Damping Local Oscillations of a Direct- Drive PMSG Wind Turbine

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1 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) Damng ocal Ocllaton o a Drect- Dre MSG Wn urbne Ayele N. egee #1, Akhay K. Saha #2, Ruren llay Caranen #3 # Dclne o Electrcal, Electronc an Comuter Engneerng, School o Engneerng, Unerty o KwaZulu- Natal, Durban 4041, South Arca @tu.ukzn.ac.za 2 aha@ukzn.ac.za 3 llayr21@ukzn.ac.za Abtract Damng local ocllaton tal n the oeraton o a rect-re ermanent magnet ynchronou generator wn turbne a well a t ntegraton nto a gr. Currently, thee ocllaton are controlle by roortonal-ntegral controller. h aer rooe the ncluon o rtual retor to mroe the erormance o the wn turbne urther. Smulaton reult how that rtual retor, connecte n ere to tator wnng, hae a ote mact on the amng o local ocllaton. hey gncantly reuce the re an ettlng tme o the rotor ee, electromagnetc torue, acte ower an reacte ower an ncreae the correonng amng rato. In contrat, the one, connecte n arallel to tator wnng, hae a negate mact on the amng o local ocllaton. h ncate that an arorate electon an connecton o rtual retor mroe the ynamc an mall-gnal erormance o a MSG wn turbne. Keywor-Damng local ocllaton, ermanent magnet ynchronou generator, wn turbne, ynamc erormance, mall-gnal tablty, rtual retor I. INRODUCION For the at two centure, raly growng oulaton an moernzaton tren hae accelerate the eman or energy. oay, the worl healy een on ol uel uch a ol, coal, an natural ga or t ncreang energy reurement. Howeer, th ol-uelle economy acng challenge nclung eleton o reere, global warmng, ecurty concern, an rng cot [1]. In tacklng thee challenge, much attenton gen to the eeloment o renewable energy, among whch harneng wn energy the cheaet alternate [2, 3]. hree tye o wn energy technologe urrel-cage nucton generator (SCIG), oubly e nucton generator (DFIG) an ermanent magnet ynchronou generator (MSG) wn turbne are wely aote n the wn nutry. Comare to SCIG, DFIG an MSG are oular or two reaon [4]. Frtly, they oer the oortunty to oerate the machne at maxmum ower or arou wn ee, an econly, they hae le mechancal tre on ther hat. hee ay, the oular arable-ee wn turbne emloye n large-cale wn ark are DFIG wn turbne [5]. Neerthele, recently, rect-re MSG wn turbne are ganng momentum among reearcher, engneer an turbne manuacturer or ther hgh ecency, low ower lo an maller ze [6 11]. he ntegraton o wn ower nto gr raly growng. In ome Euroean countre, the leel o enetraton ha reache a hgh a 21% [12]. Howeer, ue to th hgh enetraton an the ntermttent nature o the wn, there are concern uch a generaton reere, ower ytem tablty an relablty [13]. Furthermore, wn turbne themele hae tablty roblem, whch urely challenge tranmon ytem oerator. In large gr, the ablty o both local an ytem-we ower ytem ocllaton amng lay a crucal role [14]. A range o eort hae been mae to am local ocllaton o a MSG wn turbne bae on acte ower control [14 21] an reacte ower control [4, 21 25]. For examle, n [16], a mechancal metho ung rng an amer emloye to ge the MSG wn turbne a amng caablty. On the other han, [19] netgate an acte amng trategy bae on -lnk current etmaton to am toronal ocllaton. he reearcher ocu on mrong amng o ocllaton through the ower electronc conerter ung roortonal-ntegral (I) controller [14]. Howeer, the erormance o rtual retor to am local ocllaton n a MSG wn turbne, artcularly rom the generator e, ha not yet been netgate. he am o th aer, thereore, to netgate the erormance o rtual retor n amng local ocllaton o a MSG wn turbne. A the retor are rtual retor, they o not ncur ower lo. Moreoer, they can be connecte n ether arallel or ere to the tator wnng an mlemente n the generator e conerter controller. DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

2 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) h aer organze a ollow: Secton II reent the moel o the rtual retor an the erent comonent o the wn turbne, Secton III brely llutrate the moel o the ower gr, mulaton reult are reente an cue n IV, an nally, n V, a concluon rawn, an a uture work recommene. II. DYNAMIC MODEING OF A MSG WIND URBINE In recent conguraton, t common to ee wn turbne emloyng MSG, a hown n Fg. 1. he rotor re the MSG wthout a gear. he generator connecte to the ower gr through ower electronc conerter, a lter, an a tranormer, where the ower electronc conerter conert the low-reuency (10-15 Hz) oltage o the generator nto a 50 or 60 Hz oltage. Subeuently, the lter lter the harmonc create by the ower electronc conerter out whle the tranormer te u the outut oltage to the gr oltage leel [26 29]. Fg.1. A MSG wn turbne connecte to a gr he next ub-ecton cu the moel o the major comonent o the MSG wn turbne. A. Wn See Wn ee een on the clmate an geograhc locaton o the wn arm. In nature, the wn a ranom arable, contng o comonent uch a bae ee, gut, ram an backgroun noe [5]. In th tuy, the moel n (1), a Fourer ere rereentaton, conere to rereent uch behaour o the wn. w N N 0 + ak n( kt) + bk co( kt) k = 1 k = 1 ( t) = ω ω (1) w the wn ee, 0 reer to the bae comonent o the wn ee, a k an b k are ne an cone coecent reectely, the angular reuency o the kth harmonc, an N the number o harmonc. Arorate electon o the harmonc can aroxmate the gut, ram an backgroun noe comonent o the wn ee. artcularly, the harmonc are ue to tuy the mall-gnal tablty o the MSG wn turbne, relate to wn ee turbance n ecton IV. B. Dre ran he ower electronc conerter n Fg. 1 ecoule the wn turbne rom the gr. hu, the tranent n the gr coul not be een n the MSG wn turbne [30], an the re tran moele ung a one ma moel a hown n (2). ωm 1 = ( m e ) t J (2) J = J t + J g (3) J the moment o nerta o the whole re tran, J t the moment o nerta o the wn turbne, an J g the moment o nerta o the generator. ωm the rotor ee, an m the mechancal torue whch gen a, = w (4) he wn ower, wet by the blae, m ωm, een on the enty o ar, ρ, t to wn ee rato, λ, tch angle, w A r, an the wn ee. θ, area w 1 3 = ρc ( λ, θ ) Ar w (5) 2 DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

3 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) C, whch the ower coecent o the wn turbne, een on λ an C θ. It uually gen a [26], λ = θ (6) 0.5( λ 1 = λ λ θ 5.0) e θ C. tch Angle an See Controller he tch angle controller mantan the rate alue o the real ower o the MSG or wn ee, exceeng the nomnal alue. For ee le than the nomnal alue, the tch angle zero. In th work, a roortonal controller, a hown n (8), conere. Bee, the controller contan rate, uer an lower lmter. θ = K ( ωm ωmre ) (8) K a roortonal contant, an ω mre the reerence rotor ee. In (5) an (6), the wn turbne ower een on the wn ee an the tch angle. For a artcular wn ee an tch angle, there an otmum rotor ee at whch the outut ower alo otmum. he controller tre to kee the generator at th otmum ower. Euaton (9) ge the ee controller [31]. (7) re = K rate 3 otωm or ω m m > ω or ω ω m no mn al m no mn al (9) re the reerence wn turbne ower, wherea rate contant. the rate outut ower o the MSG. K ot a D. MSG Moel In lter, ocllaton an harmonc are ame ung acte amng technue, whch emloy rtual retor not to ncur ower lo [32]. In th work, the ame tren ale to am the local ocllaton o a MSG wn turbne. he rtual retor, aote here, are ulementary algorthm [33], an hence, n ractce, they are realze n the generator e conerter controller. he wnng retance o the generator can lghtly am ower an ee ocllaton, but t oe not hae a gncant mact on the amng o local ocllaton, an thu, the nee or rtual retor. Fg. 2 how the - ax an - ax eualent crcut o a MSG wth rtual retor, connecte n ere an arallel to the tator wnng. In th aer, the arallel connecton, hown n Fg. 2(c) an (), are electe a they o not ecreae the eecte retance o the generator. Alyng Krchho oltage law to the eualent crcut hown n Fg. 2 (a) an (b), the correonng new - ax an - ax oltage, whch are conere a reerence n the generator e controller, are gen n (10) an (11). * = R + ω + R (10) t * = R + + ω ( + φ ) + R (11) t R an R are armature an rtual retance reectely. an ω rotor ee n electrcal raan, an are - an - ax current, an Smlarly alyng the ame metho to Fg. 2 (c) an (), we hae are - an - ax nuctance, φ ermanent magnet lux. * = R + ( + t ) ω (12) DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

4 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) * ( + ) = R + + ω ( + φ ) (13) t where an are - an - ax rtual current. Analyng (10), (11), (12) an (13) ge the block agram n Fg. 3(a), (b), (c) an () reectely, whch are ue to analye the ynamc behaour o the MSG. In all the our agram, the nner loo rereent art o the generator tel, wherea the outer eeback loo are comonent that aear ue to the rtual retor. he electromagnetc torue eeloe n the MSG gen a [5], 3 e = ( φ + ( l ) (14) 4 the number o ole o the MSG. Fg. 2. Eualent crcut o a MSG wth rtual retor (a) - ax ere rtual retor (b) - ax ere rtual retor (c) - ax arallel rtual retor () - ax arallel rtual retor Fg. 3. Block agram o the MSG eualent crcut (a) - ax crcut wth ere rtual retance (b) - ax crcut wth ere rtual retance (c) - ax crcut wth arallel rtual retance () - ax crcut wth arallel rtual retance DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

5 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) Fg. 4. rooe generator e controller (a) controller (b) controller wth rtual retance cheme E. Generator Se Conerter Controller he generator e controller control the acte ower o the MSG through the uarature ax oltage,. h cheme am at mnmzng the ower lo. On the other han, the rect ax current ket at zero to ecreae the nonlnearty o (14) an mnmze current coulng an achee through the rect ax oltage, [31]. In th control tructure, a roortonal an two roortonal-ntegral (I) controller are emloye, a hown n Fg. 4. he I controller n Fg. 4(a) control the - ax current, wherea the one n Fg. 4(b) control the - ax current. K 1, K 2, K 3, K 1, an K 3 are the gan o the controller. he uercrt * enote a reerence alue. he negate orwar loo, wth a traner uncton hown n (15), the eect o the rtual retor. R or ere rtual retor 2 H ( ) = j + R / j j or, eenng on the eualent crcut. or arallel retor F. -nk Moel In a MSG wn turbne, ntegrate nto a gr a hown n Fg. 1, the real ower low, through the conerter an the -lnk, balance. Neglectng loe, the net real ower can be gen a, (15) + + (16) g where,, an g are the -lnk, MSG, an ower ule to the gr reectely, gen conecutely n (17), (18), an (19). (17) = = C t = + (18) g = + (19),, an C reer to the oltage, current an caactance o the - lnk caactor reectely. an are the D- an Q- ax oltage at the gr e conerter termnal. Snce the wn turbne an the gr are ecoule, they oerate at relately erent reuence, an the gr e axe are not necearly the DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

6 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) ame a the - an - axe. can be gen a, an are D- an Q- ax gr current. Fnally, the moel o the -lnk C t = (20) G. Gr Se Conerter Controller In Fg. 1, the D- an Q- ax oltage at the termnal o the gr e conerter are gen n (21) an (22) reectely [31]. = D + R + ω (21) t = + R + + ω (22) Q t D an Q are the D- an Q- ax oltage at the termnal o the lter, where R an are the lter retance an nuctance reectely, an ω the lter reuency. he gr e controller nclue our I controller, a hown n Fg. 5, whch control the - lnk oltage, the D- an Q- ax current, an the reacte ower [31]. All uantte wth uercrt * are reerence nut. Q reacte ower. K 4, K 5, K 6, K 7, K 4, K 5, K 6 an K 7 are the gan o the I controller. For unty ower actor, the reerence reacte ower mantane at zero. Fg. 5. Gr e control tructure (a) D- ax controller (b) Q- ax controller III. GRID MODE he MSG wn turbne connecte to the gr through a coulng tranormer an a tranmon lne. Fg. 6 how the ngle-lne agram o the coulng tranormer, the tranmon lne an the gr where x an x reer to the tranormer an tranmon lne reactance reectely. he reecte retance are neglecte. DQ the gr oltage. he termnal oltage o the lter conere a a reerence, an δ the angle o the gr oltage rom DQ. Fg. 6. One-lne agram o the gr, the tranmon lne an the tranormer he current, lowng rom the wn turbne to the gr, can be gen a, DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

7 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) D = DQ n 0 x + x DQ nδ DQ co0+ DQ coδ + j x + x (23) For unty ower actor, the current an the termnal oltage o the lter, the real an magnary art o the current can be exree a, DQ, are n hae. Coneuently, = x DQ nδ + x (24) = DQ DQ coδ (25) x + + x IV. SIMUAION RESUS he moel n Secton II an III were mlemente n MAAB\Smulnk to ery the alty o the rooe metho through tme-oman mulaton. he ynamc erormance o a MSG wn turbne, whch connecte to a gr, een on the ytem charactertc, the ntermttent nature o the wn, an the loa turbance n the ower ytem. artcularly, the nature o the wn make the ealuaton o the ynamc an mall-gnal erormance challengng, an n th work, large an mall wn ee turbance are conere. he erormance nce ue are re tme to meaure ee o reone, ettlng tme to ealuate tablty an ee o reone, an maxmum ercent oerhoot to ae the relate tablty o the MSG wn turbne wth an wthout the rooe cheme. he egree o ocllaton amng alo obere through amng rato. he maxmum ercent oerhoot an amng rato are relate a [34], 2 ζπ / 1 ζ M = 100e (26) where M the maxmum ercent oerhoot, an ζ the amng rato. Sub-ecton A an B reent the tme reone o the MSG wn turbne, ubjecte to large an mall turbance n the wn ee reectely. he mulaton were carre out bae on the ata n able A1 an A2. A. arge Dturbance n the Wn See In th tuy, a te ncreae o 12 m/ n the wn ee wa ue, an all the hgher orer harmonc n (1) were neglecte. hereater, the reultng ynamc erormance o the MSG wn turbne wa obere. he ollowng cae are conere: Sere rtual retance: Fg. 7 how the ee reone o the MSG wn turbne to a te wn ee when a ere rtual retance emloye n the generator e controller. A t can be obere, ee ocllaton are ucceully ame. Bee, the maxmum ercent unerhoot gncantly reuce, ncatng that the ytem ha become more table. Smlarly, the re tme reuce rom 3.5 to 0.08 econ, an the ettlng tme reuce rom 8.0 to 6.0 econ, a hown n able 1. hu, the role o the ere rtual retance n amng rotor ee ocllaton aramount or large wn ee turbance. he reult n Fg. 8, 9, 10 an ABE I llutrate that the ocllaton n real ower, reacte ower, mechancal torue, electromagnetc torue, - lnk an termnal oltage are eectely uree. hereore, or large turbance, emloyng rtual retor, connecte n ere to the tator wnng, ucceully mroe the ynamc erormance o the wn turbne. ABE I Dynamc erormance o the MSG wn turbne wth an wthout ere rtual retance Q e m Vc M (%) WOR WR S (ec) WOR WR R (ec) WOR WR WOR WR WR- wth rtual retance; WOR- wthout rtual retance; S- ettlng tme; R- rng tme; - unene DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

8 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) Fg. 7. Rotor ee wth an wthout ere rtual retance Fg. 8. ower te reone wth an wthout ere rtual retance (a) real ower (b) reacte ower Fg. 9. orue reone wth an wthout ere rtual retance (a) Mechancal torue (b) Electromagnetc torue Fg. 10. Voltage wth an wthout ere rtual retance (a) - lnk oltage (b) ermnal oltage Fg. 11: Eect o arallel rtual retance on (a) rotor ee (b) - lnk oltage arallel rtual retance: In th cae, ater actorzng, the mroer traner uncton n Fg. 3 wa mlemente n MAAB/Smulnk a the erence o a erate an a rt-orer traner uncton. he rtual retance conere reman the ame a n the reou cae. It oun that the eect o the arallel rtual retance on the ynamc erormance o the MSG wn turbne negate. he reult how that the alcaton o rtual retor, connecte to the tator wnng, ecreae the amng o the rotor ee ocllaton. Bee, the wn turbne become le table. For th artcular cae, the ytem ha become untable, a hown n Fg. 11. DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

9 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) B. Small Dturbance n the Wn See o tuy the mall-gnal erormance o the MSG wn turbne ung tme-oman mulaton, harmonc an a uen re n the wn ee are conere, a hown n Fg. 12. At the th econ, the wn ee uenly re rom 10 to 12 m/. A a arallel rtual retance wa not eecte n the reou ecton, only a ere one conere. he mulaton reult n Fg. 13 how that rotor ee ocllaton are well ame urng mall erturbaton when a rtual retance emloye. For the turbance at the th econ, the reone o the wn turbne at an more table wth the rtual retance. Fg. 14 how that the mechancal torue nearly not aecte by the rtual retor, wherea the electromagnetc torue eectely ame. In Fg. 14 (b), ater the turbance n the th econ, the electromagnetc ocllaton wll ncreae a rtual retor not emloye. Smlarly, the ncreang ocllaton n real an reacte ower ue to the mall wn ee turbance are uree by ung the rtual retor a hown n Fg. 15. he ocllaton ue to wn ee harmonc are alo ucceully ame. In general, emloyng arorate electon an connecton o rtual retor n the generator e controller gncantly mroe both the ynamc an mall-gnal erormance o a MSG wn turbne. Fg. 12: ycal wn ee Fg. 13: Rotor ee reone or mall turbance Fg. 14: orue reone (a) mechancal (b) electromagnetc or mall turbance Fg. 15: (a) Real ower (b) reacte ower reone or mall turbance V. CONCUSION In th aer, rtual retor are rooe to mroe the amng o local ocllaton uch a rotor ee, ower, an torue ocllaton n a MSG wn turbne, whch connecte to a ower gr. he rtual retor an the erent comonent o the wn turbne are moele. Vrtual retor, connecte n ere an arallel to the tator wnng o the generator, are mlemente n the generator e conerter controller, an all the moel hae been alate n MAAB/Smulnk. Smulaton reult hae hown that rtual retor, connecte n ere to the tator wnng, ecently enhance the ynamc an mall-gnal erormance o the wn turbne by amng local ocllaton. See, ower, an torue ocllaton, caue by wn turbance, are well ame when rtual retor are emloye. Moreoer, the rtual retor gncantly mroe the -lnk an the termnal oltage reone. On the other han, rtual retor, connecte n arallel to the DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

10 Ayele N. egee et al. / Internatonal Journal o Engneerng an echnology (IJE) tator wnng, ecreae the amng erormance o the wn turbne. Hence, or a better erormance, the rtual retor houl be connecte n ere to the tator wnng. hereore, the technue rooe n th work alcable n moothng the rotor ee an the outut ower o a MSG wn turbne. In uture, the erormance o a rtual RC crcut, mlemente n aance conerter controller, on the ynamc an mallgnal erormance o a MSG wn turbne wll be tue. arameter ermnal oltage Rate ower Rate reuency Flter retance Flter nuctance -lnk oltage AENDIX ABE A1 MSG wn turbne arameter Value Number o ole ar V m 1.5 MW 60 HZ u 0.3 u 1150 V = 0.3 mh 1.48 V.. Moment o nerta 1.2 u 3.0 u ABE A2 Control arameter arameter Value arameter Value 0.14 u 0.83 u 2.72 ec 5.0 ec 0.84 u 1.0 u 0.25 u ec 200 u 8.0 u 400 ec REFERENCES [1] M. A an. Muneer, "Energy uly, t eman an ecurty ue or eeloe an emergng econome," Renewable an Sutanable Energy Reew, ol. 11, , Se [2] REN , "Renewable global tatu reort: 2009 Uate," ar: REN21 Secretarat, May [3] G. Joeln Herberta, S. Inyan, E. Sreealan an S. Rajaanan, "A reew o wn energy technologe," Renewable an Sutanable Energy Reew, ol. 11, , Aug [4] Z. Mao,. Fan, D. Oborn an S. Yuarajan, "Control o DFIG-bae wn generaton to mroe nterarea ocllaton amng," IEEE ranacton on Energy Coneron, ol. 24, no. 2, , June [5] A. Rolan, A. una, G. Vazuez, A. Danel an G. Azeeo, "Moelng o a arable ee wn turbne wth a ermanent magnet ynchronou generator," n IEEE Internatonal Symoum on Inutral Electronc, Seoul, , July [6]. B. Sh,. Kang,. Z. Yao, S. Y. Qn, R. M. Wang an J.. Zhang, "Eect o wn generaton uncertanty an olatlty on ower ytem mall-gnal Stablty," Journal o Electrcal Engneerng echnology, ol. 9, no. 1, , Jan [7] H. Een, I. Norhem an K. Uhlen, "arge-cale wn ower ntegraton n Norway an mact on amng n the Norc gr," Wn Energy, ol. 8, no. 3, , Jul [8] M. A. Mahmu, M. J. Hoan, H. R. ota an C. Zhang, "Inetgaton o crtcal actor aectng ynamc tablty o wn generaton ytem wth ermanent magnet ynchronou generator," n roc. the Internatonal Feeraton o Automatc Control, Cae own, South Arca, , Augut [9] E. Maher, A. Kheher an M. Mmoun, "he wn energy coneron ytem ung MSG controlle by ector control an SMC tratege," Internatonal Journal o Renewable Energy Reearch, ol. 3, no. 1, , [10] K. Zhao, G., B. Wang an M. Zhou, "Gr-connecte toology o MSG wn ower ytem bae on VSC-HV," n Electrc Utlty Deregulaton an Retructurng an ower echnologe (DR), Weha, Shanong, , July 201. DOI: /jet/2017/91/ Vol 9 No 1 Feb-Mar

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