Taking the Nuisance Tripping out of AFCI Useful PV

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TakingtheNuisanceTrippingoutofAFCI-UsefulChangestoTestingofPVInverters2August2016ContentsIntroduction....................................................................................................................................1CodeRequirements........................................................................................................................2ExistingTestMethodvs.RealWorld..............................................................................................2ChangingtheWayAFCICircuitsareTested....................................................................................3

ImprovingtheTestArcSignature...............................................................................................4NewMethodsofTesting.............................................................................................................5OtherSystemTopologies............................................................................................................7

HiQSolarTrueStringInvertersandAFCI.........................................................................................8CustomerTestingExample.........................................................................................................9

Summary.........................................................................................................................................9

IntroductionTheabilityofaPVinvertertodetectandinterruptseriesarcs(AFCI,orArcFaultCircuitInterrupter)isausefulsafetyfeature.IntheUStheNationalElectricalCode(NFPA70)requiresPVsystemstobecapableofdetectinghighvoltageDCarcs,whetherthroughfunctionalitybuiltintotheinverterorbyothermeans.However,thetestingofthishasbeenacontroversialtopic.1. TheNECrequiresseriesarcdetectionfunctionality,

butdoesnotdictatehowtheAFCIshouldbemeasuredandcertified.UL1699BOOI(OutlineofInvestigation)describessomesuggestedtestmethods,butisnotastandardandisnotdirectlyreferencedbytheNEC.

2. ThetestmethodsdescribedinUL1699Barenotoptimalaswe’llseelater

3. Realinstallationsaresufferingnuisancetripping,whichisleadingtofrustrationandwastedtimeandmoney.SomeinstallationsevenhavetheAFCIfunctionalityswitchedofftopreventsuchissuesfromreoccurring.

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We’lltakeabrieflookattheissueswitharcdetection,andtheprogressbeingmadeinchangingtestmethodstoimprovethesituation.WewillalsolookathowHiQ'sTrueStringinvertersareuniquelyengineeredtosolvetheissuesofAFCIfunctionality.

CodeRequirementsTheNationalElectricalCode(NFPA70)specifiestherequiredfunctionalityofanarc-detectioncapabilityin690.11.Notethatinthe2011editionthisappliedtorooftops,butin2014thiswasextendedtoincludeallarrays,notjustthosemountedonbuildings.690.11Arc-FaultCircuitProtection(DirectCurrent).Photovoltaicsystemswithdcsourcecircuits,dcoutputcircuits,orboth,onorpenetratingabuildingoperatingataPVsystemmaximumsystemvoltageof80voltsorgreater,shallbeprotectedbyalisted(dc)arc-faultcircuitinterrupter,PVtype,orothersystemcomponentslistedtoprovideequivalentprotection.ThePVarc-faultprotectionmeansshallcomplywiththefollowingrequirements:(1)Thesystemshalldetectandinterruptarcingfaultsresultingfromafailureintheintendedcontinuityofaconductor,connection,module,orothersystemcomponentinthedcPVsourceandoutputcircuits.(2)Thesystemshalldisableordisconnectoneofthefollowing:

a.Invertersorchargecontrollersconnectedtothefaultcircuitwhenthefaultisdetected

b.Systemcomponentswithinthearcingcircuit(3)Thesystemshallrequirethatthedisabledordisconnectedequipmentbemanuallyrestarted.(4)Thesystemshallhaveanannunciatorthatprovidesavisualindicationthatthecircuitinterrupterhasoperated.Thisindicationshallnotresetautomatically.ThebasicmethodsusedtoinvestigateproductsinthiscategoryarecontainedinULSubject1699B,“OutlineofInvestigationforPhotovoltaic(PV)DC-Arc-FaultCircuitProtection.”

ExistingTestMethodvs.RealWorldArcfaulttestingmethodsareimportant,andinmanywaysshortcomingsofthemethodshaveleadtothemismatchbetweencomplianceandreliableperformanceinthefield.UL1699BwasanattempttospecifytestingofarcdetectioninPVsystems,whichisnowbeingrevisedaswe’llseelater.Initscurrentform,thetestisnotveryarduousorrepresentativeofreal-worldarcs.ThetestasdescribedinUL1699Busesatestfixturethatappliesahighvoltageacrossasmallpieceofsteelwool.ThearcfixtureisinsertedinserieswiththeDCinputneartheinverterDCinputterminals.SeeFigure1(a).

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Figure1:Testpractice(a)vs.real-worldconditions(b)

Figure1(b)showsthekindofsituationaninverterAFCIsystemmightseeinthefield.Itismountedsomedistancefromthearray(i),andthePVmodulesinthearrayareaseriesstring(ii).Thewiresandmodulesaddsignificantinductanceandcapacitancetothepicture.Thearcmightoccuratanypointinthestringorwiring,theworstcasebeingatthemid-pointofthestring(iii).Itisalsolikelytohavemuchloweramplitudethanthetestarc.Inadditiontoallofthis,theinitiationofthearcisquitedifferentfromanarcinitiatedbyburningthesteelwool(iv&v).Theprimaryissueisthatintherealworld,thearcthattheinverterislookingforwillbeginmuch,muchsmallerwithadifferentinitialsignature.AsarealPVstringpresentssignificantseriesinductanceandcapacitance,thesereactivecomponentscanprovidesubstantialfilteringofthearcsignaturemakingitmoredifficulttodetectwhenanarcispresent.AddtoallofthisthefactthatmostPVsystemsusedincommercialapplicationsconnectstringsinparallel,andtheproblemjustgotevenworse.Thisstartstoexplainwhyinvertersstruggletodetectarcseventhoughtheypassthetest.Thelogicalresponsefrommostinvertercompaniesistoincreasethegainofthedetectioncircuittolookforthosesmallerarcsignatures.However,thisresultsinamuch-increasedlikelihoodoffalsealarms,whilealsomissingthepresenceofrealarcproblemsuntilitistoolate.

ChangingtheWayAFCICircuitsareTestedUL1699BTaskGroup1isworkingonupdatingthetestmethodstobemorerepresentativeofrealworldconditionsandmorerepeatable.Thissectiongivesahighleveloverviewofthedevelopmentofproposalsthatareinprocessandareusefultounderstand.WhilethisisaworkinprocesstheyarecorrectasofJuly2016.

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Problemstobetackled:1. Theoldmethodoftestingresultedinlistedproductsthatdidnotdetectsomearcsin

realworldconditions2. And,somelistedproductssufferedfromunwantednuisancetripping

Objectives:

• Redefinetestmethodandfixturetomorecloselyrepresentreal-worldconditions(discussedhere)

• Developmethodtotestunwantednuisancetripping(notdiscussedhere)

TaskGroupGuidelines

• Testshouldrepresentreal-worldconditions• Testshouldrepresentworst-case• Testmustberepeatable• Shallbetestmethodagnostic

ImprovingtheTestArcSignatureOnechangebeingdiscussedistoalterthewaythearciscreated.Insteadofaburningfilamentofsteelwooltoinitiatethearc,thenewproposalusesaballandringorpairofrodsmountedonatracksuchthatthearcgapcanbeincreasedatacontrolledrate.Theideaisthatthearcbeginsataverylowlevelandincreasesatacontrolledrate,asiscommonintherealworld.Boththeavailablearccurrentandrateofseparationarenowspecified.Incontrast,thesteelwoolmethodisnotveryrepeatable,witharccharacteristicschangingdependinguponthetypeofwoolused,theofwoolquantityandhowitisplacedinthegap.Figure2showstheoldandnewmethods.1.Showstheoldfixturewithsteelwoolinserted.2.Showsthefixturegeneratinganarc.3.Showsthespentfixture.4.Showstheproposedballandring,and5.Showsitgeneratinganarc.

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Figure2:Existing(1-3)andexampleproposed(4-5)arcgenerationfixtures

Theballandringandcopperrodswithacontrolledpull-apartaremorerepresentativeofareal-worldarc,andgivesmorerepeatabletestresults.

NewMethodsofTestingInFigure1(b)wesawthattherealworldismorecomplexthanthetestsetupbeingusedtotestperUL1699BOOI.TheblockdiagramfortheoriginaltestisshowninFigure3(a)andisbeingeliminated.However,thesetupinFigure3(c)isbeingproposedforthetestingofAFCIcircuitsin,forexample,stringinverters;conceptuallyitrelatesbacktoourPVstringascanbeseenin(b).Whilethetestingwillbeharder,theresultswillbemuchimproved,andthetestset-upisdiscussedfurtherbelow.

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Figure3:Legacy(a)andnew(b&c)arctestingapparatus

InFigure3(c)thearctestfixturewillbeplacedateachofthreepositions,designated“Arc”inthediagram.TheseriesinductancerepresentsthewirebetweentheAFCIandthePVstring.Amaximumwirelengthof250feet(80m)wasconsideredreasonable1,andwillprobablyendupbeingthedefault.PVmoduleshavenormalmodecapacitancebetween+and–terminalsandcommonmodecapacitancetoground.Thesecharacteristicscanvarywidelybetweenmoduletypesandinstallations.Groundmountedmoduleshaveverylowcapacitance(ontheorderof150pF);thinfilmmodulescanactuallybeadhesivelyattachedtoastructuresuchasametalbuilding,givingacapacitancethatcanbeseveralnFpermodule.Inaddition,thewiringwillhavecapacitancetogroundthatwillincreasewithwirelength,andishigherwhenthewireisinametalraceway.And,thereiscapacitancebetweenconductorsthatvariesbywirelength,type,andorientation.

1Itisexpectedthatmanufacturerswillbeabletoshortenthisdistancetomakethetaskofcomplianceeasier;however,theywillneedtoprovidearatingofmaximumdistanceinalldocumentationandaccordinglyrestricthowcustomersinstalltheirproductinordertomaintainAFCIcompliance.

High%Voltage%Power%Supply%

Inverter%–%Device%Under%Test%(DUT)%

Ballast%Resistor%%

(As%Needed)%

S1%

Arc%Generator%

(a)%

(c)ProposedTestSetupforStringInverters

Par7alString

Par7alString

DUT

Arc

(b)Concept

Array&lineimpedancesimulator&generator

Networkwithseriesinductance,resistanceandcapacitancetoground

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Arc

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ForinverterswithasinglestringperAFCIcircuitthesituationissimple,andthecomponentvaluesintheelectricalnetworkreflectthis.ForinvertersthatparallelstringsintoasingleAFCIcircuitthecomponentvaluesinthenetworkreflectthis,withmorecapacitance,parallelinductanceandresistanceshroudingthearcsignal,makingthetestcommensuratelyhardertopass.

OtherSystemTopologiesItcouldbearguedthatastringinverterwithAFCIfunctionalityisoneofthemorestraightforwardscenariostotest.Systemtopologiesvarywidely,andtestmethodshavebeencreatedtodealwithAFCIcircuitsembeddedinstringcombinerboxes(seeFigure4),externalAFCIcircuits,andseveraldealingwithdifferentDC/DCoptimizers(anexampleisinFigure5).AFCIcircuitsembeddedinindividualPVmodulesarenotaddressed.Asanexample,theconfigurationofa1MWcentralinverterwithstringcombinersandstringre-combinerswillbeverycomplextotest,andtodesignAFCIcircuitsthatwillpass.

Figure4:Proposedtestset-upforasysteminvolvingstringcombiners

(d)ProposedTestSetupforStringCombiners

Array&lineimpedancesimulator&generator

Arc

Arc

Arc

Arraysimulator

Arraysimulator

DUT

LineImpedanceSimulatorII

“RepresentaBve”Inverter

NetworkSource

NetworkSource

NetworkSource

NetworkSource

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Figure5:Proposedtestset-upforAFCItestingofaDC/DCoptimizersystem

The2017NECcoderemovesallreferencetodetecting,annunciating,andmanualresetrequirements.Itisleftentirelytothesafetystandard,whichwillbeUL1699Bonceitisaffirmativelyballoted.

HiQSolarTrueStringInvertersandAFCIOurTrueStringinverterstartsoffwithsomeadvantagescomparedtocommon3-phasestringinverters.Theseinclude:• TrueStringinvertersare

commonlymountedunderthearray.ThismeansthatthelengthofDCwiringbetweentheinverterandthearrayisminimal,helpingtomakedetectionmorereliable.

• TrueStringinvertershaveindividualstringinputs.Reducingtheproblemtoasinglestringoneachinputinsteadofparallelingmanymakesdetectionmucheasierandmorereliable.

• Ourinvertersweretestedwithboththesteelwoolandballandringtestfixtures.• Ourdetectionmethodisuniqueandemploystimeandfrequencydomainanalysisofthearc

signal.• Thedetectionofarcsisintegratedintotheinverter,whichhelpstominimizenuisance

tripping.WemeetandexceedtherequirementsofNEC690.11andminimizenuisancetrippingwhileremainingfocusedonthedetectionofreal-worldarcs,asillustratedinthenextsection.

(e)ProposedTestSetupforDC/DCOp4mizerSystem

PVModule

Arc

Arc

Arc

Op4mizer

Middleofstring

Inverter

LongRun

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CustomerTestingExampleAgoodillustrationofwhymountingastringinverternexttothestringsitserves,alongwiththebenefitsofanarchitecturewhereeachstringhasit’sowndedicatedAFCIcircuitryisshowninthiscustomer-suppliedexample.Thecustomerwantedtosetupvalidationtestingofagroupofdifferentstringinverters.Thetestset-upusedanarcgeneratoremployingtwocopperrodssteadilydrawnapartasdescribedpreviously;italsoplacedthearcfixtureinthemiddleofastringofPVmodulestobeasreal-worldrepresentativeaspossible.Avarietyofwell-knownstringinvertersweretested,alongwiththeHiQTrueStringinverter.Thepartofthetestset-upwiththetwocopperrodsisshownshowninFigure6.(a)showsaproductbeingtested(notethatitpassedandislistedtothecurrentUL1699BOOI);thearcburnedcontinuouslyuntilthesourcewasremoved,withtheinverterundertestunawareofdangerousarcing.TheseresultswereconsistentexceptfortheHiQproduct.(b)showsanexampletestperformedwiththeHiQTrueString–thearcisquicklydetectedandthestringsafelyshutdownasmandatedpertheNEC.

SummarySignificantimprovementstoAFCItestingarecoming.Theresultwillbemorereliabledetectionofreal-worldarcs,andthereductionoreliminationofnuisancetripping.Nuisancetrippingispartlytheconsequenceoftheoverlysimplistictestmethodrecommendedinthepast.CopyrightHiQSolar,Inc.,2016www.hiqsolar.com

Figure6:ScreenshotsofcustomerAFCItesting