A current controlled matrix converter for wind energy conversion systems based on permanent magnet synchronous generator

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Naggar

H. Saad

a

,

Ahmed

A. El-Sattar

a

,

Mohamed

I. Marei

b,∗

a_Electrical_Power_and_Machines_Department,_Faculty_of_Engineering,_Ain_Shams_University,_Cairo₁₁₅₁₇_Egypt b_Maintenance_Department,_Egyptian_Drilling_Company,_Cairo,_Egypt

Received22June2015;receivedinrevisedform6December2015;accepted3January2016 Availableonline18March2016

Abstract

Themainchallengesofwindenergyconversionsystems(WECS)aretomaximizetheenergycapturefromthewindandinjecting reactivepowerduringthefault.ThispaperpresentsacurrentcontrolledmatrixconvertertointerfacePermanentMagnetSynchronous Generators(PMSG)basedWECSwiththegrid.Toachievefastdynamicresponsewithreducedcurrentripples,ahysteresiscurrent controlisutilized.TheproposedcontrolsystemdecouplestheactiveandreactivecomponentsofthePMSGcurrenttoextractthe maximumpowerfromthewindatagivenwindvelocityandtoinjectreactivepowertothegrid.Reactivepowerinjectionduringthe faultsatisfyingthegrid-codesrequirement.TheproposedWECShasbeenmodeledandsimulatedusingPSCAD/EMTDCsoftware package.

Keywords:Hysteresiscurrentcontrol;Matrixconverter;Permanentmagnetsynchronousgenerator;Reactivepowercontrol;Windenergyconversion systems

1. Introduction

Windpowerhas become arapidlygrowing technology as asource of powergeneration sinceit isbeing most economical,cleanandemissionfreetechnology.WECSareusedtocapturetheenergyavailableinthewindtoconvert into electricalenergy. Variable-speed wind turbine has aseries of advantages; it increases the energy conversion efficiencyandreducesmechanicalstresscausedbywindgusts(OfualagbaandUbeku,2008).Thegearboxcouples thewindturbinetothegeneratorcausesmanyproblemsespeciallyduringfaultsandrequiresregularmaintenance.The reliabilityofthevariablespeedwindturbinecanbeimprovedsignificantlybyusingadirect-drivebasedonPMSG. ThePMSGhasmanyadvantagessuchashighefficiency,highpowerdensity,highpowerfactor,lownoiseandeasy

∗_{Corresponding}_author.

E-mailaddress:[email protected](M.I.Marei). PeerreviewunderresponsibilityofElectronicsResearchInstitute(ERI).

http://dx.doi.org/10.1016/j.jesit.2016.01.001

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Nomenclature

λopt optimaltipspeedratio

ωm,ωref actualandreferencegeneratorspeed(rad/s)

β pitchangle

i thecurrenterror

H thehysteresisbandwidth

idref,iqref thedirectandquadraturecurrentcomponentsofthePMSG

irref,isref,itref thereferencevaluesforthethree-phasecurrentsofthePMSG

Irated theratedcurrentofthematrixconverter Q theactivepowerfedtothegrid

P thereactivepowerfedtothegrid

R radiusoftheturbine

Vppc thevoltageatthepointofcommoncoupling

Vw windvelocity(m/s)

Vd,Vq thedirectandquadraturevoltagecomponentsofthePMSG

Va,Vb,Vc thethree-phasegridvoltageatthePCC

control(Yangetal.,2012;Inomataetal.,2013).TheseadvantagesrenderedthePMSGtoreceivemuchattentionin windenergyapplication.Duetoabsenceoftherotorwindings,ahighpowerdensitycanbeachieved,reducingthe sizeandweightofthegenerator.Inaddition,therearenorotorwindinglosseswhichleadtoareducedthermalstress ontherotor.

ThemainlimitationofthetraditionalAC–DC–ACconverteristhebulkDCcapacitorusedforenergystorage.The matrixconverterdoesnotrequireanyintermediatestorageelementasthematrixconverterprovidesacompactsolution forafourquadrantfrequencyconverter.Thekeyelementofamatrixconverteristhefullycontrolledfour-quadrant bidirectionalswitch(Lietal.,2012;Haqueetal.,2010),whichallowshighfrequencyoperation(Kangetal.,2011;

ReddyandKumar, 2013;JahangiriandRadan,2013).Variousmodulationtechniquesfor thematrix converterare

introducedinliterature(Rodriguezetal.,2012;Youm,1999;Marei,2012).TheHysteresisCurrentControl(HCC)has theadvantageofitsfastresponseandsimplicity.TheHCCisprobablyoneofthemostefficienttechniquesbecauseof itsfastresponseincontrollingthecurrentandinherentpeakcurrentlimitingcapability.

Amaximumpowerpointtrackingisessentialtodeterminetheturbinespeedtoextractthemaximumelectricpower

fromthewind(Howladeretal.,2013;KhajuriaandKaur,2012;Sunetal.,2015;Mingmingetal.,2014;Lietal.,

2014;Tangetal.,2014;Abdullahetal.,2012).Inaddition,thepitchanglecontrolisimportanttoprotecttheturbine

fromdamage(KhajuriaandKaur,2012).Thematrixconvertercontrollershouldmanagethegrid-sidequantitiessuch asgrid-sidereactivepowertoimprovethesystemstabilityandpowerquality(Sunetal.,2015;Mingmingetal.,2014). ThecontrolsystempresentedinLietal.(2014)showsgoodandfastdynamicresponseinextractingmaximumpower atvariousspeedbesidesthefullycontrolledreactivepower(Tangetal.,2014).

Inthispaper,amodifiedHCCtechniqueisusedforthematrixconvertertoreducethecurrentripplesoftheinjected powertothegrid.TheproposedcontrolusesactiveandreactivepartsofthePMSGcurrenttoincreasethecontrol capabilityoftheactiveandreactivepowerproductionatboththePMSGsideandthegridsideduringvariablespeed operation.Furthermore,adynamiclimiterisusedtocontrolthereactivepowerinjectedtothegridwithoutexceeding theratedcurrentofthematrixconverter.Thepitch-anglecontrollerisusedtopreventexceedingtheratedpowerofthe generatorathighwindspeedbylimitingthePMSG’sspeed.

2. Systemdescription

ThispaperpresentsacomprehensivestudyforthedynamicperformanceofthegearlessWECSbasedonPMSG using amatrix converterconnectedtothe gridthrough LC filter inordertoinject apuresin waveshape voltage andcurrent.ThemajorcomponentsoftheproposedWECSarethewind-turbine,thepermanentmagnetsynchronous generator,thematrixconverter,theproposedcontrolsystembasedonHCC,andthepitchcontroller.

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Fig.1.DirectmatrixconverterconfigurationforPMSGgridinterfacing. iq ref Vpcc LVRT controller -+ ωref ωm ωrefcalculator X 0.252 Wind speed PI CONTROLLER phase angle estimator dq/abc transform ωreflimiter at 3.5 rad/sec idref ir ref is ref it ref

Fig.2. BlockdiagramoftheproposedcontrolstrategyforthePMSGinterfacesystem.

Themainadvantagesoftheac–acconversion, showninFig.1,incomparisonwithac–dc–acconversionarethe lackofdc-linkcapacitorandtheloweroutput-currentrippleandinput-currentripplewhichresultsinsmallerinputand outputfiltersizes.AninputLCfilterisusedtoeliminatethehigh-frequencycomponentsoftheinputcurrentstothe matrixconverterwhichleadstosinusoidalsourcecurrents.Furthermore,atabnormalgridconditions,windturbines basedonmatrixconvertershouldbeabletoprovideappropriatereactivepowertosupportthegridvoltagewhichwill bepresentedinthispaper.

3. Theproposedcontrolsystem

Intheproposedcontrolsystem,themaximumpoweroperationofthewindturbineisachievedbykeepingthetip speedratiotoitsoptimalvalueλopt.ThemeasuredwindspeedVwisusedtoproducethegeneratorspeedreference

ωref(rad/s)accordingtotheoptimaltipspeedratioλoptasshowninFig.2.Thegeneratorspeedωmiscontrolledbythe matrixconverterandwillbeequaltoitsreferenceinsteadystate,atwhichtheMPPTisachieved.Asfortheselected windturbine,theoptimumTSRis6.3.Thefollowingequationisvalidfortheselectedmodel:

ωref =λoptVw

R =

6.3Vw

25 =0.252Vw (1)

TheindirectrotorfieldorientedcontrolisusedforthePMSG.Thespeedreferenceωref,calculatedfromEq.(1), islimitedbythemaximumangularspeedofthegenerator/turbine,3.5rad/s,whichiscorrelatedtowindvelocityof

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Fig.3.Theproposedreactivepowercontrol.

14m/s.APIcontrollerisusedtoregulatethespeedofthegenerator/turbine,ωm.TheoutputofthePIcontrolleristhe quadraturecurrentreference,iqref,forthePMSG.

TheLowVoltageRide-Through(LVRT)controllerassignsthevalueofthedirectcomponentofthestatorcurrent accordingtothePCCvoltage.Atnormalcondition,wherethePPCvoltageisabove0.9pu,zerocurrentreferenceis assigned tothedirectcomponentof thestatorcurrent,idref,toachievemaximumtorque/amperecapability.The d-andq-componentsareappliedtotheParkandClarketransformstogeneratethethree-phasereferencecurrentsfor thematrixconverter,irref,isrefanditref.TheHCC,whichispresentedinSection3.2,isutilizedtocontrolthePMSG currentbygeneratingpropergatesignalsfortheswitchesofthematrixconverter.Thefollowingsubsectionspresent thedetailsoftheproposedcontroltechniqueforthePMSGinterfacesystembasedonmatrixconverter.

3.1. Reactivepowercontrol

TheobjectiveoftheMPPTcontrollerforthewindenergyconversionsystembasedonPMSGismaximizingthe deliveredactivepoweratthepointofcommoncoupling(PCC).Furthermore,theproposedcontrollerprovidesimportant servicesuchasinjectingalimitedamountofreactivepowertothegridduringfaults.Thed-componentofthePMSG currentisutilizedtomanagethereactivepowerinjectedbytheMCinterfaceintotheutilitynetwork.

AsshowninFig.3andaccordingtogridcodesrequirement,theLVRTcontrollerdeterminesthepuvalueofthe d-componentofthePMSG,idref,tocontrolthereactivefedtothegridaccordingtoVpcc.Adynamiclimiterdefinedin

(2)isusedtosetthemaximumallowablevalueoftheactivepowercomponentduringthefault.

iqref =

I_rated2 −i2_q_ref (2)

Asidiscontrolled,thePMSGandgridreactivepoweriscontrolled.ThereactivepowerofPMSGisequaltoVqidwhich isdirectlyproportionalwithidwherethed-componentoftheterminalvoltageofthePMSGisdiminished,Vd=0.At normalconditionofthegrid,idref issettozerotominimizethePMSGcurrentandlosses.However,becauseofthe highreactanceofPMSG,itdrawsreactivepowerevenatidref=0,speciallywithincreasingthewindspeed.

3.2. Thehysteresiscurrentcontrolofthematrixconverter

Thematrixconverter,showninFig.1,isconsistingofninefour-quadrantsemiconductorsconnectedinamatrix configuration.Inthisconfiguration,anyinputphasecanbeconnectedtoanyoutputphasethroughthreebidirectional semiconductorsconnectedtothethreeoutputphases.Eachbi-directionalswitchisconsistingoftwosemiconductors connectedattheiremitterterminalsasshowninFig.4.Oneswitchiscalledforwardandtheotheriscalledreverse switch.Ifthecollectoroftheswitchisconnectedtotheinputphase,itwillbecalledforwardswitch.Ontheother hand,ifthecollectoroftheswitchisconnectedtotheoutputphase,itwillbecalledreverseswitch.

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Fig.4. Single-phasematrixconverter:1=forwardswitchand2=reverseswitch. -+ HCC Va SWITCHING TABLE I Sa1 Sa2 Sb1 Sb2 Sc1 Sc2

Region

detector

(6 regions

)

ir ir ref Vb Vc

Fig.5.TheblockdiagramoftheHCCforsingle-phasematrixconverter.

Therearetwoconstrainsforcontrollingthematrixconverter,thefirstistopreventtheinputterminalsfrombeing short-circuitedandthesecondistoavoidtheoutputphasesfrombeingopened.TheHCCwasappliedforthematrix converter.Theinputthree-phasevoltagesaredividedintosix60◦partitionsbasedonthemaximumvalueamongthree line-to-linevoltages.Atanypartition,theswitchesbetweenoneoutputphaseandthethreeinputphasesaredivided intothreegroups:on-switches,off-switchesandthehysteresismodulationswitches.

Thefirstgroup,off-switches,iscontrolledtobeintheopenstateduringthepartitioninterval.Theseswitchescould bedeterminedforanypartitionbyevaluatingthethree-phaseinputvoltagesatthegrid-side.Thephasewithvoltage neithermaximumnorminimumisnotconnectedtotheoutput-sideduringthepartition.

Thesecondgroup,hysteresismodulationswitches,iscontrolledduringanypartitiontoallowtheoutputcurrent tofollowthereferencecurrentwaveform.Thetwohysteresisswitchescouldbedeterminedataspecificpartitionby consideringtheforwardswitchconnectedtoinputphasewiththemaximumvoltageandthereverseswitchconnectedto inputphasewiththeminimumvoltage.TheHCC,presentedinFig.5forsingle-phaseofthematrixconverter,isbased onaregiondetectorandcomparingbetweentheoutputcurrentandthereferencecurrent.Theerroristhencompared withthehysteresisbandHwhichcontrolsthestateofthetwohysteresismodulationswitches.Iftheerrorbetweenthe actualandreferencecurrentsreachestheloweredgeofhysteresiscomparator,theforwardswitchisclosedtoincrease theoutputphasecurrentandthereverseswitchisopenedtoavoidshortcircuitontheinput-side.Similarly,iftheerror betweentheactualandreferencecurrentsexceedstheupperedgeofhysteresiscomparator,thereverseswitchisclosed toreducethecurrentasitisconnectedtothelowerinputphasevoltage.

Thethirdgroup,on-switches,isclosedtoinsureapathforthecurrenttoavoidspikesattheinstantsofinterchanging thehysteresismodulationswitches.The on-switchesare determinedataspecificpartitionbyselectingthereverse switchofthehighestinputphasevoltageandtheforwardswitchofthelowestphasevoltage.

Forexample,if apartitionwhereVab is themaximuminputline voltage, the on-switchesare Sa2 andSb1,the off-switchesareSc1andSc2andthehysteresismodulationswitchesareSa1andSb2.Allthepossibleswitchingstates areshowninTable1for single-phasematrixconverter.Similarswitchingstates areusedfor the othertwo output phases.AsshowninFig.5,thephasevoltageofthegridsideiscomputedtodeterminetheregion.Inaddition,the errorcurrentsarecontrolledtobeadjustedwithinthehysteresiswindowaccordingtotheswitchingtableorders.

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Table1

Switchingstatesforsingle-phasematrixconverter.

Maximuminputlinevoltage On Off i Hysteresismodulation

Vab Sa2 Sc2 >H Sa1 Sb1 Sc1 <-H Sb2 Vac Sa2 Sb2 >H Sa1 Sc1 Sb1 <-H Sc2 Vbc Sb2 Sa2 >H Sb1 Sc1 Sa1 <-H Sc2 Vba Sb2 Sc2 >H Sb1 Sa1 Sc1 <-H Sa2 Vca Sc2 Sb2 >H Sc1 Sa1 Sb1 <-H Sa2 Vcb Sc2 Sa2 >H Sc1 Sb1 Sa1 <-H Sb2

Fig.6.Thewindandgeneratorspeeds.

4. Simulationresults

Thissectionevaluatesthedynamicperformanceandtheeffectivenessoftheproposedcontrolstrategy,basedon time-domainsimulationsusingPSCAD/EMTDC.Differentstepchangesofthewindspeedareassumed,from5,8, 14to7m/sasshowninFig.6(a),forthisstudy.Withtheincreaseinwindspeed,theMPPTcontrollercalculatesthe optimumreferencespeedωref,toextractthemaximumpowerfromtheavailablewindenergy.Theproposedcontrol systemmanagedcompetentlytocontrolPMSGspeedoperatingpointatdifferentwindspeedasshowninFig.6(b).

Foranyincreaseinthewindspeed,theMPPTstarttoregulatethegeneratorspeedatoptimumspeedtoproduce maximummechanical poweras showninFig.7(a). Oncethemechanical powerincreases,the mechanicaltorque increases,asindicatedinFig.7(b).Atthemomentofspeedchange,thespeedregulatorreducestheelectromagnetic toquetozeroformaximizingtheaccelerationtorqueinordertoregulatethegeneratorspeedattheoptimumspeed. Oncethegenerator speedreachesthe optimumspeed,theaccelerationtorque isvanishedandtheelectromagnetic torquecounterbalancesthemechanicaltorqueoftheturbine.

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Fig.7.Torqueperformanceatdifferentwindspeeds.

Fig.8.MechanicalandelectricalpowersofWECS.

Attimet=20s, thewind speed raisedabove its ratedspeed (13.5m/s). As aresult,the pitch angle controller isactivatedtolimit the mechanicalpowerof the turbineby regulatingits speedatthe ratedvalueas indicatedin

Figs.6and8.Thisactionprotectstheturbinefrompossiblemechanicaldamage.ItcanbenoticedfromFig.8that

thespeedcontrolleractsinstantaneouslyasacompensatortothewindspeedandadjustthereferenceq-axisPMSG current.Therefore,thereferenceq-axisPMSGcurrentcomponentreachestoitslimitsetting,asillustratedinFig.9, andtheturbine powerandspeedtendtoincrease.Oncethewindspeedreachesitslimit,thepitchcontrollertakes actionandproducesthenewappropriatepitchangle,asshowninFig.8(b),wherethereferenceq-axisPMSGreached

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Fig.9.CurrentcontrolofWECS.

Fig.10.ThephasecurrentofthePMSGandgrid.

its limit.The pitchcontrolleris continuouslyadjusting pitchangle tokeepthe generatorspeed atits ratedspeed, 3.5rad/s.Fig.8(b)portraystheactivepowerdeliveredtothegridwhichfollowsthegeneratedactivepowerfromthe PMSG.Thelittledifferencebetweenthemisduetothesmalllossesofthematrixconverterandfilter.Itisnoteworthy mentioningthatthePMSGq-axiscurrentcomponentfollowssuccessfullyitsreference,whilethed-axisisadjusted tozero.

Fig.10(a)indicatesthefastresponseandaccurateperformanceoftheproposedHCCforthematrixconverterwhere thephasecurrentofthePMSGtrackstightlyitsreferencecurrent.ThephasecurrentofthegridisshowninFig.10(b). It canbe noticedthat theinputandoutputcurrentsof the matrixconverteraresinusoidalwithaTotal Harmonics Distortion(THD)lessthan5%,whichisallowedaccordingtotheIEEEstandards519.Fig.11exploresthereactive

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Fig.11.Gridreactivepowerperformanceatvariouswindspeed.

Fig.12.Performanceoftheproposedsystemunderreducedgridvoltage.

powerinjectedtothegridatidref=0.Thereactivepowerisincreasedwiththeincreaseofthewindspeedortheextracted powerfromtheturbine.Increasingidrefatagivenwindspeed,increasesthereactivepowerinjectedtothegrid,aswill beillustratedinthenextscenario.

ThelastscenarioisdedicatedtoevaluatetheproposedreactivepowerinjectionofthePMSGinterfacesystembased onmatrixconverter.Atwindvelocityof9m/s,thePCCvoltageishypotheticallyreducedgraduallyfromtherated value,att=11s,to0.5pu,att=15s,asindicatedinFig.12(a).AccordingtotheE.ONgrid-code,therenewableenergy resourcesarerequiredtobeconnectedtothegridduringfaultsandsupplyreactivepowertoassistthevoltagestability

(slideshare,2013).At0.5puvoltagedip,theinterfacesystemshouldsupplyreactivepowertothegridwiththerated

capacityofthePMSG.DependingonthereductionofthePCCvoltage,idrefisdetermined,asdiscussedinSection3.1, tocontrolthereactivepowerinjectedtothegridasshowninFig.12(b)and(c),respectively.Oncetheratedcurrentof thematrixconverterisreached,thecontrollerreducesiqrefbythedynamiclimitertoavoidexceedingtheratedcurrent ofthematrixconverter,asillustratedinFig.12(d).Itisnotedthatthereductioniniqref affectstheinjectedreactive

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power.Thisactionisduetothecouplingbetweenthedq-componentsofthePMSGcurrentandthegridvoltage.The correspondingactivepowerisportrayedinFig.12(e).Itisworthmentioningthatthereductioninactivepowerresults fromthereductioninthePCCvoltageandtheactivepowercomponentofthePMSGcurrent,i∗_q.Theresultsshow theadvantagesoftheproposedcontrolinterfacesystemwhichincludethesimplicity,effectiveness,andfasttransient response.

5. Conclusion

Thispaperpresentedahighperformancecurrentcontrolstrategyformatrixconverterbasedwindenergyconversion systemequippedwithPMSG.AMPPTcontrollerisdesignedtodeterminetheoptimumspeedthatresultsinextracting themaximumpowerfromwindatdifferentwindvelocities.Ithasbeenshownthattheproposedmethodhasagood dynamicresponsebycontrollingthegeneratorspeedwithunderwindvelocitieslowerthanratedvalue.Athighwind velocities,wherethegeneratorspeedexceedsitsratedvalue,apitchangelcontrollerisutilizedtoregulatethespeed andlimitingtheextractedpowerfromtheturbine.Moreover,theproposedcontrolofthematrixconvertersatisfiesthe grid-codesrequirementoffeedingacontrollablereactivepowerproportionaltothereductionofthegridvoltage.The injectedreactivepoweroftheturbinedependsonthed-componentofthePMSGcurrentandtheextractedactivepower whichisproportionaltothegenerator speed.Thesimulationresults ofthe proposedinterfacesystemdemonstrate its effectivenessand fast transient responseunder different windspeeds.Furthermore, the reactivepower feeding capabilityof the proposedinterfacesystemisdemonstratedunderdifferent PCCvoltagelevels.The model ofthe proposedinterfacesystemcanbeusedforinvestigatingthedynamicinteractionsbetweenvariable-speedWECSand thegrid.

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