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2016InternationalHighVoltageDirectCurrentConference(HVDC2016InternationalHighVoltageDirectCurrentConference(HVDCShanghai,25-27Oct.HVDCPaperHVDCPaperPageHVDCGridTopologyandLanlanJi,YananWu,CongdaHan,TingAn,JieGlobalEnergyInterconnectionResearchInstitute,ChangDistrict,Beijing102209,StateKeyLaboratoryofAdvancedPowerTransmissionTechnology，Beijing102209，—VSCbasedHVDCgridtechnologyisinitsearlystageofdevelopment,andthegridstructuredesign,HVDCgirdconfigurationsandbus/switchingarrangements,topologydesignofAC/DCconverterandDC/DCconverterinDCstationsareallbasicproblemsinHVDCgrid.Theabovethreelevelsproblemsarediscussedinthepaper.HVDCgridstructure,especiallyDCgridtopologyforoffshorewindpowerintegrationisgiveninthepaper.HVDCgridconfigurations,typicalbus/switchingarrangementsofDCstationareprovided,andthecharacteristicscomparisonareinterpreted.SeveralAC/DCconvertertopologieswithDCfaultsride-throughcapabilityareprovided,andthenumbersofdevicesfordifferenttopologiesarecompared.Atlast,someproblemsshouldbepayattentiontoingroundingdesignwerediscussed.IndexTerms—HVDCgrid,topology,bus/switchingarrangement,HVDCgridconfiguration,AC/DCconverter,DC/DCconverter.HVDCgridbasedonVSC-HVDCisregardedasthemosteffectivetechnicalproposalintheintegrationoflargescaledistributedrenewableenergy,powersupplyofisland,centralizedintegrationofoffshorewindpower,constructionofnovelurbanpowergrid,andhasbeenaresearchhotspotininternationalpowerfieldnow.DCgridtechnologyanditsconstructionhavebeenanimportantdevelopmenttrendandcomponentinfuturepowergrid.DCgridismainlyconstitutiveofconverterstation,collectionsubstation,DC/DCconverterstationandtheDClinesbetweeneachstation.KeyequipmentofDCgridisconverter,DCbreakerandDC/DCconverter.WiththedevelopmentandmaturenessofVSC,DCbreaker,DC/DCconverterandDCcable,HVDCgridwillattractmoreattentionintheaspectsofpowertransmissionandpowerdistribution,andbeasignificanttrendofsmartgridandglobalenergyinterconnectioninthefuture.TheresearchofHVDCgridstructureandincludesthreelevelsasfollows:HVDCgridstructure,bus/switchingarrangement/HVDCgridconfigurationsincludingsystemgroundingdesign,AC/DCconverterandDC/DCconvertertopologies.HVDCgridstructuredescribestheconnectionrelationshipbetweeneachDCstations,suchasringstructureandmeshedstructure.Bus/switchingarrangement/HVDCgridconfigurationsincludestwolevels.ThefirstlevelistoresearchACandDCyardofAC/DCconverterstation,collectionstation,highandlowvoltage

wiringsofDC/DCconverterstation,andtodeterminetheDCstationelectricconnectionandDCbreakerconfiguration.ThesecondlevelistodeterminetheHVDCgridconfigurationasbipolarormonopolar,andthegroundingconfigurationiscontained.TopologydesignofAC/DCconverterstationandDC/DCconverterstationisaboutthedeterminationofadoptedtopologiesinAC/DCconverterandDC/DCconverter,suchashalforfullbridgestructureinAC/DCconverter,andindirecttypeordirecttypeinDC/DCconverter.DcGridGridstructuredesignofpowergridisaseniordesignofpowersystem.SimilartoACgrid,therearelotsofdifferenttopologiesaccordingtodifferentapplicationsandvoltagelevelsforHVDCgrid.Basictopologystructuresincluderingtopologyandmeshedtopology.RingRingtopologyhashighsupportcapabilitybetweeneachconverterstation.It’sconvenienttoreceivepowerfromseveraldirectionsandadjustthestructurebybreakingoutlooporexpandingthering,asshowninfigure1.TheringtopologyhasbeenadoptedinZhangbeirenewableenergyintegrationthroughVSCbasedHVDCgriddemonstrationproject..Fig.1RingtopologyofDCMeshedFeaturesofmeshedtopologyareasfollows:shorterlines,highersupportcapabilityandstrongnetframe,beingconvenienttoreceivepowerfromseveraldirections.TypicalHVDCmeshedtopologyisshowninfigure2.Inthedevelopmentprocess,thegridtopologymaybesimplerthanthatinfigure2.2016InternationalHighVoltageDirectCurrentConference(HVDC2016InternationalHighVoltageDirectCurrentConference(HVDCShanghai,25-27Oct.HVDCPaperHVDCPaperPageFig.2MeshedtopologyofDCHVDCGridTopologyforIntegrationofOffshoreWindAtpresent,interconnectionsbetweenoffshorewind

DC

DS34海 岸

DCareveryfew,andalmostallofthewindpowerfarmsareintegratedseparay.However,inthelongterm,theoffshorewindfarmscouldoperateininterconnectionstateforthesakeofthepowersupplysecurity,availability,operationflexibilityandmaintenancestrategy.Severaldcgridtopologiesfortheintegrationofoffshorewindpowerwereproposedin[7],includingringtopology,startopology,startopologywithacentralring,windfarmsringtopologyandonshoresubstationsringtopology.Ringtopology:WFmeansoffshorewindfarm,SSmeansonshoresubstation.Allwindfarmsandonshoresubstationsareconnectedinaring,asshowninfigure3.Ifafailureoccurs,thebreakersattwosidesofthefaultpointwillbedisconnected,andthesystemwilloperateatopenloopstate.Then,onedisconnectorwillbeopenedandoneofthebreakerswillbeclosedwhenthefaultcurrentdecreasestozero,thustheshutdownwindfarmcouldbeputintooperationagain.Whenthegridoperatesatopenloopstate,somelinesneedtotransmitthepowerofwholesystem.

Fig.3RingStartopology:Allofthewindfarmsandonshoresubstationswillbeconnectedtoacentralnode.Ratedcapacityofeachlineisequaltothecapacityofconnectedwindfarmorsubstation.Maindisadvantageofthetopologyisthatfailureofcentralnodewouldleadtobreakdownofthewholesystem.AnoffshoretformorsubmarineinstallationisneededtoceDCbreakersandstarconnections.Windfarmsringtopology:ThenumberofDCbreakersusedintheprojectcanbereducedinwindfarmsringtopology,asshowninfigure4.Thewindfarmsarelinkedinaringinthistopology,andthenumberofDCbreakersontheringisequaltothenumberofwindfarms.Itissimilartothepointtopointtopology,butwithhigherflexibility.Therearemorepowertransmissionchannelsbetweenwindfarms,aswellasbetweenwindfarmsandonshoresubstations.Whenfaultoccursintheline,thenearbytwobreakersareopenedandonestationisoutofservice.Thendisconnectornearthefaultwillbeopenedtoisolatethefaultlinewhenthefaultcurrentdecreasestozero.Atlast,thetwobreakerswillbeclosed,thustheshutdownstationcouldbeputintooperationagain.Fig.4WindfarmsringOnshoresubstationsringtopology:Thetopologyissimilartowindfarmsringtopology.ThedifferencearethatwhenfaultoccursintheDCline,thewindfarmswouldbeisolatedinsteadoftheonshoresubstations,sotheonshoresubstationsringtopologyhashigherflexibilityinonshorenetwork.Toensuretheumwindpowertransmission,windfarmsringtopologyisthebetterchoice.

numberofhalfbridgesubmodulesinanarmexceedsacertainvalue,outputwaveformoftheconverterwouldbeastepwavesimilartosineandfilterisnotnecessary.ButthedisadvantageoftheHBSMisthatitcan’tachievetheisolationbetweenACandDCsystemundertheDCsidefault.However,fullbridgesubmodule(FBSM)anddoubleclampsubmodule(DCSM)MMCcouldmaketheequivalentoutputvoltageofanarmenegativeandsupportACvoltagewhenDCvoltagereducesrapidly,soitcouldachievetheinhibitingeffectonACsideshortcircuitcurrent.Additionally,insingleclampsubmodule(SCSM)in[9]andtheseriesconnecteddoublesubmodule(SDSM)in[10],theseriesdiodesinthecircuitsafterconverterblockingarebackbiasedtooff,thuscanquicklycutoffthefaultcurrent.Recently,MMCwithhybridsubmoduletopologieshasbeenproposedwhichcontainsnolessthantwodifferentsubmoduletopologiesandatleastoneofthemcaninterruptthedc-sideshort-circuitfaultcurrent.MMCwithhybridsubmoduletopologiescanachievethecompromisebetweencapitalinvestmentanddcfaultride-through Fig.5OnshoresubstationsringPerformanceofwindfarmsringtopologyisbetterbecauseitonlyuses4DCbreakersanddoesnotlosewindfarmpowerunderdifferentfaults.Ifthedistancesbetweeneachwindfarmareshort,ringtopologywouldbeabetterchoice.Theflexibilityofstartopologyisworsebecauseitwouldloseallwindfarmpowerwhenthefaultoccursinthecentralnode.ConverterAtpresent,halfbridgesubmodulemultilevelconverter(HBSMMMC)isawidelyusedVSCtopology.When

（e）TheSeriesConnectedDoubleSubModule(SDSM)Fig.6SubmoduletopologiesofmodularmultilevelconverterThedevicenumbersofdifferenttopologiesperlevelaregivenintable1.Asshownintable1,DCSM,SCSMandSDSMtopologiescanrealizeDCfaultsride-throughusingfewerdevicesthanFBSM.Tab.1Thedevicenumberofdifferent22114401013401301ResearchonHVDCGridConfigurationsandGroundingSimilartopointtopointVSC-HVDCproject,HVDCgridconfigurationsfallsintotwotypesasmonopolarconfigurationandbipolarconfiguration.Amongthem,monopolarconfigurationcouldfallintotwotypesassymmetricmonopolarandasymmetricmonopolaraccordingtotheoutputDCpolarityofconverter.Bipolarconfigurationcouldalsofallintotwotypesasbipolarconfigurationwithearthreturnandbipolarconfigurationwithmetallicreturn.MonopolarHVDCMonopolarconfigurationsofDCgridareshownin7.Symmetricmonopolarconfigurationhasfeaturesofsimplestructure,smallnumberofconverters,lowcostandlowlosses.Besides,ACgridwouldnotsupplyfaultcurrentundersinglepoletogroundfault;noDCbiasvoltageontransformerandthusnoDCgroundingcurrent.However,faultsintheconvertercouldleadtotheinterruptionofpowertransmissionandaffectthetransmissionstabilityandflexibilityofDCgridbecausethereisonlyoneconverterineachend.（b）MonopolarHVDCgridwithearth

badeffectontheenvironment;sothisconfigurationisnotpermittedinmanydistrictsaroundtheworld.MonopolarwithearthreturnconfigurationonlyneedsoneDCline,sopartofcableinvestmentcouldbesaved.ContinuousDCbiasvoltageoninterfacetransformermakesthetransformermorecomplexandexpensive.Continuousgroundingcurrentwouldcausetheproblemofanti-corrosionandenvironmentprotection.Thegroundingelectrodeneedstobedesignedparticularly,andACgridwillinjectfaultcurrentwhenfaultoccursintheDCside.Specificlowvoltageconductorisusedasthereturnlineinasymmetricmonopolarconfigurationwithmetallicreturn.Allconverterstationsareconnectedthroughlowinsulationmetallicreturnandgroundedatoneterminalorseveralpoints.Insulationofmetallicreturncouldbedesignedaccordingtoovervoltagecharacteristicinthelowvoltagereturnlineunderfaultcondition.Comparedtotheasymmetricconfigurationwithearthreturn,thisschemehashigherlossbecausetheofmetallicreturnisbiggerthanearth.Monopolarconfigurationwithmetallicreturncanbeexpandedintobipolarsysteminlaterperiod.ThereisnoDCgroundingcurrent,butthetransformerneedtotoleratetheDCbiasvoltage.BipolarHVDCSimilartotheapplicationofconventionalHVDCtechnology,VSC-HVDCcanalsoadoptbipolarconfiguration.Bipolarconfigurationfallsintotwotypesasbipolarwithearthreturnandbipolarwithmetallicreturn,asshowninfigure8.Whenthesystemoperatesatsymmetricstate,voltagesinthetwopolesaswellascurrentsareequal,andthereisnocurrentintheearthreturnormetallicreturn.Converterredundancycanbeachievedinbipolarconfiguration.Whenfaultoccursinonepole,anotherpolecankeepworking.However,transformerneedstotoleratetheDCbiasvoltageandthesystemcostwillbealittlehighercomparingtothemonopolarconfigurationwiththesametransmissioncapacity.Thegroundingelectrodeofearthreturnconfigurationneedstobedesignedparticularly,andtheeffectonenvironmentmustbeevaluated.ACsystemwouldinjectcurrentintothefaultpointwhenfaultoccursintheDCline.Metallicreturnconfigurationneedsthemetallicreturnconductorwhichincreasesconstructioncostofproject.Groundingofsymmetricmonopolarconfigurationcouldbeprovidedbyseveralmethods,suchasDCcapacitormiddlepointgrounding,ACsidegroundingtransformer,ACsidestarconnectedreactors,betweenDCbusandground,etc.Thereisnocurrentflowingthroughthegroundingpointundernormaloperationconditioninalloftheabovegroundingmodes.WhenfaultoccursinoneDCpoleline,anotherpolecouldnotusethegroundingpointasreturnline,sothenormallinecouldnotkeepcontinuousrunning.Atpresent,symmetricmonopolarconfigurationiswidelyusedinmostVSC-HVDCprojects.Groundingelectrodeofasymmetricmonopolarwithearthreturnconfigurationneedstoberatedthesameaspolecurrent.Thecontinuouscurrentinthegroundingelectrodewillhave

BipolarHVDCgridwithearth（b）BipolarHVDCgridwithmetallicreturnFig.8BipolarHVDCgridCombinedHVDCDCgridisalargesystem,whichismostlikelytobeacombinationofvarietiesofconfigurationsinthefuture,suchasexistingtwo-terminalsystemsinterconnection,orasmallsystemconnectingtoanexistinglargebipolarsystem.Inthesecondcase,thesmallsystemmaynotneedsohighvoltageorreliabilitylevelasbipolarconfigurationandthebestsolutionistodesignthesmallersystemtobeanasymmetricmonopoleconfiguration,andthenconnectittotheexistingbipolarDCgrid.Figure9(a)and(b)areschematicdiagramsofanasymmetricmonopolesystemandasymmetricalmonopolesystemconnectingtoabipolarsystem,respectively.AnotherpossibleconfigurationisthatpartofthereturnpathinabipolarHVDCgridisearth,andtheotherpartofthereturnpathismetallicreturnconductor,asshowninfigure9(c).AsymmetricmonopoleconverterstationsconnectingtobipolarHVDCSymmetricmonopoleconverterstationsconnectingtobipolarHVDC

BipolarHVDCgridwithpartmetallicreturnandpartearthreturnFig.9CombinedHVDCgridInthefuture,thesymmetricalbipolarHVDCgridisapromisingconfiguration.BothconvertersofsymmetricalandasymmetricalmonopoleconfigurationscanbeconnectedtothesymmetricalbipolarHVDCgrid.ThedisadvantageisthatwhentheDCcurrentofbipolarsystemisasymmetrical,thesymmetricalmonopolarconvertercannotadjustitselftoabsorborsupplyunbalancedDCcurrent.CollectionSubstationDCcircuits,DCdisconnectorsandbusbarscomposethecollectionsubstationinDCgrid,whichysaroleincollectionanddistributionelectricenergy.Moreover,itcanalsoisolatetheDCfaultbytriptheDCcircuitbreakerquickly,andkeepthenormaloperationoftherestofDCgrid.Therearemanykindsofbus/switchingarrangementsforcollectionsubstation,someofwhicharewellknowninACsystem,suchassinglebus,singlebuswithbypassbus,doublebussinglebreaker,breakandahalf,etc.Variousfactorssuchasreliabilityofpowersupply,flexibility,adaptationtovariouspossibleoperationmodes,cost,safeandconvenientmaintenancehavetobeconsideredinthedesignofcollectionsubstationbus/switchingarrangement.Thesamewiththebus/switchingarrangementoftheACsystem,thecollectionsubstationnotonlyhastomeetthebasicfunctionofisolatingthefaultof ingandoutgoinglines,butalsomeettheoperationrequirementsafterfaultoccursontheDCsystem.arrangementOfDC/DCConverterStationTopologyofDC/DCConverterSincethereisnounifiedvoltagestandardforDCgrid,multiplevoltagelevelsexistinrunningHVDCprojects.HighvoltageandlargecapacityDC/DCconverter,ornamedDCtransformer,isoneofthekeyequipmentinDCinterconnection.DC/DCconverterisanimportantpartinthefutureDCgridwithmultiplevoltagelevels.Accordingtotheseriesofvoltagetransformation,theDC/DCconverterisdividedintoindirectDC/AC/DCconverteranddirectDC/DCconverter.IntheDC/AC/DCindirectconverter,theDCvoltageisconvertedtoACvoltagefirstly,andthentheACvoltageisrectifiedintoDCvoltage.ThereisnoAClinkindirectDC/DCconverter,suchasBuck,Boostandotherchopcircuit. ThemodularhybridDC/DCconverteriscomposedoflowvoltageconverter,highvoltageconverterandintermediatetransformer.Itcanmeetthespecialapplicationoccasions,suchastheapplicationofhighvoltageratioofDC/DCconverterinwindpowerDCnetwork,and

typicalarrangementsofDC/DCconverterstationaregivenbelow.Inpracticalengineering,differentarrangementscanbeusedinhighvoltagesideandlowvoltagesideinonestationaccordingtoreliabilityrequirementandnumberoftheinglinesofthehighvoltagesideandthelowvoltageFig.11SinglebusarrangementofDC/DCconverterFig.12FouranglesFig.10SchematicdiagramofthemodularhybridDC/DCconverterBus/SwitchingArrangementofDC/DCConverterAsthehuboftheDCgrid,theDC/DCconverterstationneedshighreliabilityandflexibility.FortheDC/DCconverterstationwithonlyoneDC/DCconverter,thesinglebusarrangementcanbeused.Fortheapplicationofhighvoltageandlargecapacity,ifthecapacityofoneDC/DCconvertercannotmeettherequirements,twoDC/DCconverterscanbeusedinparallel.Inaddition,twoDC/DCconvertersoperatinginparallelcanimprovereliabilityandflexibilityoftheDC/DCconverterstation.FortheDC/DCconverterstationwithtwoormoreDC/DCconverters,itisnecessarytodesignthelayout

GroundingOptionsOfDcForthepointtopointDCsystem,theselectionoftopologyandgroundingmodearesimple.FortheDCgrid,therearemanykindsofconvertertopologyandgroundingmodeavailable.DifferentgroundingmodesaffecttheconfigurationofthewholeDCgrid,theequipmentconfigurationandtheratedvalueselection,operationmode,protectionstrategy,networkscalability,reliabilityandredundancyofthepowersystem,andtheprojectinvestmentandoperatingcostsarealsoThefunctionofgroundingistokeepthevoltagetogroundinpredictablelimits,andtoensurethatthefaultequipmentcanbeisolatedfromthesystem.Thegroundingandconfigurationofthesystemarecloselycoupled.Thechoiceofgroundingmodesinvolvesatrade-offbetweentheinvestmentofequipmentinsulationandthesystemprotection.buses,breakersand inglines.Thesameasthebus/switchingarrangementofACsubstation,manyaspectsneedtobeconsidered,suchasreliabilityofpowersupply,flexibility,adaptationtovariouspossiblewaysofoperationmodes,cost,safeandconvenientmaintenance,avoidingorreducingtheeffectonDCpowersystemwhencollectionbusandswitchingarefaultorinmaintenance.Theselectionprincipleofthebus/switchingarrangementofDC/DCconverterstationisthesameasthecollectionstation,and

Therearevariousofneutral-pointgroundingincludingsolidlyearthed,earthedthroughimpedance,unearthed.Forthefirstpossibility,thevoltageoneachdeviceislimitedtotheratedvoltageUNundershortcircuitconditions,andpoletogroundsteady-statefaultcurrentcanonlyberestrictedbyconductorandearth,sothefaultcurrentislarger.Inordertokeeptheequipmentfromtheovercurrent,itisnecessarytocutoffthefaultcurrentassoonaspossible,preferablebeforethesteady-statefaultcurrentisreached.HVDCgridstructure,HVDCgridconfigurations,bus/switchingarrangementsofDCstation,andtopologiesofAC/DCconverterandDC/DCconverterarediscussedinthepaper.Severalconclusionsaredrawnasfollowing:RingtopologyisasimpletopologysuitableinearlystageofHVDCgrid.ThesymmetricalbipolarHVDCgridisapromisingconfigurationinthefutureHVDCgrid,andbothconvertersofsymmetricalandasymmetricalmonopoleconfigurationcanbeconnectedtothesymmetricalbipolarHVDCgridtocombineahybridHVDCgrid.MMCsubmoduletopologiesincludingDCSM,SCSMandSDSMcanrealizeDCfaultsride-throughusingfewerdevicesthanFBSM.ThemodularhybridDC/DCconverterisapromisingDC/DCconvertertopology,whichcanrealizehighvoltageratiotomeetthespecialapplicationrequirement,andcanbescaledtovariousvoltageand/orpowerlevels.AnTingZhouXiaoxinHanCongdaetalADCGridBenarkModelforStudiesofInterconnectionofACPowerSystems[J]CSEEJournalofPowerandEnergySystemsc1YaoanhonWunWnghnetaaternysisof243(3)076020(inTanguanfuuogWauanguiterminalHVDCandDCgridecnog[]rocednsoftheEE233(1) CIGREWGB4.52,“HVDCGridFeasibilityStudy,”CIGREbrochure533,Paris,AprilBarkerCD,WhitehouseRS,“AcurrentflowcontrollerforuseinHVDCgrids”inProc.10thInt.Conf.AC-DCPowerTransmission,London,U.K.,Dec.2012,pp.1-5.BellK,CirioD,andDenisAM,“Economicandtechnicalcriteriafordesigningfutureoff-shoreHVDCgrids,”inProc.IEEEPESInnovativeSmartGridConf.Europe(ISGTEurope),Oct.2010,ElectricPowerSystemsResearch2011;81(2):271–81.WanghaangZhaohenongXuanhngAmethodforcalculatingsub-moduleredundancyconfigurationsinmodularmultilevelconverters[J]AutomationofElectricPowerSses3 ZhaoenhaanghaanXuJanhnetaAsub-moduletopologyofMMCwithDCfaultride-throughcapability[J]PowerSystemTechnog204381)413446(in ZhangJianpo,ZhaoChengyong,SunHaifeng,etal.ImprovedofModularMultilevelConverterandApplication[J].TransactionsofElectrotechnicalSociety,2014,29(8):173-179(in HaileselassieTM,UhlenK,“PowerSystemSecurityinaMeshedNorthSeaHVDCGrid,”ProceedingsoftheIEEE,vol.101,no.4,pp.978-990,April2013.AkhmatovV,CallavikM,FranckCM,“TechnicalguidelinesandprestandardizationworkforfirstHVDCgrids,”IEEETrans.PowerDel.,vol.29,no.1,pp.327-335,Feb.2014.LetermeW,TielensP,DeBoeckS,VanHertemD,“OverviewofgroundingandconfigurationoptionsformeshedHVDCGrids,”IEEETrans.PowerDel.,vol.29,no.6,pp.2467-2475,Dec.2014.HaileselassieTM,UhlenK,"PowersystemsecurityinameshedNorthSeaHVDCGrid,”inProc.IEEE,vol.101,no.4,pp.978-990,April2013.WoodTB,MacphersonDE,BanhamHD,FinneySJ,"RipplecurrentTrans.PowerDel.,vol.29,no.2,pp.926-933,April2014.

GuohunhaohengnMnaarietaneanofhybridbipolarHVDCsystemperformances[J]ProceedingsoftheEE0122()98104(in)AtighechiH,ChiniforooshS,JatskevichJ,DavoudiA,MartinezJA,et“Dynamicave