Published in: Proceedings of the 16th Conference on Power Electronics and Applications, EPE 14-ECCE Europe

Transcription

1 Aalborg Unverstet Dynamc Performance of Grd Converters usng Adatve DC Voltage Control Trnts, Ionut; Sun, Bo; Guerrero, Jose M.; Munk-Nelsen, Stg; Abrahamsen, Flemmng; Thøgersen, Paul Bach Publshed n: Proceedngs of the 16th Conference on Power Electroncs and Alcatons, EPE 14-ECCE Euroe DOI (lnk to ublcaton from Publsher): /EPE Publcaton date: 2014 Document Verson Early verson, also known as re-rnt Lnk to ublcaton from Aalborg Unversty Ctaton for ublshed verson (APA): Trnts, I., Sun, B., Guerrero, J. M., Munk-Nelsen, S., Abrahamsen, F., & Thøgersen, P. B. (2014). Dynamc Performance of Grd Converters usng Adatve DC Voltage Control. In Proceedngs of the 16th Conference on Power Electroncs and Alcatons, EPE 14-ECCE Euroe. IEEE Press. DOI: /EPE General rghts Coyrght and moral rghts for the ublcatons made accessble n the ublc ortal are retaned by the authors and/or other coyrght owners and t s a condton of accessng ublcatons that users recognse and abde by the legal requrements assocated wth these rghts.? Users may download and rnt one coy of any ublcaton from the ublc ortal for the urose of rvate study or research.? You may not further dstrbute the materal or use t for any roft-makng actvty or commercal gan? You may freely dstrbute the URL dentfyng the ublcaton n the ublc ortal? Take down olcy If you beleve that ths document breaches coyrght lease contact us at vbn@aub.aau.dk rovdng detals, and we wll remove access to the work mmedately and nvestgate your clam. Downloaded from vbn.aau.dk on: Setember 18, 2016

2 Ths document s a rernt of the fnal aer: I. Trnts, B. Sun, J. M. Guerrero, S. Munk-Nelsen, F. Abrahamsen, and P. B. Thoegersen, "Dynamc erformance of grd converters usng adatve dc voltage control," n Proc. EPE Dynamc Performance of Grd Converters usng Adatve DC Voltage Control Ionut Trnts 1,BoSun 1, Jose M. Guerrero 1, Stg Munk-Nelsen 1, Flemmng Abrahamsen 2, Paul Bach Thoegersen 2 1 Deartment of Energy Technology Aalborg Unversty Pontodanstraede 101 D-9220, Aalborg, Denmark Emal: tr@et.aau.dk URL: htt:// 2 -Electronc A/S Boegldvej 3 D-7430 Ikast, Denmark URL: htt:// Acknowledgments The authors acknowledge the fnancal suort from The Dansh Natonal Advanced Technology Foundaton under the IEPE Project. eywords <<Converter control>>, <<Effcency>>, <<Grd converter>>, <<Adatve control>>. Abstract Ths aer nvestgates a controller that ensures mnmum oeratng dc-lnk voltage of a back-to-back converter system. The dc-lnk voltage adats ts reference based on the system state, reference gven by an outer loo to the dc-lnk voltage controller. The oeratng dc-lnk voltage should be ket as low as ossble to ncrease the ower converson effcency and ncrease the relablty of converters. The dynamc erformance of the roosed controller s nvestgated by smulatons and exerments. Introducton Wnd ower generators need to slowly relace the conventonal fossl fuelled based generators. The nstallaton of onshore and offshore wnd farms s emergng worldwde, wth the rated ower er wnd turbne of u to 8 MW [1]. Inducton generator was at the ground of the wnd turbnes develoment, and the synchronous generators are ganng sgnfcant attenton n recent years. For most of the generator technologes, the controllablty and ower rocessng s realzed wth a back-to-back converter. The effcency and relablty of the wnd turbne converter are very mortant for the entre system, and new ways to mrove these factors are always welcome [2]. The generator s decouled from the grd n the dc-lnk of the wnd turbne converter. The grd converter s the comonent that controls the voltage level of the dc-lnk n order to mantan the energy balance between the two ower sources, generator and grd. The general state-of-the-art wnd turbne structure s shown n Fgure 1. The grd converter synchronzes wth the grd voltage, controls the dc-lnk voltage and the grd currents accordng to the references gven to the current controller [3]. The actve current

3 reference s gven by the dc-lnk voltage controller, and can be also nfluenced by the ancllary servces such as frequency control. Normally the dc-lnk voltage reference s fxed, and setu to a level that allows the converter oeraton over the entre ower range and can acheve certan dynamcs of the current controller. Gearbox Generator AC DC v DC v DC DC AC q Grd Fgure 1: Wnd Turbne System wth Full Scale Converter Reducng the energy stored n the dc-lnk can brng benefts n effcency and relablty of both the grd and the generator converter. The energy storage can be reduced deendng on the system state, by adatvely adjust the dc-lnk voltage reference. A method that calculates an offlne table for the dc-lnk voltage, based on the oeraton of both grd and generator converters was ntroduced n [4]. It was also shown that the relablty can be ncreased due to the reduced falure rate based on cosmc rays. Ths method s however deendent on the model of the grd and generator, whch nclude arameters that can change deendng on varous factors lke oeratng temerature of the generator, grd voltage, grd medance and grd flter medance. Adatvely adjustng the dc-lnk voltage reference onlne can elmnate the calculaton errors based on the nvolved systems modellng. Imrovements n the wnd turbne converter were shown n [5], such as a slght ncrease n effcency and ncreased relablty due to reduced thermal cyclng. For nstallatons at hgh alttude the effect of reduced oeratng dc-lnk voltage mroves more the relablty by decreasng the falure rate based on cosmc rays [6]. An addtonal control loo as outer loo to the dc-lnk voltage controller, whch adats the dc-lnk voltage reference by controllng the maxmum modulaton ndex ( controller), was ntroduced n [5]. The reducton of ower losses was roven, however wthout the exermental valdaton of the controller. Ths aer valdates the controller by smulaton of a 6 MW converter and exermentally testng on a downscaled rototye. Adatve DC Voltage Control A controller that s to be used n normal oeraton mode of the wnd turbne s descrbed n the followngs. To acheve an onlne adataton of the dc-lnk voltage reference, a control loo can be mlemented as shown n Fgure 2. Instead of settng a fxed dc-lnk voltage reference, the maxmum modulaton ndex reference s controlled takng the feedback from the nut modulaton ndex to the modulator. M Max V DC, Gen Control, Grd DC Voltage Control Current Control v VDC VDC Modulaton S Fgure 2: Proosed adatve dc-lnk voltage controller A maxmum modulaton ndex reference s gven nstead, and the outut dc-lnk voltage reference wll adat to the converter system state. In ths way the oeratng dc-lnk voltage s reduced sgnfcantly at all ower levels, and more consstently at lght load. The maxmum oeratng modulaton ndex, or the mnmum oeratng dc-lnk voltage deends on all the followng factors: grd voltage, grd current,

4 grd medance, flter medance, generator voltage (n case of a turbne), modulaton strategy, mnmum ulse wdth of the used ower devces, harmonc lmts from the aled standard [5]. Most the enumerated factors are changng durng the oeraton deendng on the grd state, transferred ower and temerature. Therefore t s very dffcult to mlement an offlne calculated/measured look-u table to otmze the oeratng dc-lnk voltage durng oeraton. Ideally the maxmum modulaton ndex reference should be unty, but a small reserve should be ket to allow oeraton wth changes n ower for a gven maxmum dp/dt. The dc-lnk voltage control dynamcs can be mroved wth a feed-forward from the generator control, to allow fast ower rams. The ower devces n the converter wll set further lmtatons on the mnmum ulse wdth. Settng a maxmum modulaton ndex reference above the voltage reserve for the mnmum ulse wdth flter can ntroduce addtonal dstorton on the outut current. Therefore the maxmum reference should be exermentally determned n order to comly wth the harmonc standards. Controller Performance A ossble mlementaton structure s shown n Fgure 3. The current control can be mlemented n a standard decouled dq rotatng reference frame [7], [8]. The nut to the control structure s the maxmum modulaton ndex reference MMax and takng the feedback from the modulator, the error s regulated by the modulaton ndex controller. - DC VoltageControl s s MMax Control dq abc d q d q Current Control s L L s Current Control v gd v d v q Fgure 3: Grd converter dq adatve control structure v gq dq abc Modulaton v a v b v c SVPWM S a S b S c Ths case assumes that the wnd turbne generator rated voltage s smaller as comared wth the grd voltage, and the modulaton ndex of the grd converter s always smaller comared wth the grd converter. The outut of the modulaton controller s the dc-lnk voltage reference adjustment δ, added to the feed-forward reference VDC, regulated by the DC voltage controller havng the measured dc-lnk voltage as feedback. Proortonal ntegral (PI) controllers are used for all control loos n ths examle. To nvestgate the oeraton of the roosed controller, a smulaton model was mlemented wth the control structure sown n Fgure 3. The converter toology s the classcal two-level converter connected to a low voltage 690 V transformer, wth the rated ower of 6 MW. The model of the outut L flter and grd medance was assumed to be 70 μh and2mω n total, and the total nstalled dc-lnk caactance of 10 mf. The controller oeraton s shown n Fgure 4, enabled at 1.5 s wth converter oeratng n steady state at 50% of the rated ower. The maxmum modulaton ndex reference was set to After enablng the controller, the detected maxmum modulaton ndex s ncreased and controlled to the reference value wth a tme constant of 1 s. The oeratng dc-lnk voltage s therefore reduced to around 1000 V, wth 50 V less as comared wth the fxed reference that s classcally used to oerate the

5 1.01 Adatve DC Voltage Control Control [ ] ,ref,meas Control [V] ,meas,ref Tme [s] Fgure 4: Controller oeraton at 50% load (3 MW) Control [ ] Dynamc Performance of Adatve DC Voltage Control 0.99,ref 0.98,meas Control [V] ,meas,ref Grd Currents [A] g,a g,b g,c Power [VA] x q Tme [s] Fgure 5: Dynamc oeraton of the roosed adatve control

6 converter over the entre ower range. The aforementoned mrovement usng reduced dc-lnk voltage also comes wth restrctons regardng dynamcs of the control. The roosed controller s desgned to oerate n a mode where no fault s resent n the grd. Moreover the dynamcs of the generator control should meet the mosed maxmum ower varaton n tme, lmt that wll ensure the back-to-back system stablty. Therefore the dynamc oeraton wll be consdered n the followngs. In Fgure 5 the controller resonse to ostve and negatve ower gradents s shown. At 0.75 s the controller s enabled and brngs the voltage to the reference value as n Fgure 4, for a ower flow of about 3 MW. The magntude of the grd currents and njected owers nto the grd are shown also. The njected ower s ostve meanng that s njected nto the grd, generated by the wnd turbne. Havng the ower gradent fxed to 1.5 MW/s, the dynamc oeraton s seen from second 2.5. At negatve gradent the controller has a ostve feedback error and decreases the voltage reference, followng the mosed maxmum modulaton ndex. Durng ostve gradent at 5 s the ower s 2.25 MW, and ramed u to 4.5 MW. It can be seen that a slght overmodulaton s resent n the case of ostve gradent wth the chosen dp/dt and controller gans, however mantanng the stablty. Exermental Investgaton The smulated control from Fgure 3 was mlemented n a dsace1006 control latform. The converter rototye s a two-level 2.2 kw, grd connected to the 400 V grd. The test setu s shown n Fgure 6. The used grd LCL flter had a total nductance of 4 mh and a delta connected caactor bank of 9 μf. Converter Cabnet Control Platform Converter n Use Grahcal User Interface Fgure 6: Exermental test setu The converter s modulated wth contnuous sace vector ulse wdth modulaton, swtchng and samlng at a frequency of 10 khz. The deadtme between the low sde and hgh sde transstors was 1 μs whle the mnmum ulse wdth flter s set to clam the outut voltage for a dutycycle less than 2 μs. The controller s mlemented wth a PI controller, wth the feedback of the maxmum modulaton ndex detected at each fundamental frequency. The oeraton of the roosed controller s shown n Fgure 7. The converter starts oeratng at a fxed dc-lnk voltage reference of 600 V, oeratng at lght load. The controller s dsabled at ths tme. The ower drecton n ths case s from the AC to DC, oeratng as an actve rectfer usng a fxed resstve load n the dc-lnk. At around 1.9 s the controller was enabled, moment at whch the maxmum modulaton ndex s regulated at ts reference of It can be seen the resonse tme s around 1 s. The dc-lnk voltage s regulated at an adatve reference of around 565 V. The oeratng dc-lnk voltage s reduced by 35 V, for the gven load, grd condton, and converter confguraton and oeratng arameters.

7 Control [ ] ,ref,meas ,meas 590 [V] Tme [s] Fgure 7: Exermental oeraton of the controller The dynamc oeraton of the controller s shown n Fgure 8. In ths case, there s no load n the dc-lnk and the ower flow s from the dc-lnk to AC grd. A current controlled ower suly s nstalled n the dc-lnk to emulate the generatng ower. At tme 0 s the controller s already enabled. Control [ ] M 0.97 Max,ref,meas [V] ,meas Grd Currents [A] Tme [s] Fgure 8: Exermental dynamc oeraton of the controller g,a g,b g,c After 1 s, the ower s ncreased n a number of arbtrary stes u to 5 s. Havng the stes of ower and not a ower ram, and the stes are faster than the controller resonse the steady state cannot be acheved between stes. At around 7-8 s n tme the controller reaches steady state and the oeratng dc-lnk voltage s ncreased to around 610 V. The dc-lnk voltage s ncreased due to the ncreased ower flow, mantanng the control over the maxmum modulaton ndex.

8 Concluson A controller that otmzes the wnd turbne converter oeraton wth resect to the swtched DC voltage was evaluated. The dc-lnk voltage s adatvely controlled deendng on the system state. The control s smle, mlemented wth a PI controller, desgned for normal oeraton modes wthout any faults. The dynamc oeraton was nvestgated, and t s concluded that the wnd turbne controller robustness t not affected when the maxmum ower gradent comles wth the lmts requred by the roosed controller. The controller was exermentally valdated on a downscaled rototye. Oeraton wth ower flow n both drectons s vald, wth a hgher reducton n dc-lnk voltage n case of an actve rectfer. Havng a control over the requred oeratng dc-lnk voltage, the losses and stress of the converter can be otmzed. References [1] Blaabjerg, F.; e Ma, Future on Power Electroncs for Wnd Turbne Systems, Emergng and Selected Tocs n Power Electroncs, IEEE Journal of, vol.1, no.3,.139,152, Set [2] Fredl, T.; olar, J.W.; Rodrguez, J.; Wheeler, P.W.;, Comaratve Evaluaton of Three-Phase ACAC Matrx Converter and Voltage DC-Lnk Back-to-Back Converter Systems, Industral Electroncs, IEEE Transactons on, vol.59, no.12, , Dec [3] Massng, J.R.; Stefanello, M.; Grundlng, H.A.; Pnhero, H., Adatve Current Control for Grd-Connected Converters Wth LCL Flter, Industral Electroncs, IEEE Transactons on, vol.59, no.12,.4681,4693, Dec [4] Dayaratne, U.I.; Tennakoon, S.B.; Shammas, N.Y.A.; nght, J.S., Investgaton of varable DC lnk voltage oeraton of a PMSG based wnd turbne wth fully rated converters at steady state, Power Electroncs and Alcatons (EPE 2011), Proceedngs of the th Euroean Conference on, vol., no.,.1,10, Aug Set [5] Trnts, I.; Munk-Nelsen, S.; Abrahamsen, F.; Thoegersen, P.B., Effcency and relablty mrovement n wnd turbne converters by grd converter adatve control, Power Electroncs and Alcatons (EPE), th Euroean Conference on, vol., no.,.1,9, 2-6 Set [6] Consentno, G.; Laudan, M.; Prvtera, G.; Pace, C.; Gordano, C.; Hernandez, J.; Mazzeo, M., Effects on ower transstors of Terrestral Cosmc Rays: Study, exermental results and analyss, Aled Power Electroncs Conference and Exoston (APEC), 2014 Twenty-Nnth Annual IEEE, vol., no.,.2582,2587, March 2014 [7] Alcala, J.; Cardenas, V.; Ramrez-Loez, A.R.; Gudno-Lau, J., Study of the bdrectonal ower flow n Back - to - Back converters by usng lnear and nonlnear control strateges, Energy Converson Congress and Exoston (ECCE), 2011 IEEE, vol., no.,.806,813, Set [8] Teodorescu, R.; Lserre, M.; Rodrguez, P.;, Grd converters for hotovoltac and wnd ower systems, vol. 29. Wley, 2011.