附錄英文翻譯1.英文文獻原文Controlofinverter-basedmicro-gridsT.C.Green,M.ProdanovicImperialCollegeLondon,DepartmentofElectricalandElectronicEngineering,ControlandPowerGroup,LondonAvailableonline28September2006AbstractThepredictedgrowthofsmall-scalenon-50/60Hzpowersourcesandthedesiretobeabletosupportloadsindependentlyofthepublicelectricitygridrequiresthedevelopmentinverter-basedmicro-grids.Powerelectronicinterfaceshaveverydifferentcharacteristicstoconventionalelectricalmachinesand,therefore,differentoperation,controlandprotectionschemesarerequired.Attentionalsoneedstobegiventothedominanceofsingle-phaseharmonicallydistortingloadsinsomenetworksandcontrolschemesputinplacethatmaintainvoltagequality.Acontrolschemethatexploitsthecontrollabilityofinverterstooperateamicro-gridandprovidegoodpowerqualityisexaminedandcomparedwithbothtraditionalpowersystemsandwithcontrolofdc/dcpowerconverters.Thelimitationsofcommunicationandcontrolbandwidtharediscussed.Experimentalresultsareusedtoillustratetheperformancethatcanbeachievedwithvariouscombinationsoflinearandnon-linear,three-phaseandsingle-phaseloads.?2006ElsevierB.V.Allrightsreserved.Keywords:Distributedgeneration;Microgrids;Inverters;Powerquality1.IntroductionMicro-gridsarethoughttobealikelydirectionforevolutionofpowersystemsthatincorporatedistributedgeneration.Thetermmicro-gridisnotstrictlydefinedandcoversawiderangeofpossiblesystems[1–3].Here,thetermistakentomeanasub-setofapowerdistributionsystemthatcontainssufficientenergysourcestosupplymostorallofthelocalload.Themicro-gridisthusabletooperatewithonlyasmallpowerexchangewiththerestofthesystemandattimescanseparatefromthepublicsystemandrunasanindependentisland.Amicro-griddiffersfromexistingislandpowersystems(bothphysicalislandsandelectricalislands,suchasoffshoreoil/gasplatforms,shipsandaircraft)inthatconnectiontoanddisconnectionfromapublicgridisaregularevent.Itisanticipatedthattheloadswillbenor-maldomesticandcommercialconsumerloads.Assuch,largenumbersofsingle-phasediode-rectifierloadsareanticipatedandthesupplycurrentisexpectedtobeunbalancedanddistorted.Insomeislandsystems,forinstanceanoilplatform,theinitiationofmajorloads(hoists,pumps,etc.)canbeanticipatedandthegenerationresourcescanbeputinreadinessforthetransient.Thismaybepossibleinsomesortsofmicro-gridbut,ifnot,loadforecastingislikelytobeimportantanddemand-sideparticipationincontrolmaybeneeded.Thegeneratingtechnologyfoundinamicro-gridmaytakemanyformsandanindividualmicro-gridislikelytoincludeamixtureofthese.Sourcesmaybesmall,suchasdomesticCHPsystemsorroof-topphotovoltaicpanelsorrelativelylarge,suchasoffice-block-scaleCHPusinggas-turbinesorreciprocatingengines.Sourcesmaybeintermittent(photovoltaicandheat-ledCHP)orfullycontrollable(electric-ledCHPorsimplediesel/gasfuelledgenerators).Tomakebestuseofintermittentsourcesand/ortoprovidesecurity,amicro-gridmayincludeastorageelement,suchasaflywheel,batteryorhydrogenelectrolysis/fuel-cellsystem.Relativelylargegenerators,especiallyexistingpeak-loppingdieselgeneratorsthatareturnedtomicro-gridduty,arelikelytobeconventionalsynchronousmachinesgeneratingat50/60Hzthree-phase.Newertechnologywillbedifferent.Somesourcesarenaturaldcsources,suchasphotovoltaic’s,fuel-cellsandbatteries.Othersarevariablefrequencyorhighfrequencyac,suchasmicro-turbinesandfly-wheels.Thenon-50/60Hzsourceswillneedtobeinterfacedtothegridviainverters.Theformthemicro-gridtakesandthetypeofloadssup-pliedwillhavealargeimpactontheoperatingandcontrolregimeofthesystem.Asyet,therearenoestablishedstandardoperatingschemesanditmayneverbepossibleordesirabletohaveauniversallyappliedmethod.Thispaperwilldescribetheoperationandcontrolofamicro-gridforaparticularsituation,namely,amicro-gridthatisformedofarelativelysmallnumberofgenerationsourcesthatareinterfacedtothegridthroughinvertersandexpectedtosupplyunbalancedanddistortingloadcurrent.2.ThepowerparkTheexamplemicro-gridsystemwillbeapowerpark.Apowerparkconsistsofseveralpowersourcesinrelativelycloseproximityusedtoimprovetheelectricityservicetoagroupofcustomers.Thecustomersmaybearetailpark,shoppingmallorgroupofofficebuildings.Ifthesearenewconsumersthepowerparkmaybemotivatedbynetworkcongestionorpowershortages.Othermotivationsmightbethewishtoinstallcogenerationortoprovidepremiumpower.Thepremiumaspectwouldbetheabilitytocontinuesupplyinghighvaluecustomersintheeventofoutageofthepublicgridorvoltagesag.Itmightalsomeanavoidanceofotherpowerqualityproblems,suchaswaveformdistortion,flickerorswell.Theimprovedpowerqualitymightcomeaboutfromaloweredsupplyimpedance,activemanagement(suchasactivefiltering)and/ordisconnectionfromthepublicgridwhenthegridisdeemedtobethesourceoftheproblem.Becausesomesystemoperatorspresentlydonotallowdistributedgeneratorstocontrolthevoltageatthepointofcommonconnection,someofthepowerqualitybenefitsmayonlybeavailablewhenthemicro-gridisrunningasanisland.Forincreasedreliability,flexibilityandefficiency(whenrunningpart-load)severalgeneratorsarerequired.Thepowerparkmightbeownedbyalandlord,abuildingser-vicescompany,theconsumersthemselvesortheexistingutilitycompany.Powerexporttothepublicgridmightormightnotbeusedwhatevertheownershipofthegeneration.Micro-gridsingeneralmayhaveavarietyofownersofgenerationbutthepowerparkvariantislikelytoseeasingleownerorasingleoperator.Further,thegeneratorswillbeingroupsthatarephysicallyandelectricallyclosetogether.Thegenerationtechnologymaybeamixoftraditionalelectricalmachinesandinverters.Thereareseveralpossibleoperatingmodesofthepowerpark:(1)Grid-connectedoperationwithfixedlocalgeneration:Thegridconnectionwillseeafluctuatingpowerflowasloadschange.Thepowerflowcouldbeineitherdirection.(2)Grid-connectedoperationwithloadfollowinglocalgeneration:Thegridwillseeafixedpowerflowwhichmightbeimport(tocovershortfalloflocalgeneration),export(tosellexcesscapacity)orzero(floatingofthemicro-gridbutwithaconnectionmaintainedtocovercontingencies).(3)Islandoperation:Thelocalgenerationwillloadfollow,possiblyusingstorageordemand-sideparticipationtoincreasesecurity.Theoperationandcontrolissuestobefacedare:(1)Howtosharesteady-stateloadbetweengeneratingsets?(2)Howtosharetransientloadbetweensets?(3)Howtoswitchseamlesslyfromgrid-connectedtoislandoperation?(4)Howtoincorporateadditionalfeatures,suchasactivemanagementofwaveformquality?3.CharacteristicsofelectricalmachinesandinvertersThehistoryofpowersystemgenerationisthehistoryofthesynchronousmachineandthecharacteristicsofthismachinehaveinfluencedthedevelopmentofotheraspectsofthesystem.Forinstance,protectionschemesrelyontheshort-termovercurrentratingofanelectricalmachine(resultingfromitslargethermalmass)toprovidethefaultcurrentnecessarytodiscriminatefaults.Inturn,therequiredclearancetimesforfaultsaredictatedbytheover-swingandlossofsynchronismcharacteristicsofmachines.Ontheotherhand,electronicpowerconvertersarewellestablishedinthefieldsofOEMpowersuppliesandindustrialdrivesbuthavenotinfluencedthedevelopmentoftraditionalpowersystems.Electronicpowerconvertershaveverydifferentcharacteristicstoelectricalmachinesandwouldhavedirectedengineerstowardaverydifferenttransmissionanddistributionsystem.Therelevantcharacteristicsofelectricalmachinesare:(1)Operateasvoltagesourceswhoseamplitudecanbeadjusted.Theadjustmentisnormallypartofaclosed-loopexcitationcontrolschemewitharelativelylowbandwidth.(2)Sine-wavevoltageisafeaturedesignedintotheconstructionofthemachine.Thetotalharmonicdistortion(THD)ofthevoltageislow.Distortionintheloadcurrentwillincreasethedistortioninthevoltagetoanextentdependentonthesourceimpedance.(3)Theshort-circuitcurrentishighbecausethesourceimpedanceislowandnocurrentlimitingisemployed.(4)Thecurrentratingissetbytemperatureriseofthewindinginsulation.Thethermaltime-constantofthewindingandsurroundingsteelisrelativelylargeandausefulshort-termratingisavailable.Faultcurrentsofasmuchas10timesthesteady-statemaximumcanbesustainedforseveralmainscycles.(5)Realpowerexchangeisdictatedbythetorqueappliedtoshaft.Steady-stateloadsharingcanbeappliedbyaclosed-loopgovernorsettingthatmakespoweroutputafunctionofthe(common)systemfrequency.Thisisadesignedinfeatureofthegovernorsystembutitissimilartothenaturaltendencyforthespeedoftheprimemovertodroopwhenelectricalloadisdrawn.Thecorrespondingcharacteristicsofinvertersare:(1)Operateasvoltagesources(althoughcurrentsourceversionsareknown)withnearinstantaneousandindependentcontrolofthemagnitudeofeachphase.(2)Sine-wavevoltagecanbeachievedthroughuseofasuitablereferencewaveformandmodulatorbutanyshapecanbeusedatwill.Alternatively,closed-loopcurrentregulationcanbeappliedtoachievevariouscurrentwaveforms.Thelow-frequencyspectrumofaninverteriswellcontrolledbuttheswitchingactionoftheinverterproduceshighfrequencydistortionthatcanonlybeaddressedthroughfiltering.(3)Potentialshort-circuitcurrentishighbutprotectionagainstitmustbeprovidedintheformofcurrentlimitingaction.(4)Thecurrentratingissetbythetemperatureriseofthesemiconductors.Thethermaltime-constantofthesemiconductorisveryshortandlargeovercurrentscausedevicefailureinconsiderablylessthan1ms.Thecoolingsystemalsohasarelativelyshortthermaltime-constantandsoevenmoderate-overcurrentscanonlybetoleratedforshortperiodsunlesstheinverterhasbeenover-ratedtoaccommodatethem.Realpowerexchangeisdictatedbythereferencesappliedtothecontrolsystem(subjecttothedc-linkbeingabletosourceorsinkthispower).Thereareseverallow-powerapplicationareaswhereelectronicpowerconvertersareusedforpowerdistribution,suchasintelecommunicationsystemsandlargecomputers[4].Thesepowersystemsoftenconsistofseveraldcpowersuppliesrunninginparallel.Thesystemsthathavedevelopedherearequitedifferentfrompublicacpowergrids.Theoperatingschemesarehierarchicalcontrol-loopsthatactivelysharepowerbetweenmodulesthroughdistributionofcurrentdemandsignals.Thecurrentdemandsignalsarearrangedtoensurethatmodulesarenotsubjectedtofaultcurrentsbeyondtheirratingsandtheover-allsystemhasalimitedfaultcurrentthatcausescollapseofthesystemvoltage.Thereareseverallow-powerapplicationareaswhereelectronicpowerconvertersareusedforpowerdistribution,suchasintelecommunicationsystemsandlargecomputers[4].Thesepowersystemsoftenconsistofseveraldcpowersuppliesrunninginparallel.Thesystemsthathavedevelopedherearequitedifferentfrompublicacpowergrids.Theoperatingschemesarehierarchicalcontrol-loopsthatactivelysharepowerbetweenmodulesthroughdistributionofcurrentdemandsignals.Thecurrentdemandsignalsarearrangedtoensurethatmodulesarenotsubjectedtofaultcurrentsbeyondtheirratingsandtheover-allsystemhasalimitedfaultcurrentthatcausescollapseofthesystemvoltage.ofthehighorderfilters,islikelytolieneartheupperlimitofthecontrollerbandwidth.Thisisproblematic.Thereisbarelyenoughcontrolbandwidthtosuppressthefilterresonance.Placingtheresonancesafelyabovethecontrollerfrequencylimitisnotfeasiblebecauseinsufficientfilterattenuationwouldbeachieved.Reducingtheresonantfrequencytowellwithinthecontrolbandwidthwillrequirephysicallylargecomponentsandwillmakethesystemresponsesluggishabovethatfrequency.4.Controlofaninverterforpowerexport4.1.Grid-connectedmodeIngrid-connectedmode,theobjectiveistoexportacontrolledamountofpowerintotheestablishedvoltage.Thismodeofoperationhasbeenextensivelyexaminedforpowerflowpredominantlyintheoppositedirectionundertheguiseofactiverectifiers[5].Mostactiverectifiersemploysimplefirst-orderfilters(becausepowerqualityinthemediumfrequencyrangemaynotbeparticularlyimportant)butsecond-orderfiltersarealsoreported.Controloftheexportedpoweristhroughcontrolofthein-phasecomponentofcurrent.Phase-lockedloop(PLL)techniquesareusedtoensuresynchronism[6].Controlindq-axis(synchronousreferenceframe)formisusuallypreferredtomagnitude-angleform.Thecurrentdemandsaregeneratedfromthepowerdemandsusingthelocalvoltagemagnitude(availablefromthePLL).Thepowerdemandsthemselvescomefromthemicro-gridsupervisor(MGS)withtheexceptionofphotovoltaicandheat-ledCHPsourceswhichgenerateautonomously.TheMGSwilloperateeithertorunthegeneratorsatfixedoutput(presumablyoptimuminsomesense)orloadfollowingsoastoregulateexportfromorimporttothemicro-grid.TheoverallcontrolstructurewillbesimilartoFig.3.Thecurrentiscontrolledinclosed-loopform.Thecurrentreferences(d-and-qaxes)aresetonthebasisoftherequiredrealandreactivepowers.Strictlyspeaking,thepowerissetopen-loopbuttheclosed-loopcurrentcontrolensuresproperoutputpowerlevelsandpowersharingbetweenparallelunits.Fig.3showssomestandardinvertercontroltechniquesinadditiontothesimplecurrent-control-loop.Thesearetheadditionofdecouplingtermstocompensatethenaturalcouplingbetweend-andq-axes(introducedbythefilterreactance)andthefeed-forwardofthecapacitorvoltageintothecurrentcontrol-loop.Forclarity,Fig.3omitstermsthatmightbeaddedtocompensatevariationofthedc-linkvoltageoftheinverter.ThegridvoltagemaynotbeavailableformonitoringsoalternativeconnectionsforthePLLareshown.Becauseeachinvertercontrolsitsoutputcurrent,currentsharingandpowersharingareassuredaccordingtothedemandsestablishedbytheMGS.Thedesignofthecontrollerandthedesignofthefilterneedtobeconsideredtogetherinordertoestablishgoodattenuationofbothhighandlow-frequencyinterference[7].Controllersimplementedindiscretetimewillalsorequireadditionaldecouplingterms[8].Publicelectricitygrids(atdistributionvoltagelevels)oftenhavesignificantharmonicdistortionofthevoltage.Choosingtocontroltheoutputcurrentoftheinverterinanexplicitcurrentcontrol-loophastheadvantageofmakingtheinverterahighimpedancepaththatpreventsflowofharmoniccurrentsduetogridvoltagedistortion.(Thefiltercapacitordoesprovideapathforharmoniccurrentsandthismustbeaccountforinselectingitsvalue.)Incontrast,avoltagesourcesynchronousmachinewillpresentalow-impedancepathtonon-triplenharmonics.optionalconnections).Thoughtmustbegivenhowthevoltagemagnitudesignalisusedinthepowercalculation.Thecurrentcalculatedasnecessarytoexportconstantpowerwillbeharmonicallydistortedifthevoltageisharmonicallydistorted.Itmaybepreferredlow-passfilterthecalculatedcurrentreferencetoremoveharmonicterms.Theconsequenceofthisisthatthepowerexportwillcontainrippleatfrequenciesrelatedtothevoltageharmonics.Inotherwords,ifthevoltageisdistortedtherearethreechoices:(i)exportundistortedcurrentbutdistortedpower,(ii)exportundistortedpowerbutdistortedcurrentor(iii)suffersomedegreeofdistortiontobothcurrentandpower.Fig.3showsthecurrentdemandfedthroughalow-passfiltertoensureundistortedcurrentreferences(option(i)).Itwouldbepossibletosetallinverterstoexportbalancedfundamental-frequency-onlycurrent.However,thesingle-phasediode-rectifierloadswillstillrequiredunbalancedandharmoniccurrentcomponentsandthesewillhavetobeprovidedbyanymachinesonthemicro-gridorbythepublicgrid.Drawingthesecomponentsthroughsignificantlineimpedanceswillworsenthevoltagedistortion.Drawingahighproportionofdistortioncurrentfromthepublicgridmaybedisallowedbytheutilityoperatororbyregulations.Toachievegoodvoltagequalityforlocalconsumersextracontrolfunctionscanbeadded.Thevoltageatthefiltercapacitorcanbesensedandfilteredtoidentifytheharmonicdistortion.Thisfeedbackisusedtogenerateacurrentreferencetoaddtotheactiveandreactivecurrentreferencesinordertocorrectthedistortion.(Thetermsreactivecurrentandreactivepowerarehereusedtorefertothequadraturecomponentoffundamentalfrequencycurrentanditsassociatedreactivepower.)Fig.4showsasuitablecontrolstructure.Itrequiresthatthereissomesignificantimpedance(atharmonicfrequencies)betweenthefilterandthevoltagesourceofthepublicgridinorderthattherecanbesomeinfluenceoverthelocalvoltage.Thisisoneapproachtoincorporateactivepowerfilterfunctionalityintoapowerexportinverter.Suchtechniquesgenerallyrequireagreaterdegreeofinstrumentationthatthesimpleinvertercontrollersandacostandcomplexitypenaltyispaidforthis.Ifcontrolofthevoltagemagnitudeatthepointofcommoncouplingisallowed(orifalocalbusseparatedfromtheutilitygridisarrangedforsupplyofpremiumloads)thencontrolofthewholeofthevoltagespectrum(uptothecontrolbandwidthlimit)ispossible.Onesuchschemehasbeenreported[9]usingatechniqueknownasrepetitivecontroltosuppressdistortion.4.2.Whereasthetaskingrid-connectedmodeistocontrolcurrentintoanestablishedvoltage,thetaskinislandmodeistoestablishthatvoltage.Fig.5showstheobviousapproachofclosingacontrol-looponthesecondorderplant.Thereareshortcomingstothisapproachthathavelongbeenrecognizedinswitch-modepowersuppliesandmachinedrives[10,11]:(1)Althoughthevoltageiscontrolled,thereisnoexplicitcontrolofcurrentandlargetransientcurrents(dangeroustothesemiconductors)canoccur.(2)Theplantisnotsimplysecondorder:itincludesadistributionnetworkandloads.Itcanbeviewedasasecondorderplantwithadisturbancecurrent.Inaddition,theplantparametersmaynotbeknownaccurately.(3)Feed-forwardofloadcurrentdisturbancescanbereadilyincorporated.Thestandardalternativeistousenestedcontrol-loops,Fig.6.Theplantisconsideredastwofirst-ordersystems.Theyarereasonablywelldecoupledbecauseoftheirdifferenttime-constants.Aninnercurrent-control-loopisformedaroundtheinductorandisarrangedtohaveafastresponse.Thissubsystembehavesasanearperfectcontrolledcurrentfromthepointofviewoftheoutervoltage-control-loop.Ifthecurrentdemandgeneratedbytheouterloopislimitedthentheinverterwillnotbesubjectedtoexcessivecurrent.Thecontrollerisagainshowninarotatingreferenceframe.Theangleusedtoconvertbetweenabcanddqcoordinatescanbeobtainedfromanoscillatorthatsetsthesystemfrequency.Furtherimprovementintransientresponsetoloadchangescanbeobtainedbyfeed-forwardoftheload(disturbance)current.Anotheradvantageofthenestedcontrol-loopapproachwithitsexplicitcurrentdemandisthatitlendsitselftocurrent(andpower)sharingbetweenparallelmodules[12].ThisisillustratedinFig.7.Thetechniquecanbereadilyappliedtopowerconvertersinverycloseproximity.However,whenthereissomedistancebetweenthemthenthecurrentdemandsignalsneedtobetransmittedoveranexplicitcommunicationchannel.Inaddition,asynchronizingsignalwillalsoneedtobecommunicatedtoallinverters.Thebandwidthofthecommunicationlinkswouldneedtobeabove100kb/stoavoiddegradingtheperformanceofthecontrollers.Analternativeapproachtoprovidingsharingbetweenparallelmodulesthatispopularinmodulardcpowersuppliesisthemaster–slaveapproach[13,14].Nestedcontrol-loopsareusedforthemasterpowerconverter(orinverter).Theoutputcurrentofthisconverter(or,alternatively,thetotalloadcurrent)ismeasuredbytheslaveunitsandusedastheircurrentreference.Thus,theslavesattempttosupplythesamecurrentasthemaster(orascaledversionofthatcurrent)andsteady-statesharingisachieved.Themaster–slaveapproachcanbereadilyappliedtoinvertersandacsystemsbyformingthecontrol-loopsinarotatingreferenceframe,Fig.8orevenastationaryreferenceframe[13].Theasymmetryofthesystemleadsthemasterconvertertotakeonmostofthetransientdutyandmostofthesupplyofunbalancedcurrent.Thisisprincipallybecauseofthelimitedbandwidthornoisefilteringinthedistributionpathofthecurrentsharingsignal.(Negative-sequenceunbalancecurrentsappearasdoublefrequencytermsinarotatingreferenceframe,apotentialdisadvantageofapplyingdq-axiscontroltounbalancedsystems.Thezero-sequencetermshavenotbeenshowninthesediagramsbutwouldneedtobecontrolledalsoiffour-leginvertersandafour-wiregeneratorbuswereused.)Thiseffectisheightenedifthebandwidthofthecommunicationchannelfortheslavecurrentdemandhasbeendeliberatelylimited.Becausethetransientcapacityofaninverterislimited(incomparisontoelectricalmachines)itwouldbehelpfultoprovidetransient-statesharingbetweencloseinverters.Thiscanbeachievedwithouthighbandwidthcommunicationlinksbypartitioningthefrequencyspectrumandapplyingdifferentcontrollerstodifferentpartitions,Fig.9.Steady-statesharingofpowerisachievedthroughcontrolofthelow-frequencyportionofthevoltageinacontrolsystemsimilartoFig.6.Thecurrentdemandfordistributiontothevariousmodulesisindq-formandlimitedinbandwidthtoaround500Hz(perhaps10kb/s).Localcontrol-loops(foreachinverter)thatsensethelocalvoltagethenacttoreducetheharmonicdistortion(atrelativelyhighfrequency)towardzero.Thisgivesrisetothetermdistributedcontrol[13].Therearesomeweaknessesinthecontrolschemethatneedtobeguardedagainstinimplementation.Thecontrolisdistributedbutreliesononecontrol-loopforthemostimportantfeatureofvoltagecontrol.Thisleavesthesystemopentoasingle-pointfailuremechanismdespiteitsmodularity.Thevoltagecontrolloopcouldbeimplementedineachinvertersuchthatanyinverterisabletoassumethisdutyshouldoneinverterfail.Asecondweaknessisthatthefeed-forwardtermassumesthatthetotalloadcurrentcanbemeasured.Inapowerparkofseveralgeneratorsincloseproximityfeedingoneortwomasterbussesthismaybepossible;inothercircumstancesitmaynot.Thefeed-forwardtermisnotessentialbutdoesimprovetransientvoltageperformanceprovingnoiseandaccuracyissuesareaddressed.4.3.MixednetworksThedistributedcontrolschemewasnotdesignedexplicitlytoincorporatetraditionalelectricalmachinesaspeersoftheinverters.Therearetwoapproachesthatcouldbeusedtoformamixednetwork.Iftheinvertersdominatethenetworkthendistributedcontrolcouldbeusedtoestablishanisochronousnetwork.Asynchronousmachinecanthenberunupandsynchronizedtoit.Themachinewillrunasaconstantpowerdevicewiththepowersetbythecombinationofitsgovernorsettingsandthechoiceofreferencefrequencyfortheinvertercontroller[15].Alternatively,iftheelectricalmachinesdominatethentheycanformamicro-gridandtheinvertercanbecontrolledingrid-connectedmode.Theinvertercanbesettoruninconstant(dispatched)powermodeorgivenapowerdemandderivedfromthesystemfrequencyinamannerthatmimicsagovernordroopcharacteristic.5.ExperimentalresultsTheexperimentalrigwasbuiltwithaninverterof10kVAandwasconnectedtothe50Hzvoltagegridviaa-Yisolatingtransformer(ratedat10-kVAandwiththe-windingconnectedtothegridandwithavoltageratioof415V/208V).Theinverterparameterswere:switchingandsamplingfrequencies,8.192kHz,currentcontrol-loopbandwidth1.9kHz;C=50;L=1.35mH.Theisolatingtransformerparameterswere:;;;.Controlsystemsandpulse-widthmodulatorswereimplementedonaTMS320LF2407indiscretetimeform.Theprocessorisalsousedtomonitorseveralsignalswithinthecontrolblocksandmaketheseavailableforrecordingbyanoscilloscopeviadigitaltoanalogueconverters.5.1.Grid-connectedmodeFig.10showstheresponseofthepowerexportedtothegridwhenthedemandsaresubjecttostepchanges.Boththerealandreactivepowersarecloselyregulatedthroughtheactionofthecurrentcontrol-loops.Becausetheloopsareindq-formandthenaturalcouplingiscancelled,therealpowerisnotperturbedwhenthereactivepowerisstepped(andviceversa).Thepublicgridvoltageavailableinthelaboratoryisslightlydistortedandunbalancedsotheexportedpowershowsasmallripple.AsnotedinSection4.1,itispossibletosuppressthisripplebutonlybyintroducingdistortiontothecurrent.Fig.11showsthepre-existingdistortionoftheavailablegridvoltageandinparticularsignificantfifthandseventhharmonicdistortion.Fig.12showsthecurrentflowingintothegrid.Ithasasimilardegreeofloworderharmonicdistortiondespitethehighimpedanceoftheinverterinthisfrequencyrange.Theharmoniccurrentisbeingdrivenbythegridvoltageharmonicsthroughthecapacitoroftheinverter’sfilter.Avoltagesourcegeneratorwouldpresentamuchlowerimpedancetovoltageharmonicsandlargerharmoniccurrentswouldflow.Fig.12revealssomeresonanceliftataround2