Arc Flash Hazards on Photovoltaic Arraysprojects-web.engr. to Partially Fulfill the Requirements of ECE 402 ... Guide to Performing ArcFlash Hazard Calculations (IEEE Standard 1584) ...

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  • ArcFlashHazardsonPhotovoltaicArrays

    EndofSemesterReport,Spring2013

    DavidSmithPreparedtoPartiallyFulfilltheRequirementsofECE402

    DepartmentofElectricalandComputerEngineering

    ColoradoStateUniversityFortCollins,Colorado80523

    ProjectAdvisor:Dr.GeorgeCollins

    IndustrySponsor:DohnSimms,PraxisCorporation

    Approvedby:DrGeorgeCollins

  • AbstractForthesecondsemesterofmyseniorproject,Ichosetofocusonthesubjectofarcflashhazardsinutilityscalephotovoltaicarrays.Asanelectrician,thissubjectcombinesmyinterestsinelectricalworkersafetyandthecontinuedrapidpaceofthedevelopmentofalternativemethodsofgeneratingelectricity.ThissubjectisofgreatpracticalinteresttoFacilityandProjectManagerswhoareresponsibleforthesafetyoftheirworkersbutmustalsobalancethisconcernwiththeneedtomaintainareliablesystemwithminimaldowntime.Anarcflashisasustainedarcingcurrentthatpropagatesthroughtheconductiveplasmacreatedbythebreakdownofagaseousdielectricmedium,typicallyair.Giventherightconditions,thecurrentwillcontinuetoflowunabateduntilinterruptedbyanupstreamovercurrentprotectivedevice.Sucharcsreleaseenormousenergy,resultinginhightemperatures,soundlevels,pressuresandtheejectionofhighspeedmoltendebris.Muchoftheexistingresearchintopredictingarcflashenergiesisspecifictoalternatingcurrent.TheempiricallyderivedequationsintheInstituteofElectricalandElectronicsEngineersGuidetoPerformingArcFlashHazardCalculations(IEEEStandard1584)appliesonlytoarcsinACsystems.Currently,noconsensusstandardexistsforcalculatingarcenergiesinDCsystems.However,the2012editionoftheNationalFireProtectionAssociationsStandardforElectricalSafetyintheWorkplace(NFPA70E)referencestwopapersthatoffertheoreticalandsemiempiricalmethodsforestimatingDCarcenergy.ThisreportpresentstheresultsobtainedinperformingacomparativeanalysisofthesetwomethodsforpredictingDCarcenergyasappliedtoanexisting30MWphotovoltaicarray.Itshouldbenotedthatnohighpowerempiricaltestingwasperformedaspartofthisproject;henceallvaluesprovidedherearepurelytheoreticalandspecifictotheSanLuisSolarFarminAlamosa,Colorado.Ingeneral,faultcurrentsonthearrayarequitelowcomparedtoACsystems,andhencearcenergiesarecorrespondinglylowintheshorttimescale.However,clearingfaultsonthearraywithtraditionaltimeovercurrentdevicesisproblematic,andundercertainconditionsitmaybepossibleforfaultstopersistformuchlongerthanthe2secondmaximumoftenassumedinarcflashstudies.At2seconds,thecalculationmethodsutilizedreturnedvaluesashighas17and8calories/cm2,respectively,highenoughtopresentasignificanthazardtoelectricalworkers.

  • ContentsAbstract.........................................................................................................................................................2

    Figures...........................................................................................................................................................3

    Introduction..................................................................................................................................................4

    SystemShortCircuitCurrent........................................................................................................................4

    FaultClearingDevices...................................................................................................................................6

    ArcingCurrentandFaultClearingTime........................................................................................................9

    ArcLength...................................................................................................................................................11

    IncidentEnergy...........................................................................................................................................13

    EnclosureMultiplyingFactor......................................................................................................................14

    ComparisonofValuesbetweenMethods..................................................................................................15

    Conclusions.................................................................................................................................................17

    FutureWork................................................................................................................................................18

    References..................................................................................................................................................19

    Acknowledgements.....................................................................................................................................19

    AppendixA:Abbreviations.........................................................................................................................20

    AppendixB:ProjectCosts..........................................................................................................................20

    AppendixC:AdditionalMaterials...............................................................................................................21

    FiguresFigure1:SunpowerModuleElectricalData..................................................................................................5Figure2:GroundFault..................................................................................................................................7Figure3:GroundFaultwithPreexistingFault.............................................................................................8Figure4:PositivetoNegativeFault..............................................................................................................8Figure5:ArcEquivalentCircuit.....................................................................................................................9Figure6:SystemforImplementingAmmermanEquation(Zg=ArcGap=90mm)....................................10Figure7:TimeCurrentCurveofInverterFuse............................................................................................10Figure8:ArcPowerasFunctionArcCurrent..............................................................................................11Figure9:ArcResistanceasaFunctionofArcGap......................................................................................12Figure10:CombinerBoxArcGap...............................................................................................................12Figure11:'k'asaFunctionofEnclosureSize..............................................................................................14Figure12:'a'asaFunctionofEnclosureSize..............................................................................................15Figure13:IEValues,MPPandAmmerman................................................................................................16Figure14:SunpowerPVModule.................................................................................................................21Figure15:InverterFuseCurve....................................................................................................................22Figure16:StringFuseCurve........................................................................................................................23

  • IntroductionWiththegrowthinrecentyearsofutilityscalesolarphotovoltaicfarmscommonlyarrangedinarraysthatproducebetween600and1000voltsDC,itisbecomingincreasinglyimportanttoexplorewhetherdangerousarcscandevelopontheDCbus,andifso,toevaluatemethodsforpredictingtheenergiesworkersmightbeexposedto.Freeburningarcsofthesortthatcanbeinadvertentlystruckbyanelectricalworkerareverycomplex,difficulttoaccuratelymodelandareevaluatedfromablackboxperspectiveinthispaper.ThetwomethodsusedtopredictarcenergyontheSanLuisSolarFarm,bothcitedinthe2012versionoftheNFPA70E,are:

    MaximumPowerMethod,developedbyDanDoaninhis2007paperArcFlashCalculationsforExposurestoDCSystems[1]

    Semiempiricalmethod,developedbyDr.RavelAmmerman,etal,intheir2009paperDCArcModelsandIncidentEnergyCalculations[2]

    ThemaximumpowermethodisessentiallyaDCversionofLeesmethod[3],andreliesonthepremisethatthemaximumpowertransferredtoaresistiveloadthearcresistanceinthiscaseoccurswhentheloadresistanceisequaltothatofthesource.Theoretically,thismethodwillprovideworstcasevalues,butitisarelativelysimpleapproachthatcomputesarcenergyasafunctionofthesystemvoltageandavailableboltedfaultcurrentonly.TheAmmermanmethodisbasedonareviewofhistoricallaboratorydatafromseveraldisparatestudiesspanningmorethanacentury.Wherethesedatasetscouldbereasonablycompared,goodagreementwasgenerallyfoundintheVoltAmpcharacteristicoftheDCarc.Anequationdescribingthenonlinearnatureofarcresistancewasdevelopedfromoneofthemostextensivesetsofdata,andistheequationusedthroughoutthispaper.Unlikethemaximumpowermethod,thisrelationshipincorporatesdifferencesinarclengthandoffersawaytopredictthearcresistanceandhencethearcingcurrent.

    SystemShortCircuitCurrentAsinanyarcflashstudy,thefirststepisbuildinganaccuratemodelofthesystemandcomputingtheavailableshortcircuitcurrentatallworkeraccessiblenodesthatwouldrequireanarcflashhazardlabel.TheSanLuisfacilityiscomposedofover100,000320Wattmodulesarrangedinseriesstringsofthirteen,withbetween222and225stringsfeeding38inverters.TheinvertersaregalvanicallyisolatedpowerelectronicdevicesthatshouldnotbecapableofsupplyingpowerfromtheACgridtotheDCbus.Hencewehavetreatedthefarmas38essentiallyidentical,isolatedarrays.Theonlydifferencebetweentheseindividual

  • arraysisthenumberofstringsandsomesmalldifferencesinconductorlengthduetotheplacementoftheinvertersrelativetotheirassociatedstrings.Wehavefocusedon1INVA(222strings)and19INVB(225strings)inordertocapturethefullrangeofcurrentsavailableonthefarm.TheVoltAmpcurveofaphotovoltaicmoduleisnonlinear,andthecellwillactaseitheraconstantcurrentoraconstantvoltagesourcedependingonregionofoperation,whichisdeterminedbytheloadresistance.Themaximumpowerthatcanbeprovidedbythemoduleoccursatthetransitionbetweenthetworegionsandisthepointatwhichthemaximumpowerpointtrackingsystemwillattempttoholdthearray.Current,andvoltagetoamuchlesserextent,dependsonsolarirradianceandtemperature.StandardTestConditionsformeasuringamodulesVoltAmpcharacteristicareasolarirradianceof1000Watts/m2and25Celsius.Themanufacturerdataforthe320WattSunpowermodulesusedontheSanLuisfarmisshownbelow:

    Figure1:SunpowerModuleElectricalData

  • Unfortunately,solarirradianceisnotconstant,noris1000Watts/m2amaximumpossiblevalue,sopredictingthecurrentthatmaybeflowinginthesystematfaultinceptionisdifficult.Intheinterestsofprovidingaworstcasevalue,ahighirradiancefactorof125%hasbeenappliedtoallcomputedcurrentsinthesystem.Thiscanbeaproblematicstrategywhenfaultsareclearedbytimeovercurrentdevices,sinceahigherfaultcurrentmayindicateafasterclearingtimeandhencealowerarcenergy.However,aswillbediscussedlater,faultsclearedbytimeovercurrentdevicesonthisfarmwilllikelybeofsuchlongdurationthatthiswillnotbeameaningfulissue.Busvoltageisdeterminedbythe13seriesmodulesand,assumingthefarmisoperatinganywherenearthemaximumpowerpoint,willstaywithinarelativelynarrowbandoveralargerangeofsolarirradiancevalues.Further,lowimpedancefaultsinthearraywillsagthevoltage,andpushtheoperatingpointbackintotheconstantcurrentregionoftheVoltAmpcurve.Hence,thevaluesforcalculationsthroughoutthispaperarebasedonabusvoltageof711V(54.7Volts*13modules)and125%oftheshortcircuitcurrentratingofnmodules,dependingonthelocation.Accountingforconductorresistance,therangeofvaluesare1,571AatCombinerBox1A11,and1,734AandInverter19INVB.

    FaultClearingDevicesUnlikefaultsinothertypesofsystems,particularlyACsystemswithrotatingmachineswherefaultcurrentsaretypicallyseveraltimesthenominaloperatingcurrent,theavailablefaultcurrentinaphotovoltaicarrayislimitedbytheamountofsolarradiationthatiscurrentlyfallingonthemodules,andthiscurrentwillvaryverylittleoverawiderangeofoperatingvoltages.Hencethefaultcurrentsinthesystemareessentiallyredirectedoperatingcurrents,andtraditionaltimeovercurrentdevicesmaynotclearfaultsquicklyoratall.OntheSanLuisfarm,thereare10Afusesinthecombinerboxesattheendofeach13modulestringand315Afusesintheinvertersattheendofeachfeederfromthecombinerboxes.ThepositivepoleofthearrayisgroundedthroughcalibratedresistorsaspartoftheGroundFaultProtectiveDevice(GFPD).Ariseinvoltageacrosstheseresistors,indicatingincreasedcurrentflowaboveasmall,constantleakagecurrent,willtriggerthedevice,rapidlyopeningthearraysconnectiontogroundandtotheACgrid.TheGFPDshouldoperateforanyrelativelylowimpedancefault,aslongasthefaultcurrentisinexcessofthenormalleakagecurrentthatflowsfromthepositivepoletogroundthroughtheGFPDresistors.Figure1belowshowsthefaultcurrentpathintheeventofagroundfault:

  • Figure2:GroundFault

    Inthiscase,thecurrentwillflowfromthegroundedpositivepoletothefaultpoint,whichhasbeenforcedtoalowerpotential,openingtheGFPDandtheDCcontactorintheinverter,leavingnoavailablepathandclearingthefault.However,inhis2012paper,TheGroundFaultProtectionBlindSpot:AsafetyConcernForLargerPhotovoltaicSystemsInTheUnitedStates[4],BillBrooksidentifiedascenariounderwhichtheGFPDmightfailtoclearagroundfault,andpostulatedthatthiswasthecauseoftworecentfiresatlargesolararrays.Inthisscenario,ahighimpedancegroundfaultcoulddevelopandpersistindefinitelyifthecurrentwasbelowtheleakagecurrentpreviouslyreferredto.Nowifalowimpedancegroundfaultweretooccur,theGFPDwouldtripasbefore,butthepreexistinghighimpedancepathcoulddevelopintoapathcapableofsupplyingmuchoftheavailablecurrenttosecondfaultpoint.ThisscenarioisshowninFigure2below,wherepointsAandBindicatedifferentpossiblelocationsforthepreexistingfault.IfatpointA,thesecondfaultwouldlikelytripthestringfuseandbeclearedrelativelyquickly.AtpointB,theclearingdevicewouldbetheinverterfusewhichrequiresmorecurrentthanisavailableinthesystemtooperatequickly:

  • Figure3:GroundFaultwithPreexistingFault

    Finally,inthecaseofapositivetonegativefault,theGFPDshouldindeedtrip,butthereisnoreasonwhythearraycouldnotcontinuetosupplycurrenttoanarcingfaultasitsload.Inthiscase,theinverterfusewouldagainbetheclearingdevice:

    Figure4:PositivetoNegativeFault

  • ArcingCurrentandFaultClearingTimeAspreviouslymentioned,thelackofsignificantcurrentriseduringafaultonaphotovoltaicarraypresentsaproblemwhenthefaultmustbeclearedbyatimeovercurrentdevice.Evenassumingaboltedfaultattheinverterterminalswiththehighestavailablecurrent,1734A,thetimecurrentcharacteristicsoftheinverterfusearesuchthatitwouldrequireapproximately4.5secondstoopen.Sincethetotalenergyinthearcincreaseslinearlywithtime,thisverylongfaultclearingtimecanleadtodangerouslyhighincidentenergies.Accuratelypredictingthepotentialenergyaworkermightbeexposedtorequirestheabilitytoestimatethearcimpedance,andhencethearcingcurrent.Thisistheprimarydrawbackofusingthemaximumpowermethod.Sinceitisassumedthatthesourceresistanceisequaltothearcresistance,thevoltagedividerequationfortheequivalentcircuitatthefaultpointreducestotheequationbelow.Now,themaximumpossiblepowerinthearcisfoundsimplyastheproductofthecurrentthroughthearcandthevoltageacrossit,andtheclearingtimewouldnecessarilybecomputedbasedonanarcingcurrentofonehalftheboltedfaultcurrent,sincethatistheonlyinformationavailablewhenusingthismodel.

    Figure5:ArcEquivalentCircuit

    Inreality,thenonlinearnatureofthearcVIcharacteristicmakesthesituationmorecomplicated.InhisthoroughsurveyofhistoricallaboratorydataonDCarcs,Dr.Ammermanandhiscolleaguesobservedgoodagreementbetweenthedatasetsthatcouldreasonablybecompared,anddescribedanequationfortheVoltAmpcharacteristicofthearc:

    20 0.534 . #7 2 , Inconjunctionwiththevoltagedividerequationfortheequivalentcircuit,thisequationprovidesasystemoftwoequationsintwounknowns.AnumericalsolutiontothissystemcanbefoundbyimplementingNewtonsMethod,buttodoso,theequationsmustfirstbeexpressedasfunctionsofourtwounknowns,arcvoltage(Va)andarcresistance(Ra).

  • Dependingonanappropriateguessfortheinitialvaluesoftheunknowns,thesolutionwillconvergerapidly:

    Figure6:SystemforImplementingAmmermanEquation(Zg=ArcGap=90mm)

    Usingthismethodatthenodesinthearraywiththehighestandlowestavailableboltedfaultcurrentyieldsarcingcurrentvaluesbetween67%and84%oftheboltedfaultcurrent.Assumingtheinverterfuseistheclearingdevice,thesevaluesresultinextremelylongclearingtimesof21to30seconds.Themaxpowermethod,witharcingcurrentassumedtobe50%oftheboltedfaultcurrent,wouldreturnclearingtimesofover100seconds,aworstcasescenariosoimpreciseastobeofverylimitedutility.TheAmmermanrelationshipprovidesanarcresistancebasedonempiricaldatathatislikelytobeclosertotheactualvalue.

    Figure7:TimeCurrentCurveofInverterFuse

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  • ArcLengthAnotheradvantageprovidedbyDr.Ammermansworkoverthemaxpowermethodistheabilitypredictarccharacteristicsbasedontheestimatedarclength.AsDr.Ammermanobserved,thevoltageacrossanarcisalmostindependentofthearcingcurrentaboveacertaintransitioncurrent,andisproportionaltothelengthofthearc.Thearcresistancewillincreasewithlength,andhencepowerisgreaterinalongerarc.Plotsofarcpowerasafunctionofarcingcurrentareshownbelowfordifferentarcgapsinsidethemaximumpowerenvelopeasgivenbyafixedsystemvoltageandresistance:

    Figure8:ArcPowerasFunctionArcCurrent

    Thesameinformationpresentedintermsofarcresistanceshowshowthepowerinthearcincreasessignificantlywitharclength:

  • Figure9:ArcResistanceasaFunctionofArcGap

    Whiletheactualarclengthisnotexactlythelengthofthegapbetweentheelectrodes,particularlyforhorizontalarcs,thisgapcanatleastapproximatethelengthofthearcandshouldbeaccuratelymeasuredgiventherelativelylargechangesinpowerfordifferentarclengths.InthecombinerboxesontheSanLuisFarm,thegapbetweenthepositiveandnegativeterminalsisapproximately90mm,muchlargerthanthe32mmthatisoftenassumedforACequipmentinthesamevoltageclass:

    Figure10:CombinerBoxArcGap

  • IncidentEnergyOncethearccurrentandarcingtimehasbeenfound,itisarelativelysimplemattertocomputethetotalenergyinthearc.Then,thearccanbetreatedasapointsourceofradiantenergyandtheenergyincidentonasphericalsurfaceperunitareacanbefoundasafunctionofdistance(D)fromthearc.ThemaximumpowermethodandtheAmmermanmethodbothusethisprinciple.Combiningequations10through12inDr.Ammermanspaper,wecanwritethefollowingequationtoobtainthesphericalenergydensityofthearcinJoulesperunitofsurfacearea:

    4 4

    TheequationsgivenintheNFPA70EStandardforfindingincidentenergyusingthemaximumpowermethodare:

    0.5

    0.01

    Thisgivestheincidentenergyatthemaximumpowerpointincaloriesperunitofsurfacearea,andcanbewrittenintermsofthesystemquantitiesas:

    0.005 0.5 0.5

    AsWalshandFontaine[3]havenoted,the0.005factorgivenintheNFPA70Eequationsisroundedupfrom0.004755,andwecanseethattheAmmermanandmaxpowerequationsareidenticalwhencorrectedforthedifferentunitsofenergy(1calorie=4.184Joules):

    14

    14

    14.184 0.004755

    Hencedifferencesinincidentenergyvaluesreturnedbyeachmethodaredueentirelytotheuseofhistoricaldatathatdescribesthenonlinearnatureofarcresistance.

  • EnclosureMultiplyingFactorTheequationsabovewillyieldanestimatefortheenergyradiatedequallyinalldirectionsbyanarcinopenair.However,wearemostinterestedarcsthatmightdevelopinthecombinerboxesandattheinverterterminals,asthesearethesystemnodeswheretechniciansaremostlikelytoberequiredtoperformworkonenergizedparts.TestingperformedbytheIEEE1584workinggroup[6]onACarcsdemonstratedthattheenergyofanarcinanenclosurewillreflectofftheinteriorsurfaces,resultinginhigherincidentenergyattheopeningoftheenclosure.AswrittenintheNFPA70EStandard[7],themaxpowermethodsimplysuggeststheopenairincidentenergybeincreasedbyafactorofthreeforarcsinenclosuresofanysize.However,usingthe1584dataforthethreeenclosuresizesthatweretested,RobertWilkinsdevelopedanequationforestimatingthearcenergyinanenclosure[5]:

    WilkinscomputedtheoptimumvaluesoftheconstantsaandksuchthatEboxmatchedthemeasuredvaluesforthethreeenclosures.AvariableReffcanbedefinedastheradiusofadiskwhichhasthesameareaastheinnersurfaceareaoftheenclosure.Thisprovidesdatapointsforeachofthethreeenclosuresizestestedbythe1584workinggroup:(Reff,a)and(Reff,k).FollowingtheworkofWalshandFontaine[8],wecanusealeastsquaresapproximationtoplotaandkasafunctionofReffinordertofindanenergymultiplyingfactorforenclosuresizesotherthanthosetested:

    Figure11:'k'asaFunctionofEnclosureSize

  • Figure12:'a'asaFunctionofEnclosureSize

    ThecombinerboxesontheSanLuisfarmhaveaninteriorsurfaceareaof2,731in2foranReffof748.9mm,correspondingtoa=519.84andk=0.35557.Hence,againfromWalshandFontaine,wecanfindthemultiplyingfactorfortheconcentratingeffectsofthecombinerboxas:

    4 1

    1.95

    ComparisonofValuesbetweenMethodsFinally,withallsystemdatacollectionandforegoinganalysiscomplete,wecancomparethevaluesreturnedbyeachmethodontheSanLuisfarm.Whilevalueswerecomputedfortheentirefarm,thetablebelowshowsvaluescomputedatthetwoarrayswheretheavailableboltedfaultcurrentwashighest(1INVA)andlowest(1INVB).TheavailableboltedfaultcurrentwasobtainedbybuildingamodelofthearraysusingESAsEasyPowersoftware,whichwillcomputecurrentsthroughoutthesystembasedonthelengthandDCresistanceoftheconductors.Aspreviouslydiscussed,thesecurrentshavebeenincreasedbya125%highirradiancefactor,andtheclearingdeviceforallfaultsisassumedtobetheinverterfuse.FaultclearingtimesweredeterminedbybuildingtimecurrentcharacteristiccurvesintheESAsoftwarebasedondataprovidedbytheinverterfusemanufacturer.ArcingcurrentandincidentenergyvalueswerecomputedusingtheequationsandmethodsdescribedaboveusingcustomMatlabfunctions.Intheinterestsofcomparingthesimplifiedmaxpower

  • methodasgivenintheNFPA70EwiththemoredetailedanalysisprovidedbytheworkofAmmerman,WilkesandWalshandFontaine,theincidentenergiesforenclosuresreflectmultiplyingfactorsof3and1.95,respectively:

    Figure13:IEValues,MPPandAmmerman

    Incomparingtheopenairvaluesatthe2secondclearingtime,weseethattheAmmermanmethodreturnsvaluesofapproximately70%ofthoseyieldedbythemaxpowermethod.Whilethisiscertainlynotadramaticdifference,itissignificant,anditjustsohappensthatthethesevaluesareonthedividinglinebetweenaHazardCategory1(PPEratedatminimumof4cal/cm2)andaHazardCategory2(PPEratedatminimumof8cal/cm2).Whenweapplytherespectiveenclosuremultiplyingfactorstotheopenairvaluesat2seconds,thevaluesdivergeevenmore,withthemoredetailedmethodreturningvaluesofapproximately45%ofthemaxpowermethod.Andfinally,whenweaccountfortheactualclearingtimeasdeterminedbytheinverterfuse,thedifferencebetweenmethodsisasmuchasafactoroften.Theextremelyhighincidentenergyvaluesreturnedbythemaxpowermethodwhendeterminingtheclearingtimefromonehalfoftheboltedfaultcurrentareessentiallymeaningless,sincethereisnoevidencetosuggestthatthethisistheactualarcingcurrent.However,takingthearcingcurrentreturnedbytheAmmermanmethodasamoreaccurateguess,westillfindverylongclearingtimes,resultinginincidentenergyvaluesabove100cal/cm2atsomenodes,whichisgreaterthanthehighestratedflashsuitavailable.

  • SectionB.1.2inAnnexBoftheIEEE1584[5]Standardallowsfortheuseof2secondsasthemaximumclearingtime,basedentirelyontheideathatapersonexposedtoanarcflashwillmoveawayquicklyifitisphysicallypossible,andthisprincipleisgenerallyacceptedthroughouttheindustry.However,asnotedinthe1584Standard,apersonwhoisinabuckettruckorapersonwhohascrawledintoequipmentwillrequiremoretimetogetaway.Thereareatleasttwosignificantproblemswiththisprinciple:first,howdowereasonablyquantifyhowmuchmoretimeisenoughtimeforworkerswhoareworkinginanonoptimalposition?Second,itisnotuncommonforvictimsofarcflashesandblaststobeknockedunconsciousandhencebeincapableofmovingawayfromthearc.Further,ifsystemconditionsarefoundtobesuchthatclearingtimesarelikelytobeinexcessof2seconds,thisisaseriousdesigndeficiencythatcouldresultincatastrophicdamagetoequipmentaswellasposingahazardtoworkers.

    ConclusionsPotentialincidentenergyontheDCbusoftheSanLuisSolarFarmatworkeraccessiblenodesascomputedbythetwomethodsishighenoughtomeritconcern,andinsomecasesmaybeabovethelevelthatcanrealisticallybemitigatedbyPPE.Asexpected,thesemiempiricalAmmermanmethodreturnslowerincidentenergyvaluesinallcases.ItisimportanttonoteagainthatthemethodsusedtoobtainthevaluespresentedinthispaperaresuggestedbytheNFPA70EStandard,andthatthereiscurrentlynocodifiedmethodologybasedonempiricaltestresultsforcomputingDCarcenergies,suchastheIEEE1584standardforACsystems.Further,widedeploymentofutilityscalephotovoltaicarraysisarelativelyrecentdevelopment,sothereisadearthofrecordedincidentsonsolarfarmsfromwhichtodrawconclusionsaboutthelikelihoodandpotentialseverityofarcflashesandblastsontheDCbus.Assuch,goodengineeringjudgmentandaconservativeapproachshouldbetakeninevaluatingDCarcflashhazardsuntilcomprehensivetestingcanbeconducted.Thatsaid,requiringworkerstodonoverlyburdensomeprotectivegearcanlowerproductivity,limitmovementandvisionandevenpotentiallyincreasethechancesofinitiatingafault.GiventheconsistencyofthehistoricaldatapresentedbyDr.Ammermanandhiscoauthors,itisreasonabletousethemethodpresentedinthatpaperasalessconservativeandlikelymoreaccurateapproach.Further,theabilitytocomputearcingcurrentandhencearcingtimeisamajorimprovementoverthemaximumpowermethod.AvailablefaultcurrentsontheSanLuisfarmarerelativelylowcomparedtothosefoundinACsystemssincetheyareessentiallyredirectedlinecurrents,soincidentenergyvaluesforshortdurationfaultswillbecorrespondinglylow.WhiletheGroundFaultProtectiveDeviceshould

  • detectmostlowimpedancefaultsonthearrayandoperateveryquickly,itisonlycapableofdisconnectingthearrayfromtheinverterandthepositivepoleofthearrayfromearthground.Undercertainconditions,theGFPDmaynotinterrupttheflowoffaultcurrentandtheinverterfusewouldhavetobetheclearingdevice.Inthesecases,thelowfaultcurrentcouldleadtolongclearingtimesandcorrespondinglyhighincidentenergy.Decidingtousethe2secondruleoranyclearingtimeotherthanthatgivenbythetimecurrentcharacteristicsoftheinverterfuseisproblematicandshouldbedoneonlyaftercarefulconsiderationofthetasksthatmaybeperformedonenergizedequipment.

    FutureWorkThereisalotofuncertaintyinherentinanyArcFlashStudy,eventhoseconductedonACsystemsusingtheempiricalformulasgivenbytheIEEE1584Standard,butthatuncertaintyisgreaterhere.Thereisaneedforcomprehensivetestinginahighpowerlabunderavarietyoftestsetups.Inparticular,attemptsshouldbemadetocreatefreeburningarcswhentheDCvoltageisrelativelyhigh(600V1000V)andtheavailablefaultcurrentisrelativelylowinorderduplicatetheconditionsfoundonatypicalphotovoltaicarray.Further,itisgenerallyassumedthatDCarcswillbelesssusceptibletoselfextinctionthanACarcssincetherearenozerocrossings;investigationintowhetherlongdurationarcscanbesustainedattheserelativelylowfaultcurrentswouldbevaluable.Whilethereisarapidlygrowingselectionoffusesdesignedspecificallyforphotovoltaicapplications,andreducingclearingtimeswithjudiciousfusechangesmaybeapossibility,itseemslikelythatlowfaultcurrentsrelativetonominalcurrentswillcontinuetomaketraditionaltimeovercurrentdevicesproblematiconsolarfarms.Investigationintonovelsystemprotectionsschemesbeyondtimeovercurrentandgroundfaultdevicescouldbefruitful.Differentialrelayingandfiberopticlightdetectorsaremethodsthathavebeenusedinotherapplicationsandmightbeappliedhere.However,itisinterruption,ratherthandetection,thatisthemainproblem.Whatisneededisaninterruptingdeviceinserieswiththeinverterfusesthatcanbeopenedbasedonatripsignalfromwhateverovercurrentdetectionschemeisused.

  • References[1]DCArcModelsandIncidentEnergyCalculations,Ammerman,Gammon,Sen,Nelson,2009

    [2]ArcFlashCalculationsforExposurestoDCSystems,DanielR.Doan,2007

    [3]TheotherElectricalHazard:ElectricArcBlastBurns,RalphH.Lee,1982

    [4]TheGroundFaultProtectionBlindSpot:ASafetyConcernforLargerPhotovoltaicSystemsIn

    theUnitedStates,BillBrooks,2012

    [5]SimpleImprovedEquationsforArcFlashHazardAnalysis,RobertWilkins,2004

    [6]GuideforPerformingArcFlashHazardCalculations,IEEE15842002,2002

    [7]StandardforElectricalSafetyintheWorkplace,NFPA70E,2012

    [8]DCArcFlashCalculationsArcinOpenAir&ArcinaBoxUsingaSimplifiedApproach

    (MultiplicationFactorMethod),MichaelFontaineandPeterWalsh,2012

    AcknowledgementsIwouldliketothankMr.SimmsandDr.CollinsfortheirguidancethroughoutthesemesterandfortheirpatienceasIslowlylearnedthebackgroundmaterialnecessaryforunderstandingthissubject.IwouldalsoliketothankJohnKolakforhisconstantoptimismandtirelessencouragementwhenitwasmostneeded.

  • AppendixA:AbbreviationsIEEE:InstituteofElectricalandElectronicsEngineers

    NFPA:NationalFireProtectionsAssociation

    SCC:ShortCircuitCurrent

    IE:IncidentEnergy

    TCC:TimeCurrentCurve

    XFMR:PowerTransformer

    AppendixB:ProjectCostsTravel:

    2tripstoAlamosafordatacollection.255mileseachway At$0.555/mi(gsa.gov) 255*4*$0.555=$566.10

    Researchpapercosts: $10eachfromIEEEexplore

    6totalfor$60Edaysposter:

    $20Totalexpenses:$646.10

  • AppendixC:AdditionalMaterials

    Figure14:SunpowerPVModule

  • Figure15:InverterFuseCurve

  • Figure16:StringFuseCurve

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