电能电气信息 外文翻译 外文文献 英文文献 电能质量监测Word文档格式.docx

电能电气信息 外文翻译 外文文献 英文文献 电能质量监测Word文档格式.docx

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PatrickColeman

Manypowerqualityproblemsarecausedbyinadequatewiringorimpropergrounding.Theseproblemscanbedetectedbysimpleexaminationofthewiringandgroundingsystems.Anotherlargepopulationofpowerqualityproblemscanbesolvedbyspotchecksofvoltage,current,orharmonicsusinghandheldmeters.Someproblems,however,areintermittentandrequirelonger-termmonitoringforsolution.

Long-termpowerqualitymonitoringislargelyaproblemofdatamanagement.IfanRMSvalueofvoltageandcurrentisrecordedeachelectricalcycle,forathree-phasesystem,about6gigabytesofdatawillbeproducedeachday.Someequipmentisdisruptedbychangesinthevoltagewaveshapethatmaynotaffectthermsvalueofthewaveform.Recordingthevoltageandcurrentwaveformswillresultinabout132gigabytesofdataperday.Whilemoderndatastoragetechnologiesmaymakeitfeasibletorecordeveryelectricalcycle,thetaskofdetectingpowerqualityproblemswithinthismassofdataisdauntingindeed.

Mostcommerciallyavailablepowerqualitymonitoringequipmentattemptstoreducetherecordeddatatomanageablelevels.Eachmanufacturerhasagenerallyproprietarydatareductionalgorithm.Itiscriticalthattheuserunderstandthealgorithmusedinordertoproperlyinterprettheresults.

1.1SelectingaMonitoringPoint

Powerqualitymonitoringisusuallydonetoeithersolveanexistingpowerqualityproblem,ortodeterminetheelectricalenvironmentpriortoinstallingnewsensitiveequipment.Fornewequipment,itiseasytoarguethatthemonitoringequipmentshouldbeinstalledatthepointnearestthepointofconnectionofthenewequipment.Forpowerqualityproblemsaffectingexistingequipment,thereisfrequentlypressuretodetermineiftheproblemisbeingcausedbysomeexternalsource,i.e.,theutility.Thisleadstotheinstallationofmonitoringequipmentattheservicepointtotrytodetectthesourceoftheproblem.Thisisusuallynottheoptimumlocationformonitoringequipment.Moststudiessuggestthat80%ofpowerqualityproblemsoriginatewithinthefacility.Amonitorinstalledontheequipmentbeingaffectedwilldetectproblemsoriginatingwithinthefacility,aswellasproblemsoriginatingontheutility.Eachtypeofeventhasdistinguishingcharacteristicstoassisttheengineerincorrectlyidentifyingthesourceofthedisturbance.

1.1.1WhattoMonitor

Atminimum,theinputvoltagetotheaffectedequipmentshouldbemonitored.Iftheequipmentissinglephase,themonitoredvoltageshouldincludeatleasttheline-to-neutralvoltageandtheneutralto-groundvoltages.Ifpossible,theline-to-groundvoltageshouldalsobemonitored.Forthree-phaseequipment,thevoltagesmayeitherbemonitoredlinetoneutral,orlinetoline.Line-to-neutralvoltages

areeasiertounderstand,butmostthree-phaseequipmentoperatesonline-to-linevoltages.Usually,itispreferabletomonitorthevoltagelinetolineforthree-phaseequipment.

Ifthemonitoringequipmenthasvoltagethresholdswhichcanbeadjusted,thethresholdsshouldbesettomatchthesensitiveequipmentvoltagerequirements.Iftherequirementsarenotknown,agoodstartingpointisusuallythenominalequipmentvoltageplusorminus10%.

Inmostsensitiveequipment,theconnectiontothesourceisarectifier,andthecriticalvoltagesareDC.Insomecases,itmaybenecessarytomonitorthecriticalDCvoltages.SomecommercialpowerqualitymonitorsarecapableofmonitoringACandDCsimultaneously,whileothersareAConly.

Itisfrequentlyusefultomonitorcurrentaswellasvoltage.Forexample,iftheproblemisbeingcausedbyvoltagesags,thereactionofthecurrentduringthesagcanhelpdeterminethesourceofthesag.Ifthecurrentdoubleswhenthevoltagesags10%,thenthecauseofthesagisontheloadsideofthecurrentmonitorpoint.Ifthecurrentincreasesordecreases10–20%duringa10%voltagesag,thenthecauseof

thesagisonthesourcesideofthecurrentmonitoringpoint.

Sensitiveequipmentcanalsobeaffectedbyotherenvironmentalfactorssuchastemperature,humidity,static,harmonics,magneticfields,radiofrequencyinterference（RFI）,andoperatorerrororsabotage.Somecommercialmonitorscanrecordsomeofthesefactors,butitmaybenecessarytoinstallmorethanonemonitortocovereverypossiblesourceofdisturbance.

Itcanalsobeusefultorecordpowerquantitydatawhilesearchingforpowerqualityproblems.Forexample,theauthorfoundashortcuttothesourceofadisturbanceaffectingawideareabyusingthepowerquantitydata.Therecordingsrevealedanincreaseindemandof2500KWimmediatelyafterthedisturbance.Askingafewquestionsquicklyledtoanearbyplantwitha2500KWswitchedloadthatwasfoundtobemalfunctioning.

1.2SelectingaMonitor

Commerciallyavailablemonitorsfallintotwobasiccategories:

linedisturbanceanalyzersandvoltagerecorders.Thelinebetweenthecategoriesisbecomingblurredasnewmodelsaredeveloped.Voltagerecordersareprimarilydesignedtorecordvoltageandcurrentstripchartdata,butsomemodelsareabletocapturewaveformsundercertaincircumstances.Linedisturbanceanalyzersaredesignedtocapturevoltageeventsthatmayaffectsensitiveequipment.Generally,linedisturbanceanalyzersarenotgoodvoltagerecorders,butnewermodelsarebetterthanpreviousdesignsatrecordingvoltagestripcharts.

Inordertoselectthebestmonitorforthejob,itisnecessarytohaveanideaofthetypeofdisturbancetoberecorded,andanideaoftheoperatingcharacteristicsoftheavailabledisturbanceanalyzers.Forexample,acommonpowerqualityproblemisnuisancetrippingofvariablespeeddrives.Variablespeeddrivesmaytripduetothewaveformdisturbancecreatedbypowerfactorcorrectioncapacitorswitching,orduetohighorlowsteadystatevoltage,or,insomecases,duetoexcessivevoltageimbalance.Ifthedrivetripsduetohighvoltageorwaveformdisturbances,thedrivediagnosticswillusuallyindicateanovervoltagecodeasthecauseofthetrip.Ifthevoltageisnotbalanced,thedrivewilldrawsignificantlyunbalancedcurrents.Thecurrentimbalancemayreachalevelthatcausesthedrivetotripforinputovercurrent.Selectingamonitorforvariablespeeddrivetrippingcanbeachallenge.Mostlinedisturbanceanalyzerscaneasilycapturethewaveshapedisturbanceofcapacitorswitching,buttheyarenotgoodvoltagerecorders,andmaynotdoagoodjobofreportinghighsteadystatevoltage.Manylinedisturbanceanalyzerscannotcapturevoltageunbalanceatall,norwilltheyrespondtocurrenteventsunlessthereisacorrespondingvoltageevent.Mostvoltageandcurrentrecorderscaneasilycapturethehighsteadystatevoltagethatleadstoadrivetrip,buttheymaynotcapturethecapacitorswitchingwaveshapedisturbance.Manyvoltagerecorderscancapturevoltageimbalance,currentimbalance,andsomeofthemwilltriggeracaptureofvoltageandcurrentduringacurrentevent,suchasthedrivetrippingoff.

Toselectthebestmonitorforthejob,itisnecessarytounderstandthecharacteristicsoftheavailablemonitors.Thefollowingsectionswilldiscussthevarioustypesofdatathatmaybeneededforapowerqualityinvestigation,andthecharacteristicsofsomecommerciallyavailablemonitors.

1.3Voltage

ThemostcommonlyrecordedparameterinpowerqualityinvestigationsistheRMSvoltagedeliveredtotheequipment.Manufacturersofrecordingequipmentuseavarietyoftechniquestoreducethevolumeofthedatarecorded.Themostcommon

methodofdatareductionistorecordMin/Max/Averagedataoversomeinterval.Figure1.1showsastripchartofrmsvoltagesrecordedonacycle-by-cyclebasis.Figure1.2showsaMin/Max/Averagechartforthesametimeperiod.Acommonrecordingperiodis1week.Typicalrecorderswillusearecordingintervalof2–5

minutes.Eachrecordingintervalwillproducethreenumbers:

thermsvoltageofthehighest1cycle,thelowest1cycle,andtheaverageofeverycycleduringtheinterval.Thisisasimple,easilyunderstoodrecordingmethod,anditiseasilyimplementedbythemanufacturer.Thereareseveraldrawbackstothismethod.Ifthereareseveraleventsduringarecordinginterval,onlytheeventwiththelargestdeviationisrecorded.Unlesstherecorderrecordstheeventinsomeothermanner,thereisnotime-stampassociatedwiththeevents,andnodurationavailable.Themostcriticaldeficiencyisthelackofavoltageprofileduringtheevent.Thevoltageprofileprovidessignificantcluestothesourceoftheevent.Forexample,iftheeventisavoltagesag,theminimumvoltagemaybethesameforaneventcausedbyadistantfaultontheutilitysystem,andforanearbylargemotorstart.Forthedistantfault,however,thevoltagewillsagnearlyinstantaneously,stayatafairlyconstantlevelfor3–10cycles,andalmostinstantlyrecovertofullvoltage,orpossiblyaslightlyhighervoltageifthefaultedsectionoftheutilitysystemisseparated.Foranearbymotorstart,thevoltagewilldropnearlyinstantaneously,andalmostimmediatelybeginagradualrecoveryover30–180cyclestoavoltagesomewhatlowerthanbefore.Figure

1.3showsacycle-by-cyclerecordingofasimulatedadjacentfeederfault,followedbyasimulationofavoltagesagcausedbyalargemotorstart.Figure1.4showsa

Min/Max/Averagerecordingofthesametwoevents.TheeventslookquitesimilarwhencapturedbytheMin/Max/Averagerecorder,whilethecycle-by-cyclerecorderrevealsthedifferenceinthevoltagerecoveryprofile.

FIGURE1.1R