Arc Flash Hazards in AC and DC Systems - engr. my senior project, I chose to focus on the subject of arc flash hazards in electrical systems ... IEEE 1584 Standard

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  • ArcFlashHazardsinACandDCSystems

    EndofSemesterReport,fall2012

    DavidSmithPreparedtoPartiallyFulfilltheRequirementsofECE401

    DepartmentofElectricalandComputerEngineering

    ColoradoStateUniversityFortCollins,Colorado80523

    ProjectAdvisor:Dr.GeorgeCollins

    IndustrySponsor:DohnSimms,PraxisCorporation

    Approvedby:DrGeorgeCollins

  • Abstract

    Formyseniorproject,Ichosetofocusonthesubjectofarcflashhazardsinelectricalsystems,specificallyinutilityscalewindfarmsandphotovoltaicarrays.Asanelectrician,thissubjectcombinesmyinterestsinelectricalworkersafetyandthecontinuedrapidpaceofthedevelopmentofalternativemethodsofgeneratingelectricity.ThissubjectisofgreatpracticalinteresttoFacilityandProjectManagerswhoareresponsibleforthesafetyoftheirworkersbutmustalsobalancethisconcernwiththeneedtomaintainareliablesystemwithminimaldowntime.Anarcflashisasustainedarcingcurrentthatpropagatesthroughtheconductiveplasmacreatedbythebreakdownofagaseousdielectricmedium,typicallyair.Giventherightconditions,thecurrentwillcontinuetoflowunabateduntilinterruptedbyanupstreamovercurrentprotectivedevice.Sucharcsreleaseenormousenergy,resultinginhightemperatures,soundlevels,pressuresandtheejectionofhighspeedmoltendebris.Arcflashesandblastsinelectricaltransmission,distributionandutilizationsystemsarealeadingcauseofinjuriesandfatalitiestoelectricalworkersandareestimatedtocosthundredsofmillionsofdollarseachyearindowntimeanddamagedequipment.Asaspecifichazardtoelectricalworkersthatneededtobeactivelymanaged,arcflashesandblastswerelargelyignoreduntilthe1985publicationofRalphLeesseminalpaperArcFlash:TheOtherElectricalHazard,whichpresentedthefirsttheoreticalmodelforpredictingincidentenergiesworkerscouldbeexposedtoasafunctionofarcingtime,availableshortcircuitcurrentanddistancefromthearc.Thismodelandmuchoftheensuingresearchintopredictingarcflashenergiesisspecifictoalternatingcurrent,however.TheempiricallyderivedequationsintheInstituteofElectricalandElectronicsEngineersGuidetoPerformingArcFlashHazardCalculations(IEEEStandard1584)arespecifictoarcsinACsystems.Currently,noconsensusstandardexistsforcalculatingarcenergiesinDCsystems.However,the2012editionoftheNationalFireProtectionAssociationsStandardforElectricalSafetyintheWorkplace(NFPA70E)referencestwopapersthatoffertheoreticalandsemiempiricalmethodsforestimatingDCarcenergy.Theprimarygoalofmyprojectwillbetwofold:first,toperformacomparativeevaluationoftheconsensusstandardsforpredictingarcenergyonACsystemsasconductedonawindfarm,andtwo,toperformacomparativeevaluationofthetwomethodsgiveninthe2012NFPA70EforpredictingDCarcenergyasconductedonaphotovoltaicarray.

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

    ListofFiguresandTables..............................................................................................................................4

    Introduction..................................................................................................................................................5

    ArcsinACSystems........................................................................................................................................6

    ShortCircuitCurrentAnalysis...................................................................................................................7

    Utility...................................................................................................................................................10

    Transformers.......................................................................................................................................11

    Turbines..............................................................................................................................................11

    Conductors..........................................................................................................................................11

    ArcEnergy...................................................................................................................................................15

    LeesMethod..........................................................................................................................................15

    IEEE1584Standard.................................................................................................................................16

    Conclusions.................................................................................................................................................23

    Plansforspring,2013Semester.................................................................................................................24

    ArcsinDCSystems..................................................................................................................................24

    References..................................................................................................................................................25

    AppendixA:Abbreviations.........................................................................................................................26

    AppendixB:ExpectedProjectCosts...........................................................................................................26

    AppendixC:ProjectOutlines......................................................................................................................27

    Acknowledgements.....................................................................................................................................29

  • ListofFiguresandTables

    SymmetricalShortCircuitCurrent............7

    AsymmetricalShortCircuitCurrent..7

    ShortCircuitCurrentDCComponent..8

    SystemDiagram..9

    ShortCircuitCurrentDiagram..9

    PerUnitImpedanceNetwork.13

    VectorDiagramsforLeesMethod.15

    ArcingCurrentvs.BoltedFaultCurrent1000V17

    IncidentEnergyvs.BoltedFaultCurrent1000V18

    IncidentEnergyDiagram20

    TimeCurrentCurvesforXFMRfuseandCollectorCircuitRelay..21

    TimeCurrentCurvesofXFMRFuseandTurbineCircuitBreaker..22

  • IntroductionThesystemofelectricaltransmission,distributionandutilizationequipmentthatweinformallyrefertoasthegridconstitutesanenormousandenormouslycomplexnetworkthatissubjecttomanytypesoffailuresandhencemusthaverobustandwelldesignedprotectionsystems.Thesesystemsmustfulfillthedualfunctionofkeepingthenetworkupandrunningasconsistentlyandreliablyaspossibleaswellasprotectingthepeoplethatmaintainandrepairit.Findingthebalancebetweenreliabilityandsafetycanbedifficult,andtoooftenthesegoalsareconsideredtobecontradictory.Becausethecostsofdowntimeforconsumersofelectricitycanbeextremelyhigh,thereisenormouspressureonelectricalworkerstoperformtheirdutiesonenergizedsystems,workthatisextremelyhazardousevenforwelltrainedandqualifiedpersonnel.Clearly,amajorhazardisthepotentialforinjuryduetoelectricshock,butthereisalsothepotentialforseriousinjuryfromelectricalarcflashesandblaststhatcanoccurwhenanunintentionallowimpedancepathbetweenphaseconductorsorbetweenaphaseconductorandgroundiscreated.Ifalowimpedancefaultescalatesintoasustainedelectricarc,tremendousenergiescanbereleasedintheformofintenseheatandconcussiveblasts.Manyofthesetypesoffaultsaretriggeredbyelectriciansandlinemenworkingonenergizedequipment,workthatnecessarilyinvolvesbeingincloseproximitytothearcandwhichoftenresultsindevastatingburns,impactsfromexplosiveblasts,fallsandahostofotherequallyseriousinjuries.Untilrelativelyrecently,injuriesfromelectricshockwereconsideredtobemostcommonandserioushazardfacedbyelectricalworkers,butitisnowrecognizedthatmanyoftheburnssufferedbyvictimsofelectricalaccidentsthatwereonceascribedtobeingpartoftheconductionpathofelectriccurrentareactuallycausedbyexposuretoarcflashesandblasts.Whilethestatisticsonhowmanyinjuriesandfatalitiesarecausedbyshockvs.arcflashesareamatterofsomedebate,itisgenerallyacceptedthatarcflasheventsarenotuncommonandconstituteasignificantportionofthe320deathsand4000majorinjuriesthatarecausedbyelectricalaccidentsonaverageeachyearintheUS[1].Whilethecostsofindividualaccidentsvarywidelyandaredifficulttocalculate,electricalaccidentsoftenhavehighercoststhanothertypesofaccidents,andonestudybytheElectricPowerResearchInstituteinPaloAltoestimatestheaveragetotaldirectandindirectcoststobeashighas15milliondollarspercase[1].Further,reducingtheintensityofanarcflashprotectsequipmentaswellasworkers.Ahighenergy,longdurationarcingfaultinsidethewiringcompartmentofanoilfilledtransformer,forexample,willtypicallydestroythewiringlugsorpaddles,thefeederconductors,andcan

  • evenrupturethetank,leadingtothecompletelossofaveryexpensivepieceofequipment,aswellascostsduetosystemdowntimeandlaborforrepairs.Lessenergeticarcswillgenerallycauselessdamage,andsystemswithlowcalculatedincidentenergiesareeasiertoproperlymaintainwithlessdowntime,sincethepersonalprotectiveequipmentthatmustbewornisproportionatelylessburdensome.

    Whileshockhazardsmustmostlybemitigatedthroughworkpracticestrainingandpermitting,personalprotectiveequipmentrequirements,andthelikearcflashhazardscanoftenbesignificantlyreducedbymakingjudiciouschangestotheelectricalsystemitself.Tocorrectlyidentifywhereandhowimprovementscanbemade,onemustbeabletoestimatewithsomedegreeofaccuracythepotentialarcflashintensityatallpointsintheelectricalsystemwhereelectricalmaintenanceorrepairislikelytooccur.

    ArcsinACSystemsFreeburningarcsinopenairarechaotic,complexandunpredictableinnature,difficulttoaccuratelymodelandaretypicallyevaluatedfromablackboxperspective.ThemostcommonlyusedtheoreticalmodelforpredictingarcenergyisLeesmethod,whichreliesonbasiccircuittheoryandthemaximumpowertransfertheorem.DevelopedbyRalphLeeinhis1982paperTheOther