InsideVoltageQualityVerification

北极星电力网技术频道作者:电力论文3 2007/12/24 18:43:49

关键词: cat LT IC

Lurkingintheshadowsofeventhemostunsuspectingoperations—fromcorporatetocommercialtoindustrialfacilities—isaconditionknownaspoorvoltagequality,ahiddenexpensethattypicallygoesundetectedandundiagnoseduntilcostlyfailuresoccur.Voltageeventsinterruptsensitiveprocesses,shortentheusefullifeofequipment,andcausecomputerandcommunicationsproblems.Oftentimes,thesedisturbancestriggerneedlessexpensesintheformofimplementedsolutionstomisdiagnosedproblems.Unlessthesourceoftheproblemiscorrectlyidentifiedanditscausefoundandrepaired,thecostsandtimerequiredtosolvetheproblemaddupquickly.

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Definedintermsofmagnitude(amplitude)andduration(length),voltageevents—appearingintheformofsags,swells,impulses,andtotalharmonicdistortion—canaffectequipmentperformanceindifferentways.Typically,theultimateimpactofsucheventsisdeterminedbythesensitivityoftheequipmentonthebranchcircuit.

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Voltagesags
Asagisaperiodoflowvoltage.Minorsagsoccurfrequently,sometimeswithoutdisturbingequipmentperformance.Majorsags,ontheotherhand,alwaysdisturbequipmentperformance.Sagsoccurformanyreasons,includingvoltagedropcausedbylongrunsofwire,switchingloads,poorwiring,andoverloadedbranchcircuits.
Voltageswells
Aswellisaperiodofhighvoltage.Swellshaveseriousimpactonequipmentfunction;however,theyarenotascommonassags.Bothminorandmajorswellsaffectequipmentperformance.
Impulses
Animpulseisashortburstofenergythatlastsforlessthanacycle.Impulsesrangeinmagnitudefromtwicethenominalvoltagetoseveralthousandvolts.Noteveryimpulsehasanimpactonequipmentperformance.However,whenimpulsesoccurrepeatedlyovertime(orwhentheenergylevelisveryhigh),animpulsecancauseequipmentdegradationorevenimmediatefailure.
Harmonics
ACvoltageisasinewavethatrepeats60timespersecond(Hertz=cycles/second).Thisisthefundamentalfrequency.Harmonicsarealternatefrequenciesthatdistortthesinusoidalwaveform.Totalharmonicdistortion(THD)ismeasuredasapercentageofthefundamentalfrequency.Equipmentrunswellonvoltagethatisaclean(orslightlydistorted)sinewave.Highlevelsofdistortionmaycauseequipmentproblems.Onsingle-phasebranchcircuits,levelsofTHDgreaterthan5to8shouldbeinvestigated.

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Inmanycases,uponfurtherinvestigationbyapowerqualityprofessional,problemsthathaveexistedforyearsaretracedbacktoundetectedvoltageeventscausedbytheculpritsoutlinedabove.Aftervoltageinterruptionsmaketheirpresenceknown,thefirstquestionend-userstypicallywantansweredis,“Howdidtheygetstartedinthefirstplace?”

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Howdovoltageeventsoccur?

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Thesourceof85ofallvoltageevents,causedbyinadequatesupply,undersizedwiring,oranoverloadedcircuit,originateswithinafacility,notontheelectricutilitysystem.Frequently,problemsarisewhenequipmentturns“on”or“off”onthesamecircuit.Environmentalchanges,suchaslightningstorms,highwinds,flooding,physicaldamage,utilityoperations,orneighboringfacilities,alsocanaffectvoltage.Whenyouconsiderthatvoltagequalityalsochangeseverytimeanewpieceofequipmentisinstalledinafacility—orinaneighboringfacility—thingsbecomeevenmorecomplicated.

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Someperceivedpowerqualityproblems,however,havenothingtodowiththesupplyofvoltage.Instead,youcantracethembacktohumanerror.Forinstance,custodialcrewsunpluggingequipmenttocleanaroundthem,andpaintingcrewsdisconnectingthegroundrod,arebothexamplesofroutineactivitiesthatcanaffectvoltagequality.

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Howdoeventsaffectequipment?

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Electricalequipmentisdesignedtoworkatnominalvoltage( /-10).Althoughequipmentmaynotfailthefirsttimeaneventoccurs,excessivestressfromrepeatedvoltageeventscancausedamageovertime.Whenvoltageisoutsideofequipmentdesignspecifications,forexample,equipmenthastoworkharder,runhotter,orinsulationmayhavetowithstandextremevoltagelevels.Forinstance,arefrigeratorisdesignedtooperatebetween108VACand132VAC—thatis,atypicalvoltagerangeforanominal120Vpieceofequipment.Ifvoltagerunsconsistentlybelow108Vonthecircuitpoweringtherefrigerator,thecompressormotorwillrunhotter,reducingitsoperationandservicelife.

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Sagsarethemostprevalentpowerqualityissueforequipment.Momentarysagsmaynotaffecttherefrigeratorreferencedabove,buttheywillcauseproblemsformoresensitiveequipment,suchascomputers.Thegreaterthevoltagesag,thegreaterthelikelihoodofdamage.Similarly,thegreaterthenumberofsagsoccurring,thegreaterthechanceoffailureordamage.

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Althoughvoltageswellsoccurlessfrequentlythansags,evenrelativelyminorswellscandamageequipment.Therefore,theyrequireimmediateattention.Thelongeraswell''sduration,themoreextensivethedamagewillbe.Anexampleherewouldbealargemotorcreatingvoltagesagsbydrawinghighinrushcurrents.Whenthemotorisstoppedabruptly,voltageswellsaregenerated.Leftuncorrected,thesesagsandswellswillleadtocomputerdisruptionsandfrequenthardwarereplacementinthefacility.

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THDcanproduceexcessiveheat,generateelectro-magneticinterferenceincommunicationscircuits,andcauseelectroniccontrolstofail.Non-linearloads,suchasPCs,copyingmachines,andvariable-frequencydrives,createharmoniccurrentsthatdistortthevoltagesinewave.Themoreelectronicdevicesonacircuit,thegreaterthelikelihoodofseverevoltagedistortion.Agoodexampleofsuchaprobleminvolvedahospitaltechnicianwhotestedacircuitfortwodaysbeforeinstallingpatientmonitoringequipment.Oneinstanceofvoltageharmonics,amountingto5.2THD,wasnoted.RecognizingthislowlevelofTHDwouldn''tcauseaproblem,thetechnicianinstalledthepatientmonitoringdevice.Withinhours,thedevicefailed.ThetechnicianreviewednewdatatofindaTHDeventreaching10.2.Furtherinvestigationusingacircuitanalyzerandlong-termrecordersfoundtherewereseveralnon-linearloadspluggedintothesamebranchcircuitasthepatientmonitoringdevice.Whencertaincombinationsoftheseloadswereonsimultaneously—alongwiththenewequipment—excessiveharmonicsflowed,causingadistortedvoltagewaveformandsporadicshutdownofthedevice.

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Howtotestvoltagequality

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Thereareavarietyofvoltagemonitoringdevicesonthemarketforyoutochoosefromtodiagnosevoltageproblems.Theserangefromhigh-endpowerqualityanalyzersyoucanuseattheserviceentrancetosimplemonitorsthatconnectatthebranchlevel.

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Equipmentoperatingonvoltagelevelsoutsideof“normal”rangeswon’toperateforlong.

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Sometesterscomewithpre-loadedmonitoringstandardsbasedonthemostwidelyusedinternationalvoltagestandards,suchasCBEMA,ITIC,IEEE519or1159.1.Differentapplicationsrequiredifferentvoltagequalitystandards.Somearequitestringentandwillrecordmanyevents;othersarelessstringent,applyingtolerancesdesignedformodern,morerobustloads,resultinginfewerout-of-toleranceeventstoberecorded(Table).Theimportantthingtonotehereisthatyouthoroughlyunderstandhowthemonitoryou"reusingworksandtheparametersit"susingtomakemeasurements.

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Whenitcomestovoltagequality,thereisnosuchthingasacoincidence.Sowhileyou''retesting,carefullyexaminethesimultaneousoperationofequipmentasalikelycauseofvoltageevents.Notedatesandtimesofnuisanceevents,andrecordtheeventrecordsaspartofnormaloperations.Finally,alwaysconsultyourlocalelectricutilityforresourcestohelpdiagnosepowerproblems,improveefficiency,andavoidusagepenalties.

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Gregorecisgroupmanager,IDEALTest&Measurement,Sycamore,Ill.

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来源：中国电能质量

InsideVoltageQualityVerification

Howdovoltageeventsoccur?

Howdoeventsaffectequipment?

Howtotestvoltagequality

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Howdovoltageeventsoccur?

Howdoeventsaffectequipment?

Howtotestvoltagequality

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