Improved FRT Control Scheme for DFIG Wind Turbine Connected to a Weak Grid Abulanwar, El-Saye Mohamed; Chen, Zhe; Iov, Florin

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1 Aalbor Univeritet Improved FRT Control Scheme for DFIG Wind Turbine Connected to a Weak Grid Abulanwar, El-Saye Mohamed; Chen, Zhe; Iov, Florin Publihed in: Proceedin of the 5th IEEE PES Aia-Pacific Power and Enery Enineerin Conference, APPEEC 2013 DOI (link to publication from Publiher): /APPEEC Publication date: 2013 Document Verion Early verion, alo known a pre-print Link to publication from Aalbor Univerity Citation for publihed verion (APA): Abulanwar, E., Chen, Z., & Iov, F. (2013). Improved FRT Control Scheme for DFIG Wind Turbine Connected to a Weak Grid. In Proceedin of the 5th IEEE PES Aia-Pacific Power and Enery Enineerin Conference, APPEEC 2013 IEEE Pre. DOI: /APPEEC General riht Copyriht and moral riht for the publication made acceible in the public portal are retained by the author and/or other copyriht owner and it i a condition of accein publication that uer reconie and abide by the leal requirement aociated with thee riht.? Uer may download and print one copy of any publication from the public portal for the purpoe of private tudy or reearch.? You may not further ditribute the material or ue it for any profit-makin activity or commercial ain? You may freely ditribute the URL identifyin the publication in the public portal? Take down policy If you believe that thi document breache copyriht pleae contact u at vbn@aub.aau.dk providin detail, and we will remove acce to the work immediately and invetiate your claim. Downloaded from vbn.aau.dk on: eptember 26, 2018

2 Improved FRT Control Scheme for DFIG Wind Turbine Connected to a Weak Grid S. Abulanwar 1,2, Graduate Student Member, IEEE, Zhe Chen 1,2, Senior Member, IEEE, F. Iov 1, Senior Member, IEEE 1 Enery Technoloy Dept., Aalbor Univerity, 2 Sino-Danih Centre for Education and Reearch Pontoppidantraede 101, Aalbor Eat, Denmark ema@et.aau.dk Abtract Thi paper preent an improved coordinated fault ridethrouh (FRT) control tratey for a doubly fed induction enerator (DFIG) baed wind turbine, (WT), in a weak rid. A technique for rid ynchronization aaint voltae excurion, i.e., a Dual Second Order Generalized Interator Frequency Locked Loop (DSOGI- FLL) i utilized to extract a robut rid voltae ynchronization inal irrepective of the main condition to enhance the overall ytem performance. Beide, a decoupled double ynchronou reference frame (DDSRF) dq current controller i devoted for the rid ide converter, (GSC), controller to counteract current ripple and tackle the DC link voltae fluctuation. Alo, a reactive power upport cheme to manae the DFIG reactive power durin continencie and fulfill the rid code obliation i preented. Moreover, additional control term are employed with the DFIG converter controller to counteract rotor a well a tator current and reulate the rotor peed. Simulation reult which aure the effectivene of the propoed control cheme i preented. Index Term DFIG, DSOGI-FLL, FRT, rid code, weak rid. I. INTRODUCTION Wind enery converion ytem (WECS) are typically located in remote zone and connected to the rid via lon feeder with lower hort circuit ratio (SCR). In the area which eoraphically rich in wind enery, the tranmiion of hue amount of wind power to the power rid i retricted due to the tranmiion network weak tructure [1]. In a weak rid, a chane in active and/or reactive power can poe coniderable voltae variation which can be a limitin factor for the interation of more wind power [2]. Beide, the impact of WT output power on the voltae quality can be another limitin factor which neceitate the incorporation of enhanced control ytem to addre uch defect [2], [3]. Owin to the teady rowth of decentralized wind power plant, (WPP) many countrie have impoed trinent reulation to the (WPP) interation which known a rid code [3]-[5]. Fi. 1.a how the low voltae ride-throuh (LVRT) reulation for the German, Scottih and Irih rid code in which the wind turbine (WT) mut tay connected when the terminal voltae lie in or above the haded area [3]. A illutrated in Fi. 1.a, E.ON LVRT demand fault ride-throuh (FRT) for voltae level down to zero with 150 m duration. Moreover, WT hould releae reactive power upport a traditional enerator to boot and recover the ac voltae. Fi.1.b depict the German and Spanih reactive power upport requirement. Baically, reactive current upport i ubtantial for voltae dip with remnant voltae le than 0.9 p.u., while full reactive current upport i neceary for evere voltae dip with remnant voltae lower than p.u [3], [4]. DFIG WT, are extenively preferred due to variable peed operation uin partial cale converter rated at 25-30% that allow for independent control of active and reactive power [4]. Neverthele, DFIG i enitive to the rid fault and thu prone to variou eriou iue due to the direct connection of it tator to the rid [2]-[4]. Subequent to a fault, the rotor ide and rid ide converter, RSC, GSC repectively are prone to overcurrent and overvoltae unle an additional protection i incorporated. Uually, a crowbar circuit i inerted in the rotor ide and activated upon fault detection to mitiate the above-mentioned hortcomin. However, trierin the crowbar reult in blockin the RSC, meanwhile the machine aborb hiher reactive power a a conventional induction enerator [3]. Uin advanced control trateie, DFIG LVRT can be dramatically achieved [2]-[5]. In [5], a tability tudy for a permanent manet WT connected to a weak rid i preented. It wa hown that the operation of WT in weak rid with SCR below 4 i viable by mean of advanced ac voltae controller. The influence of the injected reactive current control to upport the ac voltae due to ymmetrical fault in a weak rid i invetiated in [6]. It ha been concluded that, DFIG WT can readily provide reactive power upport via fat voltae control for weak rid with SCR of 4. Depite thi, further compenation circuit are neceary to upport the terminal voltae for SCR between 2 and 3. Thi paper provide an improved coordinated FRT control cheme for DFIG WT connected to a weak rid of SCR of 3 under different continencie to fulfill the rid code requirement. Typically, claical ynchronou reference frame (SRF) dq current controller render deficient performance durin unbalanced utility condition. To overcome uch hortcomin, a decoupled double ynchronou reference frame, (DDSRF) current controller i dedicated to control the GSC to tackle the double-frequency current ocillation and reulate the dc link voltae durin diturbance. A reactive power upport control cheme i preented to effectively atify the rid code reactive

3 power boot obliation. Beide, a Dual Second Order Generalized Interator Frequency Locked Loop (DSOGI-FLL) i utilized o a to furnih a robut rid voltae ynchronization inal aaint variou perturbation. For better reulation of the tranient rotor current, proportional interal plu reonant, PIR controller are employed in the RSC controller. Additional propoed block are interated with the DFIG converter controller, to mitiate the rotor and tator overcurrent durin voltae interruption. II. DFIG WT CAPABILITY LIMITS The capability limit of a 2 MW DFIG coniderin tator and rotor current heatin contraint due to Joule loe, maximum and minimum active and reactive power i hown in Fi.2 [1]. Identifyin the control abilitie of the DFIG aim at optimally dein the DFIG control ytem in order to improve the ytem operation. More detail about uch capability limit can be found in [2]. III. GRID SYNCHRONIZATION A. SRF-PLL Synchronization Amon the different apect for the control of the ridconnected converter, i the exact ynchronization with the utility voltae. Phae Locked Loop, PLL baed ynchronou reference frame, SRF-PLL i the mot intenively ued technique for detectin the manitude and poition of the poitive-equence rid voltae. The baic tructure of SRF-PLL i hown in Fi.3. Depite it ood behaviour under normal condition, SRF-PLL provide poor dynamic repone under unbalanced rid condition even with reduced bandwidth [7]. B. DSOGI-FLL Synchronization Fulfillin the rid code requirement and keepin the WT connected durin continencie entail the uae of alternative ynchronization technique. Dual Second Order Generalized Interator, DSOGI baed Frequency Locked Loop, DSOGI-FLL i an inenitive frequency-adaptive ynchronization mechanim which benefit the intantaneou ymmetrical component analyi coniderin adaptive filter [7]. Bein frequency-baed, DSOGI-FLL can efficiently ride-throuh the rid perturbation, provide a clean ynchronization inal and attenuate rid voltae hih-frequency component [8]. Fi. 4 illutrate the DSOGI-FLL tructure. Further detail about DSOGI-FLL can be obtained from [7], [8]. Performance comparion between SRF- PLL and DSOGI-FLL will be preented throuh the imulation. IV. PROPOSED DFIG FRT CONTROL SCHEME Thi ection provide an iniht into the coordinated DFIG FRT control tratey ued to improve the ytem performance and meanwhile atify the rid code requirement. V / V n V / V n I q / I n Fiure 1. Typical WT Grid code requirement, (a) LVRT (b) Reactive power upport Q min P r 2 V X Pt max Q max Fiure 2. DFIG capability limit (a) Capability limit (b) Final capability area V abc V abc V ref T e v ( a ) qv a qv b e v ( b ) V êté ù ú v ë dq û q e v ( a ) e v ( b ) V = vd w o w Fiure 3. SRF-PLL baic tructure k k ( a) ( b) w w k w V qv V b qv b ( V ) + ( V a b ) w o Fiure 4. DSOGI-FLL baic tructure 0.1 Q max Q min Q Q ref Q max Q min Q max Q refq min ò Fiure 5. Reactive power control cheme + V a + V b - V a - V b 1 T 1 T w q V abc V abc Q Q

4 A. Reactive Power Support Control Scheme A cacaded reactive power control cheme (ee Fi.5) devoted to manae the reactive power harin via DFIG tator a well GSC ide i adopted here [3]. The priority for the reactive power aid i iven to the DFIG tator ide -which proceed via the RSC control- followed by the GSC ide whenever the tator ide attain the limit - due to evere fault - which pre-defined throuh the DFIG capability limit mentioned in ection II. The reactive power upport i initiated once the rm tator voltae, lip more than 0.1 p.u (ee Fi.1) out of the reference value,. Typical limiter are incorporated to enure that the releaed reactive power doe not overtake the repective capability limit of tator and GSC each. Such manaement cheme i efficient a the GSC reactive power upport tart ubequent to the tator ide injection which allow for active power tranmiion at the fault onet and thu mitiatin the unwanted tranient DC voltae fluctuation. B. RSC Control Scheme The tator ide active and reactive power,, can be reulated via actin on the repective rotor dq voltae component. The RSC applied control cheme i demontrated in Fi.6. The RSC controller i implemented in a reference frame of which d-axi i alined with the tator voltae vector. For further improvement of the RSC tranient repone, PIR current reulator are employed in the RSC controller a hown in Fi. 7 [9]. The parallel reonant block are tuned at,2. i the cut-off frequency and i the proportional ain at the reonant frequency. Hence, the RSC dq voltae component are et a: v = PIR i - i - w L i + L i (1.a) ( - ) ( ) ( ) w ( ) dr dr ref dr lip r qr m q v qr = PIR i qr-ref - i qr + lip L r i dr + L m i d Where,, are the rotor active and reactive current component repectively., are the tator ide active and reactive current component. ω i the lip frequency., refer to rotor and manetizin inductance. The tator active and reactive power,, can be reulated via the RSC current,, and ubequently rotor dq voltae component,,. Whenever the tator voltae dip, the DFIG output power decreae a well. Accordinly, to tackle the tranient rotor a well a tator current and maintain the power balance, the MPPT i deactivated and the reference active power i et a [10]: ( ) 2 3 P = k k V V w -ref p opt o r-ref opt = r b b p-max lopt k A r C N (1.b) Where, k p i a ain factor. V 0, V refer to the tator voltae before and durin the fault repectively and i the (2) (3) reference rotor peed. With 2, tiny active power upply i utained durin evere fault a recommended by the Britih and Danih rid code [3], [10]. With the aid of the reactive power upport cheme, the RSC concurrently inject reactive current upport with the remnant rotor current to fulfill the rid code commitment and retore the ac voltae. v w b wr P Q P ref P Q ref idqr ref Q Vdq ddt w w lip i dr i dr ref iqr ref w r r lip i dr i qr i qr Fiure 6. RSC control cheme i dqr k p k i/ 2kir wc wc + w 2kirwc 2 + 2wc + 2w i abc vdr ref Vrabc vqr ref ( ) 2 lip Fiure 7. RSC PIR inner current controller C dc V abc ir abc v dqr C. Conventional GSC Control Scheme Likewie, the GSC controller i applied in a reference frame whoe d-axi i oriented with the tator voltae 0. The dc link voltae, and the reactive power exchaned between the rid and the GSC ide, can be controlled throuh actin on the dq GSC voltae component which can be expreed a: v = v + w L i -L di dt - R i (4) d d f f d f d v =-wlfid -Lf di dt - Rf i (5) Where, ubcript, v,i,ω, tand for voltae, current, and anular peed. Subcript,, inify tator and rid ide. Indexe d,q tand for direct and quadrature axe of dq reference frame. R f,l f refer to GSC interface reactor reitance and inductance. The active and reactive power flow between the GSC and the rid can be written a: P 1.5 v i (6) d d The DC link dynamic i iven by: Q 1.5 v d i (7) 2 1 C dv dc 2 dc Pr P dt In normal condition, the GSC typically operate with a unity power factor, Q 0. Fi. 8 illutrate The GSC traditional control cheme. To mitiate the dc voltae fluctuation durin interruption, a propoed term i activated and added to (8)

5 the dc link voltae control loop to reflect the tranitory variation of the GSC power. The latter can be attributed to the power imbalance between the RSC and GSC reultin from the urplu rotor power followed by the tranient voltae dip which adverely impact the dc voltae tranient repone [3]. Vdc Vdc ref Q ref Q P Vdc i d id ref i ref i P Q wli f Vd ref V ref Vabc wli f d i d dq/abc abc/dq PWM V abc i abc Fiure 8. Conventional GSC control cheme D. Propoed GSC Controller Undoubtedly, LVRT and the aociated DFIG WT converter control i a key point to atify the rid code requirement. Unbalanced rid voltae emerin from aymmetrical voltae dip can certainly caue utained ocillation in the DFIG enerated active and reactive power. Thouh thi demerit can be overcome via injectin appropriate unbalanced current by neative equence controller, the conventional SRF dq current controller cannot accurately accomplih uch objective due to luih dynamic performance [11]. Uin DDSRF baed PI reulator for controllin poitive and neative equence ha been intenively implemented and thu can meet the taret under uch unbalanced rid condition. Fi.9 how the DDSRF repective poitive and neative reference frame (, ) rotatin with the ynchronou peed ( ) with anular poition ( ) repectively. In thi reard, decouplin cell to eliminate the reciprocal double-frequency cro-coupled ripple affectin both SRF are added to enhance the controller performance durin unbalanced condition [10]. C dc Thereby, the reultant DDSRF dq current controller can be expreed a [11]: ( ) + + -j( q + -q - ) j( ) - - q + -q = + - -D -dq -dq -dq d-ref d i i e i e i i AC Term Decouplin Term j( ) j( ) dq dq dq dref d i i e i e i i AC Term Decouplin Term (9) (10) The cut-off frequency of the LPF, i elected a 2) rad/. A chematic diaram that abride (9-10) for the GSC DDSRF dq current controller i hown in Fi.10. The outer controller of the DDSRF current controller i the ame a that in [12]. i q b V V w d t w q q a -w d t -q V -w Fiure 9. DDSRF repective reference frame phaor diaram j e e j i dq j( ) e, i dq i dq j( ) e i dq f f i d ref i ref f f f f i d ref i ref f f Lf Lf Lf Lf i d i i d i j e Fiure 10. GSC DDSRF dq current controller j e V abc V. SIMULATION VERIFICATIONS A 2 MW, 0.69kV DFIG WT ytem ued for the imulation i hown in Fi.11. The ytem i modeled and imulated in MATLAB/SIMULINK environment to evaluate the effectivene of the propoed control tratey under variou excurion. The imulation tet are performed at the nominal WT wind peed (11.4m/). Whenever, a voltae dip i detected, the FRT protection tratey i enabled to afeuard the DFIG WT and the aociated converter. Throuhout the imulation, the maximum allowed dc voltae i et a 1.25 p.u. Stator and rotor current protection maxima are et to 1.5 p.u. Nonethele, the DFIG ytem converter, i.e., RSC and GSC can utain 2 p.u overcurrent for a tranitory time [4]. Furthermore, the pitch controller will be triered in cae of evere voltae dip to handle the rotor peed. Fiure 11. Schematic diaram of the ytem under tudy A. Symmetrical Fault Repone Initially, the DFIG WT i upplyin the rated output power while the delivered reactive power i et to zero (Q,Q 0). The correpondin DFIG rotor peed i ω 1.1 p.u. At t=0.7, the ytem i ubjected to a evere ymmetrical 3 fault 5 in the tranmiion line (ee Fi. 11). The fault lat

6 for 500 m and cleared at 1.2 while normal operation i retrieved later a depicted in Fi. 12. The tator voltae (Fi. 12.a) drop to 95% durin the fault, meanwhile the FRT protection i activated via a fault detection alorithm a that in [4]. Subequent to the voltae dip, hiher tator and rotor tranient current are noticed in Fi. 12.c-f. A a conequence to the tranient rotor current rowth, the GSC aborbed power -via the RSC ide- rie intantaneouly, which reult in hiher dc voltae fluctuation. However, the propoed control tratey i uperior to it counterpart in maintainin the dc link voltae within the allowed limit a een in Fi.12.k,l. Beide, the adopted RSC fault ride-throuh, FRT, tratey in in both control cheme (ee Fi.12.m,n) proved to reulate the power imbalance via curtailin the DFIG input power baed on 2 in repone to the reduction of the DFIG active power caued by the voltae dip and accordinly retrictin tator a well a rotor overcurrent (Fi.12.c-f). Moreover, the pitch controller i enaed durin the fault o a to reulate the rotor peed a een in Fi. 12.i,j which till below the threhold limit (1.3 p.u.). A reactive current aid hould be releaed a a conequence to the tator voltae dip a tipulated by the rid code (ee Fi.1.b), the aforementioned reactive power cheme (ee Fi.5) i activated to achieve thi taret in repone to the tator voltae dip. The reactive current upport led by the WT i hown in Fi.12.o,p. Thouh adoptin the ame reactive power cheme, it can be obviouly noted from Fi.12.o that the propoed control method hold teadily the maximum reactive current upport durin the fault wherea, the conventional control method experience unucceful reactive current upport a depicted in Fi.12.p. The latter can be ained to the lo of the accurate PLL ynchronization and vector control orientation, VCO, caued by the evere ymmetrical fault with 95% dip beide the voltae fluctuation led by the rid lower SCR. Fi.13.a,b how the ynchronization inal detected by the DSOGI-FLL and SRF-PLL repectively. Compared with the SRF-PLL detected inal, the DSOGI-FLL extract unaffected equiditant ynchronization inal which correpondinly improve the overall ytem performance and atifie the rid code obliation. B. Aymmetrical Fault Repone Bein accompanied with neative equence current, aymmetrical fault lead to dc voltae and electromanetic torque ripple which influence the ytem converter and the couplin haft of the WT and miht eventually trip the DFIG WT [13]. Under the ame pre-fault condition aumed in the precedin tet, the ytem repone to aymmetrical fault i alo examined. Fi. 14,15 how the ytem repone to 150 m, 90% voltae dip fault and 50% voltae dip 2 fault repectively. Thank to the control of the equence current via the GSC DDSRF current controller, the dc voltae ripple i well mitiated (Fi.14,15.k) compared with that of the conventional method (Fi.14,15.l) which exceeded the pre-defined limit with 1.32 p.u., voltae overhoot. Alo, the tranient tator and rotor current are efficiently reulated below 2 p.u. Moreover, the DFIG delivered active power and injected reactive current (Fi.14,15.n,p) durin both fault inificantly fluctuate due to the limited RSC control capability [13]. On the other hand, lower ocillation are depicted in Fi.14,15.m,o by mean of the propoed control tratey with better repone. The uperior performance of the propoed controller (Fi.14,15-A) over the conventional one, (Fi.14,15-B) i due to the comparative GSC DDSRF control effort exerted to reulate the equence current [13] and concurrently the excellent performance of the DSOGI- FLL of renderin a robut and clean ynchronization inal compared with that of the conventional SRF-PLL reardle of the unbalanced main a een in Fi.13.c-f. Fiure 12. Simulated tranient repone of the tudied ytem to a ymmetrical 3 fault ( 5 (A) propoed control cheme (B) Conventional control cheme. 3f f - Fiure 13. Trackin repone of the PLL for 3, 2 fault repectively. (A) DSOGI-FLL. (B) SRF-PLL 2f

7 VI. CONCLUSIONS A propoed coordinated control tratey ha been preented to enable a DFIG WT to operate in a weak rid under different continencie. A DDSRF dq current controller i devoted to control the GSC to mitiate the dc link voltae fluctuation and tackle current ripple. A DSOGI-FLL i dedicated not only to render robut and clean ynchronization inal detection irrepective of the utility condition, but alo improve the overall ytem performance under evere diturbance. A reactive power control cheme i ued to manae reactive power injection durin fault to atify the rid code obliation. Beide, extra term are employed with RSC and GSC controller to uppre the tranient tator and rotor current and reulate the rotor peed. The obtained reult revealed the uperiority of the propoed tratey that effectively enabled the DFIG WT to atify the rid code commitment in a weak rid. REFERENCES Fiure 14. Simulated tranient repone of the tudied ytem to inle fault ( 5 (A) propoed control cheme (B) Conventional control cheme. Fiure 15. Simulated tranient repone of the tudied ytem to 2 fault ( 5 (A) propoed control cheme (B) Conventional control cheme. [1] X. Zhao., et.al., Contraint on the effective utilization of wind power in China: An illutration from the northeat China rid Ren. Su. Enery Rev., vol. 16,no. 7, pp , Sep [2] G. Mokryani, P. Siano, A. Piccolo and Z. Chen, Improvin fault ridethrouh capability of variable peed wind turbine in ditribution network, IEEE Sy. Journal., In Pre. [3] D. Xie, et al., A comprehenive LVRT control tratey for DFIG wind turbine with enhanced reactive power upport, IEEE Tran. Power Sy., vol. 28, no. 3, pp , Au [4] S. Xiao, G. Yan, H. Zhou and H. Gen, An LVRT control tratey baed on flux linkae trackin for DFIG-baed WECS, IEEE Tran. Ind. Electron., vol. 60, no. 7, pp , July [5] N. Strachan and D. Jovcic Stability of a variable-peed permanent manet wind enerator with weak AC rid, IEEE Tran. Power Del., vol. 25, no. 4, pp , Oct [6] T. Neumann, C. Felte and I. Erlich, Repone of DFG-baed wind farm operatin on weak rid to voltae a, in Proc, IEEE PES General Meetin Conf., pp. 1-6, July [7] R. Teodorecu, M. Lierre and P. Rodriuez, Grid converter for photovoltaic and wind power ytem, Wiley IEEE Pre, [8] P. Rodríuez, A. Luna, M. Ciobotaru, R. Teodorecu, and F. Blaabjer, Advanced rid ynchronization ytem for power converter under unbalanced and ditorted operatin condition, in Proc., IEEE Ind. Elec. Conf., IECON, 2006, pp [9] J. Lian, D. F. Howard, J. A. Retrepo and R. G. Harley, Feed-forward tranient compenation control for DFIG wind turbine durin both balanced and unbalanced rid diturbance, IEEE Ind. App., vol. 49, no. 3, pp , May/Jun [10] Enerinet. Technical reulation for wind power plant with a power output reater than 11 kw; September Available at: [11] M. Reye, et al., Enhanced decoupled double ynchronou reference frame current controller for unbalanced rid-voltae condition, IEEE Tran. Power Electron., vol. 27, no. 9, pp , Sep [12] N. Jelani and M. Molina, Mitiation of aymmetrical rid fault in induction enerator-baed wind turbine uin contant power load, Enerie, vol. 6. pp , Mar [13] H. Gen, C. Liu and G. Yan, LVRT capability of DFIG-baed WECS under aymmetrical rid fault condition, IEEE Tran. Ind. Electron., vol. 60, no. 6, pp , Jun