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

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<ul><li><p>ArcFlashHazardsonPhotovoltaicArrays</p><p>EndofSemesterReport,Spring2013</p><p>DavidSmithPreparedtoPartiallyFulfilltheRequirementsofECE402</p><p>DepartmentofElectricalandComputerEngineering</p><p>ColoradoStateUniversityFortCollins,Colorado80523</p><p>ProjectAdvisor:Dr.GeorgeCollins</p><p>IndustrySponsor:DohnSimms,PraxisCorporation</p><p>Approvedby:DrGeorgeCollins</p></li><li><p>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.</p></li><li><p>ContentsAbstract.........................................................................................................................................................2</p><p>Figures...........................................................................................................................................................3</p><p>Introduction..................................................................................................................................................4</p><p>SystemShortCircuitCurrent........................................................................................................................4</p><p>FaultClearingDevices...................................................................................................................................6</p><p>ArcingCurrentandFaultClearingTime........................................................................................................9</p><p>ArcLength...................................................................................................................................................11</p><p>IncidentEnergy...........................................................................................................................................13</p><p>EnclosureMultiplyingFactor......................................................................................................................14</p><p>ComparisonofValuesbetweenMethods..................................................................................................15</p><p>Conclusions.................................................................................................................................................17</p><p>FutureWork................................................................................................................................................18</p><p>References..................................................................................................................................................19</p><p>Acknowledgements.....................................................................................................................................19</p><p>AppendixA:Abbreviations.........................................................................................................................20</p><p>AppendixB:ProjectCosts..........................................................................................................................20</p><p>AppendixC:AdditionalMaterials...............................................................................................................21</p><p>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</p></li><li><p>IntroductionWiththegrowthinrecentyearsofutilityscalesolarphotovoltaicfarmscommonlyarrangedinarraysthatproducebetween600and1000voltsDC,itisbecomingincreasinglyimportanttoexplorewhetherdangerousarcscandevelopontheDCbus,andifso,toevaluatemethodsforpredictingtheenergiesworkersmightbeexposedto.Freeburningarcsofthesortthatcanbeinadvertentlystruckbyanelectricalworkerareverycomplex,difficulttoaccuratelymodelandareevaluatedfromablackboxperspectiveinthispaper.ThetwomethodsusedtopredictarcenergyontheSanLuisSolarFarm,bothcitedinthe2012versionoftheNFPA70E,are:</p><p> MaximumPowerMethod,developedbyDanDoaninhis2007paperArcFlashCalculationsforExposurestoDCSystems[1]</p><p> Semiempiricalmethod,developedbyDr.RavelAmmerman,etal,intheir2009paperDCArcModelsandIncidentEnergyCalculations[2]</p><p>ThemaximumpowermethodisessentiallyaDCversionofLeesmethod[3],andreliesonthepremisethatthemaximumpowertransferredtoaresistiveloadthearcresistanceinthiscaseoccurswhentheloadresistanceisequaltothatofthesource.Theoretically,thismethodwillprovideworstcasevalues,butitisarelativelysimpleapproachthatcomputesarcenergyasafunctionofthesystemvoltageandavailableboltedfaultcurrentonly.TheAmmermanmethodisbasedonareviewofhistoricallaboratorydatafromseveraldisparatestudiesspanningmorethanacentury.Wherethesedatasetscouldbereasonablycompared,goodagreementwasgenerallyfoundintheVoltAmpcharacteristicoftheDCarc.Anequationdescribingthenonlinearnatureofarcresistancewasdevelopedfromoneofthemostextensivesetsofdata,andistheequationusedthroughoutthispaper.Unlikethemaximumpowermethod,thisrelationshipincorporatesdifferencesinarclengthandoffersawaytopredictthearcresistanceandhencethearcingcurrent.</p><p>SystemShortCircuitCurrentAsinanyarcflashstudy,thefirststepisbuildinganaccuratemodelofthesystemandcomputingtheavailableshortcircuitcurrentatallworkeraccessiblenodesthatwouldrequireanarcflashhazardlabel.TheSanLuisfacilityiscomposedofover100,000320Wattmodulesarrangedinseriesstringsofthirteen,withbetween222and225stringsfeeding38inverters.TheinvertersaregalvanicallyisolatedpowerelectronicdevicesthatshouldnotbecapableofsupplyingpowerfromtheACgridtotheDCbus.Hencewehavetreatedthefarmas38essentiallyidentical,isolatedarrays.Theonlydifferencebetweentheseindividual</p></li><li><p>arraysisthenumberofstringsandsomesmalldifferencesinconductorlengthduetotheplacementoftheinvertersrelativetotheirassociatedstrings.Wehavefocusedon1INVA(222strings)and19INVB(225strings)inordertocapturethefullrangeofcurrentsavailableonthefarm.TheVoltAmpcurveofaphotovoltaicmoduleisnonlinear,andthecellwillactaseitheraconstantcurrentoraconstantvoltagesourcedependingonregionofoperation,whichisdeterminedbytheloadresistance.Themaximumpowerthatcanbeprovidedbythemoduleoccursatthetransitionbetweenthetworegionsandisthepointatwhichthemaximumpowerpointtrackingsystemwillattempttoholdthearray.Current,andvoltagetoamuchlesserextent,dependsonsolarirradianceandtemperature.StandardTestConditionsformeasuringamodulesVoltAmpcharacteristicareasolarirradianceof1000Watts/m2and25Celsius.Themanufacturerdataforthe320WattSunpowermodulesusedontheSanLuisfarmisshownbelow:</p><p>Figure1:SunpowerModuleElectricalData</p></li><li><p>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.</p><p>FaultClearingDevicesUnlikefaultsinothertypesofsystems,particularlyACsystemswithrotatingmachineswherefaultcurrentsaretypicallyseveraltimesthenominaloperatingcurrent,theavailablefaultcurrentinaphotovoltaicarrayislimitedbytheamountofsolarradiationthatiscurrentlyfallingonthemodules,andthiscurrentwillvaryverylittleoverawiderangeofoperatingvoltages.Hencethefaultcurrentsinthesystemareessentiallyredirectedoperatingcurrents,andtraditionaltimeovercurrentdevicesmaynotclearfaultsquicklyoratall.OntheSanLuisfarm,thereare10Afusesinthecombinerboxesattheendofeach13modulestringand315Afusesintheinvertersattheendofeachfeederfromthecombinerboxes.ThepositivepoleofthearrayisgroundedthroughcalibratedresistorsaspartoftheGroundFaultProtectiveDevice(GFPD).Ariseinvoltageacrosstheseresistors,indicatingincreasedcurrentflowaboveasmall,constantleakagecurrent,willtriggerthedevice,rapidlyopeningthearraysconnectiontogroundandtotheACgrid.TheGFPDshouldoperateforanyrelativelylowimpedancefault,aslongasthefaultcurrentisinexcessofthenormalleakagecurrentthatflowsfromthepositivepoletogroundthroughtheGFPDresistors.Figure1belowshowsthefaultcurrentpathintheeventofagroundfault:</p></li><li><p>Figure2:GroundFault</p><p>Inthiscase,thecurrentwillflowfromthegroundedpositivepoletothefaultpoint,whichhasbeenforcedtoalowerpotential,openingtheGFPDandtheDCcontactorintheinverter,leavingnoavailablepathandclearingthefault.However,inhis2012paper,TheGroundFaultProtectionBlindSpot:AsafetyConcernForLargerPhotovoltaicSystemsInTheUnitedStates[4],BillBrooksidentifiedascenariounderwhichtheGFPDmightfailtoclearagroundfault,andpostulatedthatthiswasthecauseoftworecentfiresatlargesolararrays.Inthisscenario,ahighimpedancegroundfaultcoulddevelopandpersistindefinitelyifthecurrentwasbelowtheleakagecurrentpreviouslyreferredto.Nowifalowimpedancegroundfaultweretooccur,theGFPDwouldtripasbefore,butthepreexistinghighimpedancepathcoulddevelopintoapathcapableofsupplyingmuchoftheavailablecurrenttosecondfaultpoint.ThisscenarioisshowninFigure2below,wherepointsAandBindicatedifferentpossiblelocationsforthepreexistingfault.IfatpointA,thesecondfaultwouldlikelytripthestringfuseandbeclearedrelativelyquickly.AtpointB,theclearingdevicewouldbetheinverterfusewhichrequiresmorecurrentthanisavailableinthesystemtooperatequickly:</p></li><li><p>Figure3:GroundFaultwithPreexistingFault</p><p>Finally,inthecaseofapositivetonegativefault,theGFPDshouldindeedtrip,butthereisnoreasonwhythearraycouldnotcontinuetosupplycurrenttoanarcingfaultasitsload.Inthiscase,theinverterfusewouldagainbetheclearingdevice:</p><p>Figure4:PositivetoNegativeFault</p></li><li><p>ArcingCurrentandFaultClearingTimeAspreviouslymentioned,thelackofsignificantcurrentriseduringafaultonaphotovoltaicarraypresentsaproblemwhenthefaultmustbeclearedbyatimeovercurrentdevice.Evenassumingaboltedfaultattheinverterterminalswiththehighestavailablecurrent,1734A,thetimecurrentcharacteristicsoftheinverterfusearesuchthatitwouldrequireapproximately4.5secondstoopen.Sincethetotalenergyinthearcincreaseslinearlywithtime,thisverylongfaultclearingtimecanleadtodangerouslyhighincidentenergies.Accuratelypredictingthepotentialenergyaworkermightbeexposedtorequirestheabilitytoestimatethearcimpedance,andhencethearcingcurrent.Thisistheprimarydrawbackofusingthemaximumpowermethod.Sinceitisassumedthatthesourceresistanceisequaltothearcresistance,thevoltagedividerequationfortheequivalentcircuitatthefaultpointreducestotheequationbelow.Now,themaximumpossiblepowerinthearcisfoundsimplyastheproductofthecurrentthroughthearcandthevoltageacrossit,andtheclearingtimewouldnecessarilybec...</p></li></ul>

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