Transient Modeling and Comparison of Wind Generator Topologies

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1 Transient Modeling and Comparison of Wind Generator Topologies C. Abbey, Student Member, B. Khodabakhhn, Member, F. Zhou, Non-member, S. Dennetière, Member J. Mahseredjn, Member, and G. Joos, Senior Member Abstrat--There urrently exist rious ompeting tehnologies for wind generator systems, whose differenes lie in the omplexity, ost, and degree of ontrol over the system haraterists. This paper presents the full transient models in EMTP-RV for two of the most popular topologies, namely the doubly-fed indution generator (DFIG) and the diretly onneted indution generator (IG). In addition, the system haraterists of these two generator designs are ompared by simulation of two wind parks, one omposed of eah of the designs. As the reative power ontrol is muh greater in the ase of the DFIG, a stat ompensator (STATCOM) of idental reative power rating as the ombined DFIGs is added to the IG wind park design. In this way, the two systems are similar in reative power apability as w. ell as ost and omplexity and a omparison of the different operating haraterists is obtained. The modeling of the rious omponents is presented and the system haraterists of the two wind parks are ontrasted. Keywords: wind energy, indution generator, doubly-fed indution generator, STATCOM, voltage stability W I. INTRODUCTION IND energy has grown dramatally over the past deade and while government poly has helped to fuel this growth, inreasing utility interonnetion requirements has resulted in a number of emerging wind generator tehnologies. Strter requirements demanding aurate reative power ontrol and voltage regulation apability has led to the integration of power eletron onverter in many of the wind generator designs. This allows better overall ontrol of the generator, both in terms of reative power ontrol but also limited ontrol of real power haraterists as well. Due to the inherent nature of wind, eletral power based on this energy soure possesses similar haraterists. Furthermore, wind farms are usually interonneted at remote loations, far from entral loads Finanl support was provided in part by Natural Siene and Engineering Researh Counil (NSERC) of Canada. C. Abbey ( had.abbey@mail.mgill.a), F. Zhou, and G. Joos are with MGill University, Montréal, Canada. B. Khodabahkhn is with TransÉnergie Tehnologies (Hydro-Québe). J. Mahseredijan is with Éole Polytehnue de Montréal. S. Dennetière is with Életrité de Frane. Presented at the International Conferene on Power Systems Transients (IPST 05) in Montreal, Canada on June 9-3, 005 Paper No. IPST05-3 or onventional generation. This imposes strter requirements on the reative power ontrol of the system. The majority of wind generator topologies are based upon asynhronous mahines, whh only tends to further aggrate the situation. Various studies have been performed to ompare the performane of fixed speed and rble speed mahines, [][], however, these studies have negleted to math the reative power apabilities of the two topologies. Through omparison of a fixed speed indution generator omplemented with a stat ompensator, with the rble speed mahine, the differenes between rble speed and fixed speed mahines an be highlighted. The need to aurately model these new forms of generation and investigate their interation with the network beomes inreasingly important as the penetration of wind in ertain areas reahes signifant levels. This paper presents the transient modeling of two ompeting wind generation tehnologies and ontrasts their operating haraterists, both under an intat network and in response to transients. II. BACKGROUND Wind energy is one of the fastest growing industries at present and will ontinue to grow worldwide, as many ountries have planned future development. Following reent growth of wind, utilities have responded by developing rious interonnetion requirements to whh new wind generators must abide. Many wind generators are based on asynhronous mahines, with a great number being of the diretly onneted squirrel age mahine riety. Therefore, reative power is a major onern, not only to ompensate for the reative power requirements of the wind park itself but also to help support the system voltage. Most of the requirements define a typal power fator range that the wind park should be at least able to maintain. The methods of reative power ontrol are left up to the designers and may ome in the form of apaitor banks, stat power onverter based deves (SVC, TSC, or STATCOM) or by employing mahines apable of reative power ontrol, suh as the doubly-fed indution mahine topology. Low voltage ride-through is seen to be partularly important in terms of maintaining voltage stability,

2 espelly when there is a high loal onentration of wind generation. Premature tripping of numerous wind generators due to loal disturbanes an further risk the stability of the system, ontruting to amplifation of the effet of the disturbane. This reflets the onern on maintaining stability of the wind farm and the assoted system during normally ourring power system disturbanes. The issue of power management of wind farms is beoming more important and the trend is suh that utilities may eventually demand that wind generators behave in a manner analogous to onventional types of generation. This is a requirement that is subjet to muh debate sine wind is often looked at, perhaps unfairly, as being unrelble from a power prodution stand-point and that a large spinning reserve, almost equilent to the installed apaity needs to be maintained. However, muh of the work in Europe has been attempting to prove otherwise, perhaps resulting from stringent standards imposed upon the power ontrol requirements of wind turbines there [3][4]. A. Generator topologies Varble speed wind generators are inreasingly used, as they present higher energy apture, lower mehanal stresses, more onstant output power, and redued noise ompared with fixed speed mahines, [4][5]. Diret drive tehnology using high pole permanent magnet synhronous generators are apable of rble speed operation, where the generator is onneted to the system using a bak-tobak voltage soure onverter (VSC). The doubly-fed indution generator (DFIG) is another ommon rble speed topology whh utilizes wound-rotor indution mahines with an a-d-a onverter between the stator and rotor terminals. The most ommon wind generator arrangements are shown in Fig. Fig.. Wind generator tehnologies The three main wind generator topologies eah differ in terms of overall ost, omponent ount, omplexity, ontrol potentl, protetion, and integration osts. The requirements plaed upon the wind park installation by the utility will to a large extent determine additional equipment that is required or whether the purhase of anillary serves will be neessary. For instane, reative power ontrol is almost without exeption required as previously mentioned. Therefore, diretly oupled indution mahines require at the very least swithed apaitors, while some are moving to reative ompensation deve based upon solid-state deves. III. SYSTEM MODELS The development of full transient models of the rious wind turbine topologies is of interest in order to investigate the effet of an intermittent soure on power quality, harmon injetion by power eletron onverters, voltage ontrol haraterists, and their response to disturbanes and short iruit behavior. Here, the fixed speed topology is presented and ompared with the doubly-fed indution generator (DFIG), one of the popular rble speed generators, followed by their operating haraterists. A. Wind models For the sake of determining the effet of wind generation on the eletral haraterists, it is only neessary to translate the input wind speed into a mehanal torque, whh is then applied to the blades of the generator. The relationship is one that depends not on only the wind speed but also the speed of the generator and naturally the aerodynams of the turbine in question. Wind speed measurements ailable from [6] were used in the study to model the wind itself. The input torque is most simply represented by the following equation: 3 = πρc p( λ, β ) R vw () where C p (λ,β) is the haraterist urve whh depends on the generator in question, ρ is the air density, R is the radius of the blades, v w is the wind speed, β is the blade angle, and λ is the tip-speed ratio, i.e. the ratio of the speed at the tip of the blades to the speed of the wind, otherwise: ωmr λ = () v Upon alulation of the input torque of the generator, it is applied to the two mass model of the mahine. The model was developed for a MVA generator with blade radius of 60 m with an optimum C p = 0.4 at zero pith angle for tip-speed ratio, λ opt = 7.5. B. Indution Generator The indution generator (IG) is represented using the wind torque model, applied to the shaft of the indution mahine model as shown in Fig.. The model also inorporates a pith ontrol blok, whh hanges the angle of the blades of the mahine at high wind speeds in order to modify the torque haraterist and thus limit the output power to the rating of the mahine. Sine indution w

3 mahines need to be operating at supersynhronous speed in order to generate power, the model is initlized to -% slip. This prevents simulation of the start-up transient of the mahine. wind instpq_al Ps_grid if_grid vsf Ps Qs Wind_torque_model wind_speed omega_ blade_angle Qs_grid Pgrid S beta Fig.. Diret onneted indution generator model in EMTP-RV S ASM The unompensated mahine onsumes reative power under all modes of operation and therefore, the magnetizing branh is ompensated using shunt apaitors. However, this provides only onstant reative power and therefore, a ontrollable reative power soure may be added in order to limit voltage flutuations resulting from rble output power, while at the same time an be used to improve voltage reovery following sags. Speed C. Stat Compensator (STATCOM) In the present ase, a three-level STATCOM was modeled and onneted at the high side of the transformer in order to provide voltage support for the IG. The STATCOM uses insulated gate bipolar transistor (IGBT) based tehnology and shifted pulse width modulation (PWM) for generation of the gating signals. The outline of the ontrol is given in Fig. 3 and follows that detailed in [7]. D. Doubly-fed indution generator (DFIG) The doubly-fed indution mahine was modeled using ontrol shemes as developed in [8][9]. The power iruit of the model is shown in Fig. 4. The ontrol strategy for the rotor side onverter is well doumented in the literature and will not be presented here. The ontrol of the supply side onverter is muh the same as that of the STATCOM ontrol, whereby the onverter serves primarily as a d voltage regulator with the ability to also exhange real power with the system. d Ia_in Ib_In I_In BandPass Filter 60Hz a_f b_f _f vd_ref Vd Regulation d_measure vd_ref mid_selet sope #w# - BandPass Filter 60Hz a_f b_f _f f(u) f(u) f(u) PLL w _d Voltage Regulation _w vseq ref Reative Power Control #fs# PWM Modulation Current Transformation id i0 id sope sope _ref sope vseq ref #Vd# vd_ref _st _st _st mid_selet fs vd_div Sa Sb S Sa Sb S Fig. 3. EMTP-RV representation of STATCOM ontrol algorithm

4 STATOR igrid igrid onv onv BUS6 onv onv vs p vs #L# is ea S Stator_signals ASM N #Vbase# #Sbase# stator_side_onverter Fig. 4. EMTP-RV representation of DFIG power iruit E. EMTP-RV Solver is p4 R?m #C# #C# Slip Angle Omega_ Omega_ #p_div# d_pos rotor_side_onverter d_neg Rotor_signals Slip Angle Omega_ Omega_ The transient analysis software program [0] used in this paper allows for the representation of very large systems and produes very aurate results, enabling simulation of numerous yles to even multiple seonds, with relatively short exeution times. The ability of the software to model large systems with multiple mahines, power eletrons, and ontrol makes it ideal for modeling wind energy systems and studying the interonnetion haraterists. The omplete design is based on hierarhal (subnetworks ontaining subnetworks) bloks with masking. Hierarhy is essentl for designing large and omplex systems. The assembly of ontrol blok-dgrams is straightforward and does not require extra artifl bloks to eliminate numeral problems []. The asynhronous mahine model is a hard-oded model. It is based on an iterative method and is apable of obtaining a simultaneous solution with network equations at eah simulation time-point. Automat steady-state onditions are found from the initlly given slip data. This is partularly important for weak systems. This is used to minimize startup transients and derease omputer time. IV. SYSTEM CHARACTERISTICS Following verifation of the ontrol algorithms of the two systems, the system haraterists were ompared. The two generator designs were ompared using a typal network onnetion, where both normal operating haraterists as well as their response to voltage dips were investigated. Here the apaity was set to 0 MVA aomplish by aggregation of the MVA model but negleting the effet assoted with geographal distrution. This beomes a more onsertive approah p8 Slip Angle Speed ir ir sine the averaging effet due to different wind onditions was ignored and therefore power flutuations at the point of ommon oupling are proportional to the flutuations at eah of the generators. A. Network There are a number of possle interonnetion strutures for wind parks and thus it is not possle to over every type of network onfiguration, load, and interonnetion point of the wind park. Frequently wind parks are onneted to weak systems, as they are typally loated far from major load enters and entral generation. This is refleted in the short iruit ratio (SCR) of the interonnetion, given by: SCC SCR = (3) S base Where SCC or short iruit apaity is the short iruit power delivered from the grid for a three-phase fault at the wind park: base V SCC = (4) Z line For weak systems the SCR will usually be less than 6 and in some ases may be as low as. Fig. 5 shows the network under investigation where the wind park is onneted at 69 kv to a system with short iruit ratio (SCR) of 6. The two wind generator systems have mathed apaities in terms of real and reative power, whereby reative power support is supplied from the generator itself and from the STATCOM for the rble and fixed speed mahines, respetively. 69kV /_0 CP 75?i Fault CP Ative and Passive Load 69/0.69 Fig. 5. Interonnetion of wind park to power system (EMTP-RV shemat) B. Steady-state The wind speed, output power, and a voltage magnitude are given for the two generators for an intat network with wind speeds around 3 m/s. As an be noted, the voltage remains more or less onstant for the two mahines while the output powers flutuates with the wind speed. Also, there is a ertain delay between the peak of the wind speed and that of the power, due to the mehanal time onstant of the system. Finally one an note that the DFIG is able to extrat more energy from the wind over interl onsidered due to its rble speed apability and maximum power point traking algorithm. 75 Wind v

5 Fig. 8. Slips of indution mahines for fixed speed (with and without STATCOM) and rble speed wind generators, during yle twophase fault, SCR = 6. Fig. 6. Wind speed, output power and rms a voltage at PCC for fixed speed (with STATCOM) and rble speed topologies C. Response to Disturbanes Eah of the systems is subjeted to idental two-phase to ground faults. Fig. 7 and 8 show the response of the a voltage magnitudes and mahine slips, respetively. The rble speed reovers well, while it an be noted that the fast ontrollable reative power provided by the addition of the STATCOM definitely improves the speed of response of the voltage magnitude in the ase of the fixed speed mahines. V. CONCLUSIONS The detailed transient models for rble speed wind turbines and fixed speed tehnologies were developed in EMTP-RV. The development of transient models is inreasingly important in order to investigate many of the interonnetion issues assoted with wind energy. The two systems were ompared in terms of their steady-state behavior as well as their response to disturbanes in the loal network. The rble speed topology is apable of greater energy apture when onsidering a wide range of wind speeds. Results show that as long as the fixed speed mahine is omplemented with suffient reative power ontrol, suh as a STATCOM, the transient performane is omparable to that of the rble speed design. Fig. 7. Rms voltage at PCC for fixed speed (with and without STATCOM) and rble speed wind generators, during yle twophase fault, SCR = 6. VI. REFERENCES [] Rodriguez, J.M., J.L. Fernandez, D. Beato, R. Iturbe, J. Usaola, P. Ledesma, and J. R. Wilhelmi, Inidente on power system dynams of high penetration of fixed speed and doubly-fed wind energy systems: study of the spanish ase, IEEE Trans. Power Systems, Vol. 7, No. 4, pp , Nov. 00. [] Holdsworth, L., X. G. Wu, J.B. Ekanayake, and N. Jenkins, Comparison of fixed speed and doubly-fed indution wind turbines during power system disturbanes, IEE Pro.-Gener. Transm. Distr., Vol. 50, No. 3, May 003. [3] Eletr Systems Consulting, ABB In., Integration of Wind Energy into the Alberta Eletr System, prepared for the Alberta Eletr System Operator, [4] Enslin, J.H.R., J. Knijp, C.P.J. Jansen, and P. Bauer, Integrated approah to network stability and wind energy tehnology for onshore and offshore applations, Power Quality Proeedings, pp. 85-9, May 003. [5] Bossanyi, E., Z. Saad-saoud, and N. Jenkins, Predtion of flker produed by wind turbines, Wind Energy, Vol., Issue, pp. 35-5, 998. [6] website: [7] Hohgraf, C., and R.H. Lasseter Statom ontrols for operation with unbalaned voltages, IEEE Trans. Power Delivery, 998, Vol. 3, No., pp

6 [8] Datta, R. and V. T. Ranganathan, Varble-speed wind power generation using doubly fed wound rotor indution mahine a omparison with alternative shemes, IEEE Trans. Energy Conv., Vol. 7, No. 3, Sept, 00 [9] Akagi, H. and H. Sato, Control and performane of a doubly-fed indution mahine intended for a flywheel energy storage system, IEEE Trans. Power Ele., Vol. 7, No., Jan 00. [0] J. Mahseredjn, L. Dubé, L. Gérin-Lajoie: New Adnes in the Simulation of Transients with EMTP: Computation and visualization Tehnues. Proeedings of 7th International onferene on Modeling and Simulation of Eletr Mahines, Converters and Systems. August 8-, 00, Montreal. [] J. Mahseredjn, L. Dubé, M. Zou, S. Dennetière and G. Joos: Elimination of numeral delays in the solution of ontrol systems in EMTP. Submitted to IPST-005 (The 6th International Conferene on Power System Transients), Montréal, Canada, June 005. VII. BIOGRAPHIES Chad Abbey (S 0) reeived his degree in eletral engineering from the University of Alberta in 00. In 004, he graduated with an M. Eng degree from MGill University, Montréal where he is urrently pursuing his Ph.D. He is presently working with CANMET Energy Tehnology Centre, in Varennes, Québe where he is a Researh Engineer and helps to oordinate a joint researh program on the modeling and integration of distruted generation. His urrent researh interests inlude power eletrons, ontrol, distruted generation, wind energy, mrogrids and planned islanding. Bahram Khodabakhhn holds BS and MS (Éole Polytehnue de Montréal) degrees in eletral engineering. He has over 3 years of experienes in power system studies at Hydro-Quebe. He is urrently in harge of the marketing and sales of EMTP-RV, the new Restrutured Version of the DCG-EMTP. Fengquan Zhou reeived the B.Eng. Hons. Degree, M.Eng. Degree from Tsinghua University, Beijing, P.R. China in 993 and 996, respetively. His main researh interests are in distruted generation, power system automation, power system stability, and power eletrons. At present he is a graduate student studying in the Department of Eletral and Computer Engineering, MGill University. Sébastien Dennetière (M'04) graduated from Éole Supérieure d'eletrité (Supéle) in Frane in 00. He reeived the M.A.S. degree from Éole Polytehnue de Montréal in 003. From 00 to 004 he worked at IREQ (Hydro-Québe) on researh and development ativities related to the simulation and analysis of eletromagnet transients. In Otober 004 he joined the researh enter of EDF where his interests are urrently in the field of insulation oordination and power system simulations. Jean Mahseredjn (M'87) graduated from Éole Polytehnue de Montréal with M.A.S. (985) and Ph.D. (99). From 987 to 004 he worked at IREQ (Hydro-Québe) on researh and development ativities related to the simulation and analysis of eletromagnet transients. In Deember 004 he joined the faulty of eletral engineering at Éole Polytehnue de Montréal. Geza Joos (M'8, SM'89) graduated from MGill University, Montreal, Canada, with an M.Eng. (974) and Ph.D. (987). His employment experiene inludes ABB, the Eole de tehnologie supérieure, Conord University, and MGill University, Montreal, Canada. He is involved in fundamental and applied researh related to the applation of high-power eletrons to power onversion, inluding distruted generation, and power systems, and in onsulting ativities.

Analysis of Wound Rotor Self-Excited Induction Generators

Analysis of Wound Rotor Self-Excited Induction Generators nd WSEAS Int. Conf. on CICUITS, SYSTEMS, SIGNA and TEECOMMUNICATIONS (CISST'8)Aapulo, Mexio, January 5-7, 8 Analysis of Wound otor Self-Exited Indution Generators.S. Sandhu (Professor) & S.P.Jain (Professor)