电气工程及其自动化电压波动论文中英文资料外文翻译文献.docx
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电气工程及其自动化电压波动论文中英文资料外文翻译文献.docx
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电气工程及其自动化电压波动论文中英文资料外文翻译文献
中英文资料外文翻译文献
原文:
ASPECIALPROTECTIONSCHEMEFORVOLTAGESTABILITYPREVENTION
Abstract
Voltageinstabilityiscloselyrelatedtothemaximumload-abilityofatransmissionnetwork.Theenergyflowsonthetransmissionsystemdependonthenetworktopology,generationandloads,andontheavailabilityofsourcesthatcangeneratereactivepower.OneofthemethodsusedforthispurposeistheVoltageInstabilityPredictor(VIP).Thisrelaymeasuresvoltagesatasubstationbusandcurrentsinthecircuitconnectedtothebus.Fromthesemeasurements,itestimatestheThévenin’sequivalentofthenetworkfeedingthesubstationandtheimpedanceoftheloadbeingsuppliedfromthesubstation.ThispaperdescribesanextensiontotheVIPtechniqueinwhichmeasurementsfromadjoiningsystembusesandanticipatedchangeofloadaretakenintoconsiderationaswell.
Keywords:
Maximumloadability;Voltageinstability;VIPalgorithm.
1.Introduction
Deregulationhasforcedelectricutilitiestomakebetteruseoftheavailabletransmissionfacilitiesoftheirpowersystem.Thishasresultedinincreasedpowertransfers,reducedtransmissionmarginsanddiminishedvoltagesecuritymargins.
Tooperateapowersystemwithanadequatesecuritymargin,itisessentialtoestimatethemaximumpermissibleloadingofthesystemusinginformationaboutthecurrentoperationpoint.Themaximumloadingofasystemisnotafixedquantitybutdependsonvariousfactors,suchasnetworktopology,availabilityofreactivepowerreservesandtheirlocationetc.Determiningthemaximumpermissibleloading,withinthevoltagestabilitylimit,hasbecomeaveryimportantissueinpowersystemoperationandplanningstudies.TheconventionalP-VorV-Qcurvesareusuallyusedasatoolforassessingvoltagestabilityandhenceforfindingthemaximumloadingatthevergeofvoltagecollapse[1].Thesecurvesaregeneratedbyrunningalargenumberofloadflowcasesusing,conventionalmethods.Whilesuchprocedurescanbeautomated,theyaretime-consuminganddonotreadilyprovideinformationusefulingaininginsightintothecauseofstabilityproblems[2].
Toovercometheabovedisadvantagesseveraltechniqueshavebeenproposedintheliterature,suchasbifuricationtheory[3],energymethod[4],eigenvaluemethod[5],multipleloadflowsolutionsmethod[6]etc.
Reference[7]proposedasimplemethod,whichdoesnotrequireoff-linesimulationandtraining.TheVoltageIndicatorPredictor(VIP)methodin[7]isbasedonlocalmeasurements(voltageandcurrent)andproducesanestimateofthestrength/weaknessofthetransmissionsystemconnectedtothebus,andcomparesitwiththelocaldemand.Thecloserthelocaldemandistotheestimatedtransmissioncapacity,themoreimminentisthevoltageinstability.ThemaindisadvantageofthismethodisintheestimationoftheThévenin’sequivalent,whichisobtainedfromtwomeasurementsatdifferenttimes.Foramoreexactestimation,onerequirestwodifferentloadmeasurements.
ThispaperproposesanalgorithmtoimprovetherobustnessoftheVIPalgorithmbyincludingadditionalmeasurementsfromsurroundingloadbusesandalsotakingintoconsiderationlocalloadchangesatneighboringbuses.
2.ProposedMethodology
TheVIPalgorithmproposedinthispaperusesvoltageandcurrentmeasurementsontheloadbusesandassumesthattheimpedanceofinterconnectinglines(
)areknown,asshownin(Figure1).ThecurrentflowingfromthegeneratorbustotheloadbusisusedtoestimateThévenin’sequivalentforthesysteminthatdirection.Similarlythecurrentflowingfromotherloadbus(Figure2)isusedtoestimateThévenin’sequivalentfromotherdirection.Thisresultsinfollowingequations(Figure3).Notethatthecurrentcomingfromthesecondloadbusoverthetransmissionlinewaskeptoutofestimationinoriginal(VIP)algorithm.
[1]
[2]
[3]
[4]
Where
and
arecurrentscomingfromThéveninbusesno.1and2.Equation
(1)-(4)canbecombinedintoamatrixform:
*
[5]
Usingthefirst2rowsinthesystemEquations
(1)-(4),thevoltageonbusesnumber1and2canbefoundasshowninEquation(6)below.FromEquation(6)wecanseethatthevoltageisafunctionofimpedances.NotethatthemethodassumesthatallThévenin’sparametersareconstantatthetimeofestimation.
[6]
Where,
and
Thesystemequivalentseenfrombusno.1isshowninFigure3.Figure4(a)showstherelationshipbetweenloadadmittances(
and
)andvoltageatbusno.1.Powerdeliveredtobusno.1is(
)anditisafunctionof(
).
[7]
Equation7isplottedinfigure4(b)asa‘landscape’andthemaximumloadingpointdependsonwherethesystemtrajectory‘goesoverthehill’.
Fig.1.3-BussystemconnectionsFig.2.1-Busmodel
Fig.3.SystemequivalentasseenbytheproposedVIPrelayonbus#1(2-busmodel)
(a)VoltageProfile(b)PowerProfile
Fig.4.Voltageandpowerprofilesforbus#1
2.1.On-LineTrackingofThévenin’sParameters
Thévenin’sparametersarethemainfactorsthatdecidethemaximumloadingoftheloadbusandhencewecandetectthevoltagecollapse.InFigure3,
canbeexpressedbythefollowingequation:
[8]
VandIaredirectlyavailablefrommeasurementsatthelocalbus.Equation(8)canbeexpressedinthematrixformasshownbelow.
[9]
B=AX[10]
Theunknownparameterscanbeestimatedfromthefollowingequation:
[11]
Notethatalloftheabovequantitiesarefunctionsoftimeandarecalculatedonaslidingwindowofdiscretedatasamplesoffinite,preferablyshortlength.Thereareadditionalrequirementstomaketheestimationfeasible:
•TheremustbeasignificantchangeinloadimpedanceinthedatawindowofatleasttwosetofMeasurements.
•ForsmallchangesinThévenin’sparameterswithinaparticulardatawindow,thealgorithmcanestimateproperlybutifasuddenlargechangeoccursthentheprocessofestimationispostponeduntilthenextdatawindowcomesin.
•Themonitoringdevicebasedontheaboveprinciplecanbeusedtoimposealimitontheloadingateachbus,andshedsloadwhenthelimitisexceeded.Itcanalsobeusedtoenhanceexistingvoltagecontrollers.Coordinatedcontrolcanalsobeobtainedifcommunicationisavailable.
Oncewehavethetimesequenceofvoltageandcurrentwecanestimateunknownsbyusingparameterestimationalgorithms,suchasKalmanFilteringapproachdescribed[6].
stabilitymargin(VSM)duetoimpedancescanbeexpressedas(
);wheresubscriptzdenotestheimpedance.Thereforewehave:
[12]
Theaboveequationassumesthatbothloadimpedances(
)aredecreasingatasteadyrate,sothepowerdeliveredtobus1willincreaseaccordingtoEquation(7).Howeveronceitreachesthepointofcollapsepowerstartstodecreaseagain.
Nowassumethatbothloadsarefunctionsoftime.ThemaximumcriticalloadingpointisthengivenbyEquation(13):
[13]
Expressingvoltagestabilitymarginduetoloadapparentpoweras(
),wehave:
[14]
Notethatboth
and
arenormalizedquantitiesandtheirvaluesdecreaseastheloadincreases.
Atthevoltagecollapsepoint,boththemarginsreducetozeroandthecorrespondingloadisconsideredasthemaximumpermissibleloading.
Fig.5.VIPalgorithm
2.2.VoltageStabilityMarginsandtheMaximumPermissibleLoading
Systemreachesthemaximumloadpointwhenthecondition:
issatisfied(Figure5).ThereforethevoltagestabilityboundarycanbedefinedbyacirclewitharadiusoftheThévenin’simpedance.Fornormaloperationthe
issmallerthan
(i.e.itisoutsidethecircle)andthesystemoperatesontheupperpart(orthestableregion)ofaconventionalP-Vcurve[2].
However,when
exceeds
thesystemoperatesonthelowerpart(orunstableregion)oftheP-Vcurve,indicatingthatvoltagecollapsehasalreadyoccurred.Atthemaximumpowerpoint,theloadimpedancebecomessameastheThévenin’s(
).Therefore,foragivenloadimpedance(
),thedifferencebetween
and
canbeconsideredasasafetymargin.HencethevoltageasgiveninanIEEEsurvey,whichdescribed(111)schemesfrom(17)differentcountries[8].
Fig.6.Loadactionstopreventfromvoltageinstability
2.3.AdvantagesoftheproposedVIPalgorithm
Byincorporatingthemeasurementsfromotherloadbuses(Figure3),theproposedVIPalgorithmachievesamoreaccuratevalueof
.Theon-linetrackingof
isusedtotracksystemchanges.
TheproposedimprovementsintheVIPalgorithmwillresultinbettercontrolactionforpowersystemvoltagestabilityenhancement.Thecontrolmeasuresarenormallyshuntreactordisconnection,shuntcapacitorconnection,shuntVARcompensationbymeansofSVC’sandsynchrounscondensers,startingofgasturbines,lowpriorityloaddisconnection,andsheddingoflow-priorityload[8].Figure6showsthemostcommonlyusedremedialactions.
3.Conclusions
AnimprovedVoltageInstabilityPredictor(VIP)algorithmforimprovingthevoltagestabilityisproposedinthispaper.ThepreviousVIPmethod[7]usedmeasurementsonlyfromthebuswheretherelayisconnected.Thenewmethodusesmeasurementsfromotherloadbusesaswell.Thevoltageinstabilitymarginnotonlydependsonthepresentstateofthesystembutalsoonfuturechanges.
Therefore,theproposedalgorithmusesanon-linetrackingThévenin’sequivalentfortrackingthesystemtrajectory.Thealgorithmissimpleandeasytoimplementinanumericalrelay.TheinformationobtainedbytherelaycanbeusedforloadsheddingactivationatthebusorVARcompensation.Inaddition,thesignalmaybetransmittedtothecontrolc
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