1. Introduction: STATCOMs in wind power applications

Transcription

1 Improved STATCOM Opertion Under Trnsient Disturnces for Wind Power Applictions Ion EtxeerriOtdui (1), Uni Viscrret (1), Izskun Zmkon (2), Betriz Arenl Redondo (3), Jvier Iiricu (4) (1) IKERLANIK4 Technology Reserch Centre, Apdo. 146, E2.5 Mondrgón (Spin) Tel: , FAX: , (2) OLDAR Electrónic, S.A., Brrio Bsuntz, 2, E4814 Igorre (Spin) Tel: , Fx: , (3) JEMA, Pseo del Circuito nº1, E216 LsrteOri (Spin) Tel: , Fx: (4) Eólics de Euskdi, Prque Tecnológico de Álv, E6, oficin 39, 151, Miñno (Spin) Tel: , Fx: , Acknowledgements This work ws supported in prt y the Bsque Regionl Government in the context of the Project SICRE: Sistem de Inyección de Corriente Rectiv pr Instlciones de Producción Eléctric en Régimen Especil. Keywords STATCOM, Converter Control, Wind Energy. Astrct This pper presents n optimized STATCOM control for wind power pplictions. The trnsient ehvior of fixedspeed wind frms cn e improved y injecting lrge mounts of rective power during the fult recovery. This ppliction requires high dynmic converter, which must lso e cple of working under trnsient unlnced conditions. Here, sttionry control structure is proposed, which is sed on resonnt regultors nd DFT synchroniztion lgorithm, which llows n optimized rective power injection during voltge dips occurred in the grid. The proposed structure is vlidted experimentlly using reduced scle STATCOM prototype, showing tht it is well dpted for the concerned ppliction. 1. Introduction: STATCOMs in wind power pplictions The vision of the contriution of Sttic VAr Compenstion devices in wind power pplictions hs evolved in the lst few yers. Initilly, t the end of the 199s, it ws ssumed tht their min function ws wind power qulity improvement, i.e., fixing unity output power fctor nd thus reducing voltge drops, flicker, nd electricl losses in the network during norml opertion [1]. An exmple of this viewpoint is the 8 MVAr experimentl STATCOM tht ws instlled in 1998 in the Rejsy Hede (Denmrk) wind frm [2] in the context of Europen Commission funded project [3]. Lter on, with the development nd expnsion of douly fed induction genertors nd the generlized prctice of connecting the wind frms in sufficiently strong networks, the interest of using Sttic VAr Compenstion devices in wind frms hs een flgging. However, with the puliction of new nd more restrictive interconnection requirements they re currently drwing incresing interest [4], [5], [6], [7], [8]. Until recently wind frms were required to disconnect from the grid in cse of disturnce, losing therefore their genertion cpility t criticl moment for the system opertion. EPE 27 Alorg ISBN : P.1

2 Consequently, newly developed GridCodes [9] request them Low Voltge Ride Through (LVRT) cpility: they must e le to withstnd without disconnection voltge disturnces ove certin limit chrcteristic (defined in terms of depth nd durtion) s shown in Figure V (pu) PCC Time (s) 15 Fig. 1. Fult ridethrough chrcteristic specified y the Spnish grid opertor [9]. STATCOMs cn e employed to improve fixedspeed wind frm ehvior during the most criticl phse of griddisturnce, the voltge recovery fter the clering, y injecting the rective power required y the genertors nd voiding this wy the voltge drop provoked y the sorption of this power from the grid. Let us nlyze in detil the fult ehvior of fixedspeed wind frms. Voltge dips hve doule effect on fixedspeed wind turines: electricl nd mechnicl. The min electricl effect is the demgnetiztion of the rotor while the most significnt mechnicl one is the rotor speed increse [1]. These effects re oservle from the eginning of the dip until few seconds fter its recovery (see Fig. 2, where.2 p.u. 3 phse sg is pplied to 36 MW fixedspeed wind frm from t =.5s to t =.6s using PSCAD/EMTDC). The terminl voltge dip provokes the demgnetiztion of the sttor flux. However, the rotor flux cnnot decrese instntneously nd therefore the mchine current goes through hrd trnsient (it my rech severl p.u.) in which it delivers rective power to the network. Fig. 2. Terminl voltge, sttor RMS current nd rotor speed of fixedspeed wind turine fter.1 s shortcircuit. In ddition, the ctive power exported to the grid is significntly reduced during the fult while the input mechnicl power from the wind turine is lmost constnt (the wind speed cn e considered to e unvried nd the mechnicl power limittion systems require non negligile time pitch controlor rottionl speed increse stll control to rect). Therefore the genertor will ccelerte during the shortcircuit in order to mechniclly store the energy excess. The chieved speed must e lower thn the mximum tolerle speed to void the disconnection of the genertor. EPE 27 Alorg ISBN : P.2

3 After the clering, the genertor consumes lrge mounts of rective power due to its mgnetiztion nd to the increse of the mchine slip during the fult. This power consumption mkes it difficult the recovery of the terminl voltge. If the voltge does not recover fst enough, the mchine will continue ccelerting until its finl disconnection. The est wy to void voltge dip derived prolems on wind genertors is to control the connection point voltge y compensting voltge dips nd protecting the fcility from ny voltge imperfection. This cn e done y using series power electronic compenstor Dynmic Voltge Restorer (DVR) which injects the necessry voltge in the system in order to keep the genertor voltge constnt [11]. However, this compenstor requires n ctive power sorption cpility during fults (in order to del with the power generted y the wind frm tht cn not e exported to the grid) which its min drwck. If shunt compenstor (SVC or STATCOM) is used, little cn e done during the fult in order to improve the voltge due to the huge mount of power tht would e necessry. Nevertheless, the shunt compenstor cn e used to improve the ehvior of the system fter the clering of the fult voiding the voltge drop provoked y the sorption of this power from the grid. This is the most widely proposed compenstorsed solution for the ridethrough cpility improvement of fixedspeed wind turines [4], [5]. However, s the STATCOM will hve to e operted during unlnced fults it will hve to e designed for this kind of opertion, optimizing the rective power injected into the grid t ny condition. Therefore the min scope of the pper is to propose n improved control structure in order to optimize STATCOM opertion under trnsient disturnces for wind power pplictions 2. STATCOM opertion under unlnced trnsient disturnces In this section the opertion of generic vector current controlled PWM converter (see Figure 3) will e nlyzed under unlnced trnsient conditions. The controller includes negligile dynmics PLL (it does not rect during trnsient disturnces) in order to reject hrmonic nd negtive sequence phse disturnces. In ddition, it is considered tht the converter is not ffected y zerosequence voltge disturnces ecuse of the presence of zerosequence locking trnsformers [12]. PLL θ i R, L v VSC i DC v DC e e e c i c i v v c C Lod s c 3/2 αβ / dq e αβ 3/2 i αβ αβ / dq θ θ v c * v αβ * PWM 2/3 dq / αβ V DC Control e dq i dq v dq * Current Control i d * i q * Fig. 3. Generic Single Reference Frme Vector Current Controller. The first nd immedite effect of voltge dip is the decrese of the exchnged power nd lso of the mximum exchngele power. In generl the converter controller will try to keep the exchnged power constnt nd therefore current mplitude will increse. In ddition, phse ngle shifts () provoke synchroniztion error during fults (due to the negligile PLL dynmics). This error my hve different impct on the opertion depending on the considered ppliction. Let us nlyze 2 possile gridconnected pplictions: rectifier (PF = 1) nd STATCOM (PF = ). EPE 27 Alorg ISBN : P.3

4 Rectifier (PF = 1): synchroniztion errors results in n undesired rective component, I dq, s well s decrese of the ctive current, I dd, I = sin dq I' d ( ) ( ) I dd = I' d cos where I d is the desired ctive current (see Fig. 4 left, where dq is the rel synchronous reference frme nd dq is the estimted one). Therefore, there will e power fctor error: ( ) PF = 1 cos (2) If the system contins power feedck loop (mesured phse per phse) exchnged power error cn e detected nd used for phse ngle jump estimtion, minimizing the error in fult stedystte. STATCOM (PF = ): synchroniztion errors results in n undesired ctive component: ( ) I qd = I' q sin (3) where I q is the desired rective current (see Fig. 4 right, where dq is the rel synchronous reference frme nd dq is the estimted one). However, s the converter hs no ctive power exchnge cpility (other thn for power losses compenstion purposes) the us voltge controller will rect modifying the ctive current reference (I d ) until its rel ctive projection (I dd ) compenstes the erroneously injected ctive current (I qd ). 1 cos I q = I' q Iqq Idq = (4) cos This error increses very rpidly with the phse ngle shift vlue, reching considerle vlue (1% increse) for the mximum considered jump (6 ) [12]. In this cse the error cn lso e minimized during the time etween the voltge dip eginning nd end using the exchnged power error. (1) q q' q q' I mx I mx d I qq I q d I dd d' I d I dq I qd d' I d I dd I dq Fig. 4. Impct of phse ngle jump on rectifier (left) nd STATCOM (right) opertion. Besides the converter current, DC us voltge my lso e distorted during disturnces. In cse of lnced voltge dip, DC us voltge trnsients cn e oserved oth t the eginning nd t the end of the disturnce, ut there re no stedystte oscilltions during the disturnce due to the power lnce etween phses. The cse of unlnced voltge dips is different. They re chrcterized y decrese of the positive voltge sequence nd the ppernce of negtive voltge sequence. Thus, generlly power unlnce will tke plce etween phses, which will led to doule grid frequency oscilltions in the DC us (in ddition to fult eginning nd end trnsients). In ddition, there my lso e phse ngle EPE 27 Alorg ISBN : P.4

5 unlnce etween difference phses. However, nd supposing tht there is zerosequence locking trnsformer etween the fult origin nd the converter, there will lwys e some kind of symmetry etween phses. As result, undesired power components due to phse ngle unlnces re compensted etween phses nd do not ffect DC us voltge. Figure 5 shows this compenstion phenomenon for pure rective opertion mode nd 2 different voltge sgs (I d = I cd ). I d I q I d I I q I ' ' I I c I cd c' c' c I c I cq I cd I cq Fig. 5. Compenstion of undesired current components in unlnced voltge sgs (pure rective opertion nd 2 different voltge sgs). In ddition, depending on the implemented controller, converter current my lso e distorted nd unlnced. The min cuse of current distortion is the DC us voltge hrmonic propgtion through the current reference generted y the DC us voltge loop, nd therefore it cn e esily voided y filtering this reference. Concerning current unlnces, they re originted y the incpcity of the control to completely reject negtive sequence disturnces, nd therefore they cn e voided y improving this controller feture. Summrizing, the min effects of trnsient voltge dips on the opertion of power converters re or mye: 1. Decrese of the mximum power exchnge etween the converter nd the grid. 2. Power fctor nd/or current mplitude error (in cse of phse ngle jump). 3. DC us voltge oscilltions (t the eginning nd t the end of the disturnce s well s sttionry oscilltions in cse of power unlnce). 4. Unlnce nd distortion of the converter currents. 3. Proposed control structure In the cse of the ove mentioned wind power ppliction, it is crucil to get the most out of the converter, injecting the mximum ville rective power in order to optimize the rting of the converter. An optimized PWM current controller is composed of three min locks: ) Higher level control loop: the ojective of this control loop is to generte n optimized current reference for the converter tking into ccount ctul grid conditions. ) Current control structure: the ojective of this loop is to otin good trnsient nd stedysttes responses, nd therefore it must present pproprite ndwidth chrcteristics. c) Synchroniztion: the purpose of this lock is to estimte the voltge grid instntneous phse in order to generte n optiml current reference. I c EPE 27 Alorg ISBN : P.5

6 ) Higher level control loop: current reference genertion In the considered ppliction, the ojective of the converter is to inject the mximum ville rective power in order to minimize voltge drops t the wind frm connection point fter fult clering. Therefore, the mximum current reference must e defined y the higher control loop. The direct ppliction of mximum mplitude current reference t ech of the phses my result on homopolr current injection (in cse of phse unlnces t the grid) nd if this component is directly removed, the solution my not e optiml from the mximum power injection viewpoint. Thus, the proposed strtegy is sed on the principle of injecting higher current in those phses with higher residul voltge nd it is implemented s follows (see Figure 6). The proposed strtegy is independent with respect to the nture of the converter output power (ctive nd rective) nd therefore pure ctive injection exmple is presented t this figure, ecuse of simplicity resons. As it cn e seen, the three currents re injected in phse with distured grid voltges, injecting the mximum current in the phse with the mximum residul voltge (). ' ' φ v e p c I φ i = φ v e φ ic = φ vc p e c I p I c c φ vc c c' Fig. 6. Mximum power injection for pure ctive opertion. Grid voltge (left) nd injected current nd power (right). Current Phse Angle: The phse ngle of ech of the phses is derived from the desired power fctor: c' ϕ PF 1 P ref = tn ; φ In = φvn ϕpf (5) Qref Current Amplitude: The sum of the three phse converter currents must e zero (no homopolr sequence) nd their mplitudes must e in ccordnce with residul voltge mplitudes of ech of the phses (i.e. the mximum currents must e injected in the phses with the mximum residul voltges s shown in Figure 6). The phses with mximum nd minimum residul voltges will e determined y the disturnce chrcteriztion module. Supposing for instnce tht E = E mx then I = I mx, the mplitude of the other two currents will e clculted s follows in order to gurntee null zerosequence current (tking into ccount the possiility of phse unlnce), y ssuming tht the sum of ll current components must e zero oth in the rel nd in the imginry projections: I I c = I I = cos φ cos φ i i sin φ sin φ sin φi I sin φ ic ic ic sin φ sin φ sin φ i i i cos φ cos φ ic ic (6) EPE 27 Alorg ISBN : P.6

7 ) Current control structure Vector current control structures hve een widely proposed for the control of threephse AC systems ecuse of their good stedystte fetures (they hve theoreticlly infinite gin t the rotting frequency nd they re thus le to completely eliminte the error t this frequency). However in the lst yers new fmily of controllers hs een introduced s n interesting sttic lterntive to the use of synchronous PI controllers [13], [14], [15] for AC current control pplictions (oth in single nd three phse systems): resonnt regultors (lso known s generlized integrtors). These regultors re generlly mde up of proportionl nd n integrtor term, which contins resonnt pole imed t otining n infinite gin t the resonnce frequency (ω ): 2 ki s CR ( s) = k p 2 (7) s ω 2 This regultor cn e esily deduced from the sttionry frme equivlent regultor of synchronous frme PI y uilding its rel equivlent regultor: ki ki CIG ( s) = k p (8) s jω s jω Thus, the resonnt regultor presents symmetric frequency response nd n infinite gin for oth positive nd negtive sequences of the selected frequency. Consequently it is especilly well fitted for unlnced pplictions like the one considered in this pper. The following figure presents the lock digrm of the proposed sttionry frme current controller. In ddition to the resonnt current regultor mentioned efore, the system contins PI for the DC us voltge control nd zerosequence component removing function. V c Wind Frm 3 x single phse synchroniztion I c L cos sin cos sin cos sin ϕ ϕ ϕc I qref I dref Active Rective Homopolr Extrction Icref Resonnt Regultors Vcref PWM s c PI Vccref Vcc Fig. 7. Proposed improved current control structure. c) Synchroniztion In order to implement the ove presented sclr control structure it is necessry to use singlephse synchroniztion systems with sufficient dynmics. The dynmics otined with conventionl singlephse synchroniztion techniques re quite limited (typiclly etween 5 nd 1ms) ecuse of the dely introduced y the genertion of virtul threephse system nd the need for eliminting the 1Hz component originted y the phse detector lock. Thus, in this pper reltime DFT (Discrete Fourier Trnsform) hs een used. The implemented rel time Expression used to clculte fundmentl voltge mplitude nd instntneous phse is for ech of the grid phses is: 1 2 N 1 = v( n) N n= 2 π n j ( ) N V e (9) EPE 27 Alorg ISBN : P.7

8 4. Implementtion nd Experimentl results The experimentl tests hve een crried out t testench uilt t the IKERLANIK4 Technologicl Reserch Center in Mondrgón (Spin). The testench (see Figure 8) is siclly composed of voltge source converter connected to progrmmle grid ( commercil Profline genertor nd zero sequence locking trnsformer) through grid connection inductive filter. The min prmeters of the testench re presented in Tle I. Profline e' Trnsformer e L v VSC V DC e' e' c e e c v v c C Progrmmle Grid s c dspace Controller Bord Fig. 8. Scheme of the testench used for the experimentl improved control strtegy vlidtion. This testench permits to vlidte the trnsient rective power injection cpility of the proposed STATCOM control structure, which s it hs een shown in the literture my e crucil for the grid response improvement of fixed speed wind frms [8]. A D Type voltge sg hs een considered (ccording to [12]), compring the otined experimentl results with simultion results (otined with Mtl/Simulink). TABLE I. Min Prmeters of the Experimentl TestBench Symol Mgnitude Vlue V DC DC us voltge 4V C DC us cpcitor 235µF L Grid connection filter inductnce 3mH r Grid connection filter resistnce 2mΩ f s Smpling Frequency 5kHz f sw Switching Frequency 5kHz E Trnsformer secondry voltge 22 V LL E Trnsformer primry voltge 38 V LL I NOM Converter nominl current 15A Experimentl nd simultion results re presented in Figures 913. As it cn e oserved, the control structure permits to optimize the injected rective power y injecting higher current in the phses with higher residul voltges, with reltively smooth current trnsients (Figures 1 nd 11). Before the disturnce the converter is soring rective power, ut once the disturnce strted it chnges rpidly its opertion mode in order to generte the mximum ville rective power (Figure 12). Finlly, Figure 13 shows how DC us voltge oscilltions increse during the trnsient due to the converter three phse power unlnce (which is necessry to optimize the mount of injected power) Tensiones (V) 5 5 Tensiones (V) Fig. 9. Voltge t the converter connection point. Experimentl (left) nd simultion (right). EPE 27 Alorg ISBN : P.8

9 Corrientes (A) 5 Corrientes (A) Fig. 1. Converter current t the disturnce eginning. Experimentl (left) nd simultion (right) Corrientes (A) Corrientes (A) Fig. 11. Converter current t the disturnce end. Experimentl (left) nd simultion (right) Potencis (W VAr) Potencis (W VAr) Fig. 12. Converter ctive nd rective power (one phse). Experimentl (left) nd simultion (right) Vus (V) 5 Vus (V) Fig. 13. Converter DC us voltge. Experimentl (left) nd simultion (right). EPE 27 Alorg ISBN : P.9

10 5. Conclusion This pper presents n optimized STATCOM control for wind power pplictions. The min fetures of this control re: The use of 3 singlephse synchroniztion functions in order to del not only with mplitude unlnces ut lso with phse unlnces. The use of 3 singlephse controllers sed on resonnt regultors. Seprted genertion of ctive nd rective references which re then djusted in order to otin current reference without zerosequence component. The proposed structure hs een vlidted experimentlly nd in simultion, using reduced scle STATCOM prototype, showing tht it is well dpted for the concerned ppliction. 6. References [1] SdSoud Z, Liso M, Eknyke J, Jenkins N, Strc G. The ppliction of STATCOMS to wind frms. IEE Proceedings Genertion Trnsmission nd Distriution, Vol. 145, No 5, Sep.1998, pp [2] K. H. Sorink, N. Jenkins, F. C. A. Schettler, J. Pedersen, K. O. H. Pedersen, nd K. Bergmnn, Rective power compenstion of 24 MW wind frm using 12pulse voltge source converter, in Proc. CIGRÉ Int. Conf. Lrge High Voltge Electric Systems, [3] Improvement of Wind Frm Output Power Qulity Using Advnced Sttic VAr Compenstors NonNucler Energy R&D Component (JOULE III), , Project Reference: JOR [4] Aten M, Mrtinez J, Crtwright P.J. Fult Recovery of Wind frm with FixedSpeed Induction Genertors using STATCOM. Wind Engineering, Vol. 29, No. 4, 25. [5] Xuegung W, Atputhrjh A, Chngjing Z, Jenkins N. Appliction of Sttic Rective Power Compenstor (STATCOM) nd Dynmic Brking Resistor (DBR) for the Stility Enhncement of Lrge Wind Frm. Wind Engineering, Vol. 27, No. 2, 23. [6] W. Freits, A. Morelto, W. Xu, nd F. Sto, Impcts of c genertors nd DSTATCOM devices on the dynmic performnce of distriution systems, IEEE Trns. Power Delivery, vol. 2, no. 2, pp , Apr. 25. [7] C. Chompooinwi, C. Yingvivtnpong, K. Methpryoon, nd W. J. Lee, Rective compenstion techniques to improve the ridethrough cpility of wind turine during disturnce, IEEE Trns. Ind. Applict., vol. 41, pp , MyJun. 25. [8] H. Gztñg, I. EtxeerriOtdui, D. Ocnsu nd S. Bch, RelTime Anlysis of the Trnsient Response Improvement of FixedSpeed Wind Frms y Using ReducedScle STATCOM Prototype. IEEE Trnsctions on Power Systems, Vol. 22, No. 2, My 27. [9] PO Voltge Sg Response Requirements of Wind Power Fcilities, Opertion Procedure (in Spnish) Spnish System Opertor (REE), 26. [1] R. Grünum, P. Hlvrsson, D. Lrsson, nd P. R. Jones, Conditioning of power grids serving offshore wind frms sed on synchronous genertors, in Proc. IEE PEMD 24, 2nd Int. Conf. Power Electronics, Mchines nd Drives, Edinurgh, U.K., 24. [11] H. Gztñg, I. EtxeerriOtdui, S. Bch, D. Roye, FixedSpeed Wind Frm Opertion Improvement y Using DVR Devices IEEE ISIE 7, 27, Vigo, Spin. [12] M. H.J. Bollen, Understnding Power Qulity Prolems. Voltge Sgs nd Interruptions, IEEE Press Series on Power Engineering, ISBN , 2. [13] M. Bojrup, P. Krlsson, M. Alküll, nd L. Gertmr, A multiple rotting integrtor controller for ctive filters, in Proc. EPE Conf., Lusnne, Switzerlnd, 1999, CDROM. [14] X. Yun, J. Allmeling, W. Merk, nd H. Stemmler, Sttionry frme generlized integrtors for current control of ctive power filters with zero stedy stte error for current hrmonics of concern under unlnced nd distorted opertion conditions, in Proc. IEEEIAS Annu. Meeting, Conf. Rec., Rom, Itly, 2, pp [15] I. EtxeerriOtdui, U. Viscrret, M. Cllero, A. Rufer nd S. Bch, New Optimized PWM VSC Control Structures nd Strtegies Under Unlnced Voltge Trnsients, IEEE Trnsctions on Industril Electronics. Accepted in My 27, witing for puliction. EPE 27 Alorg ISBN : P.1