A Discussion about a Start-up Procedure of a Doubly-Fed Induction Generator System

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1 1 A Dicuion about a Stat-up Pocedue o a Doubly-Fed Induction Geneato Sytem J. L. Da Silva*, R. G. de Oliveia*, S. R Silva*, B. Rabelo** and W. Homann** *Fedeal Univeity O Mina Geai/Electic Engineeing Reeach and Development Cente, Belo Hoizonte, Bazil **Deden Univeity o Technology/Dept. o Electical Machine and Dive, Deden, Gemany Abtact The peent wok popoe a tat-up pocedue o a doubly-ed induction geneato (DFIG) diven by a wind tubine. In the DFIG topology the tato i connected diect to the gid while the oto i connected to a back-to-back convete. Thi tuctue equie ome cae duing gid ynchonization to avoid undeied oveload. The main goal o a tat-up pocedue i to educe the tee on the electical and mechanical component duing ynchonization. In thi wok, it i achieved by equalization among the induced tato voltage, the voltage ove the ilte capacito and the gid voltage. A phae-locked-loop (PLL) compute the gid voltage phae diplacement equied o the ytem contol oientation and ynchonization pocedue. Peviou to the tating-up, the output cuent et-point o the gid-ide convete and oto-ide convete ae computed in ode to poduce the ame gid voltage ove the ilte capacito and on the tato teminal. The whole pocedue i ummaized in even dieent tep: tubine acceleation by the pitch contol, DC-link pe-chage; initialization o the ilte cuent contolle; ilte ynchonization; DC-Link voltage adjut; oto cuent contolle' initialization and tato ynchonization. The popoed tat-up pocedue wa teted in a 4 kw tet bench and the deciption o each tep a well a imulation and expeimental eult ae peented thoughout the pape. Index Tem Wind powe geneation, doubly-ed induction geneato, ynchonization, gid utility. M I. INTRODUCTION ANY govenment ae adopting new enegy geneation guideline towad an ecologically utainable ociety. Thi elect in a continuou gowth o wind powe geneation aound the wold in the lat yea. Among the wind geneation concept, the advantage o the adjutable peed ae well known and can be ummaized in eduction o toque pulation (once wind gut can be abobed by the ytem), eduction o mechanical tee and inceae o enegy poduction. Nowaday, doubly-ed induction geneato (DFIG) o wind tubine ae widely ued. The main advantage o the DFIG compaed to the othe adjutable peed geneato i the act that powe electonic component only handle with a action o the geneato powe. Thi educe the acquiition cot and the loe in powe electonic device. Finally, the quality o the geneated powe i alo impoved in tem o hamonic and voltage luctuation [1], [2]. The DFIG topology i depicted in igue 1 and conit on the tato diectly connected to the gid while the oto cicuit with it vaiable voltage and equency equie a back-to-back convete o the gid connection. The convete allow the active powe low in two diection: om the gid to the oto in ub-ynchonou opeation and om the oto to the gid in ove-ynchonou mode. A LC-ilte in the output o the gidide convete i ued o educing the witching hamonic. Figue 1. Scheme o the DFIG Sytem Since the ize o the machine i inceaing towad 5 MW, the tee on the mechanical dive tain and on the electical cicuit duing connection to the upply cannot be neglected. Futhemoe, the gid code equiement om the ytem opeato on wind tubine oeee the limitation o the tat-up cuent on the net connecting point. Although, ew pape have handled the tating-up pocedue o the gid ynchonization o a DFIG. In the elated liteatue ound, an induced tato voltage equal to the gid voltage i geneated beoe the ynchonization by adjuting the oto lux [6], [7], [8], [9], [10]. Thi pocedue peom a null cuent connection with a vey low impact to the gid and the machine. The ame pinciple i aboted in thi pape. Although, a tatup pocedue o the whole DFIG ytem i popoed in even dieent tep decibed in a ytematic way. Simulation and expeimental eult ae alo peented. The popoed ynchonization poce i ot, at and can be done at any opeational peed o the wind tubine. Thi chaacteitic i vey impotant not only o the tat-up om peed zeo but alo duing gid poblem which lead to the diconnection o the geneato. Fo example, voltage dip that exceed the duation peciied by the gid code equiement. It caue the action o potective elay leading the ytem to diconnection. Iolated opeation i detected by the contol ytem and it hut down the wind tubine to pevent ove peed. A ot tat-up allow the ecloing o the ytem a oon a the ault i cleaed. Eectively, ate the gid diconnection, the wind tubine can tay in a tempoay unning-up tate. I the gid i not e-etablihed ate a cetain

2 time, the tubine i taken to a top tate [9]. Othewie, the ynchonization can be caied out a at a poible integating the geneato back to the gid. The tat-up pocedue i chaacteized in even dieent tep: 1.

2 2 time, the tubine i taken to a top tate [9]. Othewie, the ynchonization can be caied out a at a poible integating the geneato back to the gid. The tat-up pocedue i chaacteized in even dieent tep: 1. Tubine acceleation by the pitch contol; 2. DC-link capacito pe-chage; 3. Filte cuent contolle initialization; 4. Filte ynchonization; 5. DC-link voltage adjut; 6. Roto cuent contolle initialization; 7. Stato ynchonization. The baic condition o both ilte and tato ynchonization ae minimal deviation on the amplitude, equency and phae angle between the gid and the voltage aco the ilte capacito and on the tato teminal. The poce o DFIG diconnection om the gid i contay to the connecting poce. The eduction o the wind peed will gadually unload the geneato until minimum peed i eached. While appoaching to the no load condition, the geneato will be diconnected om the gid and the contolle can be witched o. A. Oientation II. THE DFIG SYSTEM CONTROL The 3-phae DFIG ytem can be mathematically tanomed in a diect and quadatue axi ytem (dq-axi). Thee new axe compoe a otating eeence ame. In thi wok, the voltage oientation i adopted, i.e. the d-axi i oiented to the diection o the gid voltage vecto. The amplitude, equency and phae diplacement o the gid voltage ae etimated by a phae-locked-loop (PLL) [3]. The coect etimation o thee paamete i undamental not only o the oientation but alo o the quality o the gid connection. Thu, the PLL cheme depicted in igue 2 i adopted ince it peom a obut way o calculating the gid angle. equency (ω N = o 50 Hz ytem) i eed owaded to impove the oveall tacking peomance o the PLL. The oto component ae oiented by the lip angle which i computed by the dieence between the tato angle (given by the PLL) and the oto angle (obtained om an encode). Accoding to the oientation adopted, diect axi (dcomponent) epeent active powe and the quadatue axi (q-component), eactive powe in the ytem. B. Contol Stategy The contol tategy i ummaized in igue 3. The contol i made by a cacaded tuctue in which the inne loop ae the cuent one. In the gid-ide convete (GSC), two oute PI contolle ae eponible o the DC-link voltage contol (d-component) and the eactive powe contol (q-component). Likewie, in the oto-ide convete (RSC), the active component o the cuent eceive the et-point om oute toque and peed contolle. The ixed et-point o the oto eactive cuent deine the level o magnetization o the machine though the oto cicuit. Space vecto modulation (SVM) i ued in ode to geneate the pule o the IGBT gate [4]. The contol tategy i vey imila to the one it popoed by Peña et. al. [5]. Figue 3. Contol loop o the DFIG Sytem III. THE DFIG SYSTEM MODEL Figue 2. Thee-phae locked loop The tato voltage oientation i eached by etting the quadatue axi eeence voltage (U NQ ) to zeo. The output o the PI contolle give the voltage equency which i integated in ode to obtain the angle. Thi angle i eedbacked and ued o abc-to-dq tanomation. The eult i ued o the new eo calculation. A peciic value o A. The machine model The geneato model i obtained om the claical equation o the induction machine. The tato and oto voltage, dψ u = R i + + jω ψ (1) dψ u = R i + + jω lipψ (2) and luxe, ψ = L i + L i (3) m ψ = L i + L i (4) m will be the bai o the mathematical manipulation o the equation. The electomagnetic toque i expeed a

3 3 3 Lm T { * e = PP I Ψ i }. (5) 2 L Finally, the mechanical equation o the machine elate the electomagnetic and load toque with the oto peed. B. The DC-link model dω Te TL = J + Bω (6) The RSC and GSC ae connected though a DC-link in a back-to-back topology. The powe low in the DC-link eult in a voltage vaiation and can be expeed a the dieence between the output active powe om the GSC and RSC. Thu, in ode to obtain the cuent in the DC-link capacito, a powe ditibution in ued o calculating the total oto (i DC ) and ilte (i ndc ) contibution o the DC-link cuent. Finally, the DC-link voltage (U DC ) i obtained accoding to the ollowing expeion, 1 U DC = ( i DC + indc ) C DC whee C DC i the DC link capacitance. C. The LC-ilte model (7) Accoding to the voltage oientation adopted (U Nq = 0) and conideing that the voltage ove the capacito i the gid one (igue 1), GSC output voltage (u nd and u nq ) ae expeed a, dind und = R ind + L L ωninq + U Nd (8) dinq unq = R inq + L + L ωnind. (9) Figue 4. Connection o the tato A good machine ynchonization elect in almot no impact in the gid and machine tate. Figue 5 how that ate the gid connection i done (t = 0), the peed i kept intact which elect the toque abence. A vey low ocillation can be obeved in the oto cuent and the tato cuent don t each value highe than 0.01 pu. IV. SIMULATION RESULTS In the gid ide convete ynchonization, the popoed tategy ue an uncontolled diode ectiie o chaging the DC-link. Thi poce i made though a eito which i eponible o deining the DC-link chage cuent and time contant. Anothe poibility would be uing a thee-phae eito in eie with the ilte. The advantage o thi appoach i to eliminate the neceity o a ectiie. In the othe hand, two othe eito would be neceay. Actually, both altenative ae vey imila in what concen the implementation implicity, cot and eectivene. The ue o the ectiie make the GSC and the DFIG connection vey imila. In both cae, the ynchonization equie that the voltage in the ilte and tato teminal ae a equal a poible to the gid. It can be accomplihed by contolling the dq-axi cuent o the oto and ilte a will be peented late. A peect tato connection i peented in igue 4 and 5. The induced voltage ove the tato i equal to the gid voltage. A high equency voltage ipple i obeved in the tato beoe the ynchonization caued by the oto cuent ipple. Figue 5. Synchonization impact in the machine peed and cuent Figue 6 how thee dieent amplitude o the tato voltage (U S ) o the gid connection. In the it cae, U S and the gid voltage (U N ) amplitude ae the ame beoe the ynchonization. Vey low cuent low in the tato (i ). In the econd tet though, only 50% o the deied amplitude i induced (U S = 0.5U N ). In thi cae, tato cuent cloe to the nominal value appea. The wot cae happen when no voltage i induced beoe the connection (U N = 0). The cuent eache amplitude o two time the nominal cuent. The tanient connection hown in igue 6 wa imulated in the geneato cut-in peed, i.e pm (70% o the ynchonou peed).

4 4 Figue 6. Stato voltage and cuent duing the ynchonization with dieent amplitude o the tato induced voltage A highe poblem duing the gid connection happen with the phae diplacement o U S compaed to U N. Figue 7 how the tato ynchonization in thee dieent ituation: 0 o, 90 o and 180 o o phae diplacement o the tato voltage. Figue 8. Stato cuent tanient o U N eo in phae and amplitude. The ame gaph can be obtained o the ilte ynchonization. Figue 9 how the eo in the capacito voltage (U C ) and the peak o the gid cuent (I N ) caued by the ynchonization. Figue 7. Stato voltage and cuent duing the ynchonization with dieent phae diplacement o tato induced voltage The phae diplacement caue highe tanient cuent than the amplitude eo. Thi tanient can each value up to ive time the geneato nominal cuent o the oto peed o 1050 pm. Once a induced voltage in the tato with exactly the ame gid voltage may not be achieved, the cuve with the tato cuent tanient o dieent eo may help in the deinition o minimum U S amplitude and phae diplacement. Once deined the maximum cuent duing the ynchonization, igue 8 allow the calculation o minimum deviation in the induced voltage. Fo example, i the tat-up cuent tanient i deined to achieve value lowe than 0.1 pu, the phae and amplitude eo mut keep value unde 2% o eo, i.e pu o voltage amplitude and 3.6 o o phae diplacement. Figue 9. Gid cuent tanient o U C eo in phae and amplitude V. START-UP STEPS AND EXPERIMENTAL RESULTS A. Tubine acceleation by the pitch contol The it tep o the ytem tat-up i the acceleation o the tubine made by the pitch contol. The aeodynamic toque exeted by the tubine blade acceleate the geneato hat. The pitch angle i contolled in a way that maximum toque i poduced and the acceleation poce i hote. Thi tep i the longet due to the inetia o the involved mae and can be emulated uing a it ode tane unction modeling the mechanical ytem dynamic. Duing thi poce, tep 2 to 5 o the ilte ynchonization can take place. With the gid-ide convete connected, tep 6 can be caied out. Ate eaching the cut-in peed the geneato can be ynchonized.

5 5 B. DC-link pe-chage The DC-link i chaged though an uncontolled ectiie and the chaging cuent i limited by a powe eito, a it i hown in igue 10. Figue 10. DC-link pe-chage In moment (A) the witch S1 (igue 1) i cloed chaging the DC-link capacito. When it voltage i tabilized, the eito (R) i by-paed with S2 (moment B). C. Filte cuent contolle initialization Ate the DC-link i chaged, the ilte cuent contolle ae initialized. The goal o thi tep i to geneate the gid voltage aco the ilte capacito. Both cuent et-point (i nd * and i nq *) ae automatically calculated taking unde conideation the ilte impedance - Z = R + j(x l - X c ) - a expeed in the equation bellow, E. DC-Link voltage adjut Figue 11. Filte ynchonization Ate the ilte i connected to the gid, the DC-link voltage can be adjuted to the nomal opeational value. The voltage eeence change a a amp making the poce mooth a hown in igue 12. ( ) * R + X l in + U Nd ind = R e Z ( ) * R + X l in + U Nd inq = Im Z whee U Nd i the gid voltage (given by the PLL). (10) (11) D. Filte ynchonization The witch S3 i cloed at the moment pointed with (A) in the igue 11. A mall tanient in the ilte cuent (i nd and i nq ) can be obeved duing the ynchonization ollowed by an inceae o the DC-link voltage (U DC ). When thi voltage deviation eache a pe-deined thehold, the DC-link and eactive powe contolle ae automatically tuned on at the moment pointed by (B) changing the dq-cuent et-point. The pe-chage cicuit i then diconnected (S1 i opened) with no obeved impact to the vaiable. Figue 12. DC-link voltage adjut F. Roto cuent contolle initialization The oto cuent contolle ae tuned on in ode to induce the voltage in the tato. The oto cuent et-point (i d * and i q *) ae calculated baed on the deied tato voltage (U N ) and the mathematical DFIG equation with opened tato a ollowed, '* i = 0 (12) d

6 6 i '* q U N d = ω L whee U Nd = U N and U Nq =0 (gid voltage oientation) and ω N i the gid equency. G. Stato ynchonization N m (13) The lat tep o the DFIG ytem tat-up i the tato ynchonization. It can be done at any machine peed. Thi i an impotant chaacteitic ince no electomagnetic toque can be ceated with the opened tato, and the pitch contol i the only eponible o peed vaiation. A low impact o the DFIG can be obeved in the behavio o the machine tate when S4 i cloed a hown in igue 13. The ynchonization bellow wa caied out nea to the ynchonou peed. When the ynchonization i caied out (t=0) no change ae noticed in the peed and oto cuent (i ). The tato cuent (i ) peented mall tanient. Figue 5 how imulation eult o gid connection in the ame opeating point. The compaion between the two gaph how that the imulated and expeimental eult ae vey imila. Figue 13. Synchonization o the DFIG When the lat tat-up tep i done, the toque and peed contolle ae initialized in the maximum powe point tacking mode and the ytem i ully integated to the electical netwok. VI. CONCLUSION Thi wok dicue and peent a tat-up pocedue o a DFIG o wind tubine application. The complete popoed ynchonization poce wa explained in even dieent tep. The whole tat-up peom a ot and at connection with low impact to the machine and gid. Simulation and expeimental eult peented howed vey good eult. A APPENDIX SYSTEM PARAMETERS Rated Powe P mec 4000 W Stato voltage U 380 V Stato cuent I 8.6 A Powe acto co φ 0.84 Mechanical peed N m 1440 pm Roto voltage U 160 V Roto cuent I 15.5 A Stato eitance R 1.5 Ω Roto eitance R 0.9 Ω Magnetizing inductance L m 139 mh Stato inductance L 148 mh Roto inductance R 141 mh Moment o Inetia J kgm 2 Fiction Coeicient B Nm Pai o pole P P 2 Filte inductance L 8 mh Filte Reitance R 0.5 Ω Filte capacitance R 69 mf REFERENCES [1] B. Rabelo and W. Homann. Powe low optimiation and gid integation o wind tubine with the doubly-ed induction geneato. IEEE 36th Powe Electonic Specialit Coneence, page , [2] A. Peteon. Analyi, Modeling and Contol o Doubly-Fed Induction Geneato o Wind Tubine. PhD thei, Chalme Univeity o Technology, Goetebog, Sweden, [3] S. Silva et. al. Pll tuctue o utility connected ytem unde ditoted utility condition. 32nd Annual Coneence on IEEE Indutial Electonic, page , [4] J. Holtz. Pulewih modulation - a uvey. 23d Annual IEEE Powe Electonic Specialit Coneence, page 11-18, [5] R. Pena, J. Clae and G. Ahe. Doubly ed induction geneato uing back-to-back PWM convete and it application to vaiable-peed windenegy geneation. IEE Poceeding in Electic Powe Application, page , [6] G. Yuan, J. Chai and Y. Li. Vecto Contol and Synchonization o Doubly Fed Induction Wind Geneato Sytem, IEEE 4th Intenational Powe Electonic and Motion Contol Coneence, page , [7] A.G. Abo-Khalil, D.-C. Lee, and S.-P. Ryu, Synchonization o DFIG Output Voltage to Utility Gid, Euopean Powe and Enegy Sytem, page 1-6, [8] X. Zhang, D. Xu, Y. Lang and H. Ma. Study on Stagewie Contol o Connecting DFIG to the Gid, IEEE 5th Intenational Powe Electonic and Motion Contol Coneence, page 1-5, [9] S. A. Gomez and J. L. R. Amenedo. Gid ynchonization o doubly ed induction geneato uing diect toque contol, IEEE 28th Annual Coneence o the Indutial Electonic Society, page , [10] K. P. Gokhale, D. W. Kaake and S. J. Heikkila. Contolle o a wound oto lip ing induction machine, United State patent, patent numbe , 2001.

7. Inverter operated induction machines

7. Inverter operated induction machines 7. Invete opeated induction machine Souce: Siemen AG, Gemany Dept. of Electical Enegy Conveion Pof. A. Binde 7/1 Invete-fed induction machine Fequency convete (invete) geneate thee-phae voltage ytem with