Deadband control of doubly-fed induction generator around synchronous speed

Transcription

1 Univeity of Wollongong Reeach Online Faculty of Engineeing and Infomation Science - Pape: Pat A Faculty of Engineeing and Infomation Science 26 Deadband contol of doubly-fed induction geneato aound ynchonou peed Yingjie Tan Univeity of Wollongong, yt86@uowmail.edu.au Kahem M. Muttaqi Univeity of Wollongong, kahem@uow.edu.au Laantha G. Meegahapola Univeity of Wollongong, laantha.meegahapola@mit.edu.au Phil Ciufo Univeity of Wollongong, ciufo@uow.edu.au Publication Detail Y. Tan, K. M. Muttaqi, L. Meegahapola & P. Ciufo, "Deadband contol of doubly-fed induction geneato aound ynchonou peed," IEEE Tanaction on Enegy Conveion, vol. 3, (4) pp. 6-62, 26. Reeach Online i the open acce intitutional epoitoy fo the Univeity of Wollongong. Fo futhe infomation contact the UOW Libay: eeach-pub@uow.edu.au

2 Deadband contol of doubly-fed induction geneato aound ynchonou peed Abtact Semiconducto device in powe electonic convete of the doubly-fed induction geneato (DFIG) ae uceptible to ignificant junction tempeatue vaiation when opeating aound ynchonou peed, theeby educing the lifetime of the convete. Thi i due to the fact that the fequency of the oto cuent in a DFIG i detemined by the tato flux fequency and oto peed, and hence will lead to low oto cuent fequency when opeating cloe to the ynchonou peed, and ultimately eult in ignificant themal te on emiconducto device. In thi pape, a multimode opeation contol tategy i popoed fo the DFIG to pevent opeating aound the ynchonou peed (within a pedefined deadband); thu, the popoed contol tategy can avoid the themal te poblem. The popoed tategy engage the exiting cowba cheme fo DFIG-baed wind enegy conveion ytem to intentionally alte the opeating mode of the geneato between DFIG and induction geneato (IG). Smooth tanition between the two opeating mode can be achieved with the upplementay contol tategie. Unity powe facto can alo be maintained in both opeating mode by uing the gid ide convete a a tatic ynchonou compenato (STATCOM) to fulfill the eactive powe equiement of the DFIG in IG mode. Dicipline Engineeing Science and Technology Studie Publication Detail Y. Tan, K. M. Muttaqi, L. Meegahapola & P. Ciufo, "Deadband contol of doubly-fed induction geneato aound ynchonou peed," IEEE Tanaction on Enegy Conveion, vol. 3, (4) pp. 6-62, 26. Thi jounal aticle i available at Reeach Online:

3 Deadband Contol of Doubly-Fed Induction Geneato aound Synchonou Speed Yingjie Tan, Student Membe, IEEE, Kahem M. Muttaqi, Senio Membe, IEEE, Laantha Meegahapola, Membe, IEEE, and Phil Ciufo, Senio Membe, IEEE Abtact Semiconducto device in powe electonic convete of the doubly-fed induction geneato (DFIG) ae uceptible to ignificant junction tempeatue vaiation when opeating aound ynchonou peed, theeby educing the lifetime of the convete. Thi i due to the fact that the fequency of the oto cuent in a DFIG i detemined by the tato flux fequency and oto peed, and hence will lead to low oto cuent fequency when opeating cloe to the ynchonou peed, and ultimately eult in ignificant themal te on emiconducto device. In thi pape, a multimode opeation contol tategy i popoed fo the DFIG to pevent opeating aound the ynchonou peed (within a pedefined deadband), thu the popoed contol tategy can avoid the themal te poblem. The popoed tategy engage the exiting cowba cheme fo DFIG baed WECS to intentionally alte the opeating mode of the geneato between DFIG and induction geneato (IG). Smooth tanition between the two opeating mode can be achieved with the upplementay contol tategie. Unity powe facto can alo be maintained in both opeating mode by uing the gid ide convete a a tatic ynchonou compenato (STATCOM) to fulfil the eactive powe equiement of the DFIG in IG mode. Index Tem Cowba, doubly-fed induction geneato (DFIG), multimode opeation, ynchonou peed, wind powe, themal analyi. D I. INTRODUCTION OUBLY-fed induction geneato (DFIG) baed wind enegy conveion ytem (WECS) ae widely ued aound the wold fo the wind enegy haveting. The eliability of the WECS i an impotant conideation which enue a high etun on the invetment. Suvey caied out in [, 2] howed that the emiconducto device ae the mot vulneable component in WECS. Powe convete of the WECS ae deigned to be in evice fo 2-25 yea, and themal pefomance i cloely elated to the eliability of emiconducto device in the convete [3]. Packaging-elated failue mechanim of emiconducto device, uch a bond wie lift-off and olde fatigue, ae affected by junction tempeatue a well a tempeatue fluctuation in powe cycling of the emiconducto device [4]. The lifetime (o the numbe of cycle to failue) of witching device i popotionally deceaed with an inceae in the amplitude Y. Tan, K. M. Muttaqi and P. Ciufo ae with the Autalian Powe Quality and Reliability Cente, School of Electical, Compute, and Telecommunication Engineeing, Univeity of Wollongong, NSW 2522, Autalia ( yt86@uowmail.edu.au; kahem@uow.edu.au; ciufo@uow.edu.au). L. Meegahapola i with the School of Electical and Compute Engineeing, Royal Melboune Intitute of Technology, Melboune, Vic. 3, Autalia ( laantha.meegahapola@mit.edu.au). of junction tempeatue fluctuation [5]. A tated in [5], the minimum opeating fequency i a citical paamete of a powe emiconducto device apat fom ovecuent, and the deigne ha to take the puling of junction tempeatue into conideation when the convete opeating fequency i low, ince the junction tempeatue ha a highe vaiation at a lowe opeating fequency and the maximum junction tempeatue i likely to be exceeded. Conventionally, powe electonic device in the utility powe induty have low tempeatue wing [], but the application of wind powe bing lage tanient and themal cycling iue. Fo example, when the oto peed appoache ynchonou peed in a DFIG, the oto cuent fequency become low a the oto cuent fequency equal to the lip fequency. Hence, highe tempeatue wing and hote life pan of powe witche in the oto ide convete (RSC) ae eulted. In [6], the vaiation of the emiconducto junction tempeatue wa hown to be eaching it maximum aound the ynchonou peed when the lip i low and the peiod of themal cycle i long. The autho in [7] alo uggeted that the maximum junction tempeatue in the IGBT of the RSC ae likely to be exceeded aound ynchonou peed. A powe cycling capability etimation method wa popoed fo the DFIG in [8] and the negative effect of low lip fequency opeation on powe cycle capability of a DFIG powe convete wa tudied. Autho in [9] concluded that the themal pefomance of emiconducto in an RSC become cucial aound the ynchonou opeating point. Theefoe, emiconducto device in the RSC ae ignificantly teed aound ynchonou peed, and it i impotant to invetigate and popoe tategie to ovecome the themal te iue. II. LIMITATIONS IN EXISTING CONTROL STRATEGIES Limiting the junction tempeatue of the emiconducto device within ated limit i tictly equied fo maintaining the longevity of the convete. By limiting the powe tanfe of the convete can egulate the junction tempeatue [], fo example, oto cuent i limited to avoid futhe tempeatue inceae when the oto winding tempeatue inceae by 86% aound ynchonou peed [], but the effectivene of intended contol tategy will be affected. A convete of highe ating and a heat ink of lowe maximum tempeatue can be a olution, but additional invetment and lage pace i equied. Autho in [2] popoed a diffeent convete topology to deceae the powe lo and thu educe the ik of oveheating, which may not be the pefeable olution fo

4 WECS manufactue due to high cot and pace containt [7]. The autho of [7] uggeted the ue of a dicontinuou PWM technique to educe powe lo of the DFIG and thu the maximum junction tempeatue, enabling the geneato to opeate within the entie peed ange. In [3], the autho found that the mean time to failue (MTTF) of the IGBT in a vaiable peed dive ytem educe damatically at highe junction tempeatue vaiation, and a witching fequency eduction tategy wa popoed in ode to educe the powe lo unde low output fequency opeation, thu impoving MTTF. With thee ophiticated PWM technique, the themal poblem of ynchonou opeation can be educed, but the eulting hamonic poblem can be difficult to adde. Fo example, the vaiation in witching fequency may intoduce difficulty in hamonic filteing, and advanced hamonic filteing technique and device ae equied []. Futhemoe, thee i a common limitation in exiting contol tategie, ince the olution ae mainly capable of educing the mean junction tempeatue but have limited impact on educing the tempeatue wing which i mainly affected by the fundamental output fequency. A mentioned in [7, ], anothe olution i to avoid ynchonou opeation in pactical application. In [4], an untable ection eplace the typical MPPT cuve aound ynchonou peed. Conequently, the oto peed lide though the untable egion when the oto peed appoache the pedefined peed ange. Howeve, mechanical toque te caued by the fat toque change aound ynchonou peed bing buden on the dive-tain and inceae wea and tea of the dive-tain. The geabox and dive-tain have the longet downtime pe failue, and enegy poduction can be ignificantly affected unde dive-tain failue [2]. In thi pape, a multimode opeation tategy i popoed to pevent the DFIG opeating within a pedeigned ange aound the ynchonou peed. Hence, the lage junction tempeatue vaiation can be avoided. The tategy utilize the exiting cowba potection cheme of a DFIG in ode to opeate it a an induction geneato (IG) when it appoache a pedefined peed ange cloe to ynchonou peed. Supplementay contol tategie ae popoed to achieve a mooth tanition between the DFIG mode and the IG mode. The back-to-back convete cheme of the DFIG i ued a a tatic-ynchonou compenato (STATCOM) to atify the eactive powe equiement of the geneato in IG mode. With the popoed tategy, neithe additional facilitie no modification to the cuent ytem configuation ae equied. Theefoe, the themal iue can be olved without deating the cuent pefomance of the DFIG and violating the gid-code tandad. Powe lo model and themal netwok fo the RSC ae etablihed to veify the effectivene of the popoed tategy. III. OPERATION OF WIND ENERGY CONVERSION SYSTEM A. Low Slip Fequency Poblem of DFIG A DFIG baed WECS i mainly compoed of a wind tubine, dive-tain, wound oto induction machine (WRIM) and back-to-back voltage ouce convete cheme a illutated in Fig.. The voltage and cuent of vaiable fequencie upplied by the RSC enable the wind tubine to opeate at vaiable peed while keeping WECS ynchonized with the gid. The contol mechanim fo RSC and the gid ide convete (GSC) ae well decibed in the publihed liteatue [5], fo bevity, will not be decibed in thi pape. Tubine Geabox Encode i a,b,c ω ω T m Toque eno T m cowba contolle P powe calculation Cowba C gate G toque contolle & uppl. contol v a,b,c i ga,b,c i a,b,c T ef [i Rd] ef θ,ω i a,b,c RSC PWM [v a,b,c] ef RSC contolle θ φ d ω tato flux etimato V DC [i gq] ef i a,b,c Fig. Contol diagam of the DFIG baed WECS. GSC PWM GSC contolle θ e [v ga,b,c] ef i ga,b,c PLL ω e v a,b,c The angula fequency of oto voltage and cuent (ω) in a doubly-fed induction machine i detemined by the angula fequency of tato flux o the angula fequency of tato voltage and cuent (ω ) and the oto otational peed (ω ) [6] (i.e. ω = ω with the being the lip). Theefoe, the angula fequency of the oto cuent become vey mall when the lip appoache zeo (i.e. the oto peed appoache the ynchonou peed). Fo the DFIG ued in thi pape, the pefomance of the geneato aound ynchonou peed i peented in Fig. 2. Since lowe fequency caue much longe tempeatue ie and fall, lage peak to peak wing will occu. Fo example, an enhanced ai cooling ytem (with an ai-flow of 2 m 3 /h and a heat ink to ambient themal eitance, Rth h-a, of.93 K/W [5]) i applied to the DFIG powe convete in thi pape, the eulting vaiation in IGBT junction tempeatue i hown by the oange cuve in Fig. 2- (c). The vaiation eache it maximum value (aound C) when the oto peed i exactly at ynchonou peed and tay at the ynchonou peed (which i unlikely to happen), becaue RSC upplie diect cuent and IGBT ae contantly tuned on o off. The lifetime of powe emiconducto deceae with an inceae in the amplitude of junction tempeatue fluctuation. With a moe ophiticated liquid cooling ytem (e.g. flow ate=8 L/min, wate/glycol atio 5%:5% [7]), the heat ink to ambient themal eitance educe to.92 K/W. The mean junction tempeatue i educed and the vaiation in junction tempeatue i naowed a hown by the blue cuve in Fig. 2-(c). Howeve, the ophiticated cooling ytem demand highe invetment and, opeation and maintenance cot alo inceae. Theefoe, the autho popoe a deadband contol fo the DFIG aound ynchonou peed to educe junction tempeatue wing without upgading the cooling ytem. Gid

5 n (pm) I oto (A) Tj _IGBT (C) Rth h-a =.93K/W Rth h-a =.92K/W (a) (b) Time () (c) Fig. 2 Opeation of a DFIG baed WECS aound ynchonou peed. (a) Roto peed. (b) Roto thee-phae cuent. (c) Junction tempeatue of an IGBT in RSC. B. Popoed Multi-Mode Opeation Sudden gid voltage ditubance, uch a voltage dip, can induce a lage inuh cuent in oto winding and could potentially damage the DFIG oto winding and emiconducto device of the RSC. A cowba ytem, a hown in Fig. 3, i widely being ued to potect the DFIG baed WECS duing gid fault and to impove the low-voltage-ide-though (LVRT) capability. The cowba conit of eito and emiconducto witche. When the witche ae tuned on, the eito ae connected in eie with the oto winding and the IGBT of the RSC ae blocked. Conequently, the geneato opeate imila to an IG. gid Wound oto induction machine Stato LVRT Contol Multimode Contol Cowba Contolle Roto 7 6 R cb R cb R cb RSC Fig. 3 Schematic diagam of a cowba fo a DFIG baed WECS. In thi pape, the cowba potection cheme i ued to facilitate the popoed multi-mode opeation (i.e. DFIG mode and IG mode a explained below). The multi-mode opeation pevent the oto peed of the DFIG fom being within a paticula ange aound ynchonou peed. That i, a pedeigned deadband [ω ε, ω +ε] i et fo DFIG opeation. The toleance ε i et accoding to the themal limit of the emiconducto device to enue the emiconducto opeating within afe junction tempeatue. A uggeted in [5], when fequencie ae lowe than 5 Hz (coeponding to oto peed of. p.u. in ytem with a nomal fequency of 5 Hz), the junction tempeatue follow powe diipation, which eult in high tempeatue fluctuation. It i alo ecommended in [] that deating of the convete i neceay when oto fequencie ae lowe than. p.u. Othewie, the elationhip between the junction tempeatue and oto cuent fequencie hould be detemined though an expeiment a outlined in [7, 8] to et the value fo ε. In thi pape, ince the DFIG i opeating at unity powe facto, the oto cuent i le than it ated value (a ued in the cae whee ε i et at. p.u.) when DFIG opeate aound ynchonou peed. Theefoe, the deadband can be malle and ε i adjuted to be at.2 p.u., which coepond to a lip fequency of Hz. ) DFIG Mode: when the oto peed i outide the deadband, the WECS opeate a a typical DFIG. The equivalent cicuit of the geneato i hown in Fig. 4-(a). 2) IG Mode: when the oto peed i le than (ω +ε) o lage than (ω ε), the cowba cicuit i activated by connecting an extenal eitance to the oto winding while RSC i diabled. A a conequence, the DFIG become an induction geneato. The equivalent cicuit of the geneato at IG mode i hown in Fig. 4-(b). Anothe tategy to ectify thi exceive heating iue aound ynchonou peed i the uboptimal powe point tacking (SOPPT) appoach popoed in [9]. In the SOPPT appoach, it imply hift the oto peed to a cetain value outide the deadband to avoid the ynchonou opeation. Howeve, the contant peed opeation can caue ignificant toque te on the dive-tain. Futhemoe, an anemomete i equied to monito the wind peed to detemine the uitable time to hift back to maximum powe point tacking (MPPT) opeation mode. The wind peed meauement accuacy diectly affect the effectivene of the tategy baed on the SOPPT. Thee two dawback can be avoided with the multimode opeation tategy. It i alo impotant to be noted that the LVRT ha highe pioity ove the deadband contol of ynchonou opeation and the ytem will acifice it component lifetime to maintain the tability of the ytem unde contingency. With the popoed contol tategy, the multi-mode opeation mechanim i only active unde nomal gid voltage condition and the mode tanition i not activated unde gid fault. The deadband contol will not be enabled until gid fault i cleaed and the LVRT opeation i eet. Hence, the conflict between deadband contol and LVRT contol can be avoided. IV. SUPPLEMENTARY CONTROL FOR MULTI-MODE OPERATION Fom the equivalent cicuit of the DFIG in Fig. 4-(a), the electomagnetic toque of the geneato unde DFIG mode can be detemined by; T DFIG 3pR I 2 3p ReV whee p i the numbe of pole pai, R i the oto eitance efeed to the tato ide, and Re(V * I ) indicate the eal pat of the multiplication of the oto voltage phao and cuent phao conjugate. A explained in [6], () can be futhe extended a (2) and the tem of (2) ae given in [6]. I * ()

6 T DFIG Lm 2 3 pf 2C C V V in A C V in B L L (2) 2 A C V in B A A V V in( B B )} Similaly, the electomagnetic toque of the geneato unde the IG mode i given by; T IG 3p R Rcb ) 2 I whee R cb i the cowba eitance efeed to the tato ide. Uing Thévenin' Theoem, (3) can be extended a (4). T IG 2 3p R Rcb ) VTH (4) 2 R R R / ( L L ) 2 TH The Thévenin voltage and impedance ae given by (5) and (6) epectively. R TH V TH ( R j L TH cb j Lm j L ) j L m TH V ( R j L ) j L ( R j L ) j L whee L m i the magnetizing inductance, L i the tato leakage inductance, and R i the tato eitance. i V i R R V jω L σ jω L σ jω L m jω L m jω L σ (a) jω L σ V TH jω L TH R TH (b) R V φ Mechanical powe R R cb Mechanical powe i i m m ( )/R ( )/V φ (-)/R cb (-)/R Fig. 4 Equivalent cicuit of the geneato. (a) DFIG mode. (b) IG mode. In DFIG mode, the machine i contollable though the RSC. A (2) how, the electomagnetic toque i contolled by adjuting the phae angle (φ) and magnitude of the oto voltage (V ). Contaily, the contollability i lot when the cowba hot-cicuit the oto. A indicated by (4), with a fixed tato teminal voltage and cowba eitance, the electomagnetic toque cannot be contolled extenally in IG mode. In ode to achieve a mooth tanition between thee two opeating mode, the electomagnetic toque in both opeating mode i expected to be moe o le the ame when the tanition i taking place. Hence, the toque tanient on the dive-tain of the DFIG can be alleviated. Theefoe, upplementay contol tategie hould be implemented in the toque contolle a (3) (5) (6) hown in Fig., in ode to povide a toque efeence to the RSC contolle and modify the electomagnetic toque to match with the mechanical toque. A. Tanition fom DFIG Mode to IG Mode Fig. 5 how the toque-peed cuve of a wind tubine at vaiable wind peed. Typically, the wind tubine i contolled to opeate at the peed whee wind enegy extaction i maximized. The tacking cuve i indicated by thick dahed line in Fig. 5, and thee i one ection within the ange [ω ε, ω +ε] which hould be avoided a explained in Section II. Thu, thi ection i et a the deadband fo the DFIG mode opeation, and the opeating mode of the WECS hould be alteed to IG mode when oto peed eache the deadband. Unde teadytate condition in IG mode, the equilibium point (which i the inteection of the toque peed cuve of the WRIM and the toque-peed chaacteitic cuve of the wind tubine at a paticula wind peed) unde IG mode opeation i dependent on the cowba eitance. Fo example, the geen dah-dot line in Fig. 5 i the toque-peed cuve with the oto winding hotcicuited (i.e. R=R ). By connecting an extenal cowba eitance into the oto cicuit, the equivalent oto eitance i inceaed (i.e. R=R +R cb). The modified toque-peed cuve of the WRIM ha the hape indicated by the blue dah-dot line in Fig. 5. Although the diect hot-cicuit of the oto ha bette opeating efficiency, additional hadwae configuation i equied fo the DFIG to enable thi opeation, and the naowe opeating peed ange due to lowe total oto eitance ignificantly tee the dive-tain. Hence, exiting cowba i applied to alte the DFIG opeation mode in thi pape. With the chaacteitic cuve of the wind tubine and WRIM, the wind peed which dive the DFIG opeation within the deadband [ω -ε, ω +ε] can be detemined. Then the inteection of the toque peed cuve (i.e. the blue cuve) of the WRIM and the toque-peed chaacteitic cuve of the wind tubine can be obtained unde the coeponding wind peed. Thu, the peed ange [ω l, ω h] fo IG mode opeation can be detemined. Toque (p.u.) DFIG deadband: [ -, +] 7 m/ 6 m/ 5 m/ 8 m/ 9 m/ 3 m/ 2 m/ m/ m/ Roto Speed (p.u.) A B IG mode: [ l, h ] R=R +R cb R=R Fig. 5 Toque peed cuve of a wind tubine at vaiable wind peed. To achieve a mooth tanition fom the DFIG mode to the IG mode, the electomagnetic toque hould be adjuted to be equal in the two diffeent mode at the time of tanition. Theefoe, when the DFIG oto peed i within the deadband,

7 the opeating point i hifted intentionally away fom the optimal point (e.g. point A) to the opeating point in IG mode (e.g. point B) befoe activating the tanition. The contol logic i illutated in the flowchat hown in Fig. 6. The DFIG i contolled by the toque egulato to change the oto peed to the IG mode opeating peed ange. To avoid fequent mode tanition, hyteei contol i ued to detect the oto peed and a moving aveage method i ued to meaue the oto peed. Stat Roto peed (ω ) ω [ω -ε,ω +ε]? ye Time delay Roto peed (ω ) ω [ω -ε,ω +ε]? ye Speed contol Roto peed (ω ) ω [ω l,ω h ]? ye Switch to IG mode End no no no Speed contol activation O ω -ε ω +ε ω Deactivate, Activate Tanition activation ω l O DFIG, IG Fig. 6 Flowchat of tanition fom DFIG mode to IG mode. B. Tanition fom IG Mode to DFIG Mode In IG mode, the geneato conume eactive powe fom othe eactive powe ouce fo excitation. Hence, additional eactive powe uppot hould be povided to impove the powe facto of the DFIG duing the IG mode opeation. Capacito ae commonly ued fo tandad IG baed WECS, but imila capacito may not be intalled fo DFIG. A uggeted in [2], the GSC in combination with the DC-link can be ued a a eactive powe ouce. In thi pape, the GSC i ued to upply eactive powe fo the excitation of the geneato itelf, thu acting a a STATCOM in IG mode to coect the powe facto een by the gid. The eactive powe capability of the GSC can be fully utilized with no active powe tanfe and it i not compomied by gid fault, ince the tanition i caied out unde nomal opeation. Powe facto contol i implemented in the GSC contolle. With the powe facto (PF) et by the wind fam ytem level contol, and the actual active powe (P ) geneated fom the geneato, the eactive powe efeence (Q ef) fo the GSC contolle can be obtained uing (7). Q ef ω h P (7) PF 2 Fo example, Q ef i et to be zeo if the geneato i expected to be opeated at unity powe facto. The eactive powe contol logic i hown in Fig. 7. The input to the VA egulato i the eo between the eactive powe efeence and actual ω DFIG ytem eactive powe output (Q ). The GSC q-axi cuent efeence (which contol the eactive powe in gid voltage oiented efeence fame with d-axi aligned with the voltage) i upplied. The tanition ignal enue the upplementay eactive powe contol i only activated unde IG mode. Q Q ef VA Regulato I ef Tanition ignal Fig. 7 Reactive powe contol uing GSC. I qef GSC Contolle PWM Moeove, gid code geneally have powe facto limitation (e.g..95 lagging to.95 leading unde nomal gid condition) fo gid-connected wind tubine geneato to uppot voltage egulation; and exta eactive powe uppot can alo be equied unde fault condition (e.g. voltage dip) [2]. If the gid demand the geneato to abob eactive powe fom the gid within the pemiible powe facto ange (i.e. lagging powe facto), the eactive powe upplied fom the GSC can be educed and the buden on the GSC i educed while the gid code can be atified. Howeve, if the geneato i expected to upply eactive powe to the gid (i.e. leading powe facto) in IG mode, the GSC hould upply eactive powe to the geneato a well a the gid and it will be highly teed o may not have enough eactive powe capability to uppot voltage egulation. Hence, highe capacity might be equied fo the GSC. Nevethele, it i impotant to note that the gid code ae nomally by the voltage uppot equiement at the point of common coupling (PCC) whee the wind fam i intefaced to the gid. The coodinated contol among wind tubine geneato in the wind fam could be able to atify the eactive powe equiement, ince it i unlikely that all wind tubine geneato in the wind fam will opeate in IG mode imultaneouly. Futhemoe, exta eactive powe eouce like STATCOM, SVC, etc. ae geneally equipped fo wind fam at the PCC to uppot voltage egulation [22]. In IG mode, when the oto peed exceed the IG opeation ange [ω l, ω h], the geneato hift back to the DFIG mode. Since the RSC loe the contol of the geneato in IG mode, the toque of the IG cannot be actively contolled. Nevethele, due to the fat epone of electomagnetic toque contol, the geneato i able to tack to the electomagnetic toque efeence apidly afte witching back to the DFIG mode. If impope toque efeence i povided, lage toque tanient i expected due to the mimatch between the mechanical toque and electomagnetic toque. Theefoe, the toque efeence povided by the toque contolle hould be able to cloely follow the mechanical toque duing the IG mode opeation and an anti-windup contol loop i added into the toque contolle a hown in Fig. 8. Without the anti-windup contol, an eo peented between the actual oto peed and oto peed efeence povided by the MPPT contol will dive the toque efeence to it limit, cauing a udden hock on the dive-tain and lage ocillation in the oto peed duing tanition.

8 When the anti-windup contol i applied, the electomagnetic toque efeence can cloely follow the mechanical toque by feeding back the eo between toque efeence and mechanical toque to the peed egulato. Hence, the high toque te on the dive-tain can be alleviated. A imila upplementay contolle i ued in [9] to povide enhanced pimay fequency epone fom the DFIG. The implementation of the upplementay contol in thi pape futhe extend the benefit of the upplementay contol of the fequency egulation. P Pm W ω* ω Fig. 8 Toque contolle with anti-windup contol. e ω Anti-windup contol e T e t T m k Speed Regulato T max T min V. THERMAL ANALYSIS OF DFIG COMPONENTS A. Themal Analyi of Semiconducto Device The pocedue of themal analyi fo emiconducto device in thi pape i peented in Fig. 9. The electical loading ignal fom the DFIG electical model a hown in Fig., uch a DC-link voltage (V DC), RSC witching fequency (f w) and RSC cuent, ae fed into an enegy lo calculation function, fom which powe lo (P lo) of the emiconducto can be calculated, and then the emiconducto junction tempeatue (T j) can be etimated with the themal netwok of the emiconducto device. The tempeatue i feedback to the lo calculation function to eflect the tempeatue dependency of the loe. DFIG electical model v RSC, i RSC V DC, f w Fig. 9 Themal analyi model. Lo calculation P lo_igbt/diode Themal netwok T ef T j_igbt/diode The enegy lo of emiconducto device ae mainly compoed of two pat: conduction lo and witching loe. The conduction lo of the IGBT and feewheeling diode can be detemined by (8) and (9) epectively [7]. E T T T con _ IGBT ( T j T ce j ce j T ) i, T ) i ( t) v i ( t dt (8) T T T Econ _ diode ( id, T j ) id ( t) v f j f j id ( t) dt (9) whee v ce and v f ae the thehold voltage of the IGBT and diode epectively, ce and f ae the bulk eitance of IGBT and diode epectively, and i T and i D ae the cuent flowing though IGBT and diode epectively. In egad to the witching loe of an IGBT and diode, it depend on the load cuent, DC link voltage and junction tempeatue a given by () and () [5, 4]. E E w _ IGBT w _ diode ( i, V DC ( i, V, T ) ( E DC j j on C, T ) E E w C off T ) j i I ef T i I ef T j ef Ki V V V V T ef DC ef DC ef Kv Kv () () whee I ef, V ef, and T ef ae the cuent, voltage, and tempeatue epectively, when the witching lo i meaued a given in the dataheet of the emiconducto device. K v and K i ae the exponent fo voltage and cuent dependency of the witching lo epectively. C w and C ae the tempeatue coefficient of witching lo fo IGBT and diode epectively. In thi tudy, the SEMIKRON IGBT module SKiiP 23 GB72-4DW V3 [7] i ued fo themal analyi. The value fo thee paamete can be found in [5, 7]. The Fote netwok [5] i ued a the model fo the themal netwok ued in thi pape to etimate the junction tempeatue which i detemined by (2); T jt / D PconT / D PwT / D ) Rth( j h) T / D ( T (2) whee P con and P w ae the conduction powe lo and witching powe lo epectively fo an IGBT o diode, Rth j-h i the themal eitance between the junction and heat ink, and T h i the tempeatue of heat ink which i given by (3). T h P Rth T (3) tot h a whee P tot i the total powe lo of the emiconducto, Rth h-a i the themal eitance between heat ink and ambient which i taken a.93 K/W in the cae tudie below. T a i the ambient tempeatue choen a 5 C [9] in thi pape. B. Themal Enegy Capacity of Cowba Accoding to cuent gid code the cowba opeation duing LVRT i limited to 3 [2]. In thi pape, the multimode opeation intend to opeate the cowba fo a longe time, o it i impeative to invetigate the themal pefomance of the cowba duing multimode opeation. The oto ovecuent unde gid fault can be up to thee time the nominal cuent [2]. Contaily, when the DFIG opeate in IG mode aound the ynchonou peed a popoed in thi pape, the oto cuent i much malle than the nominal cuent. Fo example, the.5 MW DFIG conideed in thi pape ha a ated oto cuent of 88 A, and the oto cuent unde IG mode i appoximately 35 A. Hence, the powe conumed by the cowba unde IG mode i ubtantially le than unde fault condition. The themal time contant of the cowba i geneally high enough to handle the oto fault cuent. A commecial cowba eito (model: FLWR-24 KJ/.7R) [23], which ha an enegy ating of 24 kj, eitance of.7 Ω, and opeating time of 3, ha been ued a the LVRT cowba fo the DFIG in thi tudy. Uing (4), the ated cowba cu- a h

9 ent (by auming 3 opeation of the cowba duing LVRT) i calculated to be 2 A, and thu it can be hown that the cowba i capable of opeating aound (t = 24 3 /(35 2.7)) unde IG mode without oveheating the cowba. Alo it mut be noted that the impovement in elf-cooling effect due to low cowba cuent i not conideed in thi calculation, hence the cowba can be actually opeated much longe than unde IG mode. E I (4) tr Theefoe, no hadwae modification ae equied fo exiting DFIG, uch a cooling ytem etc. to implement the deadband contol tategy popoed in thi pape. It i alo impotant to acknowledge that IG mode i jut a tempoay opeating mode conideing the continuou vaiation in wind peed. Fat cyclic IG mode opeation can be avoided a exemplified in Section VI-D. VI. PERFORMANCE OF THE MULTI-MODE OPERATION The popoed contol tategy can be ued in both gidconnected WECS and emote aea powe upply (RAPS) ytem which ae ilanded powe ytem. In thi pape, a hypothetical RAPS ytem i etablihed in the SimPoweSytem platfom to invetigate the pefomance of the multi-mode contolle, and the detail of the ytem ae given in Fig.. RAPS ytem ae contituted of mall numbe of geneato and the total capacity i uually mall. Powe management i moe challenging and tability i moe citical to manage in compaion to a gid-connected ytem. By validating new contol tategie though uch ytem can how the effectivene of the contol tategy even unde the wot cae cenaio. The load in the ytem ae mainly poweed by a dieel geneato and a DFIG. The dieel geneato etablihe the ytem by poviding fequency and voltage efeence. The DFIG i expected to uppot the ytem while opeating at unity powe facto. A imila ytem ha alo been exploed in [9] to veify the fequency epone tategy, but the contol tategie and load ae adjuted to tet the popoed contol tategy. kv/.4 kv 3.5 MVA DFIG.5 MW.4 kv S bae = MVA, V bae = kv Load.4 +j. (2.56+j.332)% (3.564+j2.66)% (6.4+j3.66)% Load.7+j.5 Fig. Achitectue of the tet ytem. Load.9 +j. (.4+j.35)% (3.976+j5.27)% kv/2.4 kv 3.5 MVA dieel geneato 3.25 MVA 2.4 kv A. Tanition fom DFIG Mode to IG Mode When the oto peed eache the deadband unde DFIG mode a detailed in Fig. 6, the opeating mode of the DFIG i alteed. A hown in Fig., a tep inceae in wind peed caue the oto peed to inceae at t = 75. If the peed egulation (ee Section IV-(A)) i not implemented, the DFIG witche to IG mode immediately, which caue lage tanient in the DFIG itelf a well a the RAPS ytem. The udden emoval of the oto voltage caue the oto cuent to decay and, the collape of the oto magnetic field eult in the lo of electomagnetic toque (ee blue dotted cuve in Fig. -(c)). Hence, the powe output fom the DFIG dop abuptly to zeo a peented by the blue dotted cuve in Fig. -(a). Futhemoe, the violent vaiation in electomagnetic toque will alo exet additional te on the dive-tain. A een in Fig. -(d), the mechanical toque deceae haply and ocillate befoe eaching new teadytate, attibuting to a udden inceae and ocillation in oto peed (ee blue dotted cuve in Fig. -(b)). Contaily, with the popoed contol tategy, the tanition fom the DFIG mode to the IG mode i moothed ignificantly. At aound t = 75, the DFIG tat to adjut the peed (thu toque) to match with the peed unde the IG mode. The DFIG witche fom the DFIG mode to the IG mode aound t = 92. At the moment of tanition, only mino ditubance ae obeved in powe output, ytem fequency, and toque. Active Powe (MW) Elec. Toque (p.u.) popoed oiginal Time () (a) Time () 95 (c) Roto peed (p.u.) Mechanical Toque (p.u.) Time () (b) Time () (d) Fig. The DFIG witche fom DFIG mode to IG mode. (a) DFIG active powe output. (b) DFIG oto peed. (c) DFIG electomagnetic toque. (d) DFIG mechanical toque. B. Tanition fom IG Mode to DFIG Mode A explained in Section IV-(B), if the popoed anti-windup contol tategy i not implemented, the eo between the actual oto peed and efeence value, will inceae the toque efeence value to it limit. The electomagnetic toque follow the efeence apidly (ee blue dotted cuve in Fig. 2- (c)) and a lage diffeence i obeved between the mechanical and electomagnetic toque, which bing enomou toque te on the dive-tain and an almot tep change in oto peed i eulted a hown by the blue dotted cuve in Fig. 2-

10 (b). A lage ocillation in mechanical toque i obeved in the zoomed ection in Fig. 2-(d). Meanwhile, an active powe uge can alo be obeved at the DFIG a indicated by the blue dotted cuve in Fig. 2-(a). With the popoed anti-windup contol, the electomagnetic toque efeence tack the mechanical toque duing the IG mode opeation. At the moment of mode tanition, the toque diffeence between the mechanical toque and electomagnetic toque i naowed. Conequently, lage tanient ae avoided and a mooth tanition fom the IG mode to the DFIG mode i achieved. Active Powe (MW) Elec. Toque (p.u.).5 popoed oiginal Time () (a) Time () (c) Roto peed (p.u.) Mechanical Toque (p.u.) Time () (b) Time () 9 (d) Fig. 2 WECS witche fom IG mode to DFIG mode; (a) DFIG active powe output, (b) DFIG oto peed, (c) DFIG electomagnetic toque, (d) DFIG mechanical toque. C. Opeation unde Vaiable Wind Speed Fo the cae tudie peented in Section VI-(A) and (B), a tep change in wind peed i ued to invetigate the dynamic pefomance of the WECS duing the tanition peiod. In ode to validate the effectivene of the popoed tategy unde a moe pactical cenaio, a 25 vaiable wind peed pofile i applied to the DFIG. The pefomance of the DFIG unde vaiable wind condition ae hown in Fig. 3. The wind peed pofile i hown in Fig. 3-(a). Initially, the DFIG i opeating in DFIG mode and the oto peed (ω ) i below ynchonou peed, a hown in Fig. 3-(f) and thu, the GSC i abobing powe (P gc) fom the gid (indicated by poitive value a hown in Fig. 3-(d)). Since the GSC i opeating unde unity powe facto, the eactive powe flow (Q gc) at the GSC i zeo (ee Fig. 3-(e)). Fig. 3-(b) and (c) how the active powe (P ) and eactive powe (Q ) inteaction epectively between the DFIG and the gid. The DFIG doe not paticipate in eactive powe uppot fo the gid, and opeate cloe to the unity powe facto to atify the powe facto equiement et by the utility. Aound t = 3 to 4, the oto peed i detected to be within the deignated deadband (i.e. [ω ε, ω +ε]) fo DFIG opeation. Hence, the peed egulation i activated; the peed egulation peiod i hadowed in Fig. 3. The oto peed inceae and powe output fom the DFIG deceae, ince the opeating point of the DFIG i hifting away fom the point et by MPPT contol. Nevethele, the financial benefit will not be compomied ignificantly, becaue the WECS tempoaily opeate in IG mode conideing the continuou vaiation in wind peed. The GSC tat to povide active powe when the oto peed exceed the ynchonou peed. At the moment when the oto peed eache the ange [ω l, ω h], the cowba i activated to hot-cicuit the oto winding and the RSC i blocked. The mode tanition take place aound t = 6 in thi cae. V wind (m/) P (MW) Q (MVA) P gc (MW) Q gc (MVA) (p.u.) (a) (b) (c) (d) DFIG Mode IG Mode Speed egulation DFIG Mode 5 (e) ω [ω -ε,ω +ε] ω [ω l,ω h] Time () (f) Fig. 3 The DFIG opeation unde vaiable peed; (a) Wind peed pofile, (b) DFIG active powe output, (c) DFIG eactive powe output, (d) GSC active powe output, (e) GSC eactive powe output. (f) DFIG oto peed. Fom Fig. 3, it can be een that abupt change only occu at the powe output fom the GSC, which indicate the mooth tanition fom the DFIG mode to the IG mode. The active powe output at the GSC uddenly deceae to zeo (ee Fig. 3-(d)) due to the zeo active powe flow fom oto winding to the DC-link when the RSC i diabled. A decibed in Section IV-B, the zeo active powe output fom the GSC leave the capacity of the GSC fo eactive powe uppot. Theefoe, the GSC opeate a a STATCOM to fulfil the eactive powe equiement of the DFIG unde the IG mode, and hence the eactive powe output fom the GSC i no longe zeo, a hown in Fig. 3-(e). Conequently, the gid doe not upply any eactive powe to the DFIG (unde IG mode) and unity powe facto opeation i maintained a hown in Fig. 3-(c).

11 The DFIG opeate a an induction geneato until the wind peed amp down at aound t = 5. It i woth to be noted that an additional advantage of the IG mode opeation i that it can enhance the fequency uppot by poviding a natual inetial epone by uing it toed kinetic enegy without any contol intevention [24]. On the declining in wind peed fom t = 5, the oto peed deceae and the demand fo witching WECS back to the DFIG mode i activated when the oto peed i detected to be out of the ange [ω l, ω h]. The cowba mechanim i deactivated and the RSC i activated. The DFIG lowly hift to teady-tate opeation unde DFIG mode. The GSC top poviding eactive powe and tat to delive active powe again a een in Fig. 3-(e) and (d) epectively. The thee phae-cuent in oto winding ae hown in Fig. 4. The uppe left zoomed figue in Fig. 4 how that the fequency of oto cuent i appoximately.5 Hz befoe the DFIG witche fom DFIG mode to IG mode. A explained in Section III, the low fequency of the oto cuent can caue themal poblem fo the emiconducto device of the RSC. The intantaneou powe lo and junction tempeatue ae hown in Fig. 5-(a) and (b) epectively fo the uppe IGBT and lowe diode in one leg of the RSC. Since the IGBT and the diode tun on and tun off imultaneouly, the intantaneou powe lo of each component i coincidental, a hown in Fig. 5-(a), and the coeponding tempeatue of each component i alo peented in the zoomed gaphic in Fig. 5-(b). Although the powe lo of the IGBT i highe than the diode between 35 to 45, the tempeatue of the IGBT i oughly the ame a the diode, which i due to the lowe junction-toheatink themal eitance of IGBT compaed to diode. An additional lowe peak in the tempeatue wavefom can be obeved between two adjacent highe peak, which i due to the themal coupling effect between the IGBT and it antipaallel diode ince the two chip ae placed on the ame bae plate and cloe to each othe. The coupling effect become tonge when the cuent fequency i low and hence futhe inceae the junction tempeatue fluctuation. The heating of the antipaallel diode in the econd half of the fundamental peiod add to the heating of the IGBT on the ame bae plate. Roto Phae Cuent (p.u.) Time ().5 Fig. 4 Thee-phae cuent in oto winding The uppe ight zoomed figue in Fig. 4 peent the oto cuent when the DFIG i opeating in IG mode, and highe fequency of oto cuent can be obeved. Hence, oto cuent of vey low fequency ae avoided. Due to the ubynchonou and upe-ynchonou opeation in DFIG and IG mode epectively, the phae equence of the oto cuent change with the mode tanition. Since the RSC i blocked in IG mode, it can be een fom Fig. 5-(b) that emiconducto device ae cooled down in IG mode. Intantaneou powe lo (kw) Tempeatue (C) IGBT 45 diode (a) IGBT 6 diode Time () (b) Fig. 5 Themal pefomance of the IGBT and diode. (a) Intantaneou powe loe. (b) Junction tempeatue. D. Riding though Synchonou Speed Long-tem opeation aound ynchonou peed can be avoided with the popoed contol tategy fo the DFIG, and the opeating mode of the DFIG i not expected to be changed if the oto peed pae the ynchonou peed apidly due to lage wind peed vaiation. Hence, the opeating mode altenation will not be activated if the DFIG can ide-though the ynchonou peed within a pedefined time peiod. Fo example, the aveage wind peed inceae fom 8.5 m/ to m/ with a amp ate of.5 m/ 2 (ee Fig. 6), the opeating mode i not changed. If the amp ate of wind peed i lowe than.5 m/ 2, the mode tanition will be activated, ince the oto peed tay within the deadband fo a longe peiod of time, and the junction tempeatue inceae may damage the emiconducto device. V wind (m/) (p.u.) amp ate.5 m/ (a) DFIG ide though ynchonou peed without alteing opeating mode Time () (b) Fig. 6 The DFIG opeation duing wind peed amp-up cenaio. (a) Wind peed pofile. (b) DFIG oto peed.

12 The time that oto peed take to ide though the deadband can be appoximated baed on the amp ate of wind peed and the mechanical dynamic of the wind tubine. Conideing the pemiible time that a emiconducto device can afely opeate at maximum junction tempeatue, the citical amp ate of wind peed hould be et to enue the ide-though time doe not exceed the pemiible opeating time. Additionally, the wind peed (i.e. m/ in thi pape) at the ated powe output of the wind tubine i well above the wind peed (i.e. aound 9 m/ in thi pape) unde which the DFIG oto peed i within the deadband. Hence, accoding to the pobability denity function of the wind peed fo a typical wind fam location, the likelihood of DFIG opeating in the IG mode i low. Theefoe, the tempeatue wing due to the RSC on and off i not fequent and the financial lo due to the non-optimal opeation unde IG mode i limited while eliability of the DFIG can be impoved with the extended evice life of the emiconducto device. VII. PROOF OF CONCEPTS USING EXPERIMENTAL TEST BED The effectivene of the popoed contol tategy i veified by extenive imulation tudie peented in Section VI. Due to the fact that the IGBT junction tempeatue and it powe lo elated to themal analyi ae not available a a diect meauement in pactical expeiment, imulation i a good altenative way to validate the popoed contol tategy. In thi ection, ome expeimental eult ae povided to pove the concept of multimode opeation uing the tet ytem a hown in Fig. 7. Dynamomete & compute inteface G WRIM Tanfome module Gid of upe-ynchonou opeation, the oto powe i abobed by a dump eito aco the DC-link. The RSC i manually contolled by the invete contol unit to vay it output fundamental fequency, and the vaiable voltage DC-link contol the oto voltage magnitude. Beide, no extenal eitance i added into the cowba becaue no fault tudy i caied out in thi pape. Conequently, the ytem opeate in the ame pinciple a a nomal DFIG apat fom the lack of phyical GSC. The geneato with oto hot-cicuited unde a wind peed of m/ i teted and the oto peed at the equivalent opeating point i detemined to be 675 pm. Then, the invete i enabled and the oto voltage i manipulated to opeate the DFIG at unity powe facto aound ynchonou peed (i.e. 5 pm). A hown in Fig. 8, the RSC upplie diect cuent to the oto winding with the fact that one phae cuent i two time the cuent of each of the othe two phaecuent, cauing unbalanced heating in IGBT. By inceaing the fequency of oto cuent with the phae equence oppoite to the oto otating diection, the oto peed inceae. A thi poce i manually contolled, fluctuation in oto and tato cuent a well a mechanical toque can be obeved in Fig. 8 and 9. The tanition fom DFIG mode to IG mode take place afte the oto peed eache the peed detemined in the fit tep, the cowba i enabled while RSC i diconnected. The oto peed and toque wavefom demontate that mooth tanition fom DFIG mode to IG mode i achieved. It can be een that the oto cuent deceae while tato cuent inceae unde IG mode becaue eactive cuent i upplied by the RSC in DFIG mode wheea the gid povide eactive powe fo the geneato excitation unde IG mode (GSC i not available to povide eactive powe uppot in thi tet.). Since the invete contol unit maintain the oiginal contol ignal (which act imilaly to the RSC contolle tacing the mechanical toque efeence duing IG mode a dicued in Section VI-B), the ytem tanit fom IG mode back to DFIG mode by connecting RSC to the oto and diabling the cowba. The tanition i alo mooth without any abupt vaiation in oto peed and toque a hown in Fig. 8 and 9 epectively. Theefoe, the tet eult validated the baic concept of the multimode opeation. Invete contol unit IGBT unit Powe upply module Fig. 7 Achitectue of the tet ytem. Phae B Smooth tanition Roto peed Roto cuent The ytem i developed baed on the LabVolt electic powe technology taining ytem with the guideline and paamete given in [25]. A wound oto induction machine module i ued a the geneato, wheea a dynamomete module emulate the wind tubine with it chaacteitic contolled by the compute inteface. Since the pupoe of the tet i to obeve the dynamic in oto cuent, tato cuent a well a oto peed and toque duing mode tanition, the back-to-back convete cheme and it contol ae implified. An IGBT baed invete unit i ued fo the RSC, while the DC-link i connected to a vaiable DC voltage ouce. In cae Diect cuent Phae A, C Synchonou opeation Speed egulation IG Mode Fig. 8 Tet eult howing oto peed and oto cuent. DFIG Mode

13 REFERENCES Synchonou opeation Speed egulation Smooth tanition IG Mode Stato cuent Mechanical toque DFIG Mode Fig. 9 Tet eult howing mechanical toque and tato cuent. VIII. CONCLUSION In thi pape, a multimode opeation contol tategy i implemented to pevent the DFIG opeating aound the ynchonou peed. Hence, lage junction tempeatue vaiation in the emiconducto device due to low lip fequency can be avoided. Supplementay contol tategie ae popoed to minimize the tanient caued by the mode tanition. Detailed imulation ae caied out to invetigate the pefomance of the popoed contol tategy in a emote aea powe upply ytem. Powe lo and themal netwok fo the RSC i etablihed to veify the effectivene of the popoed tategy. The eult validate that the deadband et fo ynchonou opeation can educe high tempeatue vaiation. Senitivity analyi how that the DFIG can ide-though the deadband to avoid unneceay mode tanition if DFIG i confonted with highly vaiable wind peed and pae though ynchonou peed. The popoed tategy doe not equie any additional hadwae and modification to the cuent ytem configuation while the gid-code tandad can be atified. The concept of multimode opeation ha alo been veified uing an expeimental etup. Futhemoe, when the DFIG opeating in induction geneato mode, it can alo enhance the fequency uppot with natual inetial epone uing it toed kinetic enegy without any contol intevention, which can be futhe exploed. APPENDIX TABLE I DFIG PARAMETERS Paamete Value Paamete Value Rated powe.5 MW Lm 2.9 p.u. Tubine inetia contant 4.32 R.23 p.u. Rated wind peed m/ L.8 p.u. Rated tato voltage 4 V R.6 p.u. Rated DC bu voltage 8 V L.6 p.u. Cowba eitance.2 Ω ω l.8 p.u. f w 35 Hz ω h. p.u. [] S. Yang, A. Byant, P. Mawby, D. Xiang, L. Ran, and P. Tavne, "An induty-baed uvey of eliability in powe electonic convete," IEEE Tan. Ind. Appl., vol. 47, no. 3, pp , 2. [2] J. Ribant and L. M. Betling, "Suvey of failue in wind powe ytem with focu on wedih wind powe plant duing ," IEEE Tan. Enegy Conve., vol. 22, no., pp , 27. [3] D. Zhou, F. Blaabjeg, M. Lau, and M. Tonne, "Themal behavio optimization in multi-mw wind powe convete by eactive powe ciculation," IEEE Tan. Ind. Appl., vol. 5, no., pp , 24. [4] S. Yang, D. Xiang, A. Byant, P. Mawby, L. Ran, and P. Tavne, "Condition monitoing fo device eliability in powe electonic convete: A eview," IEEE Tan. Powe Electon., vol. 25, no., pp , 2. [5] A. Wintich, U. Nicolai, W. Tuky, and T. Reimann, "Application manual powe emiconducto," S. I. GmbH, Ed., 2nd ed. Nuembeg, Gemany: ISLE Velag, 25. [6] T. Lei, M. Bane, and A. C. Smith, "Themal cycling evaluation fo DFIG wind tubine powe convete baed on joint modelling," in Poc. Enegy Conveion Conge and Expoition (ECCE), 23 IEEE, 23, pp [7] M. Z. Sujod, I. Elich, and S. Engelhadt, "Impoving the eactive powe capability of the DFIG -baed wind tubine duing opeation aound the ynchonou peed," IEEE Tan. Enegy Conve., vol. 28, no. 3, pp , 23. [8] L. Wei, R. J. Kekman, R. A. Lukazewki, H. Lu, and Z. Yuan, "Analyi of IGBT powe cycling capabilitie ued in doubly fed induction geneato wind powe ytem," IEEE Tan. Ind. Appl., vol. 47, no. 4, pp , 2. [9] D. Zhou, F. Blaabjeg, M. Lau, and M. Tonne, "Themal cycling oveview of multi-megawatt two-level wind powe convete at full gid code opeation," IEEJ Jounal of Induty Application, vol. 2, no. 4, pp , 23. [] S. Engelhadt, I. Elich, C. Felte, J. Ketchmann, and F. Shewaega, "Reactive powe capability of wind tubine baed on doubly fed induction geneato," IEEE Tan. Enegy Conve., vol. 26, no., pp , 2. [] J. Jung and W. Hofmann, "Invetigation of themal te in the oto of doubly-fed induction geneato at ynchonou opeating point," in Poc. Electic Machine & Dive Confeence (IEMDC), 2 IEEE Intenational, 2, pp [2] M. Z. Sujod and I. Elich, "Reactive powe capability of DFIG baed wind tubine aound ynchonou opeating point with two-level and thee-level npc convete," in Poc. PoweTech (POWERTECH), 23 IEEE Genoble, 23, pp. -6. [3] L. Wei, J. 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14 [2] A. Etxegaai, P. Eguia, E. Toe, A. Ituegi, and V. Valvede, "Review of gid connection equiement fo geneation aet in weak powe gid," Renew. Sutain. Enegy Rev., vol. 4, pp. 5-54, 25. [22] A. K. Pathak, M. P. Shama, and M. Bundele, "A citical eview of voltage and eactive powe management of wind fam," Renew. Sutain. Enegy Rev., vol. 5, pp , 25. [23] Fulintech Science & Technology Co.,Ltd. Reito fo wind enegy catalog [Online]. Available: [24] G. Lalo, A. Mullane, and M. O'Malley, "Fequency contol and wind tubine technologie," IEEE Tan. Powe Syt., vol. 2, no. 4, pp , 25. [25] Feto Didactic Ltée/Ltd. Pinciple of doubly-fed induction geneato (DFIG): Couewae ample [Online]. Available: Yingjie Tan (S 2) eceived the B.Sc. (Eng.) (Hon.) degee in electical engineeing and automation fom the China Univeity of Petoleum (Huadong), Qingdao, China, in 22. He i cuently woking towad the Ph.D. degee at the Univeity of Wollongong, Wollongong, Autalia. Hi eeach inteet ae enewable ditibuted geneation and micogid. of Technology. He i a membe of IEEE and IEEE Powe Engineeing Society (PES). Phil Ciufo (SM 7) eceived the B.E. (Hon.) and M.E. (Hon.) degee in electical engineeing fom the Univeity of Wollongong, Wollongong, Autalia, in 99 and 993, epectively, and the Ph.D. degee in electical engineeing in 22. D. Ciufo ha had vaiou tint in induty a an Electical Enginee and etuned to academia in 27. Cuently, he i an Aociate Pofeo with the Univeity of Wollongong. Hi eeach inteet include modeling and analyi of powe ditibution ytem, ditibution automation, modeling and analyi of ac machine, powe ytem hamonic, and powe ytem eliability. Kahem M. Muttaqi (M SM 5) eceived the B.Sc. degee in electical and electonic engineeing fom the Bangladeh Univeity of Engineeing and Technology, Dhaka, Bangladeh, in 993; the M.Eng.Sc. degee in electical engineeing fom the Univeity of Malaya, Kuala Lumpu, Malayia, in 996; and the Ph.D. degee in electical engineeing fom Multimedia Univeity, Selango, Malayia, in 2. He i cuently a Pofeo with the School of Electical, Compute, and Telecommunication Engineeing, and a Membe of Autalian Powe Quality and Reliability, Univeity of Wollongong, Wollongong, Autalia. He wa aociated with the Univeity of Tamania, Autalia, a a Reeach Fellow/Lectue/Senio Lectue fom 22 to 27, and with the Queenland Univeity of Technology, Autalia, a a Reeach Fellow fom 2 to 22. Peviouly, he alo woked fo Multimedia Univeity, a a Lectue fo thee yea. He ha moe than 8 yea of academic expeience, and i autho o coautho of moe than 2 pape in intenational jounal and confeence poceeding. Hi eeach inteet include ditibuted geneation, enewable enegy, electical vehicle, mat-gid, powe ytem planning, and contol. D.Muttaqi i an Aociate Edito of the IEEE TRANSACTIONS ON IN- DUSTRY APPLICATIONS. Laantha Meegahapola (S 6 M ) eceived hi BSc. Eng. degee in Electical Engineeing (Fit Cla) fom the Univeity of Moatuwa, Si Lanka in 26, and hi PhD degee fom Queen' Univeity of Belfat, UK in 2. Hi doctoal tudy wa baed on the invetigation of powe ytem tability iue with high wind penetation, and eeach wa conducted in collaboation with EiGid (Republic of Ieland-TSO). In addition, he conducted extenive eeach tudie on coodinated eactive powe dipatch duing teady-tate and dynamic/tanient condition fo netwok with high wind penetation. He wa a viiting eeache in the Electicity Reeach Cente, Univeity College Dublin, Ieland (29/2). Fom 2-24 he wa employed a a lectue at the Univeity of Wollongong (UOW) and continue a an honoay fellow at UOW. He i cuently employed a a lectue at Royal Melboune Intitute