what’s an ideal pv system...

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What’sanIdealPVSystemGranularity?15December2015,v1.0ContentsIntroduction....................................................................................................................................1NativeGranularitybyInverterType...............................................................................................2CompetingFactors..........................................................................................................................3ExampleCommercial300kWInstallation......................................................................................4PerspectivefromaCommercialInstaller........................................................................................4Summary.........................................................................................................................................5Advertising......................................................................................................................................5

Introduction:AspectsofGranularityWhatistheidealgranularityforaPVinstallation?Herewe’retalkingabouttheaddressabilityofanumberofsystemfactorsincluding:

• Monitoring(enoughdatatomaximizeenergyproduction,butnottoomuchtoconsumeresourcesunnecessarily)

• O&Mstrategy(lowestcostformaximumenergyharvest)• MPPTharvest• Powerconversionblocksize(toleranceforshading,failures,

etc)• Arcdetection(safety)

Howmuchgranularityisenough?Aswithmanyquestionslikethis,theansweris“itdepends….”GranularityimpactsinitialinstallcostandO&Mdifferently.

• Fortheinstall,asanexample,thepowerconversionblocksizeisonefactorandthiswillimpactmaterialandlaborcostsetc.

• ForlaterO&Mitcomesdowntohowmuchoftheoutputofanarraycanunderperformbeforeitimpactsrevenue-andrevenueisimpactedbyenergyproductionandO&Mcosts.Forexample,ifonemoduleinaresidentialinstallof16modulesdies,a17%dropinproductionwillbenoticeableandresultinanurgentneedtofixit.Ifthatsamemoduleisoneof10,000atautilityscaleinstallthereislessurgencyoverfixinga0.01%hit.GranularityhasasignificanteffectontheimpactofPVmoduleissues.

• Asanotherexample,ifyouhaveper-stringMPPTandaplantwith100stringsof10moduleseach,amoduleissuewillimpactlessthan1%ofplantoutput.Thatsameplantwithper-modulepowerconversion(microinverterorpoweroptimizer),theimpactwillbe0.1%oftotalplantoutput.Sopowerconversioncomesintoplay.

Sowhynotjustmonitor/powerconverteverymodule?Aswe’llsee,forsmallinstallsthatcouldbetherightanswerbutthere’smoretoit.

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NativeGranularitybyInverterTypeThechoiceofinverterhasabigimpactonmonitoringandMPPTgranularity,andisusuallydriveneitherbyafocusonthelowestfirstcostorlowestinstalledcost.Figure1givesanexampleofhowthenativegranularityvarieswithinvertertype,highlightedwiththeorangemodules.Granularitycanbeenhancedthroughtheadditionofmodulelevelpowerelectronicssuchasoptimizers,butatextracostandareductioninreliability.Also,theeventualfailureofelectronicsmountedunderthePVmoduleresultsinsignificantlyhigherrepaircost.Ingeneralthelowestinstalledcostformanycommercialprojectscomesfromthelargest3-phasestringinvertersbutthegranularityofbothmonitoringandMPPTgetscoarserasthepowerratingclimbs.

Figure1:Granularitybyinvertertype

Figure2:Competingfactorsaffectingthedegreeofgranularity

Central(with(zone(monitoring( Large(32P(string( TrueString( Micro(

Pushing(for(more(granularity(

Pushing(for(less(granularity(

Large(group(of(strings(

Module(level(

Several(strings(

String(level(

•  Be8er(performance(visibility(

•  Be8er(pinpoin<ng(for(troubleshoo<ng((

•  Be8er(MPPT(granularity/(harvest(

•  Visual(and(analy<c(data(overload(

•  Customer(visibility(of(minor(issues(

•  Cell(modem(data(bandwidth(

•  Failure(of(components(under(modules(

•  Lower(weight(on(rooCops(

•  Cost(

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CompetingFactorsThehigherthegranularity,thebettertheoutcome,right?Figure2showsthatwhilethisistrueuptoapoint,therearefactorspushingintheotherdirectionalso.Someoftheissuesinbothdirectionsare:

• Monitoringisexpensive,andmoredataismoreexpensive–hardwaretodoit,mechanismsforgatheringthedata,datastorage,timeandprocessingtoanalyzeit,etc.YouwanttheminimumthatallowsyoutoperformyourjobandoptimizePVplantenergygeneration.

• Notethattheminimumaccuracyforrevenuegrademetering(RGM)is+/-2%,althoughrelativemeasurementscomparingrelativeoutputsofstringsorinvertersdaytodayareusuallymoreusefulanddonotrequireirradianceandtemperaturedatatobemeaningful,assumingnoshading.And,revenuegrademeteringisusuallyonlyusedfortotalPVplantoutput,sothegranularityofthismeasurementisinsufficienttoidentifyproblemsatamoduleorstringlevel.

• Argumentsaboundonthetopicoffailures–isitbettertohavefewerpointsoffailurethataremoresignificantiftheydie,ormanymorebutwithanyonefailurehavinglessimpactonenergyproduction?Thisiswherethepowerconversionblocksizecomesintoplay–howmuchofthesystemneedstobereplacedwhensomethinggoeswrong,andhowbigajobisittotransportandfitareplacement?Onepieceoffeedbackfrominstallerswehearisthat,eitherway,leveringupindividualPVmodulesinanarraytoreplaceelectronicsistimeconsuming,expensiveandasafetyrisk.Ifyourmodule-levelmonitoringisprimarilyusedtolocatefailedmodulelevelelectronics,ittendstodefeatthepurpose(andinitialexpense).

• Weightonaroofcanbeanissue.Centralinvertersareheavy,butwhenaddedup,thetotalweightofanarrayofmicroinvertersofthesametotalkWisactuallyheavier,justmoredistributed.

• Inplantswhereoutputismeasuredandgovernedbyacontractguaranteeingtheenergyproduction(PPA),thegranularityshouldbesmallenoughsothatanissuecanbenoticedfarenoughinadvanceofacrisisthatmaintenancecanbescheduledratherthanbecominganemergency.Forexample,oftenmodulestaketimetodie,andsoilingcanbequantifiedsowashingofthearraycanbescheduledoptimally.

• Troubleshootingisvastlyeasierwhenitcanbepinpointedtoassmallanareaaspossible.Forlargerplantsthepositionofindividualmodulesisunlikelytohavebeenrecorded,butisolatingtheissuetostringlevelisaveryhelpfulandpracticalplacetostartfrom.Thisresultsinreducedmaintenancetimeandcost.

• WithlowMPPTgranularity(manystringsinparallel)ifonemodulefails,itcanaffecttheoutputofparallelstrings.And,evendiscoveringtheproblemcanbeachallenge,letalonefindingthespecificfailureandfixingit.

• Thereisabigleapintheamountofdatathatisgatheredandprocessedforeachstepmovingbetweenarray,group,stringandmodulelevel.Manyplantsarestartingtousecellularmodemsasthemostreliableformofbackhaulcommunication,buttheeconomicviabilitypartlydependsonkeepingmonthlydatausagelow.

• ThereisalsotheseparateissueaboutthegranularitythatthecompanyresponsibleforO&Mmightwishfor,andthe(usuallymuchlessdetailed)informationtheywishtheendcustomertoseetoavoidexcessivenuisancecallsaboutonePVmoduleoutputpowerbeinglowerthananeighboringmoduleoutput.

• Installedcostisdirectlyaffectedbygranularity.

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ExampleCommercial300kWInstallationFora300kWinstall,300Wmodulesand16modulestrings,we’dhavethefollowing:

• ModuleLevel(microoroptimizersystem)–permodule,0.1%or0.3kWgranularityformonitoringandMPPT

• StringLevel(forexamplesmallstringinverter,HiQTrueString™)–62strings,1.6%or4.8kWgranularityformonitoringandMPPT

• ParalleledStrings(largestringinverter,canbe2-12stringsinparallel)–say21groupswith3stringsparalleledtogether,4.8%or15kWgranularity

• Largegroups(centralinverter,say300kW)–1MPPT,6monitoringzones–17%or50kWmonitoringgranularity

ImplementationsofthesetopologiesaredescribedfurtherinTable1. Power

ConversionBlockSize(Nom.)

MPPT Monitoring ArcDetection

300kWCentral 300kW(100%) 300kW(100%) 50kW(17%) AlmostimpossiblewithoutadditionalAFCIequipment

LargeString 27.6kW(9%) 15kW(5%) 15kW(5%) 15kW(5%)TrueString 8kW(2.7%) 4.8kW(1.6%) 4.8kW(1.6%) 4.8kW(1.6%)Micro/Optimizer

0.25kW(0.08%) 0.3kW(0.1%) 0.3kW(0.1%) Notrequiredfor<80V

Table1:Granularitiesfortheexample300kWsystem

Inthisexample,stringlevelorsub-stringlevelwithmodulesgroupedtogetherwouldmakethemostsenseasacompromisebetweenvisibility,modemchargesandclarity.Thecalculuswouldbeverydifferentfora5MWora5kWsystembuttheideasholdtrue.Thegranularitythatisacceptableinagiveninstallwillvary,startingpointsforeachoftheparametersareshowninTable2. Power

ConversionBlockSize

MPPT Monitoring ArcDetection

SuggestedRanges

1-10% 1-5% 0.5-2% Singlestringlevelorbetter

Table2:Suggestedidealgranularityrangesfordifferentplantparameters

PerspectivefromaCommercialInstallerAsoneexperiencedcommercialinstallerinCaliforniasaidtousrecently:“Ihavenouseformodulelevelmonitoring–ifIwasdoingmostlyresidentialitmightbedifferent.Anythingmodulelevel–Ihavenoneedforit,Idon’twantit…..It’stoomuchdata–Ihatehavingthatmuchdata,Ijustgetburiedinit,ittakestoomuchcellmodembandwidth,andit’sjusttoomuchvisualnoise;Icertainlydon’twantmycustomerslookingatit…..andreliabilitywillbeanissueeventually,too.Stringlevelisgreat.”

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SummaryTherearemanyfactorsthataffecttheidealgranularityofaparticularsystem.Aswithallotheraspectsofsystemdesigntheanswerisacompromisebetweeninstallcost,maintainabilityandmanyotherfactorsaswe’veseen.Theidealgranularitycouldbedifferentforpowerconversionblocksize,MPPT,monitoringandarcdetection,andwe’vesuggestedsomestartingrangesinTable2.

AdvertisingObviouslywebelievethatsinglestringmonitoring,arcdetection,andMPPTforcommercialinstallsmakessomuchsensewedesignedtheHiQSolarTrueString™inverterfamilywiththisinmind.Itiscostcompetitivewithlargestringinverterswhileofferingmuchbettermonitoring,MPPTgranularity(increasedenergyharvest)andsafetyfunctionality.However,theprinciplesdiscussedhereapplybroadly.

Figure3:Individualstringperformancefromacommercialrooftopwitheast/weststrings

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