V dc - i gd PI PI ' V gd - V d 2/3 PWM i gq ' V gq V q + - PI - - v Tansfome ω e L f V gd αβ/dq v actan v g g 3/2 V gabc ω e L f v i gd αβ/dq 3/2 i gq i gabc Figue 2.6. Gid-side PWM convete fo gid-connected DFIG Figue 2.6 illustates the schematic diagam of the gid-side vecto contol PWM convete. The DC-link voltage is contolled by egulating d-axis cuent i gd while the q-axis cuent demand i gq detemines the eactive powe flow between the gid 28

41 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine and the gid-side convete. In ode to ensue unity powe facto, i gq is kept zeo. Theefoe the eactive powe demand is supplied by DFIG magnetization. This situation is used thoughout the thesis. Figue 2.7 shows the cascade contol of the gid-side convete. Whee K 3m, DC 4 C m and C ae PWM modulation depth of the gid-side convete and DFIG DC-link capacitance espectively. Inne Cuent Loop V DC + - k ip s a s ip igd + - k i s ai s ' V gd 1 L s f R f igd K DC s V DC PI Contolle PI Contolle Plant Plant Figue 2.7. Schematic diagam of the contol scheme of the gid-side convete The deivation of contol plants and design of PI contolles fo the voltage and cuent loops ae explained in the liteatue [43, 48, 49] using diffeent methods. In Appendix A Pole-placement and Chaacteistic Equation methods ae used to design the PI contolle fo a 1000MVA DFIG (with paametes given in Appendix B) using Mathcad softwae. Appendix A can be used to obtain the popotional and integal gains fo the DFIG s PI contolles of any atings by changing the appopiate paametes Contol of DFIG using Indiect Stato Flux Oientation Unlike DSFO, the stato flux is no longe detemined by gid voltage and is contolled by oto excitation cuent [44]. The next two subsections explain the contol of the oto- and stato-side convetes Vecto contol scheme of oto-side PWM voltage souce convete Figue 2.8 shows the contol of oto-side convete fo ISFO. 29

42 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine L i s q DC- Link i ms i d L s i mm ms L i d - V d + - PI i q + - PI V q 2/3 PWM + - PI L s i q i L o i sq i d 3/2 + - ω DFIG s f 50 Hz 3/2 1 L o sd Stato flux Calculation i s V s Figue 2.8. Contol of oto-side PWM convete fo ISFO As Figue 2.8 illustates, the stato flux angle θ s is not deived fom the voltage measuement (since thee is initially no voltage souce) but is set though fee unning integation of the efeence stato voltage fequency (50Hz). The stato voltage is contolled by the magnetizing cuent i ms though egulating the oto d- axis cuent i d. The magnetizing cuent efeence value is set as snom enom whee V s-nom is nominal stato voltage (1kV) and ω e-nom is the nominal angula fequency (314.16ad/s). Aligning the d-axis of the efeence fame along the stato flux vecto position gives L i sq s sq L i o q 0 i q L L 0 s i sq i ms V L0 (2.32) 30

43 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine sd L i s o ms (2.33) Equation (2.32) is the oientation condition [44] i.e. (2.32) is used to foce the oientation of the efeence fame along the stato flux vecto position. Equation (2.32) also means that i q can not be used to contol the electical toque since it is detemined by i sq and this is why the ISFO is suitable fo isolated load (standalone) stuctue. The main advantage of this stuctue is that the DFIG can be used to contol the local gid voltage and fequency. The design of magnetizing cuent PI contolle is given in [44] which is also used in Appendix A Vecto contol scheme of gid-side PWM voltage souce convete The contol of the gid-side convete is exactly the same as that of the gidconnected one (Figue 2.6). In a gid connected application, the gid voltage may be assumed to be fee of hamonics and the θ v is deived though voltage measuements. In the standalone case thee is no low impedance voltage souce and stato voltage hamonics will aise fom the stato convete. Fotunately, θ v can be deived fom: v s 2 [44] whee θ s is stato flux angle which is set fom the integation of the fequency efeence. The ISFO has eceived less attention to date, and is the main focus of this thesis. 2.3 Wind tubine geneato contol Figue 2.9 shows the wind tubine connected to a DFIG though a mechanical dive tain. The wind tubine oto blades convet some of the available wind powe to mechanical powe acting on shaft inetia. Neglecting mechanical fiction and losses, the shaft speed is T m T J e whee T m, T e and J ae the mechanical toque, electical toque and shaft inetia. 31

44 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine β Dive tain T e contol T e V w Wind tubine Eq.(2.34) T m T m T J e ω DFIG T e ω Figue 2.9. Schematic diagam of wind tubine connected to DFIG though mechanical dive tain In a DSFO-contolled DFIG, the efeence electical toque T e is contolled by I q though the oto-side convete. In an ISFO-contolled DFIG, howeve, an extenal mechanism, which will be investigated in late chaptes, is needed to set the efeence toque. In the next subsection the PSCAD wind tubine model, which is used thoughout this thesis, is explained and then diffeent wind tubine contol methods ae discussed Wind tubine model The mechanical powe extacted fom wind speed P t is a function of wind speed V w, shaft speed ω and pitch angle β. This function is called powe coefficient C p. T m P 0.5AV t Pt 3 w C p (2.34) Thee ae a numbe of diffeent equations fo the powe coefficient. The PSCAD one, which is used thoughout this thesis, is descibed by [53]: C p e NV w 0.17 (2.35) 32

45 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine whee ρ = ai density, A = oto blade aea λ = tip speed atio, N = gea atio Equation (2.34) explains that the aeodynamic powe captued by wind tubine is a function of wind speed cubed, blade aea and powe coefficient. C p is a function of pitch angle. The bigge the pitch angles the smalle the powe coefficient and tubine powe. Pactically C p is less than 0.4 which happens when β=0. In ode to extact the maximum enegy fom the wind, not only should the pitch angle be kept at zeo, but also the geneato powe should be a cubic function of shaft speed. The latte is achieved by contolling the electical toque as T e K opt 2. This method is called Maximum Powe Tacking (MPT). Thee ae howeve othe methods that can be used to contol electical toque. Two methods have eceived attention in papes [54]: Constant Powe Mode (CPM) and Constant Toque Mode (CTM). Othe methods may also be defined as T e =f(ω ). The main diffeence of these methods is the degee to that they exploit the shaft inetia as an enegy stoe. In pinciple, the moe enegy stoed in inetia, the smoothe the output powe is fo a given wind speed pofile. As a esult, a smalle extenal ES is needed to smooth the output powe to a specific level. The thee main methods ae explained in following sections Contol of wind tubine geneato unde Maximum Powe Tacking Fo a given wind speed, thee is only one shaft speed at which the captued wind powe is maximum which is called optimum shaft speed ω opt [54]. Figue 2.10.a shows the MPT chaacteistic fo diffeent wind speeds. The contol is done by contolling the electical toque as T e K opt 2 (whee K opt is given fo a wind tubine) which is shown in Figue 2.10.b. In this way the shaft speed is diven to the optimum shaft speed and thus the captued powe is maximum fo each wind 33

46 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine speed. In MPT a pitch angle contol is equied in ode to keep geneato powe at 1pu wheneve the wind speed is moe than the ated (ated wind speed is the wind speed at which the tubine powe is 1pu). In othe wods, when tubine powe becomes moe than 1pu, the pitch angle inceases to educe the enegy captued and maintain powe at 1pu. This pitch angle contolled is called standad pitch contol in this thesis. The MPT is the standad contol method fo wind tubines. The main advantage of the MPT method, in compaison with othe toque contol methods, is that it is always stable fo any shaft speed [21, 54]. It also minimizes the enegy equied fom any extenal geneato souce by maximizing the wind enegy captued. On the othe hand, the maximum enegy captued means that a lage extenal Enegy Stoage (ES) is equied to smooth the output powe to a desied level. P t V w =12 T m V w =12 Stable egion Stable egion V w =6 V w =6 (a) ω (b) ω Figue Maximum Powe Tacking is shown in (a) tubine powe-shaft speed chaacteistic (b) mechanical toque-shaft speed chaacteistic Contol of wind tubine geneato unde Constant Powe Mode Figue 2.11 shows the CPM chaacteistic defined by T e P whee P is the demand powe. In CPM, as its name suggests, the output powe is constant and equal to a demand powe which means that theoetically no extenal ES is needed. Theefoe all wind powe fluctuations ae stoed in the shaft inetia which in tun 34

47 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine causes the widest shaft speed vaiations, thus maximizing inetial ES usage. The main disadvantage of CPM is its unstable egion shown in Figue The P /ω cuve is moe likely to coss the T m -ω cuves fo a given wind pofile due to the wide shaft speed vaiation [21, 54, 55]. Figue 2.12 compaes demand powe with extactable wind powe i.e. tubine powe with β=0. When the demand powe is less than the extactable powe (Figue 2.12.a), CPM is possible. This case is studied in Chapte 5 whee a pitch contol is designed to keep the shaft speed in the stable egion. P t T m Stable egion V w =12 Stable egion P V w =12 V w =6 V w =6 (a) ω (b) ω Figue Constant Powe Mode is shown in (a) tubine powe-shaft speed chaacteistic (b) mechanical toque-shaft speed chaacteistic Howeve, when the demand powe appoaches the aveage of the extactable wind P ave powe (Figue 2.12.b); ES and/o an extenal enegy souce is equied to compensate fo the shotage of enegy wheneve the demand powe is moe than the extactable wind powe. This implies that CPM is not possible in such situations. P ext (β=0) P ext (β=0) P ave P P ave P (a) (b) 35

48 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine Figue Diffeent situations of demand powe in espect to extactable wind powe P ext As mentioned befoe, an ISFO-contolled DFIG equies an extenal mechanism to set the efeence electical toque. Theefoe, a DFIG contolled unde ISFO contol, which is supplying a constant powe load, is inheently in CPM unless its electical toque is contolled extenally Contol of wind tubine geneato unde Constant Toque Mode Fo the Constant Toque Mode (CTM), as its name indicates, the electical toque is constant P g T e ; the geneato powe is hence a linea function of shaft speed (Figue 2.13). Unde CTM the shaft speed vaiation fo a given wind Te T e pofile is less than that fo the CPM but moe than that fo the MPT, so it epesents a compomise in the degee to which it exploits the tubine inetia as an enegy stoe. Instability is still possible but at much lowe shaft speed which means much lage wind petubation is equied (compaed to CPM) fo the instability egion to be appoached [54, 56]. In CTM the choice of T e is vey impotant by which the instability poblem can be addessed. P t T m V w =12 T e Stable egion V w =12 Stable egion V w =6 V w =6 (a) ω Figue Constant Toque Mode is shown in (a) tubine powe-shaft speed chaacteistic (b) mechanical toque-shaft speed chaacteistic (b) ω 36

49 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine The contol of a wind tubine-es system unde CTM in ode to delive a constant powe demanded by load is the subject of Chapte Repesentation of wind pofile in PSCAD A wind model is equied that can popely simulate the spatial effect of wind behaviou including gusting, apid (amp) changes, and backgound noise (tubulence) [45]. The PSCAD wind model is a fou-component model and defined by [53]: V w V wb V wg whee V wb, V wg, V w and V wn ae base (aveage) wind speed, gust wind, amp wind and noise wind components espectively. The noise component which is used to simulate the tubulence in wind is explained below. The fluctuation in the wind speed can be epesented as a mean value and a continuous spectum o spectal density function. In tem of powe the spectal content is called Powe Spectal Density (PSD) [45]. A numbe of PSD functions ae used as models in wind enegy engineeing. The PSD used in PSCAD wind model is [53]: S( ) i V ω i is the fequency of i th component and is defined as: ω i =(i-0.5) W. W is noise amplitude contolling paamete (0.5-2ad/s) K is suface dag coefficient (0.0192) w V 2 2KF F is tubulence scale (600m) μ is mean wind speed at efeence height (m/s) wn 2 2 F i 1 i 4 3 (2.36) The PSD is a function of fequency and the poblem is to tanslate the PSD into a time sequence of values with the given spectal density. To solve the poblem PSCAD uses a method as follows. The PSD is used to deive infomation about the amplitude of a signal component with a given fequency (which is defined by 37

50 2 Contol of Doubly-Fed Induction Geneato and Wind Tubine W and i). Then, a lage numbe (N) of sines waves with a andom initial phase angle (Ф i ) and amplitude calculated fom PSD ae added fo each time step. Equation (2.37) descibes the noise component based on the explained method [53]: V wn N 0.5 S ( i ) W cos it i 2 i1 (2.37) In this thesis all poposed contol methods ae validated using eal (measued) wind pofile. Howeve in some cases, fo sake of explanation, constant wind speed o simulated wind speed using the model above is used. 38

51 3 Contol of wind tubines-es system with extenal voltage and fequency souce 3. Contol of wind tubines-es system with extenal voltage and fequency souce As discussed in Chapte 2, thee ae geneally two methods of contolling the DFIG: Diect Stato Flux Oientation (DSFO) and Indiect Stato Flux Oientation (ISFO). The DSFO-contolled DFIG needs to be connected to an extenal voltage and fequency souce. In othe wods, a DFIG contolled unde DSFO is acting as a powe souce. This chapte deals with DSFO-contolled DFIG. In this case, the wind fam can be eithe diectly connected to the main gid o via AC-DC-AC convetes. Both scenaios ae discussed in this chapte. Pio to this, two stategies egading Enegy Stoage (ES) contol in wind fam ae explained in the next subsection. 3.1 Stategies of contolling ES Geneally two ES contol stategies can be defined [56]: Powe Smoothing Contol (PSC) and Powe Demand Contol (PDC). In a PSC stategy, as its name suggests, ES is utilized to smooth a havested wind powe. It means that the powe deliveed to the gid (load) is a smoothed vesion of the wind powe. Fo example in [20, 21, 55] the wind tubine shaft inetia has been exploited as an enegy stoe to absob wind powe fluctuations. Obviously having an extenal ES can enhance powe smoothing. This has been addessed in many papes such as [5, 16, 25-27]. Taditionally powe netwok opeatos ae obliged to buy all the wind enegy poduced by wind fams and othe powe geneatos have to balance the geneation with demand by inceasing o deceasing thei geneation. Unde PSC, the best scenaio is when the unwanted wind powe fequencies ae filteed out by ES in ode to povide a smoothe powe to the gid to potect the netwok fom unwanted fequencies. Howeve, the aveage powe deliveed to the gid is still detemined by the wind and this is why PSC may not be the optimum delivey fo a powe netwok with a high penetation of wind powe, especially if the ability of 39

52 3 Contol of wind tubines-es system with extenal voltage and fequency souce othe geneatos to balance the demand is limited. As wind enegy penetation inceases, the capacity of othe geneatos to balance geneation and demand is educed and this is why PDC may be viable option. Unde PDC, the combination of ES and the wind tubines delives a constant powe detemined by the load athe than wind. In othe wods, unde PDC, ES is used to absob powe vaiations between wind powe and a efeence powe P detemined by the system opeato. Obviously if the wind is the only souce of geneation, the demand powe can neve be moe than the aveage of the extactable wind powe P ave. In pactice, howeve, a PDC will always equie an auxiliay geneation souce. The aim of the PDC contol egime is to maximise the enewable geneation and minimize the auxiliay powe. The PDC stategy, which has eceived almost no attention to date, is the main focus of this thesis. Both PSC and PDC stategies can be applied on both DSFO- and IFSO-contolled DFIGs. Howeve as a geneal ule, DSFO is moe appopiate fo PSC while ISFO seems to be moe suitable fo PDC. This will be discussed thoughout the thesis. This chapte consides the DSFO-contolled DFIG using both PSC and PDC stategies. The objective of this chapte is to biefly study the PSC and PDC in a DSFO-contolled wind fam which is eithe diectly connected to the AC gid o via a HVDC link. The simulation of all of the possible scenaios is out of the scope of this thesis. Howeve a PSC stategy fo an aggegated DSFO-contolled wind fam connected to a LCC-HVDC link will be simulated, as an example. The ISFOcontolled DFIGs ae studied in the following chaptes. 3.2 DSFO-contolled DFIGs-ES system with diect AC gid inteface The DSFO is the standad DFIG contol in which the geneato is field oientated off an extenal voltage and fequency souce. Theefoe i and i ae used to contol the stato active and eactive powe espectively. This section studies ES contol stategies in a wind fam based on DSFO-contolled DFIGs which ae q d 40

53 3 Contol of wind tubines-es system with extenal voltage and fequency souce diectly connected to AC gid. This case does not seem appopiate fo a weak gid. If the wind fam is integated within a micogid which is islanded fom the main gid, it is necessay to contol the local gid s voltage and fequency using, fo example, a STATCOM. In the next two subsections, it is assumed that the voltage and fequency contol is povided by the main gid Powe Smoothing Contol fo wind tubine-es connected diectly to the AC gid The PSC stategy, which is shown in Figue 3.1, has been addessed in many papes [5, 16, 25-27]. In this stuctue DFIGs ae conventionally contolled in the Maximum Powe Tacking (MPT) mode. The ES can be eithe aggegated on to the collecto bus of the wind fam o distibuted and integated with each DFIG. The total output powe of the wind fam is likely to be smoothe than the powe geneated by each wind tubine due to the possible phase displacement of individual tubine powes. Theefoe in the aggegated ES, the equied enegy capacity of the ES in ode to smooth the wind powe to a cetain level; tends to be smalle than that of the total distibuted ES. Hence, the aggegated ES seems to be the appopiate choice in this case. Howeve in lage offshoe wind fams, accommodating such a lage aggegated ES can be quite an engineeing challenge. In such cases, distibuting ES, fo example, in the space available in the tubines towe athe than aggegating on a huge cental platfom; might be pactically and economically beneficial. A numbe of available ES technologies wee explained in Chapte 1. This eseach concentates on the shot to medium-tem ES such as: flywheel, SMES and Supecapacitos. The ES, in this thesis, is simulated by a DC voltage souce which is intefaced to the AC system via an AC/DC convete. The convete is called the ESI o ES Inteface. The ES powe can be contolled by egulating the d- (eal) component of the ESI cuent I d es (see Figue 3.1). The cuent contol is the standad cuent contol which is identical to the DFIG s cuent loops explained in Chapte 2. 41

54 3 Contol of wind tubines-es system with extenal voltage and fequency souce Gid P g1,q g1 ~.... MPT P g2,q g2 s 1s HPF I d-es ES ESI MPT Figue 3.1. PSC in a DSFO-based wind fam with diect AC gid inteface Studies have shown that powe system is moe sensitive to medium fequency wind powe fluctuations (0.01-1Hz) [16] since the powe density of wind speed educes as the wind fequency inceases. Theefoe the ES is contolled to filte out these fequencies in ode to shield the powe system. This can be done by subjecting the geneated wind powe to a High Pass Filte (HPF), the output of which sets the efeence ES d-axis cuent I d-es. The time constant of the HPF is used to achieve diffeent cut-off fequencies Powe Demand Contol fo wind tubine-es connected diectly to AC gid In a PDC stategy, geneally, the demand powe P must be imposed on the wind fam. The easiest way to impose the demand powe, in a wind fam diectly connected to the AC gid, is the ES (see Figue 3.2). The ES is contolled in ode to balance the powe geneated by the wind tubine(s) with the demand. In a DSFO-contolled wind fam, communication is equied to make the wind fam geneation as close as possible to P in ode to minimize the ES. Figue 3.2 illustates a PDC stategy in a DSFO-contolled wind fam which is diectly connected to the AC gid. In this case a Supevisoy Wind Fam Contol (SWFC) unit [57-59] is used to detemine the efeence demand powe and/o 42

55 3 Contol of wind tubines-es system with extenal voltage and fequency souce pitch angle fo each wind tubine based on the total demand powe P and wind speed associated with each wind tubine. The aim hee is to minimize the ES. The total demand powe is set by the system opeato. It is noted that if P P, 1 2 P no ES is needed which implies that that wind tubines ae contolled unde Constant Powe Mode (CPM). As discussed in section 2.5.3, CPM is not possible when the demand powe appoaches the aveage of the extactable wind powe. Wheneve P 1 and P 2 ae not detemined by MPT, stability issues must be taken into account. The ES powe is contolled using the ESI-eal cuent in ode to absob/inject the diffeence between the wind fam powe and P. If the ES is distibuted within the wind tubines, the output of each individual DFIG-ES is smooth. β 1 P P=P Gid g1,q g1 ~.... P 1 - P es ES ESI β 2 P g2,q g2 P P 2 SWFC V w1 V w2 Figue 3.2. PDC in a DSFO-based wind fam with diect AC gid inteface In [60] a pitch angle contol, with efeence powe deived fom the aveage wind speed, is used to smooth the output powe of wind tubine. A simila pitch contol can be used hee with the efeence powe given by total demand powe consideing the associated wind speed in ode to educe the size of ES. 43

56 3 Contol of wind tubines-es system with extenal voltage and fequency souce 3.3 DSFO-contolled DFIGs-ES system with AC/DC/AC inteface This section studies both PSC and PDC stategies fo a DSFO-contolled DFIGs connected to the main gid though a High Voltage DC (HVDC)-link. Two existing HVDC technologies will be investigated. The HVDC connections ae usually used to tansfe lage powes ove lage distances. They can also be used fo connecting two powe netwoks with diffeent fequencies. One of the main applications fo HVDC connections is cuently fo lage offshoe wind fams. The HVDC-link divides the powe system into two pats: main gid and wind fam gid. The challenge hee is to contol the voltage and fequency of wind fam gid. Consideing a micogid application, the wind fam gid is connected to the local gid though the HVDC-link. If the local gid is islanded fom the main gid, the contol of the wind fam gid s voltage and fequency can be even moe demanding; especially with a high wind enegy penetation. The contol of the wind fam gid will be studied in this section fo the two HVDC technologies. The HVDC-link can also be used to impose the demand powe in a PDC stategy, which is discussed in this section Intoduction of HVDC technologies This section studies the two available HVDC technologies and compaes them with one anothe. In ode to explain the HVDC technologies, an application to lage offshoe wind fam is consideed. Howeve, the technologies ae applicable to othe applications as well. Tansfeing lage powes (e.g. 1000MVA) poduced by lage wind fams, ove lage distance (e.g. 100km), ceates an engineeing challenge fo system opeatos. Wind fams may be connected to the main gid by eithe High Voltage AC (HVAC) o HVDC. HVAC is an economic connection fo medium ange wind fams (up to a few hundeds MVA) with tansmission distances less than 50-75km [61]. Fo distances moe than 50-75km, dynamic eactive powe compensation will be equied in ode to meet the connection ageement equiements [62]. In 44

57 3 Contol of wind tubines-es system with extenal voltage and fequency souce such cases HVDC tansmission offes advantages such as: fully defined and contolled powe flow, lowe cable losses than AC connection, and independent contol of sending- and eceiving-end fequencies [4, 61-63]. In addition the pesent capacity of a thee-coe AC submaine cable is limited to 200MVA [63] which means that fo lage powe multiple cable ae equied. Theefoe the HVDC connection has technical, economical and envionmental advantages fo lage wind fams with long distance to the main gid, which is usually the case fo offshoe wind fams. Cuently thee ae two options fo HVDC connection: Voltage Souce Convete (VSC) and Line Commutated Convete (LCC). LCC utilizes thyistos while VSC uses eithe Gate-Tun-Off thyistos (GTOs) o Insulated Gate Bipola Tansistos (IGBTs). The main diffeence between the LCC and VSC solutions is that the VSC is self-commutated and, unlike LCC, does not need an active voltage souce fo commutation. This enables a VSC convete to geneate an AC theephase voltage [62, 64], thus contolling the voltage and fequency of the wind fam gid. Theefoe, independent contol of active and eactive powe and connection to weak gid ae also possible. The VSC solution equies smalle convete platfom as the equied AC filtes ae smalle that the LCC one. The VSC connection used to be citicised fo its limited ating and high switching losses compaed to LCC. Howeve both Siemens and ABB have developed a VSC connection with ating up to 1200MVA and 320kV using new switching topologies which can significantly educe the convete losses [65]. Consideing the ecent impovement in VSC-HVDC topologies and thei contol, VSC can be an economical solution fo HVDC tansmission. Howeve thei eliability is yet to be fully poved in pactice especially fo high powe ating, compaed to the LCC which has been in opeation fo moe than 30 yeas. The following sections study a DSFO-contolled DFIGs-ES system connected to HVDC-link. Diffeent topologies fo both PSC and PDC stategies fo both VSC and LCC solutions ae biefly discussed. 45

58 3 Contol of wind tubines-es system with extenal voltage and fequency souce DSFO-contolled DFIGs-ES system connected to VSC- HVDC In a VSC-HVDC connection, conventionally the sending end convete (the one connected to wind fam) is esponsible fo collecting enegy fom the wind fam while the eceiving end convete (the one connected to the gid) is esponsible fo maintaining the HVDC DC-link voltage constant. Constant DC-link voltage indicates the balance of active powe exchanged between the two sides. The contol of the eceiving end convete is simila to the DFIG s gid side convete i.e. a otating dq fame can be used with the d-axis fixed to the gid voltage. Theefoe, the d-axis cuent is used to contol the DC-link voltage (active powe) while the q-axis cuent contols the eactive powe and can be used to suppot the gid voltage. The pime objective of the sending end convete contol is to collect the powe poduced by wind fam. The sending end convete is contolled to povide a voltage souce with constant voltage magnitude and fequency. Doing so, as is the case fo wind fams diectly connected to AC gid, the powe poduced by the wind fam is absobed by the sending end convete and tansfeed to the gid though the eceiving end convete. Diffeent contol methods fo the sending end convete ae explained in papes [62, 64, 66, 67]. In all of them the sending end convete contols the voltage and fequency of the wind fam gid. In the following subsections the integation of the ES unde both PSC and PDC with a VSC-HVDC link is studied Powe Smoothing Contol fo wind tubine-es connected to a VSC-HVDC link Figue 3.3 illustates a possible PSC topology fo a DSFO-contolled DFIGs-ES system connected to the gid though a VSC-HVDC link. The DFIG contol is identical to the one explained in section 3.2. The sending end convete contols 46

59 3 Contol of wind tubines-es system with extenal voltage and fequency souce the voltage and fequency of the wind fam gid. The Wind powe fluctuations ae eflected on the DC-link voltage V DC. In this scheme both eceiving end convete and ES ae esponsible fo contolling the DC-link voltage. As Figue 3.3 shows, the V DC eo is subject to a HPF, the output of which is used to egulate the ES cuent I d-es (which is popotional to ES powe assuming a constant voltage). Theefoe the high fequency fluctuations ae absobed by the ES and the low fequency fluctuations ae tansfeed to the gid by egulating the d-axis cuent of the eceiving end convete I d-con. In this stuctue the ESI would be a DC/DC convete. Howeve placing the ES in the HVDC DC-link may not be pactical due to the high voltages involved. This may also incease the cost of insulations. Sending end Receiving end.... MPT V, f V DC - ESI ES I d-es PI PI I d-con Gid MPT s 1s HPF - Figue 3.3. PSC in a DSFO-based wind fam connected to VSC-HVDC link An altenative stuctue is to place the ES between the wind fam and the VSC- HVDC and contol it in the same way as that of Figue 3.1. In this stuctue the powe into the VSC is aleady smoothed by ES and the VSC is contolled conventionally. In this case the ESI is an AD/DC convete Powe Demand Contol fo wind tubine-es connected to VSC-HVDC link A numbe of diffeent stuctues ae possible to impose the demand powe in a DSFO-contolled wind fam connected to a VSC-HVDC system: One way is to place the ES between the wind fam and the VSC-HVDC link. The ES is 47

60 3 Contol of wind tubines-es system with extenal voltage and fequency souce contolled identical to the one shown in Figue 3.2 i.e. the ESI is used to impose the demand powe P. The wind fam gid voltage and fequency is contolled by the VSC sending end convete. Theefoe, the powe to the VSC is constant and equal to P. Again in this stuctue the ESI is an AC/DC convete. Sending end Receiving end β P 1 V, f V DC - ESI I d-con k Gid β 2 ES P P 2 SWFC V w1 V w2 Figue 3.4. PDC in a DSFO-based wind fam connected to VSC-HVDC link A second possible stuctue is shown in Figue 3.4 in which the ES is placed in the VSC DC-link and the demand powe is imposed by the d-axis cuent of the eceiving end convete I d-con (whee k=1/v DC ). ES contols the DC-link voltage. The ESI would be again DC/DC convete; howeve, this stuctue may not be the best one due to the high voltage of the HVDC DC-link. A thid stuctue is shown in Figue 3.5 in which the demand powe is imposed by the d-axis cuent of the eceiving end convete while the d-axis cuent of the sending end convete is used to contol the HVDC DC-link voltage. The wind fam gid voltage and fequency is contolled by the ESI. Theefoe the powe into the VSC is constant and equal to the demand powe P. In this stuctue the ESI also opeates as STATCOM [12, 68]. The ESI is an AC/DC convete, contolling gid voltage and fequency by ESI. Howeve, this stuctue may not make the best use of VSC-HVDC fom powe electonic point of view. 48

61 3 Contol of wind tubines-es system with extenal voltage and fequency souce Sending end Receiving end β 1 β P 1 ES V, f I d-con PI - k V DC I d-con Gid P P 2 SWFC V w1 V w2 Figue 3.5. Altenative PDC stuctue fo DFO-based wind fam connected to VSC-HVDC link DSFO-contolled DFIGs-ES system connected to LCC- HVDC link Unlike VSC, the LCC uses thyistos and is not self-commutated. The LCC equies an extenal voltage souce to foce commutation. The extenal voltage souce can be eithe a synchonous compensato [63] o STATCOM [4, 69]. The STATCOM has faste contol and lowe losses than a synchonous compensato [69, 70]. The LCC convetes absob lage amount of eactive powe that is mainly povided by the AC filtes. Theefoe the LCC convetes need much bigge filtes than the VSC type. The STATCOM is used to contol the wind fam gid voltage and fequency, balance the active and eactive powe and povide the commutation voltage needed by the LCC-HVDC link. The active powe flow though the HVDC is contolled by the ectifie fiing angle α [4]. The invete is esponsible fo keeping the DC-link voltage constant and unde nomal conditions has almost no effect on the ectifie contol egime [4, 48]. The contol design of the STATCOM connected to the LCC-HVDC link is explained in Appendix C. 49

62 3 Contol of wind tubines-es system with extenal voltage and fequency souce In the following subsections the integation of the ES unde both PSC and PDC with a LCC-HVDC link is studied Powe Smoothing Contol fo wind tubines-es connected to LCC-HVDC link Figue 3.6 illustates the PSC stategy in a DSFO-based wind fam connected to ESI-STATCOM [12, 68] and a LCC-HVDC link. The ESI, which also opeates as a STATCOM, contols the wind fam gid voltage and fequency using the contol scheme explained in Appendix C and [4]. Constant voltage and fequency implies that any unbalance in powe is eflected on the DC-link voltage of the ESI- STATCOM. In [4] the STATCOM DC-link voltage vaiations is used to egulated the ectifie fiing angle α, hence tansfeing the powe geneated by wind fam though the HVDC link. Hee, the ESI-STATCOM DC-link voltage is kept constant by the ES. The powe geneated by wind fam is measued and filteed by a Low Pass Filte (LPF). The output of the LPF is used to set the efeence cuent of HVDC link I 0 whee k=1/e 0. Theefoe the smoothed vesion of the wind fam powe is tansfeed to the main gid though the HVDC link while the high powe fequencies ae absobed by ES. Rectifie Invete I MPT V, f E 0 - Gid ES MPT k 1s LPF I 0 - I 0 PI α Figue 3.6. PSC fo DSFO-contolled DFIGs connected to ESI-STATCOM and LCC HVDC link Powe gids ae moe sensitive to the medium fequencies of wind powe fluctuations (0.01-1Hz) [16]. Theefoe the ES is usually designed to absob these 50

63 3 Contol of wind tubines-es system with extenal voltage and fequency souce fequencies. This can be achieved by adjusting the time constant τ of the LPF. The HVDC cuent loop contol design is out of the focus of this thesis and is explained in [4]. Simulation esults This section simulates an aggegated DSFO-contolled DFIG connected to a LCC- HVDC link and ESI-STATCOM, as shown in Figue 3.6. The contol of the ESI- STATCOM is given in Appendix C. This case is simulated as an example of standad PSC stategy which has moe o less similaly been used in many papes such as [5, 16, 25]. The esults will also be used as benchmak to be compaed with the late esults fom a poposed PDC stategy. Figue 3.7shows the PSCAD simulation esults of a 1000MVA DSFO-contolled DFIG connected to the STATCOM-ES and LCC-HVDC link shown in Figue 3.6. The DFIG s paametes ae given in Appendix B. The ated wind speed fo all wind tubines used in this thesis is kept almost aound 12.5m/s. Theefoe, the ating of DFIG does not make any diffeence in the esults (in pu), since the paametes of all DFIGs ae the same in pu. The DFIG is contolled unde conventional MPT. The time constant of the LPF is set at 16sec in ode to filte out the fequencies moe than 0.01Hz [16]. The ES, fo the sake of simplicity, is simulated by a 35kV DC voltage souce. AC-filtes (not shown in the Figue) ae tuned fo the ated HVDC powe (1000MVA), the fine eactive powe balance is clealy povided by the ESI-STATCOM. The HVDC invete, fo the sake of simplicity, is eplaced by a DC voltage souce (E 0 =490kV). The pitch angle contol (not shown in the Figue) is the standad MPT pitch contol which contols the output powe at 1pu fo wind speeds above the ated wind speed. The wind pofile, shown in Figue 3.7.a, is a eal (measued) wind speed with aveage and standad deviation of appoximately 11.5m/s and 1.39, espectively. The petubation of the wind is elatively lage. Figue 3.7.b1 shows the DFIG output powe unde MPT contol while Figue 3.7.b2 is the powe into HVDC. The time constant of the LPF (τ=16sec) detemines the smoothness of the HVDC powe. The lage the time constant, the smoothe the HVDC powe; and thus the lage 51

64 3 Contol of wind tubines-es system with extenal voltage and fequency souce ES would be. Figue 3.7.c is the powe absobed fom/injected into the wind fam gid by the ES which is the diffeence between the wind fam output powe and the HVDC powe. Figue 3.7.d shows the enegy of the ES which is deived by integating the ES powe. Figue 3.7.e depicts the wind fam gid voltage (132kV) and fequency (50Hz) which ae contolled by the ESI-STATCOM. Figue 3.7.Results of a PSC stategy fo a 1000MVA DSFO-contolled DFIG connected to ESI- STATCOM and LCC HVDC link 52

65 3 Contol of wind tubines-es system with extenal voltage and fequency souce The size of ES is chaacteized by its powe ating and its enegy capacity. Hee it was assumed that the size of ES is infinite. Howeve, in pactice it will have a finite value and a pitch angle contol can be used to pevent ES fom hitting its maximum limit. A slow pitch contol can be also applied to make the aveage of ES powe zeo; this will educe the powe ating of ES. These esults demonstate that in ode to filte out the powe fequencies moe than 0.01Hz, the equied ES capacity is about 20pu while the powe ating of ES is just less than 0.5pu. The equied ES capacity and powe ating ae a function of the wind speed petubation, shaft inetia and the LPF time constant. Appendix D gives a pespective of the size of the equied ES. In ode to maintain the enegy of ES within its limits, Enegy Management System (EMS) is usually equied. These methods will be discussed in late chaptes. The next section consides Powe Demand Contol (PDC) fo LCC-HVDC connection Powe Demand Contol fo wind tubines-es connected to LCC-HVDC link One stuctue fo implementing a PDC fo a DSFO contolled wind fam connected to the ESI-STATCOM and LCC HVDC link is shown in Figue 3.8. Rectifie Invete β 1 I 0 + β P 1 ES V, f E 0 - Gid P k I 0 - PI α P 2 SWFC V w1 I 0 V w2 Figue 3.8. PDC fo DSFO-contolled DFIGs connected to STATCOM-ES and LCC HVDC link 53

66 3 Contol of wind tubines-es system with extenal voltage and fequency souce The demand powe P is imposed by the HVDC ectifie fiing angle α (whee k=1/e 0 ). The Supevisoy Wind Fam Contol (SWFC) unit [57, 58] adjusts the pitch angle and the efeence powe of each wind tubine in ode to minimize the extenal ES. If the efeence powes of the DFIGs ae not detemined by the MPT, stability issues must be taken into account. If the demand powe is less than the extactable wind powe, CPM is possible which means that no extenal ES is needed. This is the subject of Chapte Discussions and conclusions This chapte intoduced two stategies of contolling a wind geneatos-es system: Powe Smoothing Contol (PSC) and Powe Demand Contol (PDC). The application of the two stategies in a DSFO-contolled wind fam connected to both AC gid and HVDC link has been discussed. The diffeent wind fam gid voltage and fequency contol stuctues in each case was illustated. Though PSCAD simulation it was illustated that in a PSC, the powe into the gid is a smoothed vesion of the wind powe. Theefoe the PSC stategy may not be appopiate fo a powe system with a high penetation of wind enegy. This is because the system opeato cannot detemine the wind powe geneation, and the ability of othe geneatos to balance the powe may be limited. In the PDC stuctues fo a DSFO-contolled DFIGs-ES system, communication between the system opeato and the wind fam is necessay in ode to detemine the efeence powe and pitch angle fo each wind tubine. Moeove the wind tubine cannot contol the wind fam gid voltage and fequency. In futue, an incease in both electical enegy and powe quality demanded, beside the apid ise in enewable enegy penetation may necessitate the e-designing of T&D systems. Small-scale distibuted powe geneation combined with powe electonic systems lead to the concept of futue netwok technologies such as the micogid which seems to be a pomising solution fo such poblems. The micogid is an intentionally islanded powe system including Distibuted Enegy Resouces (DER), contollable loads and ES which can opeate in paallel with, o isolated fom the main gid [33, 35]. Micogids ae designed to povide the local 54

67 3 Contol of wind tubines-es system with extenal voltage and fequency souce costume with uninteuptible powe, enhance local gid eliability and impove powe quality by suppoting the local gid voltage and fequency. To achieve this functionality each active component must be able to change its opeating point based on local voltage and fequency vaiations. The most obust way to do so is the use of classical fequency and voltage doops. This chapte has poposed some stuctues fo a PDC in a DSFO-contolled wind geneato. In all of them, fast communication between the system opeato and each wind geneato unit is necessay in ode to shae the demand powe (i.e. Powe Management Communication). The existence of the cental communication unit may educe the system eliability. Futhemoe, a DSFO-contolled DFIG cannot contibute to local gid contol since they ae field oientated off the gid voltage. Theefoe a method involving doop chaacteistics cannot be applied to DSFO-contolled DFIGs. This means that they may not be so suitable fo integation into a micogid. Howeve DFIGs contolled unde Indiect Stato Flux Oientation (ISFO) ae able to contol the local gid voltage and fequency and can be augmented with doop chaacteistics. These popeties make the ISFOcontolled DFIGs a moe pope and flexible choice fo micogids. In the next chapte an aay of ISFO-contolled DFIGs ae augmented with classical fequency and voltage doops in ode to shae, espectively, the active and eactive powe demanded by the load. The doop chaacteistics ae initially a function of the DFIGs atings. The doop chaacteistics ae adjusted in Chapte 5 in ode to shae the load accoding not only to the atings of the DFIGs but also the available wind powe. 55

68 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs 4. Local gid voltage and fequency contol using ISFO-contolled DFIGs 4.1 Intoduction The Indiect Stato Flux Oientation (ISFO) contol of DFIG is explained in [44] and has eceived little attention. The stuctue is suitable fo isolated load since the q-axis of the stato cuent i sq is imposed by the load athe than the wind. Theefoe the q-axis of the oto cuent i q has to be kept popotional to i sq in ode to maintain the field oientation of the DFIG. This stuctue does not equie an extenal voltage souce. The voltage magnitude is contolled by the magnetising cuent i ms though egulating the d-component of the oto cuent i d while the voltage fequency is imposed by the stato flux angle though fee unning integation of the efeence stato voltage fequency. The contol stuctue is given in section Theefoe, a DFIG unde ISFO contol opeates as a voltage and fequency souce athe than a powe souce (which is the case in the DSFOcontolled DFIG). As a esult, the ISFO stuctue can be augmented by voltage and fequency doop chaacteistics which is explained in this chapte. Moeove, the ISFO seems to be a suitable option fo integation within a micogid as they can suppot local gid voltage and fequency contol and hence enhance the powe quality. The main dawback of the ISFO stuctue is that i q cannot be used to contol electical toque T e, since i q has to be kept popotional to i sq. In this stuctue the electical toque is imposed by the load, hence, an extenal mechanism is needed to contol T e. In [44] an auxiliay load is used to contol T e in the Maximum Powe Tacking (MPT) mode. In this thesis, the cuent component coesponding to the ES powe will be used to contol the electical toque wheneve equied. Without an extenal toque contol mechanism, the electical toque is diectly detemined by the load powe. This implies that the DFIG is unde Constant Powe Mode (CPM), assuming a constant load. Figue 4.1 compaes the implementation of the CPM in an ISFO-contolled DFIG with that in 56

69 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs a DSFO-contolled DFIG. Figue 4.1b illustates that in an ISFO-contolled DFIG the local gid voltage is egulated by the DFIG and the DFIG output powe is equal to the powe demanded by the load. Gid ~ P=P P PL=P P (a) i q =ki sq (b) Figue 4.1. Constant Powe Mode in (a) DSFO- (b) ISFO-contolled DFIG Howeve in a DSFO-contolled DFIG the voltage is set by an extenal voltage souce (e.g. gid) and, in ode to get a smooth output powe equal to P, the powe loop efeence must be set at P.Hence communication is needed Intoduction to technical equiements fo wind fams With inceasing wind enegy penetation, the impacts of vaiable wind enegy on the T&D system become moe significant, necessitating sticte equiements fom the wind geneatos [3, 5, 14]. Cuently, the equiements ae intended to limit the distubances of wind enegy geneation on gid, fo example, fault ide though capability. With inceasing penetation of wind enegy, especially at distibution level, wind fams may be equied to povide gid contol functions nomally associated with conventional powe geneation units [16, 25, 57]. This poblem becomes moe sevee in an isolated powe system like that of a small island which has poo capability of powe egulation. Theefoe, a moden wind fam may in futue be equested to povide advanced gid suppot such as contol functions fo both active powe-fequency and eactive powe-voltage contol. These functionalities equied fom a wind fam can be summaised as follow [22, 58]: Balance contol means that the wind fam can incease o decease its active powe geneation in ode to balance the demand. 57

70 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs Delta contol means that the wind fam is equied to geneate less than the maximum powe such that a eseved available powe can be used fo a limited fequency contol action. Fequency contol means that the wind fam inceases o deceases its active powe geneation in ode to compensate fo fequency fluctuations. Reactive powe contol means that the wind fam is equied to absob o inject a cetain amount of eactive powe. Voltage contol means that the wind fam contols the voltage of the point of common coupling by absobing o injecting eactive powe. The next section eviews diffeent conventional methods fo suppoting wind fam gid voltage and fequency contol. In a DSFO-contolled DFIG, unlike the ISFO one, an extenal voltage and fequency souce is always needed as the DFIGs ae seen as powe souce by the powe system. The ISFO-contolled DFIGs, howeve, appea as voltage and fequency souces and it will be shown in late chaptes that they can maintain gid voltage and fequency when even thee is no wind. This chapte consides ISFO-contolled DFIGs and augments them with classical fequency and voltage doop chaacteistics in ode to shae active and eactive powe demanded by the load accoding to thei atings. The philosophy is to make the wind fams opeate as a conventional powe plant, fo example, like a synchonous geneato. In Chapte 5, the doops will be adjusted to shae the powe accoding to both the available wind powe and the ating of DFIGs. 4.2 Conventional methods fo suppoting gid voltage and fequency In a DFIG based wind fam, the eactive powe-voltage suppot can be achieved by supplying eactive powe fom the DFIG stato [56, 71-73] and/o by egulating the q-axis cuent of the gid-side convete. The balance contol can be done by egulating the pitch angle and/o using ES. In ode to suppot the gid fequency contol in a DSFO-contolled DFIG, it is equied to have enough eseve active 58

71 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs powe (delta contol). In othe wods, it is geneally necessay to foce the wind tubine to opeate in a non-maximum powe point which is commonly efeed to as de-loading [18]. Regading paticipation in fequency contol fo a DSFOcontolled DFIG, two diffeent types of opeation can be defined [22]: Paticipation at full load, when wind speed is highe than ated wind speed: in this case a pitch contol educes the active powe to a value less than the wind tubine ated value. Theefoe the active powe eseved can be deliveed to the gid in ode to contol the fequency when equied [18]. Paticipation at patial load: Again pitch angle can be utilized to de-load the geneato to a value less that the maximum available wind powe and use the eseve powe to egulate fequency. An altenative way is to set the shaft speed at a non-optimum speed esulting in a powe geneation less than the maximum one. Theefoe, the gid fequency can be egulated though the use of the powe eseved [22]. It is also possible to suppot the gid fequency though using the enegy stoed in an extenal ES [19, 59]. In all of these methods, howeve, an extenal voltage and fequency souce is still equied. The pevious chapte consides a STATCOM (which also opeates as ES inteface) contol fo a DSFO-contolled wind fam connected to a LCC-HVDC link. In the liteatue, howeve, some STATCOM-less solutions have been suggested which ae biefly eviewed below STATCOM-less solutions fo LCC-HVDC connected wind fams to contol wind fam gid In [48, 72, 74] a wind fam connected to a LCC HVDC link is consideed. The active powe flow though the HVDC link is contolled by egulating the ectifie fiing angle. As discussed in the pevious chapte, fo a DSFO-based wind fam 59

72 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs connected to a LCC HVDC link, an extenal voltage and fequency souce such as a STATCOM is usually needed. Howeve, [48] poposed thee STATCOM-less solutions which ae summaized as follows. The gid fequency contol method [72] in which the wind fam gid fequency is contolled by egulating the ectifie fiing angle. The fequency fluctuation is used to egulate the ectifie fiing angle in ode to tansfe the powe geneated by the wind fam to the main gid though the HVDC link. With a contolled gid fequency, the DFIGs ae contolled to set the magnitude of gid voltage by contolling the stato flux though egulating magnetizing cuent i ms. The DFIGs ae contolled such that the oto d-axis cuent demand i d is set by the magnetizing cuent while i q is set by a MPT scheme. Howeve, this contol scheme is not valid fo system stat-up due to the absence of the stato voltage fequency fo the field oientation. Theefoe, the wind fam has to be initiated unde the ISFO contol scheme in which a fequency demand is imposed on the system and i q is kept popotional to i sq. Afte the stat-up, i q is switched ove to a MPT scheme while the wind fam gid fequency is contolled by egulating the ectifie fiing angle. In this case the wind fam is not contibuting to the main gid fequency contol since all the wind powe geneated is tansfeed to the main gid. Theefoe, lage wind powe fluctuations may esult in lage fequency fluctuations in the main gid especially in case of a weak gid. In [74] it was shown that if the main gid fequency is too high o too low, active powe flow though the HVDC link can be deceased o inceased by intoducing a doop at the ectifie contol loop. At the same time the pitch angle will incease o decease to educe o incease the powe geneated by the wind fam. The gid contol via classical doop method which is intended to make wind tubines opeate like a conventional synchonous geneato. This appoach allows the DFIGs to contibute to a shaed contol of the gid voltage and fequency. In ode to achieve this, each oto side convete imposes a stato voltage fequency and stato flux level on each of the DFIG statos. The autho, as a membe of the eseach team, utilized the doop method in [48] in ode to povide ide-though in case of loss of the main gid. Howeve the pesent thesis is intended to apply the 60

73 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs doop chaacteistics in both nomal and fault ide-though opeations. This is the subject of this chapte. The maste-slave appoach in which one DFIG contols the wind fam gid voltage and fequency (the maste) and the othes (slaves) opeate unde standad DSFO contol. The maste one is contolled unde ISFO method in ode to set the wind fam gid voltage and fequency while the slaves ae field oientated off the gid voltage (DSFO-contolled). In this stuctue the HVDC ectifie fiing angle is egulated to contol the electical toque of the maste DFIG. Theefoe, communication between the HVDC link and the maste DFIG is equied. In the maste-slave method, simila to the gid fequency contol appoach, the wind powe fluctuations ae tansfeed to the main gid via the HVDC link and may cause lage fequency vaiations paticulaly in the case of a weak gid. Moeove, the powe into the gid is detemined by the wind athe than the system opeato. Theefoe these appoaches may not be appopiate fo a powe system with a lage penetation of wind enegy, no fo integation into a micogid. In the methods mentioned above, some level of active powe eseve is needed to contibute to fequency contol. This implies that the fequency contol is not possible unde MPT mode. Futhemoe, the degee to which the fequency contol can be done is limited to the level of the active powe eseved. As a esult the wind fam cannot be the only voltage and fequency souce of the system. Howeve, ISFO-contolled DFIGs enable full contol of the local gid voltage and fequency egadless of the wind speed and the demand powe. This can be achieved by applying the doop chaacteistics on the wind geneatos. As a esult, the wind fam can be the only voltage and fequency souce in the system and can be well-integated into a micogid. This is the subject of this chapte. 4.3 Wind fams and micogids Micogids ae supposed to opeate in both gid connected and islanded modes. In the gid connected mode, most of the system dynamics ae imposed by the main gid due to the elatively small size of the micogid. Howeve the challenge is in the islanded mode [42, 75] since the micogid has to meet the local enegy 61

74 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs demand and povide the powe quality equied by the local loads. These equiements can be achieved only if all active units suppot the local gid voltage and fequency and be able to adjust thei opeating points vey fast accoding to the demand. These functionalities ae nomal fo the conventional powe plants equipped with doops chaacteistics. The equiements become hade to meet as the wind enegy penetation inceases since the ability of othe powe geneatos may become limited. Moeove, the DSFO-contolled wind tubines have limited capacity to suppot the local gid. Two diffeent stategies fo contolling a wind fam-es system wee identified in the pevious chapte [56]: Powe Demand Contol (PDC) and Powe Smoothing Contol (PSC). The latte is the conventional one unde which the ES filtes out high wind powe fequencies in ode to povide the main gid with a smoothed powe. The powe into the gid is still a measue of the wind powe. This may cause difficulties fo othe powe geneation units to meet the demand especially in a powe system with a high wind enegy penetation. In the PDC stategy, the combination of wind tubine(s)-es is contolled in ode to delive a constant demand powe to the gid. This stategy seems to be moe suitable fo a micogid with a high wind enegy penetation. In the pevious chapte some contol stuctues fo the PDC stategy using the DSFO-contolled DFIGs have been discussed. In those stuctues a cental Supevisoy Wind Fam Contol (SWFC) unit is equied which detemines the efeence powe and pitch angle fo each wind tubine. This is called Powe Management System in this thesis. Beside the fact that the DSFO-contolled DFIG has limited capacity fo the local gid suppot, the need fo a cental contol unit to detemine the powe efeence fo each individual DFIG may undemine the system eliability. It is noted that in the PDC stategy using the DSFO-contolled DFIGs, an Enegy Management System (EMS) is also necessay in ode to pevent the ES fom hitting its enegy limits in addition to the Powe Management System. Howeve, the ISFO-contolled DFIGs equipped with the doop chaacteistics enable the full contol of the local gid voltage and fequency and incease the system eliability by eliminating the need 62

75 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs fo a SWFC unit. The EMS, howeve, is still equied which is quite acceptable fo micogids [42] ISFO-contolled wind fam and micogids In an ISFO-contolled DFIG the electical toque cannot be contolled by i q, and is imposed by the load. In the case of an ISFO-contolled wind fam integated into a micogid, two scenaios can be consideed: with o without an extenal toque contol mechanism. ISFO-contolled wind fam without extenal toque contol mechanism Figue 4.2 illustates an example of an ISFO-contolled wind fam integated into a micogid without an extenal toque contolle. SS Main Gid Local Gid Local load Local load V,f ISFO- Wind Fam EMS AC/DC/AC ~ AG AC/DC DL Othe powe geneatos Figue 4.2. ISFO-contolled wind fam integated within micogid without extenal toque contol mechanism The micogid is assumed to contain the ISFO-contolled wind fam, an Auxiliay Geneato (AG e.g. diesel geneato), Dispatchable Loads (DL) and othe powe geneatos (such as PV, mico-tubines, combined heat and powe, etc). The AG and the DL ae used in the EMS which will be explained late. It is assumed in this 63

76 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs thesis that the local gid voltage and fequency is fully contolled by the ISFOcontolled wind fam, howeve in pactice, othe geneatos can also contibute to contol the voltage and fequency. Thee is no extenal mechanism to contol the electical toque and assuming a constant local load means that the DFIGs ae unde Constant Powe Mode (CPM). This implies that the wind powe vaiations ae eflected on the shaft speed of the DFIGs which inceases the isk of instability. In the next chapte a pitch contol is designed to keep the shaft speed within the stable egion when the demand powe is less than the extactable wind powe. If the demand powe appoaches the aveage of the extactable wind powe, the shotage of enegy is compensated by the enegy stoed in the shaft inetia causing a eduction in shaft speed. In this case the EMS is esponsible fo keeping the shaft speed within the stable egion. If the shaft speed deceases moe than a cetain theshold, the AG injects enegy in ode to maintain the demand powe. If the demand powe is too low, the shaft speed may inceases moe than a cetain level which is used to egulate the DL in ode to absob the excess of enegy. SS Main Gid Local load Local load Local Gid V,f ISFO- Wind Fam EMS AC/DC/AC ~ AG AC/DC Hydogen Geneation DL AC/DC Fuel Cell Othe powe geneatos Figue 4.3. ISFO-contolled wind fam integated within micogid without extenal toque contol mechanism with the DL as a hydogen geneation station 64

77 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs The DL can be anything fom esisto sets to hydogen geneation station o an iigation system. Figue 4.3 shows the DL as a hydogen geneation unit. In such a case the DL can also opeate as a long-tem ES. Fo instance, wheneve the local load is low (compaed to the extactable wind powe); the DL can geneate hydogen which late can be used in fuel cells (i.e. as an auxiliay geneato) in ode to minimize the fuel consumption of the AG when the demand is aised. The exta enegy can also be shed by the pitch contol. Thee is a tade-off between the powe ating of the DL and the maximum slew ate of the pitch contol. It will be shown in late chaptes that a slow pitch contol may esult in a elatively lage DL whilst a nomal pitch contol can eliminate the need fo a DL. It is noted that even in a standad DSFO-contolled DFIG unde MPT mode, the pitch angle is used to maintain the powe at the ated value and pevent the shaft speed fom exceeding the maximum limit fo wind speeds above ated. SS Main Gid Local load Local load Local Gid V,f ISFO- Wind Fam T e contol AC/DC Enegy Stoage AC/DC/AC ~ AG AC/DC DL Othe powe geneatos EMS Figue 4.4. ISFO-contolled wind fam integated within micogid with extenal toque contol mechanism (i.e. ES) 65

78 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs ISFO-contolled wind fam with extenal toque contol mechanism The second scenaio is shown in Figue 4.4 whee an ISFO-contolled wind fam is integated into a micogid with an extenal ES. The ES is used to contol the DFIGs electical toques. This can be eithe aggegated on the local gid o distibuted within individual wind tubines. The EMS, which is used to pevent the ES fom hitting its enegy limits, is based on the same logic as the fist scenaio. This stuctue enables the contol of DFIGs unde the standad MPT mode which is investigated in Chapte 7. In Chapte 6, which can be consideed as a tansition step fom Chapte 5 (CPM) to Chapte 7 (MPT), the micogid is the same as that of Figue 4.4 but without any extenal enegy souce (i.e. AG). Although this may sounds impactical, it is a suitable case study fo DFIGs unde a non-maximum powe tacking contol such as Constant Toque Mode (CTM). In the next section the ISFO-contolled DFIGs will be augmented by the classical fequency and voltage doops in ode to shae a demand active and eactive powe accoding to thei atings. 4.4 Voltage and fequency contol using classical doop chaacteistics Intoduction and applications The doop method, which has been used fo many yeas, is based on a well known concept in powe netwoks that consists of educing the fequency of the synchonous geneato when its output powe inceases [32]. The concept is also adopted in the contol of paallel invetes such as Uninteuptable Powe Supplies (UPS) [76-79]. The advantages of such a powe configuation include high eliability and no estiction on the physical location of the UPS units [80] since thee is no communication and units use only local vaiables. Similaly, the doop chaacteistics can be applied on Distibuted Enegy Resouces, including wind tubines, in ode to incease the system eliability and obtain autonomous opeation. This section discusses some diffeent applications involving a doop- 66

79 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs contolled wind fam. In section 4.4.2, an aay of wind geneatos ae augmented with doop chaacteistics in ode to shae a vaiable active and eactive load. The ISFO-contolled DFIGs augmented with doops do not equie a SWFC unit to set the efeence powe fo each DFIG; unlike the DSFO-contolled ones (section ). In othe wods, thee is no need fo powe management system; howeve, EMS is still equied which will be investigated in the following chaptes. Figue 4.5 shows an aay of ISFO-contolled DFIGs integated into a micogid. It is assumed, in this thesis, that the wind fam is the main powe supply of the system and is esponsible fo contolling the local gid voltage and fequency. Howeve, othe geneatos can also contibute to the local gid contol (using the doop chaacteistics), if equested. As Figue 4.5 illustates, the eactive powevoltage doop detemines the stato voltage magnitudes which in tun sets the efeence magnetising cuents i ms. The magnetising cuent is contolled by egulating i d (not shown in the figue). The active powe-fequency doop detemines the stato voltage fequency efeence f s which sets the stato flux angles θ s. 67

80 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs ES P g1,q g1 Main Gid ES SS.... i ms θ s 1/2πf s L 0 V s f s V s f s Q P Q 1 P 1 ~ AG DL P g2,q g2 ES Local gid P &Q Load θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 Figue 4.5. Local gid contol by an aay of ISFO-contolled DFIGs integated within a micogid The ES can be aggegated on to the local gid o distibuted and integated with the geneatos, eithe as shown in Figue 4.5 o affixed to the DC-link of each DFIG. The DL and AG ae contolled in ode to keep the enegy level of the ES within its limits. This chapte is intended to study an aay of ISFO-contolled DFIGs augmented with classical doops and assumes that the DFIGs ae diven at a constant shaft speed. This implies that the load powe is always less than the hypothetical available wind powe. Theefoe at this stage, the contol of the ES, DL and AG ae not consideed. Figue 4.6 shows anothe application in which an ISFO-contolled wind fam is buffeed fom the main gid by a set of powe electonic convetes. These ae effectively Powe Flow Contolles (PFC). The PFCs can contol the active powe flow to/fom the main gid in ode to pevent the enegy level of the ES fom hitting its uppe/lowe limits. In this case, thee is theoetically no need fo the DL 68

81 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs and AG. Howeve, the AG could be equied if the aim was to minimize the powe fom the main gid due to a highe pice of the main gid enegy. P g1,q g1.... θ s i ms 1/2πf s L 0 V s f s V s f s ES Q P Q 1 P 1 PFC P &Q Main Gid Load P g2,q g2 ES Local gid i ms θ s 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 Figue 4.6. A doop-contolled wind fam buffeed fom the main gid by Powe Flow Contolle (PFC) The method is also applicable to an ISFO-contolled wind fam connected to the gid though a HVDC-link. Both VSC- and LCC-HVDC connections ae possible. Figue 4.7 shows an aay of ISFO-contolled DFIGs connected to a VSC-HVDC link. In this scenaio the wind fam gid is contolled by the DFIGs athe than the sending end convete of the VSC-HVDC link. The demand powe is imposed by the d-axis cuent of the eceiving end convete as I, whee V DC is the voltage of the HVDC DC-link. The HVDC DC-link voltage is contolled by the d-axis cuent of the sending end convete which is the evese of the standad contol of VSC. d con P V DC 69

82 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs P g1,q g1 ES Sending end Receiving end.... θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 1 P 1 DL AG I d-con PI I d-con - 1/V DC Main gid P g2,q g2 ES Local gid V DC P θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 Figue 4.7. An ISFO-contolled wind fam equipped with classical doop chaacteistics and connected to the main gid though a VSC-HVDC link Figue 4.8 illustates an ISFO-contolled wind fam connected to the gid though a LCC-HVDC link. The local (wind fam) gid is contolled by the DFIGs utilising doop chaacteistics. Theefoe, neithe an extenal voltage souce, such as a STATCOM, no a SWFC unit (compaed to the DSFO-contolled wind fam explained in section ) is equied. The EMS, howeve, is still needed to contol the enegy level of ES within its limits using the DL and AG. The demand powe sets the efeence cuent of the HVDC DC-link as I, whee E 0 is the voltage of the HVDC link. The HVDC cuent is contolled by egulating the ectifie fiing angle α. The HVDC DC-link voltage is contolled by the HVDC invete. This case will be simulated late in this chapte to illustate the doop contol. 0 P E0 70

83 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs Rectifie Invete P g1,q g1 I i ms θ s 1/2πf s L 0 V s f s V s f s ES Q P Q 1 P 1 P DL AG 1/E 0 I 0 - α PI E 0 - Main gid P g2,q g2 ES Local gid I 0 i ms θ s 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 Figue 4.8. An ISFO-contolled wind fam equipped with classical doop chaacteistics and connected to the main gid though a LCC-HVDC link Pio to applying the doop method on an aay of ISFO-contolled DFIGs, the following subsection eviews the theoy behind the doop chaacteistics in bief Voltage and fequency doops Each geneato (in this case DFIG), can be epesented as a voltage souce connected to a common bus (the local gid) though a decoupling impedance Z as shown in Figue 4.9 [32, 79]. S P jq Z V 1 V 2 0 Figue 4.9. Geneato connected to the local gid though an impedance The complex powe flowing fom the geneato to the common bus can be witten as [77]: 71

84 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs S P V e jq V Ze V V V Z j j( ) 2 e j (4.1) whee V 1 and V 2 ae the amplitude of the geneato voltage and the common bus voltage, δ is the powe angle, and Z and θ ae the magnitude and the phase of the geneato output impedance. Using (4.1), one can wite: 2 V2 Z e j V1V P Z 2 V1V Q Z 2 V2 cos( ) Z V2 sin( ) Z 2 2 cos sin (4.2) Equation (4.2) shows that the output impedance affects the elation between the voltage amplitude o phase diffeence and the active and eactive powe components ciculating between the geneatos [81]. Assuming a mainly inductive output impedance (i.e. Z=jX, θ=90º), the following well-known expessions of active and eactive powe can be deived fom (4.2) [77, 78]: V1V P X 2 sin V1V 2 cos V2 Q X 2 (4.3) Fom (4.3), and assuming small powe angles ( cos 1 and sin ), it can be seen that the active powe is stongly dependent on the powe angle δ, and the eactive powe is mainly dependent on the diffeence between the voltage magnitudes [79]. Obviously the contol of the fequency dynamically contols the powe angle. Theefoe, the fequency can be contolled by active powe while the geneato voltage amplitude can be contolled by eactive powe. This concept leads to the well-known fequency and voltage doop equations: f V f V 0 0 mp nq (4.4) 72

85 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs whee f 0 and V 0 ae the output voltage amplitude and fequency at no load (set point), and m and n ae the fequency and voltage doop coefficients espectively. As a geneal ule, the lage the doop coefficients, the bette the powe shaing would be at the expense of degading the voltage and fequency egulation. Usually, the maximum acceptable deviation of voltage and fequency ae ΔV=5% and Δf=2% espectively [79]. If the voltage o fequency dops moe than this level, thei set point values can be inceased (using communication o a vey slow bandwidth contol) in ode to compensate fo the eduction. f V f 0 V 0 f V (a) P max P (b) Q max Q Figue Doop chaacteistics (a) fequency-active powe (b) voltage-eactive powe Figue 4.10 illustates the doop chaacteistics basing on (4.4). The doop coefficients can be calculated as: m f P max V n Q max (4.5) whee P max and Q max ae the maximum active and eactive powe (usually the geneato ated value), and Δf and ΔV ae the maximum acceptable fequency and voltage deviations. Usually, Δf and ΔV ae chosen to be equal fo all the units: m P 1 n Q 1 max1 max1 m P n Q 2 2 max 2 max 2... f... V (4.6) 73

86 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs Choosing the doop coefficients accoding to (4.6) ensues that the active and eactive powes dawn fom each geneato ae shaed by them accoding to thei atings [76]. It should be noted that (4.4) is only tuly valid fo mainly inductive output impedance, which is usually the case. This is because of the lage filteinductance and small esistance of ovehead lines [32, 81]. Howeve, in the case of esistive o capacitive output impedance, it can be shown that the active powe becomes mainly dependent on voltage while eactive powe can be egulated by fequency [80] i.e. the evese of the inductive impedance case. In this thesis the output impedance is assumed to be inductive, howeve, the esistive o capacitive ones seem applicable as well. Active powe-fequency doop: The stato voltage fequency is contolled by the stato flux angle θ s though the fee unning integation of the fequency efeence value f s. Substituting the efeence fequency with the f-p doop equation given in (4.4), yields: f s dt 2 ( f s 2 0 mp) dt (4.7) Using (4.7), the active powe-fequency doop contol scheme is shown in Figue P (measued) m - f s 2π θ s f 0 =50Hz Figue Active powe-fequency doop contol scheme In this thesis the fequency set point will be always set at 50Hz, howeve, in pactice the set point can incease in ode to compensate fo the fequency dop. Reactive powe-voltage doop: The stato voltage magnitude is contolled by egulating the magnetising cuent i ms which in tun is contolled by i d. Neglecting the stato esistance, the stato 74

87 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs voltage can be witten as Vs e s whee ω e and φ s ae the electical angula fequency and the stato flux. Since s L 0 i ms, one can wite Vs el0ims. Using the Q-V doop equation given in (4.4), one can deive: i ms i ms0 n L e 0 Q (4.8) Figue 4.12 illustates the eactive powe-voltage doop contol scheme basing on (4.8). Q (measued) n e L 0 - i ms i ms0 V L0 snom enom Figue Reactive powe-voltage doop contol scheme whee i ms0 is the magnetising cuent at no load which is usually set as i ms0 V L0 snom enom whee V s-nom and ω e-nom ae the nominal values of the stato voltage magnitude and the electical angula fequency espectively. Howeve if necessay, i ms can become moe than the nominal value in ode to compensate fo the voltage eduction. Although i ms0 fo DFIGs with diffeent ating is diffeent, choosing doop coefficients using (4.6) leads to the same stato voltage fo any given eactive powe. In the next section the doop chaacteistics will be applied on two ISFOcontolled DFIGs connected to a LCC-HVDC link. 75

88 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs Simulation of a doop-contolled wind fam connected to a LCC-HVDC link This section simulates two ISFO-contolled DFIGs equipped with the doop chaacteistics as shown in Figue pu Local gid (33kV) Rectifie Invete I 0 4-step filte α E pu θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 1 P 1 θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 P 1/E 0 0.2pu S N F I 0 - I 0 PI Figue A ISFO-contolled wind fam augmented with doop chaacteistics and connected to the gid though a LCC-HVDC link The wind fam is connected to the gid though a LCC-HVDC link. The fist DFIG is 0.66pu and the second one is 0.34pu with paametes given in Appendix B. It should be noted that pu epesent the pe unit value based on the total wind fam ating. Fom now on in the thesis, pu gen will stand fo the pe unit value based on the ating of the associated DFIG in multi DFIGs systems. The demand powe is imposed by egulating the HVDC DC-link cuent I 0 which in tun is contolled by α. In the nomal opeation I 0 =P /E 0. The paametes of the HVDC DC-link cable ae given in Appendix B. The contol of the HVDC cuent loop is explained in [4] and is beyond the scope of this thesis. The ated value of the HVDC DC-link cuent is 2kA with the ated voltage of 500kV. The invete contols the HVDC DC-link voltage and is eplaced by a DC-voltage souce [4, 48] E 0 =490kV. A 4- step (0.25, 0.5, 0.75, 1pu) switchable AC-filte is designed, with paametes given 76

89 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs in Appendix B, in ode to absob the 11 th and 13 th hamonic cuents and also compensate fo the ectifie eactive powe demand. In this simulation, in ode to investigate the effect of the doop chaacteistics, the DFIGs ae diven in a constant shaft speed. This implies that the demand powe is always less than the hypothetical wind powe. Howeve, eal wind speed pofiles will be applied thoughout the following chaptes. Both nomal and fault ide-though opeations ae simulated. In the nomal opeation the switch S, shown in Figue 4.13, is in position N while in the fault opeation the switch is in position F. This will be explained below. Simulation esults of nomal opeation This section compaes the active and eactive powe shaing with and without the doop chaacteistics explained above. Figue Compaison between active and eactive powe shaing with and without doop chaacteistics fo an ISFO-contolled wind fam connected to a LCC-HVDC link 77

90 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs In both cases the active powe demand is inceased fom 0 to 1pu in fou steps, as shown in Figue 4.14a. The eactive powe demand by the ectifie is Q =P tan(α) which is mainly supplied by the AC-filtes. The eactive powe demanded fom the wind fam (Figue 4.14b) mainly coves the eactive powe absoptions in the tansfomes which incease as the demand active powe ises. Figue 4.14c and Figue 4.14d illustate the active and eactive powe shaing without doop. As it can be seen, befoe 1sec, duing which no powe is demanded, the output powe of the DFIGs ae non-zeo. This implies that thee is ciculating active and eactive powe between DFIGs. Moeove, Figue 4.14c shows that the fist DFIG is oveloaded, when the demand powe is 1pu, while the second DFIG geneates fa less than its ated value. The situation fo eactive powe shaing without doop is even wose and the second geneato geneates moe than the fist one until 3sec. Figue 4.14e and Figue 4.14f show the active and eactive powe shaing using the poposed doop chaacteistics with the doop coefficients given intable 4.1. Rating, pu m, Hz/pu n, kv/pu DFIG DFIG Table 4.1. Doop chaacteistics coefficient of DFIGs It can be seen that thee is no ciculating active and eactive cuent. Futhemoe, applying the doops, the active and eactive powe ae shaed in popotion to thei atings: P 1 P2 Q1 Q These impovements have been achieved at the expense of a vey small fequency and voltage deviations as is shown in Figue 4.14g and Figue 4.14h, espectively. The tansient voltage and fequency fluctuations ae due to the switching effects of the AC-filtes. In this simulation, the eactive powe fom the DFIGs coves only the losses in the tansfomes eactance. Howeve in pactice, the DFIGs may need to cove the excess/shotage of eactive powe between each filte level and its uppe/lowe level. Assuming the 4-level filte of Figue 4.13, the eactive powe unbalance will be always less than 0.15pu. Theefoe in the simulations caied out in the following chaptes the eactive powe demanded by the load is set at 0.15pu. 78

91 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs Fault ide-though opeation This section consides an ISFO-contolled wind fam connected to a LCC-HVDC link as shown in Figue The ide-though of loss of the AC-main gid, in which the powe has no whee to go, is simulated. The fault, as illustated in Figue 4.13, is simulated by a shot cicuit on the HVDC at the invete end i.e. the voltage of the DC-voltage souce epesenting the invete dops to 0kV. The fault lasts fo 150msec. The objective of this simulation is to demonstate that the ISFOcontolled wind fam equipped with the doops have the ability to ide-though without any communication. Figue Fault ide-though fo an ISFO-contolled wind fam augmented with doops Since the AC-filtes (see Figue 4.13) cannot be switched fast enough following the fault, maintaining some DC cuent demand is useful in ode to keep the eactive powe demand of the ectifie. Theefoe, following the fault, the switch S in Figue 4.13 is switched to position F which imposes a DC-cuent efeence of 79

92 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs 0.2pu. Howeve, this is not a mandatoy equiement and will only affect the dynamic of the ide-though esponse and not the natue of it. It is noted that opeation with constant shaft speed may affect the dynamics of the ide-though esponses as the powe unbalance is only eflected on the voltage and fequency. The simulation esults ae given in Figue Befoe the fault occus (at 3sec), the demand powe is 1pu. It can be seen that afte the fault is cleaed the active (Figue 4.15a) and the eactive (Figue 4.15b) powe ecove to thei pe-fault values with no communication. The active and eactive powe shaing using doops also etuns to nomal afte fault cleaance. Figue 4.15e and Figue 4.15f show that the fequency and voltage contols ae estoed while thei vaiations duing the fault ae about 2% and 6% espectively, which ae acceptable. In the late chaptes a simila fault ide-though scenaio will be simulated (without the HVDC-link) while a eal wind speed pofile is applied. 4.5 Discussions and conclusions This chapte eviewed the diffeent functionalities that may be equied fom a wind fam including the active powe-fequency and the eactive powe-voltage contol. Cuent methods fo suppoting the local gid voltage and fequency have been discussed. In all of these methods, an extenal voltage and fequency souce is still equied. It was emphasised that a conventional DSFO-contolled wind fam has limited capacity fo suppoting the local gid fequency accoding to the level of the active powe eseved. Howeve, an ISFO-contolled wind fam augmented with doop chaacteistics has the potential to fully contol the local (wind fam) gid voltage and fequency. The diffeent applications of a doop-contolled wind fam including AC gid connection, HVDC connection, and integation within a micogid has been discussed. This chapte has addessed the contol of an aay of ISFO-contolled DFIGs with classical doop chaacteistics. The active and eactive powe shaing using doops has been compaed with that of without doops. It was demonstated, using PSCAD simulations, that the doops eliminate the active and eactive powe ciculations and foce the DFIGs to shae the load in popotion to thei atings. It 80

93 4 Local gid voltage and fequency contol using ISFO-contolled DFIGs was shown though PSCAD simulation that a doop-contolled wind fam is inheently able to ide-though loss of the gid with no need fo communication. It is poposed that an aay of ISFO-contolled DFIGs augmented with doops is a moe suitable option fo integation into a micogid than that contolled unde DSFO. This is due to thei ability to contol the local gid voltage and fequency and adjust thei opeating point using only local measuements, with no cental powe contol communication. Howeve, enegy contol communication is still needed in ode to contol the DFIGs shaft speed and/o to keep the enegy level of the ES within its limits. This is the subject of the following chaptes. This chapte assumed that the demand powe is always less than the extactable P ext wind powe. In pactice the demand powe may be moe than P ext. In that case an ES and/o extenal enegy souce is needed to compensate fo the enegy shotfall. The next chapte poposes an enegy management system using an extenal enegy souce while the contol of ES will be intoduced in Chapte 6. An enegy management system consisting of both ES and an extenal enegy souce is consideed in Chapte 7. The DFIGs equipped with the doop contol in this chapte shae the demand powe in popotion to thei atings, egadless of the available wind powe. The next chapte adjusts the doops in ode to take the available wind powe into account. It will be shown that the impoved doop can significantly educe the enegy equied fom an extenal souce since it enables the DFIGs with sufficient extactable wind enegy to compensate fo those with insufficient extactable wind enegy. 81

94 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5. Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5.1 Intoduction This chapte studies an aay of ISFO-contolled DFIGs equipped with doop chaacteistics. The diffeent applications of such a contol method, including AC gid connection, HVDC connection, and integation into a micogid, wee investigated in the pevious chapte. This chapte consides the micogid application without a diect toque contol mechanism, as discussed in section and shown in Figue 5.1. SS Main Gid Local Gid Local load Local load V,f ISFO- Wind Fam EMS AC/DC/AC ~ AG AC/DC DL Othe powe geneatos Figue 5.1. Integation of a doop-contolled wind fam into a micogid without extenal toque contol mechanism In this scenaio the DFIGs contol the local gid voltage and fequency and shae the local load though the doop method. The wind tubine inetia opeates as a shot-tem ES and thee is no extenal ES. Theefoe, the shaft speed vaies accoding to the demand load and the available wind powe. This means that the 82

95 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES shaft speed can be consideed as an indicato fo the excess o shotfall of enegy. In ode to keep the shaft speed within its limits, an Enegy Management System (EMS) is equied which will be explained in this chapte. It is noted that the EMS is not an exta equiement fo this system and is also needed fo the standad DSFO-contolled wind fam with ES. The EMS consists of an Auxiliay Geneato (AG) and a contollable o Dispatchable Load (DL). The AG is used to compensate fo the lack of the enegy when the shaft speed deceases too much. On the othe hand, if the shaft speed inceases too much, the DL is utilized to absob the exta enegy. The DL can be sets of esistos, an iigation system, a hydogen geneation station, etc. It will be shown in Chapte 7 that using a pitch angle contol with slew ate up to 3-5º/sec can eliminate the need fo a DL. Howeve, the DL as hydogen geneato combined with fuel cells, fo example, can be used as a long-tem ES to educe the fuel consumption of the AG (Figue 5.1). This last scenaio is out of the scope of this thesis. It will be shown in Chapte 8, that the DL can also be distibuted among the DFIGs which might be a bette place fo esistive DLs Constant Powe Mode (CPM) contol It was discussed in Chapte 2 that ISFO-contolled DFIGs usually equie an extenal mechanism to contol thei electical toque T e. In [44] an auxiliay load is used to contol T e and the next chapte will contol the electical toque by egulating the ES inteface (ESI) eal cuent. This chapte deals with the case that thee is no extenal toque contol mechanism; hence the electical toque is imposed diectly by the load. Such a case is depicted in Figue 5.2. Assuming that no enegy is injected/absobed by the AG/DL shown in Figue 5.2, the DFIGs ae unde CPM fo a constant load demand. As agued in section 2.3.3, the CPM has the widest shaft speed vaiation but the system may be exposed to instability. A pitch angle contol will be designed in this chapte to contol the shaft speed within the stable egion fo demand powes which ae always less than the extactable wind powe (Figue 5.1a). P ext 83

96 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES P 1,Q 1 Main Gid SS.... i ms θ s 1/2πf s L 0 V s f s V f Q P Q 1 P 1 PFC ~ AG DL P 2,Q 2 P &Q Local gid Load θ s i ms 1/2πf s L 0 V s f s V f Q P Q 2 P 2 Figue 5.2. ISFO-contolled DFIGs contolling local gid using doop chaacteistics If the demand powe appoaches the aveage of the extactable wind powe P ave (Figue 5.3b), ES and/o an AG is equied to inject the enegy shotfall wheneve P ext the demand powe is moe than. P ext (β=0) P ext (β=0) P ave P P ave P (a) (b) Figue 5.3. Diffeent situations of demand powe in espect to extactable wind powe P ext This chapte consides a system that uses an AG to addess the shotfall. The integation of the ES will be intoduced in Chapte 6. 84

97 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5.2 Constant demand powe delivey using pitch contol This chapte designs a pitch contolle in ode to delive a constant powe demanded by load as shown in Figue 5.4. It will be designed fo one DFIG and validated with two DFIGs equipped with doop. V w β CPM Pitch contol P=P ω V w P &Q i ms 1/2πf s L 0 V s Vaiable Load θ s f s Figue 5.4. Pitch angle contol fo deliveing a constant demand powe It is emphasized again that the method of pitch contol alone is only applicable if P is less than P ext (Figue 5.3a). In [82] a pitch contol was designed in ode to smooth the output powe. The efeence powe fo the pitch contolle is deived fom the aveage wind speed calculation. Howeve in this thesis the efeence powe is imposed by the load. 85

98 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES P t P ave V w V w Stable egion P β=0 β β opt ω stb ω opt ω ω Figue 5.5. P t -ω chaacteistics fo a constant wind speed and diffeent pitch angle Assuming a constant wind speed, Figue 5.5 illustates the tubine powe vs shaft speed chaacteistics fo diffeent pitch angles. Fo a given demand powe thee is only one pitch angle β opt which is tangential with P. The shaft speed associated with β opt and P is called the optimum shaft speed ω opt. If P P ave, thee is only one option fo the efeence shaft speed ω which is its coesponding ω opt. Howeve in pactice (i.e. eal wind speed) the demand powe cannot appoach P ave unless an ES and/o an AG is used. The contol stuctue using an AG is coveed in section 5.3. As Figue 5.5 illustates, the intesection point of the demand powe and the efeence shaft speed lines detemines the efeence pitch angle β. The calculation of β fo a given P and ω is explained late in this section. The choice of the efeence shaft speed will be discussed late in this chapte. The minimum stable shaft speed fo a given demand powe and wind speed is denoted by ω stb. Since thee is no diect toque contol mechanism, the electical toque is imposed by the load, hence the electical toque effectively is system can be simplified as depicted in Figue 5.6. T P e. Theefoe the 86

99 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES V w Wind tubine Eq(5.1) T m T m T J e ω β T e / R β P Blade sevo system 1 1 s - β β Cal. δβ ω β cmd Pitch contolle δω - Figue 5.6. Simplified block diagam of system fo contol pupose As shown in Figue 5.6, the pitch angle contol system consists of thee main pats: β calculation, pitch contolle, and sevo system. The β calculation unit, as its name suggests, is esponsible fo calculating the efeence pitch angle fo a given demand powe and efeence shaft speed. The Pitch contolle is based on the wind tubine lineaized model and contols the states of the system to thei desied values. The sevo system follows the pitch angle command β cmd povided by the pitch contolle. The sevo system is modelled by a fist ode lag [82, 83] and a ate limite in ode to make sue that the pitch angle cannot vay faste than the allowed ate R β. The next two subsections explain the β calculation and pitch contolle units. 87

100 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Calculation of the efeence pitch angle In ode to calculate the efeence pitch angle, a wind tubine model is needed. In this thesis the PSCAD wind tubine model, which was explained in section 2.3.1, is used. The model is descibed by (5.1): C p e NV w 0.17 (5.1) whee C p and λ ae the powe coefficient and tip speed atio espectively. As Figue 5.5 shows, fo and, the tubine powe is equal to the demand powe: P P t 0.5AV 3 w C p C p P 0.5AV 3 w (5.2) The C p is called efeence powe coefficient. Similaly the tip speed atio coesponding to the efeence shaft speed can be called efeence tip speed atio λ : NV w (5.3) Substituting (5.2) and (5.3) into the powe coefficient equation given in (5.1) and then simplifying it fo β, yields: e 2C p 0.17 (5.4) 88

101 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Pitch angle contolle The wind tubine chaacteistic is nonlinea. In ode to design a contolle the wind tubine model needs to be lineaized. This appoach is based on the assumption that small signal stability acoss the opeational envelope esults in global stability. Although this is not necessaily tue fo all nonlinea system, it is nomally the case fo nonlinea systems without discontinuities. One can lineaize (5.1) as: N V 0 C p w NV e ( ) e 0 w0 0 0 (5.5) whee vaiables with suffix 0 epesent the lineaization point. The tubine powe and tubine toque equations can also be lineaized as follows: P 0.5AC V T t m p Pt T 3 w m P 1 t AC V V 0.5 AV p0 P P t t0 2 0 w0 w w0 C p (5.6) Equations (5.5) and (5.6) can be simplified as: K V C t T p m 1 w K K P K V 5 3 w K P K 7 t K 2 4 K C 6 8 p (5.7) Equations (5.7) can be futhe simplified as: Tm M1Vw M 2 M 3 (5.8) whee M ( K K K K K 1), M K K K K ) and K Equation (5.8) is the wind tubine lineaized model. 2 ( K8 M 3 K7K6K4 89

102 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 90 As Figue 5.6 shows, the system is second ode. Thee ae two states vaiables which ae the pitch angle and the shaft speed. Assuming that aound the opeating point the change of the pitch angle is less than R β, the ate limite can be neglected. Theefoe the blade diffeential equation would be: (5.9) The second diffeential equation is: (5.10) Equation (5.10) can be lineaized as: (5.11) Substituting (5.8) into (5.11) and simplifying that, gives: (5.12) Using (5.9) and (5.12), one can wite the space state equations as: (5.13) Whee the state vaiables and input vaiables ae espectively: (5.14) The state matix and the input matix ae espectively: cmd cmd s 1 1 m P T e m P T J T T J e 1 1 m P P J T J P J M J P J M V J M w u D x C y u B x A x w cmd V P u x, 0 A 0 B

103 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES A 0 B 0 1 M 3 J 1 0 M J 0 1 J 2 0 J 0 P 2 J0 0 0 M 1 0 (5.15) By choosing shaft speed as output, the output matix C 0 and the diect tansmission matix D 0 would be: 0 0 1, C D 0 (5.16) The contol and efeence inputs ae δβ cmd and δω, espectively. The demand powe δp and wind speed δv w ae distubances and theefoe ae not included in the contol law given by: K f (5.17) The state feedback matix is K f K f1 K f2 and is designed to place the system closed loop poles at thei desied place. Figue 5.7 shows the pitch angle contolle (of Figue 5.6) using the contol law given in (5.17). δβ K f1 δβ cmd β cmd δω K f2 β Figue 5.7. Pitch angle contolle 91

104 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES In Figue 5.7, K f1 and K f2 ae the sate feedback gains which ae detemined in ode to place the system closed loop poles at thei desied locations. Since all the states vaiables ae measuable, thee is no need fo an obseve. The necessay and sufficient condition that the closed loop poles can be placed at any abitay locations in the s plane is that the system be completely state contollable [84]. Geneally a system descibed by x Ax Bu whee A is an n n matix and u matix is an -vecto, is completely state contollable if the vectos) [84]. In this case the that it is state contollable. 92 contollability is of ank n, (i.e. contain n linealy independent column contollability matix is of ank 2 which means The system open loop poles, which ae the eigenvalues of the matix, may change fo diffeent lineaization points. Fo this study it is felt sufficient to deive a single contol law that will give acceptable pefomance fo all the lineaization points (the altenative appoach would be to have a gain-scheduling pocedue). Pio to choosing the desied closed loop pole, the wost-case lineaization point is detemined. Theefoe, fist the vaiations of the system eigenvalues fo the diffeent lineaization points ae studied in ode to find the wost case fo lineaization. BAB... A n1 B 28 Thee ae two open loop poles: one is associated with the pitch angle and is always at 1 and the othe is associated with the shaft speed and vaies with the lineaization points. So the fist eigenvalue is always stable. The lineaization point is defined by P 0, V w0, and ω 0. Figue 5.8 shows the movement of the open loop poles when P 0 inceases fom 0.2 to 1pu (in fou steps), V w0 =13m/sec, ω 0 =1.1pu, and τ β =1. It can be seen that as powe inceases the system tends towads instability and the system is unstable fo powe moe than appoximately 0.75pu. It is noted that even fo P 0 =1pu, the pitch angle β 0 4º (using (5.4)) which implies that the closed loop system can be stable using a pope contol design. In othe wods, all these lineaization points ae within the stable egion shown in Figue 5.5. Fo unstable egion (ω <ω stb ), pitch angle contol cannot be used to stabilize the system since that egion coesponds to Figue 5.3b i.e. when the extactable wind powe is less than the demand powe. n n A 0

105 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue 5.8. Movement of system open loop poles fo vaiation of P 0 fom 0.2 to 1pu while V w0 =13m/s, ω 0 =1.1pu and τ β =1 Figue 5.9 shows the open loop poles movements when V w0 vaies fom 10 to 16m/sec, P 0 =0.5pu, ω 0 =1pu, and τ β =1. Figue 5.9. Movement of system open loop poles fo vaiation of V w0 fom 10 to 16m/sec while P 0 =0.5pu, ω 0 =1pu and τ β =1 Figue 5.9 illustates that as the wind speed deceases the second eigenvalue moves towad instability. Again, closed loop contol can addess this povided that the extactable wind powe is not less than the demand powe. 93

106 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Movement of system open loop poles fo vaiation of ω 0 fom 0.7 to 1.3pu while P 0 =0.5pu, V w0 =12.5m/sec and τ β =1 Finally the movements of the open loop eigenvalues when ω 0 changes fom 0.7 to 1.3pu (in 6 steps), P 0 =0.5pu, V w0 =12.5m/sec and τ β =1 is shown in Figue As can be seen, the second open loop pole is unstable fo shaft speed less than appoximately 0.95pu. Figues 5.8, 5.9 and 5.10 demonstate that as P 0 inceases and V w0 and ω 0 decease, the system tends towad instability. Theefoe one can set the lineaization point as: P 0 =1pu, ω 0 =1.2pu, V w0 =12.5m/sec and hence β 0 =1.6º. At this point since P, and the pitch angle is small. Theefoe this P 0 ave 0 opt point can be consideed as the wost case. It is emphasized again that all the lineaization points consideed above ae within the stable egion of Figue 5.5. Theefoe the closed loop pitch angle contol can make them stable. The points outside the stable egion (shown in Figue 5.5) coespond to the case when the demand powe is moe than P ext which will be addessed in Section 5.3 using an AG. Knowing the lineaization point, the desied closed loop pole can be detemined and hence the pope state feedback gains can be found. The state feedback gains, of couse, vay fo DFIGs with diffeent atings. In the next subsection two DFIGs with diffeent atings ae consideed. 94

107 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES The state feedback gains ae detemined accoding to the desied closed loop poles. As a geneal ule, the system esponse becomes faste as the dominant closed loop poles ae placed futhe fom the jω axis. It is noted that a high-speed esponse equies lage amount of contol enegy which means that heavie actuato is needed. Thee ae a numbe of methods fo finding pope K f1 and K f2 including, the pole-placement and the quadatic optimal egulato methods. The latte has the advantage of detemining the desied closed loop poles such that it balance the acceptable esponse and the amount of the contol enegy equied [84]. Howeve, in this case the pitch angle vaiation is ate limited. It will be shown that it neve vaies faste than 5º/sec, which is a decent maximum ate of change fo the blades. This implies that the equied contol enegy is acceptable. Theefoe in this eseach the pole-placement method is used. The next subsection demonstates the contol stuctue using PSCAD simulations Simulation esults of constant demand powe delivey using pitch angle contol This section consists of thee pats. The fist pat validates the poposed pitch angle contolle using one ISFO- contolled DFIG and constant wind speed. The second simulation consides anothe ISFO-contolled DFIG with a diffeent ating, but this time with eal wind speed pofile. The thid pat simulates the two DFIGs (of the two pevious simulations) equipped with doops and the pitch contol scheme explained above while eal wind speed pofiles ae applied. Simulation 1: This section consides one ISFO-contolled DFIG connected to a vaiable load as shown in Figue 5.4. The voltage and fequency is contolled by the DFIG. The DFIG1 with paametes povided in Appendix B is simulated in this section. The objective of this simulation is to validate the poposed contol stuctue. Theefoe constant wind speeds ae used in ode to be able to cove a wide ange of cases. 95

108 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES t 0-5s 5-12s 12-20s 20-28s 28-36s 36-46s 46-57s 57-75s V w, m/s P, pu ω ef, pu Table 5.1. Sequence of events of the simulation shown in Figue 5.11 Using the lineaization point explained above (i.e. P 0 =1pu, ω 0 =1.2pu, V w0 =12.5m/sec) and setting τ β =1 [82, 83], the open loop poles ae -1 and The desied closed loop poles should be detemined such that they assue a welldamped esponse without hitting the maximum slew ate which is R β =3º/sec. Tial and eo shows that setting the closed loop poles at and -1.5 gives a easonably fast and well-damped esponse. Theefoe state feedback gains K f1 and K f2 ae set espectively at 0.65 and using MATLAB pole-placement command. Table 5.1 explains the sequence of events of the simulation esults shown in Figue In each column, the bolded numbe epesents the paamete which has been changed compaed to the pevious peiod. It can be seen that a vey wide ange of diffeent possible situations has been simulated. In each situation the pitch angle (Figue 5.11c) is adjusted in ode to contol the shaft speed (Figue 5.11.d), hence maintaining the demand powe (Figue 5.11b). The tansient fluctuations in the pitch angle, which is still less than 3º/sec, ae due to the step changes in the wind speed and efeence shaft speed. In the case of the demand powe (Figue 5.11b) changes (at 20s and 46s), a fist ode filte with time constant of τ=0.5 is intoduced to educe the ate of the change in powe demand. This esults in smoothe pitch angle vaiations. Obviously in pactice, step changes in wind speed and shaft speed ae not possible. The last pat of the simulation (57-75s) coesponds to the lineaization point (i.e. P =1pu, V w =12.5m/sec and ω =1.2pu). It can be seen that the pitch angle is < 2º which means that the demand powe is almost equal to the extactable wind powe. 96

109 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Validation of the pitch contol fo constant powe delivey In a case with eal wind pofile, howeve, it is not possible to appoach this close to P ave unless using ES and/o an extenal enegy souce. This is shown in the next simulation esults. Simulation 2 The model simulated in this section is shown in Figue 5.4 but this time a eal wind pofile is consideed. In this pat the DFIG2 with paametes given in Appendix B is used. The diffeent ating (compaed to the pevious simulation) esults in diffeent state feedback gains fo the same lineaization point. The lineaization point is again P 0 =1pu, ω 0 =1.2pu, and V w0 =12.5m/sec. Setting the 97

110 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES desied closed loop poles at -1.5 and leads to the state feedback gains K f1 and K f2 equal to 0.65 and -0.59, espectively. The efeence shaft speed is 1.1pu (the choice of which will be discussed late in this chapte). The maximum pitch angle slew ate is R β =3º/sec. The PSCAD simulation esults ae given in Figue Figue One ISFO-contolled DFIG deliveing a constant demand powe while eal wind speed is applied and the demand powe appoaching the aveage of the extactable wind powe The wind speed pofile (Figue 5.12a) is a eal (measued) wind speed with P ave aveage of appoximately 12.5m/s ( 1pu ) and standad deviation of 1.39 (which is elatively lage petubation). The extactable wind powe (i.e. with β=0) is shown in Figue 5.12b1. The demand powe (Figue 5.12b2) is 0.6pu and it P ext can be seen that P ext is occasionally less than the demand powe. Wheneve the demand powe is moe than P ext, the pitch angle (Figue 5.12c) becomes zeo; 98

111 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES hence the shaft speed contol is lost. Theefoe, the shaft speed (Figue 5.12d) educes in ode to compensate fo the enegy shotage. Although in this simulation the shaft speed ecoves as the wind speed inceases, this is not a secue opeation since failue will occu if the shaft speed keeps dopping. This case will be addessed in section 5.3. It can be seen that in nomal opeation (demand powe less than P ext is vey well contolled. ), the pitch angle vaiation baely hits the 3º/sec while the shaft speed Simulation 3 This pat simulates the two ISFO-contolled DFIGs of the two pevious simulations as shown in Figue The wind fam supplies a vaiable load and contols the voltage and fequency of the local gid using the doop chaacteistics explained in the pevious chapte. 99

112 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES V w1 β 1 CPM Pitch contol P 1 ω 1 V w1 P 1,Q 1 i ms θ s 1/2πf s L 0 V s f s V s f s Q P Q 1 P 1 P &Q Vaiable Load V w2 β 2 CPM Pitch contol P 2 ω 2 V w2 P 2,Q 2 Local gid (33kV) θ s i ms 1/2πf s L 0 V s f s V s f s Q P Q 2 P 2 Figue Doop-contolled DFIGs opeating in CPM The objective the simulation is to validate the poposed pitch contol while the DFIGs ae unde doop contol and eal wind speed pofiles ae applied. The doop coefficients ae given in Table 5.2: Rating, pu m, Hz/pu n, kv/pu DFIG DFIG Table 5.2 Doop chaacteistics coefficient of DFIGs The DFIGs shae a vaiable load which is simulated by a vaiable cuent souce. The ating of the fist DFIG is 0.66pu and that of the second one is 0.34pu. The pitch contolles of the DFIGs wee explained in the two pevious simulations. The simulation esults ae given in Figue The aveage of the two eal wind speed 100

113 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES pofiles (Figue 5.14a) is appoximately 12.5m/s which coespond to P ave 1pu gen. The standad deviations of the fist and the second wind pofiles ae, espectively, 1.28 and 1.39 which ae elatively lage petubations. Appendix E explains how the chaacteistics of the wind speed pofiles can vay. Figue 5.14b shows the active powe shaing when the demand powe is P =0.1, 0.3, and 0.5pu (total wind fam pu). It can be seen that the powe demanded by the load is shaed popotional to the ating of DFIGs while the local gid fequency (Figue 5.14f2) is vey well egulated at appoximately 50Hz. The fist DFIG pitch angle (Figue 5.14e1) neve hits the maximum slew ate of 3º/sec and this is why its shaft speed (Figue 5.14d1) is vey well contolled. When the demand powe is 0.5pu, howeve, the pitch angle of the second DFIG (Figue 5.14e2) becomes occasionally almost zeo. This implies that the powe geneated fom the second DFIG becomes vey close to (o even slightly moe than) its associated P ext. This causes the pitch angle to hit the 3º/sec limit which in tuns causes the lage shaft speed vaiation. Although the shaft speed is still within the acceptable bounday fo DFIGs, it suggests that the demand powe cannot incease anymoe unless an extenal ES and/o enegy souce is intoduced. This is the subject of the next section. Figue 5.14c shows that the eactive powe demanded by the load (Q =0.15pu) is shaed by the DFIGs in popotion to thei atings while the local gid voltage is vey well contolled aound 33kV. 101

114 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Results of two doop-contolled DFIG opeating in CPM and supplying a vaiable load The next section poposes an Enegy Management System (EMS) in ode to contol the shaft speed of the DFIGs. In section 5.3.2, the doop method will be adjusted in ode to take the available wind powe into account, in addition to the ating of the DFIGs. 102

115 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5.3 Constant demand powe delivey using extenal enegy souce Enegy Management System (EMS) It was shown in the pevious section (Figue 5.12) that when the demand powe becomes moe than P ext, the kinetic enegy stoed in the shaft inetia compensates fo the lack of the enegy which obviously causes eduction in the shaft speed. If the shaft speed deceases below ω stb (Figue 5.5), the system will become unstable. Theefoe this situation must be avoided. Figue 5.15 poposes an EMS in ode to maintain the shaft speed within the stable egion. V w β CPM Pitch contol P g ω V w P L =P P g P DL P DG i ms θ s 1/2πf s L 0 V s f s PFC Vaiable Load ω I DL DL ~ ω I AG AG Figue EMS fo an ISFO-contolled DFIG opeating unde CPM As Figue 5.15 illustates, if the shaft speed dops below a low-theshold, a cuent demand I AG is sent to the Powe Flow Contolle (PFC) of the AG. The PFC is a convete which contols the powe demanded fom the AG. Hence I AG can be consideed to be the eal o d-axis component of the convete cuent in phase with the local gid voltage. Assuming that the voltage is well-egulated by the DFIG, I AG is equivalent to the instantaneous AG powe P AG. Theefoe, the AG supplies the enegy shotfall between the wind geneated powe P g and the demand 103

116 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES powe P. Hence the shaft speed will ecove. In anothe situation the demand powe might be low while the wind powe is high. This may cause the shaft speed to incease moe than the DFIG maximum limit (i.e. 1.3pu which was discussed in Chapte 2), thus the wind powe needs to be shed. The wind powe can be shed eithe by the pitch contol o a Dispatchable Load (DL). A tade-off mechanism is possible in which a slow pitch contol (o none) esults in a highe DL ating, whilst a nomal pitch contol slew ate (3-5º/sec) may esults in the DL being unnecessay. Assuming that the pitch contol is not fast enough to pevent the shaft speed fom exceeding its maximum limit, a DL would be equied. If the shaft speed hits a high-theshold, the DL is tuned on via a convete (eal) cuent demand I DL which absobs the excess wind geneation. This chapte deals with the cases in which the pitch angle is fast enough to pevent the shaft speed fom exceeding 1.3pu. It is noted that in the standad DFIG contol, also, the pitch angle is used to contol the shaft speed fo the wind speeds above ated. Now the shaft speed high- and low-thesholds must be chosen. The high-theshold must be chosen as (1.3-ε)pu, fo example 1.25pu. The closeness of the hightheshold value to 1.3pu depends on the dynamics of the DL and the pitch contol. Fo choosing the low-theshold, two scenaios can be consideed based on whethe the ω stb is less than the DFIG minimum shaft speed limit (i.e. 0.7pu) o not. If ω stb <0.7, the low-theshold must be chosen at (0.7+ε)pu. The second case is when 0.7< ω theshold < ω stb. In such a case, when the shaft speed becomes less than ω stb, the system becomes unstable which causes futhe eduction in shaft speed. As a esult, the shaft speed dops below ω theshold, hence, the AG is tuned on and injects the shotage of enegy in ode to ecove the shaft speed. This will be shown in the simulation caied out in the next subsection. This contol stuctue is, theefoe, a self-ecovey system and the low-theshold value can be even less than ω stb (this will be shown by simulation). Thus, the low-theshold value can always be chosen at (0.7+ε)pu. The closeness of the low-theshold to 0.7 depends on the dynamics of the AG. The study of the dynamic of the AG is beyond the scope of this thesis. In this chapte the low-theshold is set at 0.85pu, howeve, close values ae also possible which equie lage gains, hence faste esponse 104

117 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES fom the AG. This thesis is only intended to validate the poposed contol scheme and a detailed engineeing design is beyond the scope of the eseach Simulation esults of EMS fo constant demand powe delivey This section undetakes two simulations: the fist validates the poposed EMS fo one DFIG while the second one simulates two DFIGs with the EMS. In this eseach the shaft speed low theshold is set at 0.85pu. In the both simulations fo ω <0.85pu, I AG =-Kω (K=-150). The communication delay between the EMS and the AG is neglected. It is noted that if thee is a delay in the powe tansfe of the AG, it may be necessay to incease the low-theshold value of the shaft speed o incease the gain K to compensate fo the delay. Simulation 1: The model simulated in this pat is shown in Figue The DFIG1 with paamete given in Appendix B is simulated in this chapte. The pitch contolle is the same as befoe with the maximum slew ate of 3º/sec. The AG is simulated by a DC-voltage souce connected to a convete (PFC) which contols the powe demanded fom the DC-voltage souce. The objective of this simulation is to validate the poposed EMS. The simulation esults ae shown in Figue The aveage and the standad deviation of the eal wind speed (Figue 5.16a) ae 12.5m/s and 1.28, espectively. The esults can be divided into fou pats which ae specified by capital lettes A, B, C and D on Figue 5.16b. Ove pat A the demand powe (Figue 5.16b1) is 1pu while the wind speed is above the ated (12.5m/sec). Theefoe the pitch angle (Figue 5.16e) contolle is able to contol the shaft speed (Figue 5.16d) and no powe fom the AG (Figue 5.16c) is needed. 105

118 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Results of the EMS fo a hybid wind geneato and AG system Then ove pat B the wind speed dops below ated while the load demand powe is still 1pu. The pitch angle becomes zeo in ode to extact the maximum wind powe which is still less than the 1pu demand. Thus the shaft speed educes to compensate fo the lack of the enegy. It can be seen that just afte pat B stats the wind speed vaies fom appoximately m/s. Figue 5.17 shows the mechanical toque vs shaft speed chaacteistic of the wind tubine fo wind vaiation fom m/s. Figue 5.17 also depicts the electical toque of the DFIG ove pats A and B. It can be seen that ove pat A the electical toque is unde CPM and since the wind speed is always moe than the ated wind speed (12.5m/s), the system is stable. The wind speed eduction duing pat B causes the 106

119 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES shaft speed to decease. Even assuming that the wind speed is 13.5m/s, ω stb is aound 0.95pu which is moe than the low-theshold of shaft speed (0.85pu). Figue The electical toque associated with pats A and B of the simulation esults shown in Figue 5.16 on the T m vs ω chaacteistics of the wind tubine fo diffeent wind speed Howeve as the shaft speed dops below 0.85pu, the AG is tuned on and injects the enegy shotfall in ode to pevent the shaft speed fom futhe eduction and maintain the demand powe. Since the DFIG output powe (Figue 5.16b2) is no longe 1pu, and vaying accoding to the available wind powe, the stability issue is no longe a concen. Figue 5.17 demonstates that the system becomes tempoay unstable but will ecove automatically. Ove pat C the wind speed inceases above the ated wind speed. As a esult, the shaft speed becomes moe than 0.85 and the AG is switched off. Thus, the DFIG supplies once again the total 1pu powe demanded by the load. In pat D the demand powe dops down to 107

120 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 0.5pu. It can be seen that the shaft speed inceases, the AG is tuned off and the total demand is supplied by the DFIG. The pitch angle tansiently hits the 3º/sec slew ate limit which in tun causes the shaft speed spike. Howeve this is due to the fact that the demand powe educes by 0.5pu in one step which is impactical. Pats C and D demonstate the ability of the poposed contol scheme to enable the DFIG to take ove the total supply of the demand powe due to an incease in the wind powe o a eduction in the demand. The efeence shaft speed in this simulation is 1.1pu. If the wind speed was constant (like Figue 5.11) the choice of the efeence shaft speed would be vey impotant. Howeve in pactice, the wind speed is fluctuating. Theefoe two scenaios can be defined. Fist scenaio is when P ext is moe than P (pats A, C and D). Such cases imply that the pu is patially o fully within the stable egion. Obviously, the lage the shaft speed efeence, the lage the enegy stoed in oto inetia which can be used late if eithe wind speed educes o the demand inceases. Howeve, too lage shaft speed efeence equied elatively faste pitch angle to keep the shaft speed away fom the maximum limit. Thus, choosing the efeence shaft speed fom pu is the best as a compomise between stoed inetial enegy and pitch contol. When the P is moe than P ext (pat B), the shaft speed dops; egadless of the efeence shaft speed. The simulation esults demonstate that the system is automatically able to ecove fom possible instability. So the choice of the efeence shaft speed is not impotant and in this chapte is always set at 1.1pu. Simulation 2 The objective of this simulation is to demonstate the poposed EMS fo a multi- DFIG system. The model simulated in this section, which is shown in Figue 5.18, consists of two doop-contolled DFIGs equipped with the EMS and the CPM pitch contolle explained above. Again the atings of the DFIGs ae 0.66pu and 0.34pu with the doop coefficients given in Table 5.2. As can be seen fom Figue 5.18, the AG and the DL ae aggegated on the local gid, the voltage and 108

121 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES fequency of which ae contolled by the DFIGs. Each DFIG has its own EMS which poduces I AG and I DL. Theefoe the summations I DL and I AG ae fomed and communicated ove communication links to the AG and the DL. In this section only two DFIGs ae simulated, howeve, the concept is applicable to moe DFIGs. (Section 5.4 simulates fou DFIGs using a educed model ). The DL and the AG ae simulated by DC-voltage souces buffeed fom the local gid by a VSC convete. The convetes contol the cuent fom/to AG/DL. It will be shown in late chaptes that the DL can also be distibuted within individual wind geneatos, if equied. In such case, obviously, no communication is needed fo the DL. β 1 CPM Pitch contol P 1 ω 1 V w1 V w1 P 1,Q 1 DL I DL EMS I DL1 I AG1 β 2 i ms θ s CPM Pitch contol 1/2πf s L 0 V s f s P 2 ω 2 V w2 V f Q P Q 1 P 1 AG I AG P &Q Load V w2 P 2,Q 2 I DL2 EMS I AG2 θ s i ms 1/2πf s L 0 V s f s V f Q P Q 2 P 2 Local gid (33kV) Figue EMS fo doop-contolled DFIGs The esults ae shown in Figue The aveage of the two eal wind speed pofiles (Figue 5.19a) ae about 12.5m/sec which coesponds to 1pu gen. The 109

122 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES standad deviations of the fist and the second wind pofiles ae, espectively, 1.28 and 1.39 which ae elatively lage petubations. Figue 5.19b shows the active powe shaing when the demand powe is 0.5, 0.75 and 1pu. Fo the fist 200sec, the demand powe is 0.5pu which is less than, hence the pitch angles (Figue 5.19e) contol the shaft speeds (Figue 5.19d) and no powe is demanded fom the AG (Figue 5.19c). The output powe of each DFIG is constant and popotional to its ating. Duing sec, the demand powe is 0.75pu and it can be seen that the shaft speeds occasionally dop below 0.85pu gen as the powe demanded fom eithe of the DFIG becomes moe than the associated extactable wind powe. When eithe of the shaft speed dops below 0.85pu the AG is tuned on and injects the enegy shotfall in ode to maintain the demand powe and pevent the shaft speed fom futhe eduction. As Figue 5.19b shows wheneve the demand powe is available, the AG is switched off and the demand is shaed by the DFIGs in popotion to thei atings. Ove the last 200sec, the demand powe is inceased to P ext 1pu. As it can be seen P is moe than P ext fo almost the entie peiod. Theefoe the AG compensates fo the shotage of enegy. Figue 5.19h shows that the local gid fequency is vey well egulated by the DFIGs using doop chaacteistics. Figue 5.19f shows that the eactive powe demanded by the load (0.15pu) is shaed by the DFIGs accoding to thei atings while the local gid voltage (Figue 5.19g) is vey well contolled though the doops. Figue 5.19d shows that the pitch angle contol with slew ate of 3º/sec is able to pevent the shaft speed fom exceeding 1.3pu, hence no DL is needed. Next section adjusts the doop chaacteistics in ode to shae the demand powe accoding not only to the atings of the DFIGs but also the available wind powe. 110

123 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue EMS fo two doop-contolled DFIGs 111

124 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Vaiable doop gain contol Using the standad doop chaacteistics, the demand powe is shaed by the geneatos popotional to thei atings. This is because the fequency-powe doop coefficient is set as m f, whee P max is the ating of the associated geneato and Δf is the maximum allowed fequency deviation which is kept the same fo all geneatos. This is an acceptable shaing appoach in case of synchonous geneatos fed by a pime move in which the input powe can be epesented by, fo example, natual gas. Howeve in an intemittent geneation, like wind enegy, this is not necessaily the best shaing method. To explain this, conside two DFIGs shaing a load in popotion to thei atings using the standad doop method (Figue 5.18). If the extactable wind powe in one of the DFIG dops below the contibution equied fom the DFIG, the output powe of the DFIG will educe accodingly. This eduction is acceptable and of couse inevitable. Howeve, the standad doop will foce the othe DFIG to decease its output powe too. The eason of this behaviou is illustated in Figue f P max f op f op P2 1 P P P1 2 P Figue f-p doops when wind speed of one f the DFIG dops Let us assume that the opeational fequency pio to the wind speed eduction is f op. Hence, the output powes of the fist and the second DFIGs ae P 1 and P 2, espectively. Following the wind speed eduction in eithe of the DFIGs, the opeational fequency inceases to f op due to the DFIG output powe eduction. As a esult, the othe DFIG has to educe its output powe in ode to comply with the new opeational fequency. This is because of the fixed doop coefficients. If 112

125 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES the doop coefficients could vay accoding to the available wind powe, the output powe of the second DFIG could stay the same o even incease (subject to the wind powe availability) following the wind powe eduction in the fist one. This can be achieved though making the doop coefficients vay as a cubic function of the shaft speed i.e. m f 3, whee k opt is a given constant fo maximum powe tacking of wind powe. This is due to the fact that the maximum powe of wind is a cubic function of shaft speed. The main advantage of the vaiable doop gain is that the powe equied fom the AG significantly deceases since the DFIGs equipped with the vaiable doop can compensate fo one anothe. This will be shown though the following PSCAD simulations. Howeve the vaiable doop gain method can affect the system stability which will be discussed late in this section Simulation esults of the vaiable doop gain method k opt This section includes two simulations. The model simulated in the both cases is shown in Figue The model consists of two ISFO-contolled DFIGs augmented with the vaiable doop contol explained above with Δf=0.1Hz and k opt =0.5pu. The voltage-eactive powe doop and the pitch contol ae the same as befoe. The atings of the DFIGs ae 0.66pu and 0.34pu with paametes given in Appendix B. The fist simulation uses constant wind speed while in the second one eal wind speed pofiles ae applied. Simulation 1 The objective of this simulation is to illustate the poposed vaiable gain doop and to compae that with the standad doop. Fo the sake of explanation, constant wind speeds ae used. The simulation esults, which ae shown in Figue 5.21, consist of pats A and B. The pat A illustates the esults with the standad doop while the pat B shows those with the vaiable doop. In the both cases the demand powe is 0.78pu. The wind speed of the both DFIGs is 12.5m/sec until 20sec when the wind speed of the second DFIG dops to 10m/sec (coesponding to extactable wind powe of appoximately 0.5pu gen ). 113

126 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Compaing the behavio of two doop-contolled DFIGs when the wind speed of the second one dops fom 12.5 to 10m/s (A) standad doop (B) vaiable gain doop The wind speed eduction causes the output powe of the second DFIG to educe (Figue 5.21c). As explained, the standad doop foces the fist DFIG to educe its output powe too. With the efeence shaft speed set at 1.1pu, the fist DFIG pitch angle (Figue 5.21e) inceases to contol its shaft speed (Figue 5.21g). Since the contibution demanded fom the second DFIG is moe than its, the pitch angle of the second DFIG becomes zeo. This causes the shaft speed to dop below 0.85pu which tuns on the AG. The AG injects powe (Figue 5.21i) in ode to compensate fo the shotfall and pevent the shaft speed fom futhe eduction. It is noted that the pitch angle of the fist DFIG is about 13º which means that the DFIG is geneating fa less than its available wind powe. Howeve using the vaiable doop gain method, the output powe of the fist DFIG inceases (Figue P ext 114

127 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5.21d), causing a decease in its pitch angle (Figue 5.21f). In othe wods, the fist DFIG compensates fo the output powe eduction of the second one. This leads to no powe demanded fom the AG (Figue 5.21j). It can be seen fom Figue 5.21d that until 20sec, duing which the demand powe is less than the extactable wind powe, the demand powe is shaed by the DFIGs accoding to thei atings, even though the vaiable doop gain is used. Simulation 2 The model simulated in this section is the same as befoe (Figue 5.18). The objective of this simulation is to validate the vaiable doop gain method with eal wind pofiles and compae its esults (Figue 5.22) with those of the standad doop shown in Figue Theefoe the same pocedues as Figue 5.19 ae simulated hee. The aveages of the eal wind speed pofiles (Figue 5.22a) ae appoximately 12.5m/sec with the standad deviations of 1.28 and 1.39 espectively. Ove the fist 200sec, the demand powe (Figue 5.22b) is 0.5pu which is less than P ext of the both DFIGs. Because of this the demand powe is shaed in popotion to the DFIG atings and the output powe of each DFIG is almost smooth. The pitch angles (Figue 5.22e) contol thei associated shaft speeds (Figue 5.22d) and no powe fom the AG is needed. It can be seen that in this pat the situation is vey simila to the Figue Fo the second 200sec, the demand powe inceases to 0.75pu which is occasionally moe than P ext. It is shown in Figue 5.22b that the output powes of the DFIGs ae not smoothed anymoe and in fact ae vaying accoding to the wind speeds. This helps the DFIGs to compensate fo the enegy shotage of each othe when the demand powe is moe than P ext. As a esult, unlike the one with the standad doop (Figue 5.19c), no powe is demanded fom the AG (Figue 5.22c). It is noted that the powe geneated fom the total wind fam is smoothed and of couse equal to the demand, in spite of the vaying output powe of each DFIG. Duing the last P ext 200sec, the demand powe is 1pu which is almost always moe than. 115

128 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Theefoe, both shaft speeds educe and the AG is switched on to compensate fo the shotfall. Since the demand powe is almost always moe than, the amount of the enegy saved using the vaiable doop gain method is not significant. Howeve it can be seen fom Figue 5.22c that the powe fom the AG becomes tempoaily zeo duing the last 200sec which does not happen using the standad doop (Figue 5.19c). Although the vaiation of the local gid fequency (Figue 5.22h) is slightly moe than that of the standad doop (Figue 5.19h), it is still less than 0.3Hz. The eactive powe demanded fom the wind fam is 0.15pu which is shaed in popotion to the ating of the DFIGs duing the fist 200sec. Fom 200sec onwads, as the fequency vaiation inceases due to the vaiable doop, the output eactive powe of each DFIG vaies accodingly. Howeve, the total eactive powe fom the wind fam is still smooth and equal to the demand eactive powe. Figue 5.22g shows that the local gid voltage is still vey well egulated. P ext The simulations above demonstate the ability of the vaiable doop gain method to foce the DFIGs to compensate fo one anothe and significantly educe the enegy equied fom the AG while both voltage and fequency ae contolled. 116

129 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Shaing with the vaiable doop gain method 117

130 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES The main dawback of the vaiable doop method is that the system stability may be affected. A simila scenaio was conside in [42] in which thee 10kW invetes feed an islanded micogid. The doops in [42] wee a function of the dispatched powe being fed though each invete. It was shown in [42] that in the wost case scenaio the lagest possible f-p doop gain is Hz/W (o 0.31Hz/pu). This is due to the fact that the system eigenvalues vaies with the f-p doop gain and in fact the eigenvalues move towad instability as the f-p doop gain inceases[42, 75]. Tanslating thei findings to the pesent study means that thee is a minimum shaft speed fo the vaiable doop gain method which is aound 0.32pu. This means that the system will become unstable fo wind speeds less than the cut-in wind speed. This will be illustated in Chapte 8. Theefoe, it seems that the system is stable within the allowed shaft speed egion fo DFIGs (i.e pu). Having said that, the autho admits that a full system stability study is equied that is beyond the scope of this thesis. It is noted that the poposed EMS woks with the standad doop and the vaiable doop gain method is suggested in ode to impove the system efficiency though educing the enegy equied fom the AG. 5 The next section poposes a simplified model fo ISFO-contolled DFIG in ode to be able to simulate moe numbes of DFIGs. 5.4 Simplified ISFO-contolled DFIG model PSCAD simulation of a multi-dfig system including the wind tubine, pitch angle contolle and eal wind pofile connected to the AG and DL fo a long time (600sec) takes seveal hous. This causes difficulties fo simulating diffeent scenaios especially fo models with moe than two DFIGs. Theefoe, this section poposes a simplified model which will be used to simulate a system consisting of a lage numbe of DFIGs in ode to validate the EMS. Both standad and vaiable gain doops ae simulated. 118

131 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Explaining the simplified model It was discussed that an ISFO-contolled DFIG appeas like a voltage souce in the system. Theefoe the DFIG can be epesented by an AC voltage souce while its voltage and fequency ae set though doop chaacteistics. Figue 5.23 shows the simplified model of two DFIGs contolling the local gid voltage and fequency using doop chaacteistics. The electical toque of each DFIG is deived though dividing its measued output powe by the shaft speed. It can be seen that the full model of the wind tubine is consideed. It was shown in [54] that the stable egion of the DFIG (the geneato itself) is much wide that that of the wind tubine. Theefoe the stability chaacteistic of this simplified model is the same as that of the one with the full DFIG model since the full wind tubine model is consideed in the simplified model. The pitch angle contol and the EMS ae the same as befoe. 119

132 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES P 1 Q 1 Doops f s V s P 1 (measued) DL I DL / AG I AG V w1 β 1 Wind tubine Eq(5.1) T e1 T m1 Tm Te J ω 1 EMS I AG1 I DL1 P &Q Load P 2 Q 2 Doops f s V s P 2 (measued) Local gid (33kV) / V w2 β 2 Wind tubine Eq(5.1) T e2 T m2 Tm Te J ω 2 EMS I AG2 I DL2 Figue Simplified model of two DFIGs contolling the local gid voltage and fequency Now the simplified model will be use to simulate fou doop-contolled DFIGs using PSCAD Simulation esults of fou doop-contolled DFIGs using the simplified model The model simulated in this section consists of fou simplified ISFO-contolled DFIGs supplying a local load, as is shown in Figue In Figue 5.23, fo the sake of simplicity, only two DFIGs ae depicted. Howeve in the model simulated, fou DFIGs ae used. The pitch angle contol (not shown in the figue), EMS and AG ae exactly the same as befoe. Two simulations ae undetaken. The objective of the simulations is to validate the EMS fo fou DFIGs using both the standad doop and the vaiable doop gain methods. The atings of the fist and the thid 120

133 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES DFIGs ae 0.33pu while those of the second and the fouth DFIGs ae 0.17pu. Thee ae two eal wind speed pofiles with the aveage of appoximately 12.5m/sec and the standad deviations of 1.28 and The fist wind speed pofile is applied to the fist and the fouth DFIGs while the second wind speed pofile is applied to the second and the thid DFIGs. As a esult, the DFIGs with the same ating ae applied diffeent wind speed pofiles. Figue 5.24 shows the esults with the standad doop chaacteistics. The doop gains ae those used in section Figue 5.24b shows the active powe shaing. The demand powe, fo the fist 200sec, is 1pu which is equal to the aveage of the extactable wind powe. Theefoe, when the demand is moe than P ext of any of the DFIGs, its shaft speed (Figue 5.24f&g) educes and tuns on the AG (Figue 5.24c). Since the output powe of each DFIG cannot be moe than 1pu gen, the pitch angles (Figue 5.24d&e) ae not always at zeo. In othe wods, the pitch angles inceases to shed powe moe than 1pu gen. Ove the second 200sec, the demand powe educes to 0.75pu which is less than the aveage of the extactable wind powe. As a esult, the AG is tuned on in only thee occasions. Duing the last 200sec, in which the demand is 0.5pu, no powe is needed fom the AG. The eactive powe demand (not shown in the esults) is 0.15pu which is shaed popotional to the atings of the DFIGs. It is noted that the output powe of the fist and the thid DFIGs, which have the same ating, ae identical. This is also the case fo the second and the fouth DFIGs. This is due to the standad doop which shaes the powe accoding to the atings of the DFIGs egadless of the wind speed. Figue 5.24h shows that the local gid voltage and fequency ae well-egulated. Figue 5.25 shows the esults using the poposed vaiable doop gain method while the same pocedues as the one with standad doop ae simulated. Fo the fist 200sec the demand powe is 1pu. As Figue 5.25b shows the output powes of the DFIGs with the same atings ae not identical anymoe. This is because of the vaiable doop gain method which makes the DFIGs shae the demand accoding to both thei atings and the available wind powe. As a esult, less active powe is needed fom the AG (Figue 5.25c) than that of with the standad doop (Figue 5.24c). Duing the second 200sec, the demand powe dops to 0.75pu. In this case 121

134 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES the powe fom the AG is zeo, unlike the case using the standad doop. Finally fo the last 200sec the demand powe is 0.5 which is less than. Theefoe, the demand powe is shaed almost popotionally to the atings of the DFIGs, simila to the case with the standad doop. Figue 5.25h shows that the local gid voltage and fequency ae still vey well-contolled. These esults demonstate that the poposed EMS and the pitch contolles ae able to maintain the shaft speeds within its acceptable egion in a multi-dfig system. The esults in Figue 5.24 show the functionality of the standad doop contol in a multi-dfig system. Figue 5.25 shows that the poposed vaiable doop gain contol method is able to opeate is a multi-dfig system without violating the system stability. P ext 122

135 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Powe shaing of fou simplified ISFO-contolled DIFGs using standad doop 123

136 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES Figue Powe shaing of fou simplified ISFO-contolled DIFGs the vaiable doop gain 124

137 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES 5.5 Discussions and Conclusions This chapte dealt with an ISFO-contolled wind fam without an extenal toque contolle fo the DFIGs. An application of such a stuctue involving a micogid has been taken into consideation. A pitch angle contol has been designed and validated to maintain the shaft speed within its limits when the demand powe is less than P ext. The pitch angle maximum slew ate is always kept at 3º/sec. The pitch angle contol is also illustated fo a two-dfig system while the demand powe is shaed by the DFIGs using doop chaacteistics. When the demand powe appoaches the aveage of the extactable wind powe, the enegy stoed in the shaft inetia compensates fo the shotage of the enegy wheneve the demand becomes moe than. This causes shaft speed eduction. If the shaft speed educes to less than a cetain theshold, an EMS is used to switch on an AG in ode to compensate fo the enegy shotage and pevent the shaft speed fom the futhe eduction. The EMS using standad doop chaacteistics has been validated in a system with two DFIGs while eal wind speed pofiles ae used. It has been shown that using the standad doop, if of one DFIG dops below its contibution to the demand powe, the output powe of the othe DFIGs will dop accodingly in ode to opeate at the new fequency foced by the fist DFIG. A vaiable doop gain contol paadigm has been poposed to ovecome this poblem with the standad doop. It was shown that using the poposed vaiable doop gain contol method, the powe demanded fom the AG can be significantly educed. This is because that the DFIGs equipped with vaiable doop gain contol ae able to compensate fo one anothe. The dawback of the vaiable doop gain is the fact that the lage doop gains can affect the system stability [42, 75]. This can occu when the shaft speed is too low which happens when the demand powe is moe than P ext P ext. Howeve, it was shown in [42] that instability occus only fo vey lage doop gains. The vey lage doop gains seem to coespond to wind speed less than the cut-in wind speed in case of the multi-dfig system equipped with the P ext 125

138 5 Doop-contolled wind fam deliveing a constant demand powe without an extenal ES vaiable doop. In othe wods, the DFIGs augmented with the vaiable doop contol seem to be stable within the opeating shaft speed egion. The stability study of the vaiable doop gain contol is out of the scope of this thesis. It is noted that the poposed EMS woks with the standad doop contol. In ode to validate the poposed EMS in a system with moe than two DFIGs, a simplified DFIG model has been poposed. The simplified model still consides the full model of the wind tubine. Theefoe the stable egion of the simplified model is identical to the system with the full DFIG model. This is due to the fact that the stable egion of a DFIG is much wide than that of a wind tubine [54]. A model consisting of fou simplified DFIGs has been simulated using both standad doop and the vaiable doop gain methods. This chapte focused on constant demand powe delivey methods without an extenal ES. This educes the cost of the system significantly. Howeve, since the DFIGs ae not unde MPT contol, the powe fom the AG is not necessaily minimized. This is definitely the case when the standad doop is used. Moeove, using an extenal ES may inceases the efficiency of the AG though deceasing the incident of powe demanding fom the AG. Constant demand powe delivey methods using an extenal ES ae the subjects of the next chaptes. The following chaptes deal with an ISFO-contolled wind fam using the ES as an extenal geneato toque contolle. The next chapte consides CTM contol. The MPT mode is studied in Chapte

139 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato 6. Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato 6.1 Intoduction The pevious chapte investigated the contol stategies involving an ISFOcontolled wind fam and an Auxiliay Geneato (AG) in ode to delive a constant powe demanded by the load. It was shown that if the demand powe is less than the extactable wind powe, the poposed pitch angle contol with a slew ate of 3-5º/sec can contol the shaft speed within its stable egion. If the demand powe becomes moe than the extactable wind powe, the shaft speed educes in ode to compensate fo the enegy shotfall. This may expose the system to instability. In ode to avoid such situations, fo shaft speeds less than a cetain theshold ( pu), the AG is tuned on and injects the enegy needed in ode to pevent shaft speed fom futhe eduction and maintain the demand powe. No extenal ES was consideed in the pevious chapte. In Chapte 7 a full model consisting of both an AG and an extenal ES will be studied. Chapte 6, howeve, studies the contol of an ISFO-contolled DFIG with an extenal ES while no AG is consideed. The ES powe will be egulated to contol the electical toque of the DFIG. Not having an AG is obviously impactical due to the limited capacity of ES systems especially when the demand powe appoaches the aveage of the extactable wind powe P ave. Howeve this chapte is included, in ode to explain and design the geneato toque contol loop and to study the Constant Toque Contol (CTM) mode. In Chapte 7, the DFIGs ae contolled unde standad Maximum Powe Tacking (MPT) mode in ode to minimize the enegy fom the AG when the demand powe is moe than the extactable wind powe. This chapte, since no AG is consideed, is a pope case study fo DFIGs unde a non-mpt contol such as CTM. Consideing an isolated load fed by wind tubines with no AG, CTM (o in fact any non-mpt 127

140 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato contol) has the advantage of educing the equied extenal ES compaed to the MPT mode. This is because of the fact that the shaft speed vaiation of CTM is moe than that of MPT fo a given wind speed and demand powe [54]. In papes such as [54], the CTM is named as a possible method of contolling a DFIG. Howeve, as fa as the autho is awae, no publication has consideed the stability issue of CTM contol. One of the objectives of this chapte is to study the instability poblem of the CTM contol and attempt to addess the issue without using an AG. The MPT seems to be the best choice, at least when the demand powe is moe than the extactable wind powe to minimize the powe fom the AG. This chapte consists of two main pats. The fist pat poposes a geneato electical toque contol scheme though egulating the ES powe. The poposed contol scheme will be validated fo both CTM and MPT contols. The same contol stuctue will be used in the following chaptes to contol the toque of the geneato. The second pat of this chapte attempts to deive a mathematical expession fo the size of the equied ES fo a given wind pofile. The second pat coves wok which was discontinued, but it is included hee fo completeness and it may also have achival value. 6.2 Electical toque contol by egulating ES powe In an ISFO-contolled DFIG, i q is kept popotional to i sq in ode to maintain the field oientation of the DFIG [44]. Theefoe the electical toque must be contolled extenally. In [44] an auxiliay load is used to contol the DFIG unde MPT mode. This section poposes a toque contol scheme using the ES powe. The contol stuctue will be validated using PSCAD simulations fo both CTM and MPT contols. Figue 6.2 illustates the poposed electical toque contol scheme fo the ISFO-contolled DFIG. 128

141 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato β R β β Slow PI P L =P P g,q g P es V s 1/2πf s L 0 i ms Toque contol T e method - I I d-es ESI f s θ s Te L0imsiq 3 2 ES Figue 6.1. Poposed electical toque contol stuctue fo an ISFO-contolled DFIG The load powe P L is equal to the demand powe P which is detemined by the system opeato. In a multi-dfig system, the demand powe is shaed by the DFIGs using doop chaacteistics, as explained in the pevious chaptes. It will be shown in Chapte 8 that the ES can also be aggegated on to the wind fam collecto bus which necessitates communication between wind fam and the ES. The geneato powe is P g =P es +P L, whee P es is the ES powe. The electical toque of the DFIG effectively is T e =P g /ω. Theefoe, the electical toque can be contolled by egulating P g though P es. Since the local gid voltage is contolled by the DFIG, the ES eal component cuent I d-es is equivalent to the instantaneous ES powe P es. As a esult, the electical toque can be contolled though egulating I d-es. The ES inteface (ESI) is an AD/DC PWM convete. The q- component of the ES cuent I q-es is set at zeo but it can also be used to suppot the local gid voltage V g if the demand eactive powe is too lage fo the DFIG. The ESI cuents loops (not shown in the figue) ae the standad cuent contolles identical to those of the DFIG s gid side convete explained in Chapte 2 and [48, 49]. An integal contolle is shown to be sufficient to contol the electical toque. The efeence electical toque T e is detemined accoding to the equied toque contol method (i.e. MPT, CTM, etc). The fed-back electical 129

142 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato toque is calculated fom Te L0imsiq 3 2 [44], whee ρ and L 0 ae the numbe of the pole pais and mutual inductance espectively. A pitch angle contol can be utilized to educe the aveage of the ES powe towads zeo though using a slow PI contolle, as shown in Figue 6.1. The output of the pitch angle PI contolle will dive any DC o low fequency components (within the contolle bandwidth) of the ES powe towads zeo. Since the contolle aveages the ES powe towad zeo, this has the effect of educing the powe ating of the ES. The next subsection deives the contol plant and designs the integal contolle of the poposed toque contol scheme Toque contol loop design This section consides the electical toque loop design which is also used in Chaptes 7 and 8. Accoding to Figue 6.1, one can wite: P g P es P L (6.1) Assuming a otating dq fame with the d-axis fixed to the gid voltage, P es can be witten as: P es V gdides VgqIqes VgdIdes 2 (6.2) Substituting (6.2) and P g T e into (6.1) and simplifying it, yields: T e 3 I 2 d es P V L gd Vgd (6.3) The gid voltage V gd can be consideed constant and well egulated by the DFIG. The load demand powe P L is an extenal distubance. Accoding to (6.3), the contol stuctue is given in Figue 6.2: 130

143 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato Inne Cuent Loop 1 PL V gd T e 3 k i I d es Vd ESI - 2 s - s LT s RT I Contolle k ( s a) PI Contolle 1 Plant I d es V gd Plant T e Figue 6.2. Electical toque contolle stuctue The inne cuent loop contols the cuent flowing though the ES tansfome with inductance L T and esistance R T given in Appendix B. The load powe P L is an extenal distubance and can be neglected in the contol design. The gid voltage is constant and well-contolled by the DFIG. Theefoe, the contol plant is a vaiable gain that vaies with only ω. Due to the oto inetia, ω vaies smoothly. Hence, it is expected that T e also vaies quite smoothly. As a esult, the oute toque loop bandwidth can be 15-30Hz (this is simila to the powe loop of a standad DSFO-contolled DFIG). Since the bandwidth of the inne cuent loop is 250Hz which is much faste than the oute toque contol loop, the closed loop tansfe function of the inne loop can be assumed to be 1. This means that the inne cuent loop can be neglected fo designing the oute toque contol loop. As mentioned, the contol plant is a gain vaying with ω. Howeve, since the opeating egion fo a DFIG shaft speed is limited to only pu, the contol plant is assumed a constant gain with ω =1pu. Because the plant can be epesented as a gain fo the fequencies of inteest, an integal contolle is sufficient. Theefoe the chaacteistic equation of the oute toque loop is: 3 k Vgd 3 V i gd 1 0 s ki 0 2 s 2 (6.4) Thus 131

144 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato 3 Vgd 2 k i 2 Te T ki e 3V gd (6.5) whee is the bandwidth of the oute toque contol loop. If, ω and V gd ae T e 15Hz, 1pu and 33kV (line-line ms), k i is Pefomance of the electical toque contol loop This section validates the toque contol loop using PSCAD simulations. In the fist pat the electical toque is contolled unde CTM while in the second pat MPT mode is consideed. T e CTM contol Unde CTM the efeence electical toque is constant fo a given demand powe. The efeence electical toque can be defined as: T e P L (6.6) whee is called the aveage shaft speed. The choice of the aveage shaft speed will be discussed late in this chapte. In a multi-dfig system, P L is the powe detemined by the doop chaacteistics. Assuming a constant aveage shaft speed, T e will change as the demand powe changes. Simulation esults The objective of this simulation is to validate the poposed toque contol scheme fo CTM contol. The model simulated in PSCAD is shown in Figue 6.1. The DFIG1 with paametes given in Appendix B is used fo this simulation. Figue 6.3 shows the simulation esults. The aveage of the eal wind speed pofile (Figue 6.3a) is appoximately 12.5m/s ( P ave 1pu ) with the standad deviation of 132

145 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato The degee to which P can appoach without violating the system stability will be discussed late. P ave Figue 6.3. CTM contol using the poposed contol stuctue The efeence electical toque is given in (6.6) with the aveage shaft speed of 1.1pu which is kept constant duing the simulation. The demand powe (Figue 6.3b) is inceased fom 0.2pu to 0.5pu in a step change. As a esult, the electical toque efeence (Figue 6.3c1) inceased fom 0.2/1.1=0.18pu to 0.5/1.1=0.45pu. Figue 6.3c2 shows the electical toque following its constant efeence. Figue 6.3 demonstates the ability of the poposed electical toque contol scheme via ES powe egulation to contol a DFIG unde CTM. 133

146 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato MPT contol Unde MPT mode, the DFIG output powe P g is a cubic function of the shaft speed. Theefoe, the efeence electical toque must vay as a quadatic function of the shaft speed: T e k opt 2 (6.7) whee k opt is a constant coefficient given fo each wind tubine. Simulation esults The model simulated is shown in Figue 6.1 with the same DFIG ating as the pevious simulation. This time, the efeence electical toque is given in (6.7) with k opt =0.52pu. The simulation esults ae illustated in Figue 6.4. The aveage of the eal wind speed pofile (Figue 6.4a) is about 10.5m/sec with the standad deviation of The demand powe (Figue 6.4b) inceased fom 0.5pu to 0.8pu at 50sec. Figue 6.4c shows that the electical toque follows its efeence with a small tansient eo. Howeve Figue 6.4c shows only that T e follows its efeence and does not pove that the DFIG is unde MPT contol. In ode to show that the DFIG is unde MPT contol, Figue 6.5 depicts the DFIG output powe P g vs the shaft speed on the tubine powe vs shaft speed chaacteistic fo diffeent wind speeds. Figue 6.5 clealy shows that the DFIG output powe is following the maximum extactable wind powes. These esults demonstate the ability of the poposed toque contol scheme to contol a DFIG unde MPT mode via egulating the ES powe. 134

147 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato Figue 6.4. MPT contol using the poposed contol stuctue Figue 6.5. MPT contol using the poposed contol stuctue on P t -ω chaacteistic 135

148 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato 6.3 Mathematical deivation of the size of ES fo a given wind pofile Intoduction This section attempts to deive a mathematical expession fo the powe ating and the enegy capacity of the equied ES. Obviously, this can be done fo a known wind speed pofile. The following assumptions ae made: The DFIG is contolled unde CTM (howeve, the method can be adopted fo MPT in futue). The wind speed pofile is known and has the aveage of, the wind speed vaiation of δv w and an abitay fequency of f w. V w The idea behind choosing a fequency f w was that it was assumed that the pitch contol could be used to smooth powe fequencies less than f w. Hence the ES could be used to absob fequencies moe than f w. Consideing such a assumption, the ES enegy capacity ating would be detemined by the (lowest) fequency f w. Howeve the appoach was not found to be effective and (as stated) was discontinued. Howeve the wok is included as it deives the elationship between ES capacity and wind powe fequency which may have achival value and find application in futue. The deived mathematical expession will be validated using PSCAD simulations consideing the above assumptions into account Mathematical deivation of ES ating It was discussed in Chapte 2 that the main dawback of the CTM is its instability as shown in Figue 6.6a. Howeve, the stable egion of the CTM is wide than that of the CPM [54]. Theefoe, the method explained in the pevious chapte fo CPM, in which an AG is used to dive the DFIG to its stable egion, is quite applicable fo CTM as well. Howeve, this chapte concentates on constant demand powe delivey without an AG. In fact the assumption that thee is no AG 136

149 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato justifies the contol of the DFIG unde CTM. Othewise the MPT contol is the pope choice in ode to minimize the enegy demanded fom the AG. As mentioned befoe, unde CTM, the electical toque is T e PL. The choice of the is vey impotant and can help the system to keep away fom instability. This will be discussed late. In ode to pedict instability the shaft speed vaiation δω (see Figue 6.6a and equation (6.11)) must itself be pedicted. The calculation of the shaft speed vaiation leads to the enegy and powe ating of the ES. T m P Te Te Stable egion V w Vw0 V β=0 δω β 0 w P t P 0 =P P ave V w Vw0 V β=0 w β 0 β opt ω stb (a) 0 ω ω opt (b) 0 ω Figue 6.6. Wind tubine chaacteistics (a) T m -ω chaacteistic showing CTM stable egion (b) P t - ω chaacteistic defining the aveage shaft speed and lineaization point In ode to calculate δω, the wind tubine model needs to be lineaized. This was caied out in Chapte 5 and the lineaized model can be expessed as: Tm M1Vw M 2 M 3 (6.8) whee M 1, M 2 and M 3 ae functions of the lineaization point which is P 0 =P, V and (see Figue 6.6). w0 V w 0 Assuming a wind vaiation at the sinusoidal fequency f w : V w V w V cos 2f w w t (6.9) 137

150 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato the tubine mechanical toque can be appoximated by: T m T e T cos 2f m w t (6.10) Theefoe, the shaft speed is: sin 2f w t (6.11) Thee is a small phase shift between the mechanical toque and the shaft speed due to the mechanical loss which is neglected in (6.11). In CTM, Te T e, so one can wite: T m T e J d dt (6.12) whee J is the combined geneato and tubine inetia as seen fom the geneato. Substituting (6.10) and (6.11) into (6.12) and simplifying, yields: m 2f wt 2f wj cos2 f wt Tm f wj T cos 2 (6.13) Equating (6.8) with (6.13) and assuming that at steady state δβ 0, the shaft speed vaiation can be deived as: M1Vw w 2 f J M 2 (6.14) Assuming that at steady state the aveage of the geneato powe is P, the amplitude of the ES powe P es is the shaft speed vaiation multiplied by the electical toque: P es T e P (6.15) Given the time peiod of the selected wind fequency T w =1/f w, each half peiod of ES powe can be appoximated by a tiangle with a base of T w /2 and height of P es 138

151 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato given in (6.15). The aea of this tiangle epesents the equied enegy fom the ES (E es ) to be exchanged with the gid: E es P es T 4 w (6.16) Equations (6.14), (6.15) and (6.16) deive espectively shaft speed vaiation, ES powe and ES enegy fo a given J, P, f w, δv w, V and. This will be validated using PSCAD simulations late in this chapte Stability study w Figue 6.6a shows the stable egion fo a given P, V and. As Figue 6.6a w illustates, the choice of assists the system to etain stability. If an AG is available, an Enegy Management System (EMS) identical to the one explained in Chapte 5 can be used in ode to ecove the system fom possible instability. In such a scheme the AG is tuned on when the shaft speed dops below a cetain theshold and injects the enegy shotage to pevent the shaft speed fom futhe eduction. It was shown in Chapte 5 that the shaft speed theshold can be less than ω stb. In such a scenaio, the choice of is not impotant and can always be set at 1-1.1pu. Howeve, assuming a system in which no AG is available, the choice of becomes vey impotant. Figue 6.6a illustates that as inceases, the system is futhe fom the instability shaft speed ω stb. Howeve Figue 6.6b shows that as P appoaches P, the choice of is limited. If P P, the only ave ave choice fo is the associated optimum shaft speed ω opt. In ode to pedict instability, fist a value of ω stb needs to be deived. Accoding to Figue 6.6a, at ω =ω stb the following holds: T m P t 0.5AC p V w 3 T e (6.17) 139

152 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato In the PSCAD wind tubine model, the powe coefficient C p is defined as: C p e (6.18) The shaft speed as a function of Tip Speed Ratio (λ) is: 2.237NVw NV w (6.19) Substituting C p fom (6.18) and ω fom (6.19) into (6.17) and simplifying it, gives: e 0.17 Ke T (6.20) whee K AV N w 2. As Figue 6.6a shows, two λ values satisfy (6.20). One coesponds to and the desied one to ω stb. Equation (6.20) is not easily solved although a seach solution is quite feasible. Since the wok was discontinued the seach algoithm will not be epoted hee. Once ω stb and δω ae calculated, the instability can be pedicted. In ode to enhance the safety magin of the calculations, δω can be inceased by 10-30%. Theefoe, if 1. 3 stb, the system is consideed too close to the instability egion fo the given P and. The algoithm of choosing can be summaized as follows: P ave If P, and in this case if 1. 3, the opt demand powe educes to pevent instability. If P < P, a seach solution fo is equied whee fo each opeational ave shaft speed (i.e pu), ω stb and δω ae calculated. The seach loop should continue until the stability citeia is achieved (i.e > stb 140

153 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato stb ). If no opeating shaft speed satisfies the stability citeia, the demand powe needs to be educed. It means that the degee to which P can appoach P ave (without violating the stability citeia) depends on δω. As δω inceases, the degee to which P can appoach P deceases. Accoding to (6.14), fo a given P and, δω mainly ave depends on δv w and f w. As δv w inceases and/o f w deceases, δω inceases. This is because moe enegy is stoed in the oto inetia. Theefoe, the maximum degee to which P can appoach to P ave, educes. This thesis is not intended to design the seach loop fo choosing. The next section caied out some simulations, in ode to illustate the calculated ES powe ating and enegy capacity Simulation esults fo ES enegy and powe ating The objective of these simulations is to validate the mathematical expessions fo the equied ES powe ating and enegy capacity given by (6.15) and (6.16) espectively. The model simulated is shown in Figue 6.1. The DFIG 1 with paametes povided in Appendix B is used fo simulations. Since the esults ae given in pu, the ating of the DFIG is not impotant and fo DFIGs with diffeent atings but with the same paametes in pu (i.e. ating wind speed, inetia, etc), the esults will be the same. The paametes of the pitch angle PI contolle ae: k p =0.01 and k i = The ES is again simulated by a DC-voltage souce connected to the local gid though an AC/DC convete. The wind speed pofiles used in these simulations ae made by PSCAD standad wind model explained in Chapte 2. The wind pofile consists of a mean value added to 14 sinusoidal components detemined with the equation (2.37). The total wind speed vaiation is δv w =2.86m/sec (which is a nomal wind petubation) and f w =0.0833Hz. These values of δv w and f w ae chosen in ode to be able to incease the demand powe 141

154 ES Powe, pu Enegy of ES, pu 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato P up to P ave and hence conside a wide ange of cases. Figue 6.7 compaes the simulation (solid lines) and calculation (dashed lines) values of P es (bottom) and E es (top) fo diffeent demand powes and aveage wind speeds. The aveage wind speeds ae 12.5, 11.5, 10 and 8m/sec and fo each of them the demand powe is inceased up to its associated P ave while δv w and f w ae kept the same Aveage wind=8m/s Aveage Wind=10m/s Aveage Wind=11.5m/s Aveage Wind=12.5m/s Demand Powe, pu Figue 6.7. Compaison of simulation (solid lines) and calculation (dashed lines) values of P es (bottom) and E es (top) fo diffeent demand powes and aveage wind speeds 142

155 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato Fo each aveage wind speed, is kept at its ω opt fo P =. The calculation of ω opt fo a given demand powe and wind speed is given in Appendix F. Figue 6.7 shows that the calculation values fo P es and E es (given by (6.15) and (6.16)) ae acceptably close to thei simulation values. The eo between the calculation and simulation values fo E es is, geneally, moe than that of the P es. This is simply because of the fact than in calculating E es moe simplifications ae made. It can be seen that as P inceases the eo inceases and the wost case is P ave when P = =1pu. Since depicting all of the simulation cases shown in Figue 6.7 ae not possible, Figue 6.8 shows fou cases with diffeent values of P and P ave V w. The wind speed pofile (Figue 6.8a), as explained, is simulated by the PSCAD standad wind model with δv w and f w given above. Table 6.1 illustates the sequence of the events simulated in Figue 6.8. The bold numbes epesent the paamete changed compaed to the pevious event. V w Time 0-100sec sec sec sec, m/sec P, pu Table 6.1. Sequence of events of the simulation esults shown in Figue 6.8 It can be seen that the ES powe (Figue 6.8c) and the ES enegy (Figue 6.8d) ae the same as those given in Figue 6.7. Figue 6.8c shows that the pitch angle (Figue 6.8e) contol makes the aveage of the ES powe zeo. 143

156 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato Figue 6.8. Simulation esults fo validating P es and E es given by (6.15) and (6.16) The esults shown in Figues 6.7 and 6.8 validate the mathematical expessions of P es and E es fo a wind speed pofile consisting of seveal sinusoidal components. Howeve, the deived equations do not pedict P es and E es satisfactoily in the case of a eal wind speed pofile. This is because of the fact that in a eal wind pofile f w, δv w and V w ae continuously vaying. This causes tansient vaiations in P es and E es which ae not pedictable. It is noted that even in Figue 6.8 the tansient values, caused by changing V w o P, ae not pedicted in the mathematical equations. Howeve, it might be possible to find out an expeimental coefficient to 144

157 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato adjust the esults given by the equations in ode to make them moe appopiate fo cases with eal wind speed pofiles. This is out of the scope of this eseach. 6.4 Discussions and conclusions This chapte consides a wind geneato-es system deliveing a constant powe demanded by the load while no AG is available. Such a scenaio is a case study fo a DFIG unde CTM contol. This is because any non-mpt contol (like CTM) stoes moe wind enegy in the oto inetia and hence educes the equied extenal ES. When an AG is pesent, thee is no advantage to non-mpt methods since in pactice the demand powe will exceed the momentay extactable wind powe which equies the MPT contol in ode to minimize the enegy demanded fom the AG. This chapte descibes an electical toque contol scheme by egulating the ES eal cuent. The toque contol scheme has been illustated fo CTM and MPT contol using PSCAD simulations. Howeve, the contol stuctue can be used to contol the electical toque in any othe contol method as well. The MPT contol will be used in the following chaptes. This chapte attempts to deive mathematically the powe ating and enegy capacity of the equied ES fo a given wind pofile and demand powe. The mathematical esults ae quite close to the simulation ones in case of a sinusoidal base wind speed pofile. Although this not the case fo a eal wind speed pofile, a simila appoach might be adopted in futue eseach to obtain satisfactoy esults in case of eal wind speed as well. As mentioned befoe the ES ating did not wok fo eal wind pofiles. This was because the pitch contol did not wok to effect a minimum powe fequency seen by the ES. The values fo f w tend to be vey low fo eal winds which esults in lage ES capacity equied. Due to the technological constaints, the capacity of the ES systems is limited. Theefoe, in the next two chaptes the capacity of the ES system is assumed to be a given paamete. Appendix D calculates the physical size of a hypothetical flywheel ES system fo the enegy capacity of 1, 5, and 20pu fo a 3MVA DFIG. It shows that a 5pu ES system epesents a vey easonable 145

158 6 Wind tubine-es system deliveing a constant demand powe without an auxiliay geneato size of the otating mass. Thus, the capacities of the ES systems in the next two chaptes ae set at 5pu. Chapte 7 will conside a full system consisting of doop-contolled DFIGs, extenal ES, AG and Dispatchable Load (DL). Two diffeent contol appoaches will be discussed. Chapte 8 will study diffeent stuctues and scenaios involved such as fault ide-though, no wind scenaio, etc. 146

159 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato 7. Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato 7.1 Intoduction In Chapte 5 a micogid consisting of a doop-contolled wind fam, an Auxiliay Geneato (AG) and a contollable o Dispatchable Load (DL) was studied while no extenal ES is consideed. It was shown that in such a scenaio the shaft speeds of DFIGs indicate the shotage o excess of the wind enegy fo a given demand powe. If the shaft speed exceeds a high-theshold, the DL is tuned on in ode to shed the excess enegy. Similaly, if the shaft speed dops below a low-theshold, the AG is switched on to inject the enegy shotfall and maintain the demand powe. It was shown that the low-theshold can be even less than the shaft speed at which the instability occus ω stb (Figue 5.5) since the poposed contol stuctue pevents the shaft speed fom futhe eduction and inheently ecoves the system into the stable egion (Figue 5.17). Theefoe, the high- and low-thesholds ae chosen to keep the shaft speed within the opeating egion (i.e pu). Chapte 6 consides a wind geneato-es system when no AG is available. A toque contol scheme using the ES eal cuent was poposed and validated using PSCAD simulations. It was seen that such a scenaio (i.e. no AG) is a pope case study fo a non-maximum Powe Tacking (MPT) contol. Theefoe, Constant Toque Mode (CTM) contol was studied. It was explained that as wind petubation inceases, the shaft speed vaiation fo a given tubine inetia inceases which in tun educes the extent to which P can appoach P ave (the aveage of the extactable wind powe P ext ) without violating the system stability. This chapte studies a full model consisting of doop-contolled wind fam, extenal ES, AG and DL, as shown in Figue 7.1. This wok assumes that the local 147

160 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato gid voltage and fequency ae contolled only by the wind fam using doop chaacteistics. Howeve in pactice, othe geneating units can also paticipate in the voltage and fequency contol. SS Main Gid Local load Local load Local Gid V,f ISFO- Wind Fam AC/DC AC/DC/AC AC/DC T e contol OR ω Enegy Stoage ~ AG DL EMS Figue 7.1. Micogid consisting of doop-contolled wind fam, extenal ES, AG and DL Two appoaches fo contolling the ES ae identified in this chapte: The fist is simila to the contol scheme explained in Chapte 5 in which the shaft inetia is consideed only as an ES mechanism. In Chapte 5 the shaft speed of the DFIG is used to actuate the AG and the DL. In the cuent chapte, howeve, the ES is actuated by the shaft speed. In othe wods, if the shaft speed inceases/deceases too much, the ES is used to absob/inject the excess/shotage of enegy in ode to keep the shaft speed within its opeating egion. This method will be discussed in section 7.3. An altenative appoach, which was explained in Chapte 6, is to use the ES powe to contol the electical toque of the DFIGs. This appoach, which is the main focus of this chapte, is consideed in section 7.4. Chapte 8 will also use the toque contol appoach and will investigate diffeent possible stuctues and scenaios. 148

161 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Each ES system has an enegy capacity limit which is imposed by the ES technology and the physical constaints. Theefoe, it is necessay to ensue that the enegy level of the ES does not exceed the maximum limit, no becomes less than zeo (since negative enegy has no physical meaning). As a esult, in any ES contol method, including the two contol appoaches consideed in this chapte, an Enegy Management System (EMS) is equied to pevent the ES fom satuation. This is explained in the next section. This chapte utilizes the vaiable doop contol method explained in Chapte 5, howeve, the poposed contol schemes in this chapte ae also applicable with the standad doop as well. 7.2 Enegy Management System fo ES The EMS and the pitch angle contol ae illustated in Figue 7.2. Fist the maximum limit of the ES must be detemined. As discussed in the pevious chapte, sizing of the equied ES is not easily possible due to the andom natue of eal wind speed pofiles. Appendix D shows that a 5pu ES fo a 3MVA DFIG epesents a vey modest otating mass, assuming a flywheel ES. Theefoe, in this chapte the maximum enegy capacity of the ES is chosen to be 5pu gen. Howeve, the poposed contol stuctue, shown in Figue 7.2, is quite applicable fo ES systems with othe enegy atings. When the demand powe P is moe than the extactable wind powe (i.e. powe tansmitted to the shaft with β=0) P ext, the enegy stoed in the ES educes to compensate fo the enegy shotage. If the enegy level of the ES dops below a cetain low-theshold E es-low (e.g pu), the AG is tuned on via a eal cuent demand I d-ag in ode to supply the shotfall between P and P ext and pevent the enegy of the ES fom futhe eduction. It is noted that since the local gid voltage is egulated by the DFIG, the AG powe P AG is popotional to I AG contolled by the Powe Flow Contolle (PFC). 149

162 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato β max β R β β β Pes β Ees P g,q g PI Slow P L =P P es P DL PAG E es V s f s 1/2πf s L 0 i ms θ s T e contol OR ω I d-es ES ESI I d-dl I d-ag DL PFC ~ AG Figue 7.2. Enegy Management System and pitch angle contol schemes In anothe situation the demand powe can be too low compaed to the available wind powe. Theefoe, the exta enegy is absobed by the ES and causes E es to incease. If E es exceeds a high-theshold (E es-high1 ), the pitch angle is inceased though the signal β Ees in ode to educe the wind enegy captued. If the maximum slew ate of the pitch angle R β is not fast enough, E es keeps ising until it exceeds a highe-theshold E es-high2 >E es-high1. The DL is then switched on via a convete eal cuent demand I d-dl and absobs the exta wind enegy to pevent the ES fom satuation. In such a case a tade-off mechanism is possible in which a slow pitch angle may esult in a lage DL. It will be shown in this chapte that a pitch contol with a slew ate of maximum 5º/sec, (which is a nomal ate), esults in elimination of the need fo a DL. It is emphasized that the concept of using pitch angle in ode to shed the output powe is quite acceptable and is applied on cuent wind geneatos Pitch angle contol The pitch angle contol, which is shown in Figue 7.2, consists of thee pats: Pes Ees max. The β Pes, which was intoduced in Chapte 6, is used to educe the powe ating of the ES though contolling the aveage of P es towad zeo using a vey slow PI contolle. The bandwidth of the PI contolle is not 150

163 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato citical as system can opeate without this pat. The β Ees is pat of the EMS explained above and can be used to educe the ating of the DL. Finally, the β max is the standad pitch angle contol fo wind geneatos and is used to keep the output powe aound 1pu and to pevent the shaft speed fom exceeding its maximum limit (i.e. 1.3pu) fo wind speeds above ated. The summation of the thee pats is ate-limited to make sue that the pitch angle cannot vay too fast. The next two sections studies the two ES contol appoaches mentioned above. 7.3 ES actuated by shaft speed The idea behind this contol appoach is simila to the one used in Chapte 5 in which the shaft speed is contolled within its opeating egion (i.e pu) using the AG and the DL. In a simila way, the ES can be used to contol the shaft speed within its opeating egion, as is shown in Figue 7.3. β max β R β β β Pes β Ees P g,q g PI Slow P L =P P es P DL PAG E es i ms θ s 1/2πf s L 0 V s f s ω I d-es ES ESI I d-dl I d-ag DL PFC ~ AG E es E es E es Figue 7.3. ISFO-contolled DFIG with extenal ES while ES is actuated by the shaft speed When P is less than P ext, the shaft speed inceases. If ω exceeds a high-theshold value, the ES absobs the exta enegy though a eal cuent demand I d-es which is contolled by the ES inteface (ESI). Theefoe, ω stops ising. Since the local 151

164 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato gid voltage is contolled by the DFIG, the ES powe P es is popotional to I d-es. Fo P moe than P ext, the shaft speed educes. If ω dops below a low-theshold value, the ES injects the enegy shotfall to pevent the shaft speed fom futhe eduction. Compaing this stuctue with the one poposed in Chapte 5 (i.e. without ES), the ES opeates as a buffe between the wind fam, on one side, and the AG and DL, on the othe. This may help to educe the fequency of events in which the AG and the DL ae tuned on. Reducing the tun on and off events is an advantage if fo example the AG is a diesel geneato. As shown in Chapte 5, the low-theshold value can be less than ω stb (see Figue 5.17). As discussed in Chapte 2, the opeating egion of DFIGs shaft speed is pu. Consideing at least 0.1pu fo a safety magin (0.05pu fo each side), the maximum inetial enegy can be stoed is E J =0.5J( )=1.75pu fo J=3.5pu, which is not a vey lage amount of enegy. Theefoe, it is expected that, as wind speed vaies, the shaft speed vaies fom the low-theshold to the hightheshold fequently which in tun causes the ES to switch on and off quite fequently (this will be shown though simulation). It is noted that in Chapte 5 the pitch angle contols the shaft speed. Howeve, since thee is no diect shaft speed contol in this stuctue (i.e. a specific shaft speed efeence), ω vaies as wind speed changes. The fequent tuning on and off of the ES may not be desiable fo cetain ES technologies. Theefoe, the best contol scenaio is to make the shaft speed stay aound the high-theshold value fo P <, and stay aound the lowtheshold value fo P > P ext. This makes the best use of the shaft inetia as an ES mechanism, because the enegy level of the tubine inetia (which is a function of ω ) is aound the maximum limit (high-theshold value) when thee is excess of enegy, while when thee is lack of enegy, ω is aound the minimum (low- P ext theshold) value. In ode to achieve this, the tem E es E es E es is added to the output of the shaft speed limit contolle (Figue 7.3). The E es value is half of the ES capacity and E es is the instantaneous enegy of ES. Theefoe, the tem E es is 152

165 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato P ext always positive fo P >, and is always negative fo P <. Using this method the ω and E es ae synchonized in the sense that, fo P > E es appoach thei maximum limits togethe and fo P <, both ω and, they both stay aound thei minimum limits. In othe wods, the shaft inetia appeas solely as an ES mechanism fo the system. Although this contol stuctue makes the best use of the shaft inetia as an ES mechanism, it will be shown in section 7.4 that this is not necessaily the best wind geneato contol method. The next section validates the poposed contol stuctue using PSCAD simulations. P ext Simulation esults fo ES actuated by shaft speed P ext P ext This section consists of two simulations. The tem E es is consideed in the fist simulation esults while it is not consideed in the second simulation. The objective of the simulation is to validate the poposed contol stuctue. The simulated model, which is shown in Figue 7.4, consists of two vaiable doopcontolled DFIGs with the ES system distibuted within individual DFIGs. In Figue 7.4 the AG and the DL ae aggegated on to the local gid while the ES is distibuted amongst the tubines. Howeve, the DL can also be distibuted within individual DFIGs while the ES can be also aggegated on to the local gid. These stuctues will be investigated in Chapte

166 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato V w1 P g1,q g1 P 1,Q 1 I d-es1 ES β Ees1 EMS DL I DL i ms θ s ω limit contol 1/2πf s L 0 V s f s V f Q P I DL1 Q 1 P 1 I AG1 AG I AG P &Q Load V w2 P g2,q g2 P 2,Q 2 I d-es2 ES β Ees2 EMS Local gid (33kV) i ms θ s ω limit contol 1/2πf s L 0 V s f s V f Q P I DL2 Q 2 P 2 I AG2 Figue 7.4. Doop-contolled wind fam with distibuted ES within individual wind geneatos while the ES is actuated by the shaft speed As it can be seen fom Figue 7.4, each ES system is equipped with the ω limit contol and the EMS explained above. The pitch angle contol (not shown in the Figue) is the same as the one depicted in Figue 7.3 with maximum slew ate of 3º/sec. The paametes of the pitch angles PI contolles ae: k p =0.01 and k i = Each EMS poduces the cuent efeences fo the DL and the AG. The summations I DL and I AG ae fomed and tansfeed ove a communication link since these ae aggegated. Obviously, the communication is local fo the distibuted elements. The atings of the fist and the second DFIGs ae 0.66pu and 0.34pu with the paametes given in Appendix B. Although the vaiable doop method is consideed hee, the standad doop is applicable too. The ES, AG and DL ae simulated by DC-voltage souces connected to the local gid though an AC/DC convete. The enegy capacity of each ES is 5pu gen. Fo ω > 1.2pu, I d es 130ω and fo ω <0.8pu, I d-es =-150ω. Fo E es >3.3pu, β Ees =20E es, fo E es >4.3pu, I DL =50E es and fo E es <0.7pu, I AG =-80E es. These gains ae detemined 154

167 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato by tial and eo, howeve, in pactice they must be designed taking the dynamics of the ES, AG and the DL into account. The slowe the dynamic, gains with lage absolute values ae needed. Moeove, if thee is a delay in tuning on the ES, AG and DL, the high/low-theshold values may need to be deceased/inceased in ode to compensate fo the delay. This thesis is only intended to illustate the poposed contol stuctue and a full-detailed engineeing design is beyond the scope of the eseach. The ES, AG and DL cuent contol loops ae identical to the DFIG gid side convete explained in Chapte 2. The load is simulated by a vaiable cuent souce demanding active and eactive powe detemined by the system opeato. Simulation esults1: E es is consideed This pat simulates the model illustated in Figue 7.4 while the tem E es is included in the ω limit contolles of Figue 7.4. Since the ES capacity is 5pu gen, E es 2.5. The simulation esults ae illustated in Figue 7.5. The aveage of the eal wind speed pofiles (Figue 7.5a) is appoximately 12.5m/sec which coesponds to P ave 1pu gen. The standad deviations of the fist and the second wind pofile ae 1.28 and 1.39 espectively which ae elatively lage petubations. Ove the fist 200sec, the demand powe (Figue 7.5b) is 0.5pu which is less that P ext. Theefoe, the exta wind enegy inceases the shaft speeds (Figue 7.5c). Howeve, since the inetial ES of the tubine is not sufficient, fo ω >1.2pu, the ES systems absob the excess of the wind enegy, hence the E es value (Figue 7.5d) is inceased. As a esult, the shaft speeds stop inceasing. Fo E es >3.3pu gen, the pitch angles (Figue 7.5i) incease in ode to educe the wind enegy captued. It can be seen that with the pitch angle contol with a maximum slew ate of 3º/sec the DL (Figue 7.5h) is not activated. Slowe pitch angle slew ate will be consideed fo the toque-contol ES in the section

168 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue 7.5. Simulation esults of model shown in Figue 7.4 with Ees E es included 156

169 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Duing the second 200sec the demand powe is inceased to 0.75pu which is occasionally moe than P ext. This causes the shaft speeds to educe occasionally. Howeve, since the shaft speeds neve dop to less than 0.8pu, the enegy levels of the ES (Figue 7.5d) do not decease. Ove the last 200sec the demand powe is aised to 1pu which is moe than P ext fo almost the entie peiod. Theefoe, the both shaft speeds dop below 0.8pu which cause the enegy level of the ES to educe. Since the enegy stoed in the ES systems is not enough, the enegy level of the ES systems becomes less than 0.7pu which in tun switches the AG on. The total enegy demanded fom the AG (Figue 7.5g), which is deived by integating P AG (Figue 7.5f), is 50pu. Povided that the wind enegy is enough to fill the ES, inceasing the capacity of the ES can educe the total enegy demanded fom the AG. The ES powes (Figue 7.5e) ae always less than ±0.6pu gen. This means that the ES systems can be connected to the DFIG DC-link, hence educing the equied powe electonic devices. Figue 7.5j shows the eactive powe shaing fo Q =0.15pu. It can be seen fom Figue 7.5k that the local gid voltage and fequency ae well-egulated by the wind geneato though doop contols. E es Simulation esults1: not consideed This pat also simulates the model shown in Figue 7.4. But this time the tem E es is not included in the ω limit contolles. The main objective of this E es simulation is to compae these esults with those with the tem (Figue 7.5). The esults ae shown in Figue 7.6. The same wind pofiles and simulation scenaio as Figue 7.5 is consideed hee. As expected, unlike Figue 7.5, the shaft speeds (Figue 7.6c) vaies fom the low-theshold to the high-theshold quite fequently. This is because in this case the shaft speed is not synchonized with E es (Figue 7.6d) i.e. does not incease o decease with an incease o decease of E es, unlike the one with (Figue 7.5c &d). The method without E equies E es the ES systems to tun on and off vey fequently (as can be seen fom Figue 7.6e) which may not be desied fo some ES technologies. Howeve, inceasing es 157

170 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato the tubine inetia can esult in a smoothe shaft speed vaiation which in tun can educe the ES tun on and off incidents. The eactive powe shaing, pitch angle contol and the local gid voltage and fequency ae identical to those of Figue 7.5 and not shown hee. The esults of Figue 7.6 show that the system can still E es opeate without the tem. Figue 7.6. Simulation esults of model shown in Figue 7.4 while Ees E es is not included In the poposed contol stuctue in this section, in which the ES is actuated by the shaft speed, the tubine inetia is being exploited as an ES mechanism which can have the advantage of educing the extenal ES equied. The main dawback of this appoach is moe evident fo P < P ext. In such cases, the wind geneato is 158

171 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato expected to geneate the maximum wind powe in ode to minimize the enegy demanded fom the AG. It implies that the DFIG must be contolled unde the Maximum Powe Tacking (MPT) mode, which is not the case in the poposed contol stuctue. In the contol scheme poposed in Chapte 5, it is not possible to contol the electical toque T e diectly, as thee is no extenal ES. Howeve in this chapte, the ES eal cuent can be used to contol T e unde MPT, as shown in Chapte 6. This is the subject of the next section. 7.4 ES powe egulating the DFIG electical toque Chapte 6 poposes a T e contol scheme though egulating ES eal cuent. The contol scheme was validated using PSCAD simulation fo contolling the DFIG unde MPT mode. The same contol stuctue, as shown in Figue 7.7, is used in this section. β max β R β β β Pes β Ees P g,q g PI Slow P L =P P es P DL PAG E es i ms θ s 1/2πf s L 0 V s f s T 2 I d-es e =k opt ω I - ES ESI I d-dl I d-ag DL PFC ~ AG Te L0imsiq 3 2 Figue 7.7. Poposed scheme to contol an ISFO-contolled DFIG unde MPT mode The pitch contol and the EMS ae identical to those explained in section 7.2. The toque contol loop design was explained and validated in Chapte 6. In Chapte 6 no AG and DL wee consideed. Howeve, it was shown that the load demand powe appeas as a distubance and can be ignoed in the contol design. Similaly, 159

172 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato the AG and DL powes can be neglected fo designing the toque contol loop. The effectiveness of the poposed contol loop in a multi-dfig system including the AG and the DL will be validated in this section though PSCAD simulations Simulation esults fo ES contolling the DFIG electical toque This section is intended to validate the poposed toque contol scheme, the EMS and the pitch contol in a multi-dfig system. Two simulations ae undetaken in this section. The fist simulation consides a pitch angle contol with nomal slew ate while the second one consides a slow pitch contol. The objective hee is to show the tade-off between the pitch angle slew ate and the size of the equied DL. The model simulated in this section is depicted in Figue 7.8. Although the ES systems ae distibuted within individual DFIGs, it will be shown in Chapte 8 that the ES can be aggegated on to the local gid which necessitates communications. The atings of the fist and the second DFIGs ae 0.66pu and 0.34pu with paametes given in Appendix B. The pitch angle contols (not shown in the figue) and the EMSs ae identical to the one shown in Figue 7.7. The theshold values fo the EMSs ae those explained in section It was explained in Chapte 6 that an integal contol is sufficient fo the toque contol loop since the contolled plant is modeled by a gain. Fo a toque contol with bandwidth of 15Hz, the integal contol gain is 240, which is used in the two simulations caied out in this section. 160

173 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato V w1 P g1,q g1 P 1,Q 1 I d-es1 ES β Ees1 EMS DL I DL T 2 e =k opt ω 1 I - i ms θ s 1/2πf s L 0 V s T e1 f s V f Q P I DL1 Q 1 P 1 I AG1 AG I AG P &Q Load V w2 P g2,q g2 P 2,Q 2 I d-es2 ES β Ees2 EMS Local gid (33kV) T e =k opt ω I I DL2 I AG2 i ms θ s 1/2πf s L 0 V s T e2 f s V f Q P Q 2 P 2 Figue 7.8. Doop-contolled wind fam while distibuted ES within individual DFIGs contols the electical toque of the associated DFIG Simulation esults1: Nomal slew ate fo pitch contolles The simulated model is shown in Figue 7.8. The pitch angle maximum slew ate R β =5º/sec. It is mentioned in [45] that the pitch speed can exceed 10º/sec duing emegencies. Theefoe, 5º/sec seems to be a nomal slew ate. The simulation esults ae shown in Figue 7.9. The wind speed and the simulation scenaio ae identical to those of Figue 7.5 (i.e. ES actuated by the shaft speeds). Duing the fist 200sec, P =0.5pu which is less than P ext. As a esult, E es (Figue 7.9d) inceases. Fo E es >3.3pu gen, the pitch angles (Figue 7.9i) ise to pevent E es fom futhe incease and hence intoducing the DL. It is noted that the shaft speeds (Figue 7.9c), unlike Figue 7.5c, do not incease to thei maximum limit. This is because in this stuctue the shaft speeds ae contolled (indiectly) in ode to extact the maximum wind powe. Consequently, a slightly faste pitch contol (5º/sec) than the one used in Figue 7.5i (i.e. 3º/sec) is equied in ode to 161

174 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato eliminate the need fo a DL. Simila to Figue 7.5, ove the second 200sec duing which P =0.75pu, no AG and DL is needed. Duing the last 200sec, demand powe is inceased to 1pu. As in Figue 7.5, the enegy levels of the ES educe to less than 0.7pu gen which tuns on the AG (Figue 7.9f). Howeve, unlike Figue 7.5c, the shaft speeds (Figue 7.9c) do not decease. This is because in Figue 7.5 the oto inetia is exploited as an ES mechanism and its kinetic enegy (ω ) educes when thee is lack of enegy (i.e. P > ). Howeve in Figue 7.9, the shaft speeds stay at elatively high values in ode to tack the maximum wind powe. As a esult only 25pu enegy is demanded fom the AG (Figue 7.9g) which is half that in the case of the shaft speed-actuated ES system (Figue 7.5g). Figue 7.10 shows the DFIGs output powes vs thei shaft speeds on the P t -ω chaacteistics fo diffeent wind speeds. Figue 7.10 demonstates the effectiveness of the poposed toque contol scheme in a multi-dfig system including an AG and a DL. It can be concluded that the advantage of this scheme is that it significantly educes the equied enegy fom the AG fo P >.This advantage comes with a slightly faste pitch angle equiements in ode to eliminate the need fo a DL fo P <. Howeve, consideing the DL as a long-tem ES (such as hydogen geneation station, compessed ai, etc), geneating maximum wind powe can be consideed as an advantage athe than a disadvantage. An altenative appoach is to opeate in a non-mpt mode (e.g. CTM o the shaft speed-actuated ES contol method explained above) fo E es > (when P < ) in ode to educe the DL powe, and also to switch ove to the MPT mode fo E es < (when P > ) in ode to educe the AG powe. This appoach, howeve, is not consideed in this wok. P ext E es P ext P ext P ext E es P ext 162

175 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue 7.9. Simulation esults of model shown in Figue 7.8 with pitch angle slew ate of 5º/sec 163

176 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue DFIGs output powes vs thei shaft speeds on P t -ω chaacteistics fo diffeent wind speeds Simulation esults2: Slow slew ate fo pitch contolles This section also simulates the model shown in Figue 7.8. But this time the maximum pitch angle slew ate is 1º/sec. The simulation esults ae shown in Figue The wind speed pofiles, simulation scenaio, DFIGs atings, EMS thesholds and the toque contols ae the same as the pevious simulation (i.e. with 5º/sec pitch angle slew ate). The slow pitch angle (Figue 7.11e) slew ate esults in the intoduction of the DL (Figue 7.11g) duing the fist 200sec in which P =0.5pu. This is not the case fo the nomal pitch angle slew ate (Figue 7.9h). The slow pitch angle also causes the AG (Figue 7.11f) to inject enegy fo P =0.5 and P =0.75pu. Note that no enegy fom the AG is needed fo P =0.5 and P =0.75pu in the case of a nomal pitch angle slew ate (Figue 7.9f). This is simply because the pitch angle can etun to smalle values fast enough in ode to incease the output DFIG powe and to pevent the intoduction of the AG. 164

177 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue Simulation esults of model shown in Figue 7.8 with pitch angle slew ate of 1º/sec It is noted that the incease in the AG powe is an inevitable consequence of the slow pitch angle contol which is also the case fo the standad DSFO-contolled DFIG. Figue 7.11c shows that despite the slow pitch contol, the ES powes ae 165

178 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato still less than ±0.6pu gn which suggest that the ES can be connected to the DFIGs DC-link. The eactive powe shaing, local gid voltage and fequency ae identical to the pevious simulation and not shown hee. The esults given in Figue 7.9 and Figue 7.11 demonstate the tade-off between the pitch angle slew ate and the size of the DL. The faste the pitch angle, the smalle the DL is equied. Obviously, it is possible to have DL with nomal slew pitch contol by educing (o even eliminating) β Ees (Figue 7.7), if equied. 7.5 Toque contolling-es system using simplified DFIG model PSCAD simulation of a multi-dfig system including wind tubine, pitch contolles, eal wind speed pofiles and ES; connected to the AG and DL fo the 600s simulation takes seveal hous. A simplified ISFO-contolled DFIG was intoduced in section 5.4 in ode to incease the numbe of the simulated DFIGs. The same concept is also used in Figue 7.12 in ode to simplify a doopcontolled DFIG equipped with T e -contolling ES scheme. In the simplified model the DFIG is epesented by a doop-contolled voltage souce while its electical toque is deived by T e =P g /ω, whee P g is the DFIG output powe. The full wind tubine model is, howeve, included. The next section used the simplified model to simulate an aay of doop-contolled DFIGs using the vaiable doop method intoduced in Chapte

179 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato P 1 Q 1 Doops f s V s P g1 β Ees1 DL I DL V w1 β 1 Wind tubine Eq(5.1) T e1 T m1 / Tm Te J ω 1 T e - I ES EMS I DL1 I AG1 AG P &Q Load I AG P 2 Q 2 Doops f s V s P g2 β Ees2 V w2 β 2 Wind tubine Eq(5.1) T e2 T m2 / Tm Te J ω 2 T e - I ES I DL2 EMS I AG2 Local gid (33kV) Figue Simplified doop-contolled DFIGs equipped with toque contolling-es systems Simulation esults of fou simplified doop-contolled DFIGs equipped with distibuted T e -contolling ES systems The model simulated in this section consists of fou simplified doop-contolled DFIGs with distibuted ES systems contolling the associated DFIG s electical toque, as is shown in Figue In Figue 7.12, fo the sake of simplicity, only two DFIGs ae depicted. Howeve in the model simulated, fou DFIGs ae used. The doop chaacteistics, toque contol loop, EMS and pitch contols (not shown in the figue) ae the same as befoe. The objective of this simulation is to validate the EMS and the toque contol scheme in a system consisting of moe than two DFIGs. The ating of the fist and the thid DFIGs ae 0.33pu while those of the second and the foth DFIGs ae 0.17pu. Thee ae two eal wind speed pofiles (Figue 7.13a) with the aveage of appoximately 12.5m/s ( P ave ) and the 1pu gen 167

180 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato standad deviations of 1.28 and The fist wind speed pofile is applied to the fist and the fouth DFIGs while the second wind speed pofile is applied to the second and the thid DFIGs. Theefoe, the DFIGs with the same ating ae applied with diffeent wind speed pofiles. The simulation esults ae show in Figue Fo the fist 200sec, P =1pu (Figue 7.13b). Theefoe the enegy levels of the ESs (Figue 7.13c&d) dop below 0.7pu gen which tuns on the AG (Figue 7.13k). Ove the second 200sec, P =0.75pu. Since the maximum pitch angle (Figue 7.13i&j) slew ates ae 3º/sec, the AG and DL (Figue 7.13l) powes ae zeo. And finally duing the last 200sec, P =0.75pu which is less than. Since the pitch angle is not fast enough, some DL powe is equied. The 3º/sec slew ate is chosen intentionally in ode to test both AG and DL. It can be shown that using a slew ate of 5º/sec will make the pesence of a DL unnecessay. It can be seen that the ES powes (Figue 7.13e&f) ae much less than ±0.6pu gen which implies that the ES systems can be connected to the DFIGs DC-link. Figue 7.13m shows the eactive powe shaing fo Q =0.15pu. Figue 7.13n illustates that the local gid P ext voltage and fequency ae well-contolled by the doop-contolled DFIGs. 168

181 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue 7.13 (Pat 1): A micogid including fou doop-contolled DFIGs with distibuted T e - contolling ES systems, AG and DL 169

182 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato Figue (pat 2) A micogid including fou doop-contolled DFIGs with distibuted T e - contolling ES systems, AG and DL 170

183 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato 7.6 Discussions and conclusions This chapte consides a micogid consisting of doop-contolled DFIGs, ES, AG and DL while the local gid voltage and fequency ae fully contolled by the DFIGs. Two methods fo contolling the ES system have been investigated. The fist method exploits the tubine inetia as an ES mechanism. Hence, the shaft speed indicates the lack o excess of enegy. In this scheme when ω is less/moe than a cetain theshold, the ES injects/absobs the enegy shotfall/excess. Howeve in the second appoach the ES powe is egulated in ode to contol the electical toque of the DFIG unde MPT mode. Fo both appoaches a pitch angle contol consisting of thee components has been poposed. The fist component is activated fo E es >E es-high1 in ode to educe the wind enegy captued. The second component is the standad pitch contol which is used in ode to maintain the output powe at 1pu fo wind speeds above ated. The thid component, the pesence of which is not citical fo the system, is included to educe the powe ating of the ES though contolling the aveage of P es towads zeo using a slow PI contolle. It is noted that the pitch angle contolle poposed in Chapte 5 cannot be used hee as it pevents the ES fom chaging up fo P < P ext. Howeve, the poposed pitch contolle in this chapte allows the ES to chage up fo P < P ext which late can be used to compensate fo the enegy shotage in case of P > P ext. An EMS has been also suggested fo the ES in which an AG/DL is used to inject/absob the enegy shotage/excess if E es dops/aises below/above E es-low /E es-high2 (E es-high2 >E es-high1 ). The EMS is needed in ode to pevent the ES fom satuation. The poposed EMS and pitch contolle ae validated fo the both ES contol appoaches using PSCAD simulations. It was shown that thee is a tade-off between the pitch angle slew ate and the powe ating of the DL in which a nomal slew ate (3-5º/sec) can lead to the DL being unnecessay. It was also illustated that egulating the ES powe in ode to contol the DFIG unde MPT mode can significantly educe the equied enegy fom the AG. Howeve, this 171

184 7 Doop-contolled wind fam deliveing a constant demand powe with extenal ES and auxiliay geneato comes at the expense of a slightly faste pitch contol (5º/sec) equied than that of the ω -actuated ES contol (3º/sec) to eliminate the need fo a DL. This is simply because the DFIG is tacking the maximum wind powe which implies that eithe a faste pitch contol o a lage DL is equied fo P < P ext. A lage DL, howeve, is not necessaily a disadvantage, when the DL is a long-tem ES (e.g. a hydogen geneation station). Finally, a simplified DFIG-ES system has been poposed in ode to illustate the EMS, toque contol stuctue and pitch contol in a multi-dfig system with moe than two DFIGs. The next chapte studies the diffeent scenaios and possible stuctues fo the poposed contol scheme. 172

185 8 Studying diffeent system stuctues and opeational scenaios 8. Studying diffeent system stuctues and opeational scenaios 8.1 Intoduction In the pevious chapte a micogid consists of an aay of doop-contolled DFIGs, ES systems, AG, and DL was consideed, as shown in Figue 8.1. SS Main Gid Local load Local load Local Gid V,f ISFO- Wind Fam AC/DC AC/DC/AC AC/DC T e contol Enegy Stoage ~ AG DL EMS Figue 8.1. A micogid consists of an aay of doop-contolled DFIGs, ES, AG, and DL In Chapte 7 the ES systems wee distibuted within individual DFIGs (unlike Figue 8.1) while the DL and the AG wee aggegated on to the local gid. Unlike the AG, the DL can also be distibuted within each DIFG which may to be a bette place fo a DL acting as a esistive dump load. Likewise, the ES system can be eithe distibuted within individual DFIGs o aggegated on to the local gid. The cuent chapte consides the micogid with the same components as that of the Chapte 7 and is intended to investigate the possible system stuctues egading 173

186 8 Studying diffeent system stuctues and opeational scenaios the ES and DL placement. This is the subject of the sections 8.3 and 8.4. Befoe this the fault ide-though and no wind powe scenaios ae investigated fo the micogid stuctue used in the pevious chapte. 8.2 Zeo wind speed and fault ide-though scenaios In this section two scenaios ae studied. The fist consides the ide-though stategy in case of zeo wind speed and the second studies the ide-though in case of a 3-phase fault on the local gid. V w1 P g1,q g1 P 1,Q 1 I d-es1 ES β Ees1 EMS DL I DL T e =k opt ω I I DL1 I AG1 AG I AG i ms θ s 1/2πf s L 0 V s T e1 f s V f Q P Q 1 P 1 P &Q Load V w2 P g2,q g2 P 2,Q 2 I d-es2 ES β Ees2 EMS Local gid (33kV) T e =k opt ω I I DL2 I AG2 i ms θ s T e2 1/2πf s L 0 V s f s V f Q P Q 2 P 2 Figue 8.2. An aay of doop-contolled DFIGs with distibuted ES within each DFIG and aggegated DL on to the local gid In both scenaios the system will have a DL aggegated on to the local gid while the ES systems ae distibuted within individual DFIGs. This stuctue, which is shown in Figue 8.2, is actually the one studied in the pevious chapte. 174

187 8 Studying diffeent system stuctues and opeational scenaios Zeo wind speed ide-though In a standad DSFO-contolled wind fam, fo wind speeds less than cut-in, the wind tubines simply stop geneation. Howeve, the challenge in a doopcontolled wind fam is that the wind geneatos also contol the local gid voltage and fequency. The eactive powe-voltage contol is not the main challenge hee since, fo example, the DFIGs gid side convetes can also be used to suppot the local gid voltage, apat fom the machines statos. The active powe-fequency contol can, howeve, be quite a challenge. One possible solution is to make the AG take ove the local gid fequency contol as the DFIGs out put powes die out. It may, howeve, estict the choice fo the AG. Anothe altenative solution is to contol the DFIGs gid side convetes like STATCOM units in ode to contol the local gid fequency, as well as its voltage. Howeve, this may equie a sophisticated contol because the contol scheme must switch fom contolling the eal powe fom the machine oto to a STATCOM contol such as the one explained in Appendix C. This method is not studied in this thesis. Theefoe, the main question hee is whethe the doop-contolled DFIGs can still contol the local gid fequency with no wind powe. The PSCAD simulations show that one DFIG (i.e. without doop) can contol the local gid voltage and fequency even when the wind speed becomes zeo. Howeve, it becomes complicated in the case of a multi-dfig system equipped with the vaiable doop contol due to the fact that the vaiable doop gains vaies accoding to the wind powe. It was discussed in Chapte 5 that in a multi-dfig system contolled unde the standad doop, if the wind speed of one DFIG dops, the output powes of the othe DFIGs will also educe to comply with the new opeating fequency imposed by the fist DFIG. This poblem was addessed by the vaiable doop method explained in Chapte 5. The vaiable doop contol method vaies the f-p doop gains accoding to the available wind powe, not just the DFIGs atings. It was shown that the vaiable doop method significantly educes the enegy equied fom the AG. Although the vaiable doop method was used in Chaptes 6 and 7, it can be shown that the poposed contol schemes can wok with the standad doop at the expense of moe enegy demanded fom the AG. Having said that, the wost 175

188 8 Studying diffeent system stuctues and opeational scenaios case scenaio fo the standad doop is when the wind speed of one of the DFIGs dops just above the cut-in wind seed and stays thee fo a long time. As a esult, all DFIGs will geneate accoding to the lowest wind powe which will significantly incease the enegy demanded fom the AG. To avoid such a situation the vaiable doop contol is used. The poblem with the vaiable doop gain method is that the system may become unstable fo lage doop gains (i.e. small shaft speed). Howeve, it will be illustated in this section that a system contolled unde the vaiable doop gain method only becomes unstable fo shaft speed consideably less than 0.7pu which is the minimum DFIG opeating shaft speed. Following zeo wind speed in a DFIG, its shaft speed dops. In ode to avoid instability, fo ω < (0.7-Δω)pu, the DFGI s contol is switched fom the doop contol to the standad DSFO contol i.e.: The stato flux angle is deived by voltage and cuent measuements using, fo example, a PLL (instead of the integation of the efeence fequency). The q-component of the oto cuent contols the active powe with P =0 Ls (instead of iq isq). L 0 The integal contol of the T e contol scheme is eset i.e. I d-es =0. Obviously, in ode to contol the local gid voltage and fequency, the final DFIG must stay unde ISFO contol. Howeve this DFIG (i.e. the ISFO-contolled one) must opeate eithe without the doop contol o with the standad doop (not the vaiable doop so as to avoid the instability when its shaft speed becomes vey small). This will equie a communication between DFIGs though a cental contol unit. Howeve, this cental contol unit is much simple than those equied in a DSFO-contolled wind fam geneating a demand powe. 176

189 8 Studying diffeent system stuctues and opeational scenaios Simulation esults of zeo wind speed ide-though This section is intended to illustate the zeo wind speed ide-though pocedue explained above. The model simulated is shown in Figue 8.2. The ating of the fist DFIG is 0.66pu while that of the second one is 0.34pu. The toque contol, EMS and pitch contol (not shown in the figue) ae identical to those explained in the pevious chapte. The local gid voltage and fequency ae egulated by the DFIGs though the vaiable doop contol intoduced in Chapte 5. The simulation esults ae shown in Figue 8.3. At 3sec, the wind speed (Figue 8.3a) of the fist DFIG (lage one) dops to zeo which causes its output powe (Figue 8.3b1) and shaft speed (Figue 8.3c1) to educe. It is noted that if the system was contolled unde the standad doop, the second DFIG output powe (Figue 8.3b2) and shaft speed (Figue 8.3c3) would also educe. Since the demand powe (Figue 8.3b) is 1pu (which is moe than the extactable wind powe P ext ), the enegy level of the ES (Figue 8.4d) becomes less than 1pu which tuns on the AG (Figue 8.3f) to compensate fo the enegy shotfall. It can be seen fom Figue 8.3c1 that the fist DFIG shaft speed becomes less than 0.7pu while the vaiable doop-contolled DFIGs ae still stable. When the shaft speed of the fist DFIG becomes 0.3pu, the DIG contol is switched ove to the standad DSFO contol with P =0. The 0.3pu shaft speed theshold is chosen in ode to show that the system contolled unde vaiable doop method is still stable fo a shaft speed much less than 0.7pu. In pactice the theshold can be just below 0.7pu. In the simulation, in ode to make the shaft speed zeo, the PSCAD induction machine model is switched fom toque mode to shaft speed mode with ω =0. Theefoe, the shaft speed and the output powe of the fist DFIG becomes zeo while the local gid voltage and fequency ae contolled by the second DFIG (smalle one). At 28sec, the wind speed of the second DFIG also becomes zeo which causes eductions in its output powe and shaft speed. Consequently, the AG powe inceases to maintain the demand powe. Since DFIG 2 is the final DFIG, when its shaft speed dops to 0.3pu, the DFIG will still be contolled unde ISFO mode but the doop contols ae emoved. 177

190 8 Studying diffeent system stuctues and opeational scenaios Figue 8.3. Zeo wind speed opeation of an aay of doop-contolled DFIGs 178

191 8 Studying diffeent system stuctues and opeational scenaios As a esult, the local gid fequency (Figue 8.3h) is povided by the second DFIG (which is 0.34pu) even though its wind speed is zeo. It is noted that the eactive powe-voltage doop is still active in both DFIGs. The total active powe demand is supplied by the AG. Simila to the fist DFIG, when the shaft speed of the second (final) DFIG dops to 0.3pu, the integal contol of its T e contol scheme is eset which causes the small tansient fluctuation in eactive powe shaing (Figue 8.3g). Theefoe, the zeo wind powe ide-though can be summaized as follows: Fo all DFIGs befoe the final, if ω < 0.3pu (o a value less than 0.7pu), the DFIG contol scheme is switched fom ISFO to DSFO method with P =0 and I d-es =0. Fo the final DFIG, if ω < 0.3pu (o a value less than 0.7pu), the doop contols ae emoved and I d-es =0. This simulation esults illustate the zeo wind speed ide-though pocedue explained above and also demonstate the ability of the ISFO-contolled DFIGs to contol the local gid voltage and fequency even when the wind speed dops to zeo. It also shows that a vaiable doop-contolled wind fam is stable fo shaft speed consideably less than 0.7pu (i.e. down to 0.3pu). In othe wods, it seems that a multi-dfig system contolled unde the vaiable doop method is stable within the opeating egion of the DFIG. Howeve, the autho admits that a full system stability studies fo the vaiable doop contol method is still equied which is out of the scope of this thesis Fault ide-though to a balanced gid fault A majo challenge fo wind enegy geneation, especially fo those with DFIGs, is thei opeation duing gid faults [85]. New gid code equies the wind tubines to ide-though voltage sags which means that nomal powe poduction should be e-initiated once the nominal gid voltage has been ecoveed [50]. Anothe challenge, which is moe cucial fo lage wind fams, is to pomptly educe the 179

192 8 Studying diffeent system stuctues and opeational scenaios wind powe geneation towad zeo afte a fault on the gid since thee is no place fo the wind powe to flow. In [48] an offshoe wind fam connected to a LCC- HVDC link is consideed and the autho, as a membe of the eseach team, utilized the doop method in ode to ide-though a balance fault on the AC gid. In [48], upon fault detection, the DFIGs ae switched fom the DSFO-contolled to doop-contolled and hence the wind powe geneation automatically dops to zeo. It was shown in [48] that using the doop method a vey good fault idethough pefomance is achieved. Moeove, the doop method eliminates the inteacting cuent flowing between the DFIGs following the fault. Howeve, the dawback is the communication equied fom the AC gid to the DFIGs in ode to switch to fault mode. It was found in [48] that a communication delay moe than 20ms would make the wind fam gid voltage and fequency go outside the acceptable ange. This eseach poposes a contol stuctue which uses the doop chaacteistics duing nomal opeation. Theefoe it is expected that the poposed system contol to ide though a balanced fault on the local gid without the need fo communication as both the nomal and fault opeations ae based on doop method. This is the subject of this section. This study is esticted to a balanced 3- phase fault since it is the wost case fault fo a DFIG based wind fam [48, 50] Simulation esults of fault ide-though on local gid This section investigates a 3-phase fault ide-though scenaio on the local gid as is shown in Figue 8.4. The fault is simulated by the PSCAD standad 3-phase fault component and lasts 150ms. It is assumed that afte 150ms, the potection elays emove the fault and the load is supplied by anothe paallel connection line. The line inductance between the fault location (Figue 8.4) and the wind fam gid is 0.05mH. The fault location is chosen to be close to the wind fam gid in ode to make the wind fam gid voltage dops to zeo following the fault which is the wost case scenaio. 180

193 8 Studying diffeent system stuctues and opeational scenaios V w1 P g1,q g1 P 1,Q 1 I d-es1 ES β Ees1 EMS DL I DL T e =k opt ω I I DL1 I AG1 AG I AG i ms θ s 1/2πf s L 0 V s T e1 f s V f Q P Q 1 P 1 P &Q Load V w2 P g2,q g2 P 2,Q 2 I d-es2 ES β Ees2 EMS Local gid (33kV) T e =k opt ω I I DL2 I AG2 i ms θ s T e2 1/2πf s L 0 V s f s V f Q P Q 2 P 2 Figue 8.4. A 3-phase fault on the local gid of a doop-contolled wind fam The atings of the DFIGs ae 0.66pu and 0.34pu with paametes given in Appendix B. The toque contolles, EMSs and pitch contolles (not shown in the figue) ae identical to those explained in the pevious chapte. The simulation esults ae shown in Figue 8.5. The balanced fault occus at 3sec. The constant wind speed (Figue 8.5a) is 12.55m/sec (coesponding to almost 1pu gen extactable wind powe). It is noted that since the fault lasts only 150ms, the wind speed vaiations do not matte. The active (Figue 8.5b) and eactive (Figue 8.5c) powe demands befoe fault ae 1pu and 0.1pu, espectively. It can be seen that the local gid voltage (Figue 8.5f) and fequency (Figue 8.5g) dop to almost 0kV and 42Hz, espectively. As a esult, the measued active and eactive demand powe become zeo and so do the active and eactive powe fom each DFIG. Howeve within 0.5sec afte fault cleaance, the local gid voltage and fequency ae estoed to thei pe-fault values and so ae the active and eactive powes. 181

194 8 Studying diffeent system stuctues and opeational scenaios Figue 8.5. Results of the fault ide-though of the model shown in Figue

195 8 Studying diffeent system stuctues and opeational scenaios Figue 8.5d shows that the shaft speed vaiations of both DFIGs ae within the opeational egion (note that it is the wost case scenaio as the demand powe is 1pu). Figue 8.5e shows that the ES powe vaiations ae less than ±0.6pu gen. These esults demonstate the ability of the poposed contol scheme to idethough a fault on the local gid with no need fo communications. It is noted that the standad DFIG potection schemes ae still applicable. 8.3 System studies when ES and DL ae distibuted The DL can be esisto banks, a hydogen geneation station o an iigation system. Fo bulky DL, such as hydogen geneation, distibuting the DL within individual DFIGs is pobably not possible. Howeve, fo DL like esisto sets, distibuting the DL within each DFIG seems not only possible but also easonable since it only equies local communication between the DFIG and its associated DL. This section consides an aay of doop contolled DFIGs while the ES and DL ae distibuted within individual DFIGs, as shown in Figue 8.6. The contol paadigms of the EMS, toque contol and pitch contol ae identical to Chapte 7. Although both the standad and vaiable doop methods ae possible, only vaiable doop is simulated hee. 183

196 8 Studying diffeent system stuctues and opeational scenaios V w1 P g1,q g1 P 1,Q 1 T 2 e =k opt ω 1 I - i ms θ s 1/2πf s L 0 V s T e1 I DL1 f s DL I d-es1 V Q f P ES Q 1 P 1 β Ees1 EMS I DL1 I AG1 AG I AG P &Q Load V P g2,q g2 P 2,Q 2 w2 T 2 e =k opt ω 2 I - i ms θ s 1/2πf s L 0 V s T e2 I DL2 f s DL I d-es2 V Q f P ES Q 2 P 2 β Ees2 EMS I DL2 I AG2 Local gid (33kV) Figue 8.6. An aay of doop contolled DFIGs with distibuted ES and DL within each DFIG Simulation esults of distibuted ES and DL The model simulated hee is shown in Figue 8.6. The objective of this simulation is to illustate the functionality of the doop-contolled wind fam, EMS, toque contol and pitch contol fo the distibuted DL and ES. The capacity of the ES systems is 5pu gen. The thesholds of the EMSs and the pitch contol (not shown in the figue) ae the same as those used in Chapte 7.The pitch angle slew ate is 3º/sec. Since the DL is distibuted, the communication is local. Howeve, the communication fom the DFIGs to the AG is still needed. The atings of the fist and the second DFIGs ae 0.66pu and 0.34pu espectively. The simulation esults ae shown in Figue 8.7. The aveage of the two eal wind speed pofiles (Figue 8.7a) is 12.5m/sec ( 1.39, espectively. P ave 1pu gen ) while thei standad deviations ae 1.28 and 184

197 8 Studying diffeent system stuctues and opeational scenaios Figue 8.7. Simulation esults of distibuted ES and DL 185