TakingtheNuisance
TrippingoutofAFCI-Useful
ChangestoTestingofPVInverters
2August2016
Contents
Introduction....................................................................................................................................1
CodeRequirements........................................................................................................................2
ExistingTestMethodvs.RealWorld..............................................................................................2
ChangingtheWayAFCICircuitsareTested....................................................................................3
ImprovingtheTestArcSignature...............................................................................................4
NewMethodsofTesting.............................................................................................................5
OtherSystemTopologies............................................................................................................7
HiQSolarTrueStringInvertersandAFCI.........................................................................................8
CustomerTestingExample.........................................................................................................9
Summary.........................................................................................................................................9
Introduction
TheabilityofaPVinvertertodetectandinterruptseries
arcs(AFCI,orArcFaultCircuitInterrupter)isauseful
safetyfeature.IntheUStheNationalElectricalCode
(NFPA70)requiresPVsystemstobecapableofdetecting
highvoltageDCarcs,whetherthroughfunctionalitybuilt
intotheinverterorbyothermeans.However,thetesting
ofthishasbeenacontroversialtopic.
1. TheNECrequiresseriesarcdetectionfunctionality,
butdoesnotdictatehowtheAFCIshouldbe
measuredandcertified.UL1699BOOI(Outlineof
Investigation)describessomesuggestedtest
methods,butisnotastandardandisnotdirectly
referencedbytheNEC.
2. ThetestmethodsdescribedinUL1699Barenotoptimalaswe’llseelater
3. Realinstallationsaresufferingnuisancetripping,whichisleadingtofrustrationandwasted
timeandmoney.SomeinstallationsevenhavetheAFCIfunctionalityswitchedofftoprevent
suchissuesfromreoccurring.
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We’lltakeabrieflookattheissueswitharcdetection,andtheprogressbeingmadeinchanging
testmethodstoimprovethesituation.WewillalsolookathowHiQ'sTrueStringinvertersare
uniquelyengineeredtosolvetheissuesofAFCIfunctionality.
CodeRequirements
TheNationalElectricalCode(NFPA70)specifiestherequiredfunctionalityofanarc-detection
capabilityin690.11.Notethatinthe2011editionthisappliedtorooftops,butin2014thiswas
extendedtoincludeallarrays,notjustthosemountedonbuildings.
690.11Arc-FaultCircuitProtection(DirectCurrent).
Photovoltaicsystemswithdcsourcecircuits,dcoutputcircuits,orboth,onorpenetratinga
buildingoperatingataPVsystemmaximumsystemvoltageof80voltsorgreater,shallbe
protectedbyalisted(dc)arc-faultcircuitinterrupter,PVtype,orothersystemcomponentslisted
toprovideequivalentprotection.ThePVarc-faultprotectionmeansshallcomplywiththe
followingrequirements:
(1)Thesystemshalldetectandinterruptarcingfaultsresultingfromafailureintheintended
continuityofaconductor,connection,module,orothersystemcomponentinthedcPVsource
andoutputcircuits.
(2)Thesystemshalldisableordisconnectoneofthefollowing:
a.Invertersorchargecontrollersconnectedtothefaultcircuitwhenthefaultisdetected
b.Systemcomponentswithinthearcingcircuit
(3)Thesystemshallrequirethatthedisabledordisconnectedequipmentbemanuallyrestarted.
(4)Thesystemshallhaveanannunciatorthatprovidesavisualindicationthatthecircuit
interrupterhasoperated.Thisindicationshallnotresetautomatically.
ThebasicmethodsusedtoinvestigateproductsinthiscategoryarecontainedinULSubject
1699B,“OutlineofInvestigationforPhotovoltaic(PV)DC-Arc-FaultCircuitProtection.”
ExistingTestMethodvs.RealWorld
Arcfaulttestingmethodsareimportant,andinmanywaysshortcomingsofthemethodshave
leadtothemismatchbetweencomplianceandreliableperformanceinthefield.UL1699Bwas
anattempttospecifytestingofarcdetectioninPVsystems,whichisnowbeingrevisedaswe’ll
seelater.Initscurrentform,thetestisnotveryarduousorrepresentativeofreal-worldarcs.
ThetestasdescribedinUL1699Busesatestfixturethatappliesahighvoltageacrossasmall
pieceofsteelwool.ThearcfixtureisinsertedinserieswiththeDCinputneartheinverterDC
inputterminals.SeeFigure1(a).
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ChangestoAFCITesting
V,'I'
2August2016
t'
(v)'
a)'
Inverter'
Wool'
PSU'
V,'I' (iv)'
t'
(ii)'
b)'
Inverter'
(i)'
L'
(iii)'
C'
Figure1:Testpractice(a)vs.real-worldconditions(b)
Figure1(b)showsthekindofsituationaninverterAFCIsystemmightseeinthefield.Itis
mountedsomedistancefromthearray(i),andthePVmodulesinthearrayareaseriesstring
(ii).Thewiresandmodulesaddsignificantinductanceandcapacitancetothepicture.Thearc
mightoccuratanypointinthestringorwiring,theworstcasebeingatthemid-pointofthe
string(iii).Itisalsolikelytohavemuchloweramplitudethanthetestarc.Inadditiontoallof
this,theinitiationofthearcisquitedifferentfromanarcinitiatedbyburningthesteelwool(iv
&v).
Theprimaryissueisthatintherealworld,thearcthattheinverterislookingforwillbegin
much,muchsmallerwithadifferentinitialsignature.AsarealPVstringpresentssignificant
seriesinductanceandcapacitance,thesereactivecomponentscanprovidesubstantialfiltering
ofthearcsignaturemakingitmoredifficulttodetectwhenanarcispresent.Addtoallofthis
thefactthatmostPVsystemsusedincommercialapplicationsconnectstringsinparallel,and
theproblemjustgotevenworse.
Thisstartstoexplainwhyinvertersstruggletodetectarcseventhoughtheypassthetest.The
logicalresponsefrommostinvertercompaniesistoincreasethegainofthedetectioncircuitto
lookforthosesmallerarcsignatures.However,thisresultsinamuch-increasedlikelihoodof
falsealarms,whilealsomissingthepresenceofrealarcproblemsuntilitistoolate.
ChangingtheWayAFCICircuitsareTested
UL1699BTaskGroup1isworkingonupdatingthetestmethodstobemorerepresentativeof
realworldconditionsandmorerepeatable.Thissectiongivesahighleveloverviewofthe
developmentofproposalsthatareinprocessandareusefultounderstand.Whilethisisawork
inprocesstheyarecorrectasofJuly2016.
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Problemstobetackled:
1. Theoldmethodoftestingresultedinlistedproductsthatdidnotdetectsomearcsin
realworldconditions
2. And,somelistedproductssufferedfromunwantednuisancetripping
Objectives:
• Redefinetestmethodandfixturetomorecloselyrepresentreal-worldconditions
(discussedhere)
• Developmethodtotestunwantednuisancetripping
(notdiscussedhere)
TaskGroupGuidelines
• Testshouldrepresentreal-worldconditions
• Testshouldrepresentworst-case
• Testmustberepeatable
• Shallbetestmethodagnostic
ImprovingtheTestArcSignature
Onechangebeingdiscussedistoalterthewaythearciscreated.Insteadofaburningfilament
ofsteelwooltoinitiatethearc,thenewproposalusesaballandringorpairofrodsmountedon
atracksuchthatthearcgapcanbeincreasedatacontrolledrate.Theideaisthatthearcbegins
ataverylowlevelandincreasesatacontrolledrate,asiscommonintherealworld.Boththe
availablearccurrentandrateofseparationarenowspecified.Incontrast,thesteelwool
methodisnotveryrepeatable,witharccharacteristicschangingdependinguponthetypeof
woolused,theofwoolquantityandhowitisplacedinthegap.Figure2showstheoldandnew
methods.1.Showstheoldfixturewithsteelwoolinserted.2.Showsthefixturegeneratingan
arc.3.Showsthespentfixture.4.Showstheproposedballandring,and5.Showsitgenerating
anarc.
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Figure2:Existing(1-3)andexampleproposed(4-5)arcgenerationfixtures
Theballandringandcopperrodswithacontrolledpull-apartaremorerepresentativeofarealworldarc,andgivesmorerepeatabletestresults.
NewMethodsofTesting
InFigure1(b)wesawthattherealworldismorecomplexthanthetestsetupbeingusedtotest
perUL1699BOOI.TheblockdiagramfortheoriginaltestisshowninFigure3(a)andisbeing
eliminated.However,thesetupinFigure3(c)isbeingproposedforthetestingofAFCIcircuits
in,forexample,stringinverters;conceptuallyitrelatesbacktoourPVstringascanbeseenin
(b).Whilethetestingwillbeharder,theresultswillbemuchimproved,andthetestset-upis
discussedfurtherbelow.
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S1%
(a)%
Ballast%
Resistor%%
(As%Needed)%
Arc%
Generator%
Inverter%–%
Device%Under%
Test%(DUT)%
High%Voltage%
Power%Supply%
(b)Concept
Arc
(c)ProposedTestSetupforStringInverters
Arc
DUT
Par7al
String
Networkwithseries
inductance,resistanceand
capacitancetoground
DC
Source
Arc
Arc
Par7al
String
DUT
DC
Source
(Asabove)
Arc
Arc
Array&lineimpedancesimulator&generator
Figure3:Legacy(a)andnew(b&c)arctestingapparatus
InFigure3(c)thearctestfixturewillbeplacedateachofthreepositions,designated“Arc”in
thediagram.
TheseriesinductancerepresentsthewirebetweentheAFCIandthePVstring.Amaximumwire
lengthof250feet(80m)wasconsideredreasonable1,andwillprobablyendupbeingthe
default.PVmoduleshavenormalmodecapacitancebetween+and–terminalsandcommon
modecapacitancetoground.Thesecharacteristicscanvarywidelybetweenmoduletypesand
installations.Groundmountedmoduleshaveverylowcapacitance(ontheorderof150pF);thin
filmmodulescanactuallybeadhesivelyattachedtoastructuresuchasametalbuilding,givinga
capacitancethatcanbeseveralnFpermodule.Inaddition,thewiringwillhavecapacitanceto
groundthatwillincreasewithwirelength,andishigherwhenthewireisinametalraceway.
And,thereiscapacitancebetweenconductorsthatvariesbywirelength,type,andorientation.
1
Itisexpectedthatmanufacturerswillbeabletoshortenthisdistancetomakethetaskofcompliance
easier;however,theywillneedtoprovidearatingofmaximumdistanceinalldocumentationand
accordinglyrestricthowcustomersinstalltheirproductinordertomaintainAFCIcompliance.
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ForinverterswithasinglestringperAFCIcircuitthesituationissimple,andthecomponent
valuesintheelectricalnetworkreflectthis.ForinvertersthatparallelstringsintoasingleAFCI
circuitthecomponentvaluesinthenetworkreflectthis,withmorecapacitance,parallel
inductanceandresistanceshroudingthearcsignal,makingthetestcommensuratelyharderto
pass.
OtherSystemTopologies
ItcouldbearguedthatastringinverterwithAFCIfunctionalityisoneofthemore
straightforwardscenariostotest.Systemtopologiesvarywidely,andtestmethodshavebeen
createdtodealwithAFCIcircuitsembeddedinstringcombinerboxes(seeFigure4),external
AFCIcircuits,andseveraldealingwithdifferentDC/DCoptimizers(anexampleisinFigure5).
AFCIcircuitsembeddedinindividualPVmodulesarenotaddressed.
Asanexample,theconfigurationofa1MWcentralinverterwithstringcombinersandstringrecombinerswillbeverycomplextotest,andtodesignAFCIcircuitsthatwillpass.
(d)ProposedTestSetupforStringCombiners
Arc
Array&line
impedance
simulator&
generator
Source
Network
Arc
Source
Network
DUT
Arc
Array
simulator
Source
“RepresentaBve”
Inverter
Network
Line
Impedance
SimulatorII
Array
simulator
Source
Network
Figure4:Proposedtestset-upforasysteminvolvingstringcombiners
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(e)ProposedTestSetupforDC/DCOp4mizerSystem
PVModule
Arc
Op4mizer
Arc
Inverter
Arc
Middleof
string
LongRun
Figure5:Proposedtestset-upforAFCItestingofaDC/DCoptimizersystem
The2017NECcoderemovesallreferencetodetecting,annunciating,andmanualreset
requirements.Itisleftentirelytothesafetystandard,whichwillbeUL1699Bonceitis
affirmativelyballoted.
HiQSolarTrueStringInvertersandAFCI
OurTrueStringinverterstartsoff
withsomeadvantagescompared
tocommon3-phasestring
inverters.Theseinclude:
• TrueStringinvertersare
commonlymountedunderthe
array.Thismeansthatthe
lengthofDCwiringbetween
theinverterandthearrayis
minimal,helpingtomake
detectionmorereliable.
• TrueStringinvertershave
individualstringinputs.
Reducingtheproblemtoa
singlestringoneachinputinsteadofparallelingmanymakesdetectionmucheasierand
morereliable.
• Ourinvertersweretestedwithboththesteelwoolandballandringtestfixtures.
• Ourdetectionmethodisuniqueandemploystimeandfrequencydomainanalysisofthearc
signal.
• Thedetectionofarcsisintegratedintotheinverter,whichhelpstominimizenuisance
tripping.
WemeetandexceedtherequirementsofNEC690.11andminimizenuisancetrippingwhile
remainingfocusedonthedetectionofreal-worldarcs,asillustratedinthenextsection.
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CustomerTestingExample
Agoodillustrationofwhymounting
astringinverternexttothestrings
itserves,alongwiththebenefitsof
anarchitecturewhereeachstring
hasit’sowndedicatedAFCIcircuitry
isshowninthiscustomer-supplied
example.Thecustomerwantedto
setupvalidationtestingofagroup
ofdifferentstringinverters.
Thetestset-upusedanarc
generatoremployingtwocopper
rodssteadilydrawnapartas
describedpreviously;italsoplaced
thearcfixtureinthemiddleofa
stringofPVmodulestobeasrealworldrepresentativeaspossible.A
varietyofwell-knownstring
invertersweretested,alongwith
theHiQTrueStringinverter.
Thepartofthetestset-upwiththe
twocopperrodsisshownshownin
Figure6.(a)showsaproductbeing
tested(notethatitpassedandis
listedtothecurrentUL1699BOOI);
thearcburnedcontinuouslyuntil
thesourcewasremoved,withthe
Figure6:ScreenshotsofcustomerAFCItesting
inverterundertestunawareof
dangerousarcing.TheseresultswereconsistentexceptfortheHiQproduct.(b)showsan
exampletestperformedwiththeHiQTrueString–thearcisquicklydetectedandthestring
safelyshutdownasmandatedpertheNEC.
Summary
SignificantimprovementstoAFCItestingarecoming.Theresultwillbemorereliabledetection
ofreal-worldarcs,andthereductionoreliminationofnuisancetripping.Nuisancetrippingis
partlytheconsequenceoftheoverlysimplistictestmethodrecommendedinthepast.
CopyrightHiQSolar,Inc.,2016
www.hiqsolar.com
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