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
DUT
Arc
DCSource
DCSource (Asabove)
Arc
Arc Arc
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
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