output-filter-design-for-a-grid-interconnected-thr

OutputFilterDesignforaGrid-interconnectedThree-phaseInverterTimothyCYWang,ZhihongYe,GautamSinha,XiaomingYuanGEGlobalResearchCenterOneResearchCircleNiskayuna,NY12309,USAAbstract-Traditionally,LCfilterisusedforaninverterpowersupply.Agrid-interconnectedinverter,however,hassomeuniquerequirementsthatanLCfiltermaynotbesufficient.Thispapercomprehensivelydiscussesthedesignconsiderationsoftheoutputfilterforthegrid-interconnectedinverter.Differentpassivedampingfdtersolutionsarecomparedandtheoptimizeddesignguidelinesarealsoproposed.Simulationresultsareprovidedtovalidatethedesign.I.INTRODUCTIONTraditionally,LCfilterisusedforaninverterpowersupply.Agrid-interconnectedinverterusedindistributedgeneration(DG),however,hassomeuniquerequirementsthatanLCfiltermaynotbesufficient.Thispapercomprehensivelydiscussesthedesignconsiderationsoftheoutputfilterforthegrid-interconnectedinverter.ThedesignisvalidatedbySabersimulations.APWMconverterwithhigherswitchingfrequencywillresultinsmallerLCfiltersize.However,switchingfiequencyisgenerallylimitedinhighpowerapplications.Asanalternativesolution,LCLfilterismoreattractive[1]-[3]foritworeasons:First,ithasbetterattenuationthanLCfiltergiventhesimilarsize.Second,LCLfilterprovidesinductiveoutputatthegridinterconnectionpointtopreventinrushcurrentcomparedtoLCfilter.SimilartoLCfilter,oneconsiderationofthefilterdesignisdamping.Activedampingisgenerallymoreattractivethanpassivedamping.However,thecontrolbandwidthisquitelimitedinhighpowerconvertersduetotheirlowswitchingfiequency.Inmostcases,passivedampingisconsidered[4]-Inthispaper,theoutputfilterdesign,includingLCLfilterdesignprocedureandthedampingfilterdesignwillbediscussedsystematically.171.11.LCLFILTERDESIGNThesystemdiagramisshowninFig.1.Thefilterhasthreeunknowns,L1,CandL2.Inthefollowing,threeconsiderationsthatleadtothreeequationsfordeterminingtheAttenuationrequirementfromIEEE519subjectto0.3%currentlimit,asfiamedinTableI.Thatis,thecurrentrippleat5kHzshouldbelessthan0.3%oftheratedcurrent.ThefirstdesignconsiderationfortheLCLfilteristomeetIEEEStd.519requirements.DuetoPWMswitching,theinverterwillinjectripplecurrentintothegrid.Theswitchingfrequencyoftheinverteris5mZatthegivenpowerlevelThecurrentiscausedbythepulsedvbrduetotheinverter'WM.DuringtheswitchingOnandOff,Vbrvariesfromtovdc(Or-vdcl2to+vdc/2,dependingOnreferencepoint).ThetransferfunctionfromVbrtoiL27is(1)1L,(L2+L,)Cs3+R,CL.,s2+(L,+L,+L,)s+R,(175kW)andthedevices(IGBTs)used.Sincethegrid-shownin(1)*interconnectedinverterispowergenerationequipment,itis-42--AVbrThisworkwassupportedbyUSDepartmentofEnergy(DOE)underawardnumberDE-FC02-00CHI1063.0-7803-7754-0/03/$17.0002003IEEE779Authorizedlicenseduselimitedto:EastChinaUnivofScienceandTech.DownloadedonApril21,2010at10:33:43UTCfromIEEEXplore.Restrictionsapply.Generally,thegridimpedanceL,and&arevarying,dependingonthegridlocationwheretheinverterisconnected.Foratypicalgridlocationwith500kVAtransformer,theimpedanceis5.55%-6.5%withX,iRg=7-10.,IIII8iL1ripplecurrentrequirement1-UTheselectionoftheripplecurrentisatrade-offamonginductorL1size,IGBTswitchingandconductionlosses,andinductorcoilandcorelosses.Thesmallertheripplecurrent,thelowertheIGBTswitchingandconductionlosses,butthelargertheinductor,resultinginlargercoilandcorelosses.Typically,theripplecurrentcanbechosenas15%-25%ofratedcurrent.Forthiscase,theripplecurrentisselectedas20%oftheratedcurrent.Themaximumcurrentripplecanbederivedasin(2)[8]:TABLEICURRENTDISTORTIONLIMITSFORGENERALDISTRIBUTIONSYSTEMS(120Vthrough69kV)P*ktsdmun,HatmodieCurrentDlskmkloninPemtofTLIndividualMannoniEOrder(OddHsrmics)(3)BaseonEquations(1)-(3),L1,L2andCcanbedetermined.Takeasystemwith175kWasanexample,theparametersofLCLfiltercanbedeterminedL1450uH(12.9%impedance);C=270uF;L2=300uH(8.4%impedance).111.PASSIVEDAMPINGFILTERDESIGNIIIIIIIII8ReactivepowerofthecapacitorCTheselectionofthecapacitorisatrade-offbetweenreactivepowerinCandL1inductance.Themorecapacitance,themorereactivepowerflowingintothecapacitor,andthemorecurrentdemandfromtheL1andtheswitches.Asaresult,theefficiencywillbelower.Thecapacitancecannotbetoosmalleither.Otherwise,theinductancewillbelargeinordertomeettheattenuationrequirements.ThelargerinductanceL1resultedfromsmallercapacitanceleadstohighervoltagedropacrosstheinductorLI.Inthisdesign,thereactivepowerischosenas15%oftheratedpower.I11IIIII1IIIIII1IIII1IIIIII4001$00maiw80.31okIDLio.oklomfiw(b)Bodefigureofk/VgFig.2.LCLfilterwithoutanypassivedamping.B.AlternativesofpassivedampingsolutionsHere,severalpassivedampingfiltertopologiesarecompared.Thecriteriaforthefollowingcomparisonareeffectiveresonancesuppressionwithoutdeterioratingtheattenuationatswitchingfrequency.TheBodeplotsareusedtoanalyzetheperformanceofdampingfilters.Thebodeplotsarenotshowninthedigest,andwillbeincludedinthefullpaper.780Authorizedlicenseduselimitedto:EastChinaUnivofScienceandTech.DownloadedonApril21,2010at10:33:43UTCfromIEEEXplore.Restrictionsapply..Solution1Fig.3showsoneofthealternatives.TheBodefiguresofoutputcurrentvs.bothvoltagesourcesareshowninFig.4.Itisobservedthat,l