电能电气信息 外文翻译 外文文献 英文文献 电能质量监测.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.docx》由会员分享,可在线阅读,更多相关《电能电气信息 外文翻译 外文文献 英文文献 电能质量监测.docx(13页珍藏版)》请在冰豆网上搜索。
电能电气信息外文翻译外文文献英文文献电能质量监测
电能电气信息外文翻译外文文献英文文献电能质量监测
1PowerQualityMonitoring
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