Methods for Voltage Stability Enhancement
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This document discusses various methods for enhancing voltage stability in power systems, including using lower power factor generators, capacitor banks, controlling transformer tap changers, undervoltage load shedding, coordination of protections and controls, control of network voltage and generator reactive output, use of FACTS devices, artificial neural networks, fuzzy logic, excitation control, booster transformers, phase shifting transformers, secondary voltage regulation, series capacitors, and must-run generation. It provides details on how each method can be implemented and their effectiveness.
Methods for Voltage Stability Enhancement
- 1. By-RAVI ANAND MTech. (PowerSystem)-1922001 Methods for Voltage Stability Enhancement 1} Lower PowerFactor Generators: Where new generationisclose enoughtoreactive-shortareasor areas thatmay occasionally demand large reactive reserves,a0.80 or 0.85 powerfactorgeneratormaysometimesbe appropriate.However, shuntcapacitors with a higherpowerfactor generatorhavingreactive overloadcapability may be more flexible andeconomical. 2} Capacitor Banks : Shuntcapacitor banksshouldusually be located on the regulatedside of LTC voltage regulators.The shuntcapacitor bankare thenconstantreactive powersources. Control of voltage by switchedshuntcapacitors or seriescapacitors,rather than LTC transformersand distribution voltage regulators,will improve voltage stability. 3} Control of transformer tap changers : Tap changerscan be controlled,eitherlocally or centrally, so as to reduce the risk of voltage collapse. Microprocessor-basedOLTCcontrolsofferalmostunlimitedflexibility for implementingULTCcontrol strategiessoas totake advantage of the load characteristics. 4} Undervoltage load shedding: For unplannedorextreme situations,itmay be necessarytouse undervoltage load-sheddingschemes. Load sheddingschemesshouldbe designedsoasto differentiate betweenfaults,transientvoltage dips and low voltage conditionsleading to voltage collapse. 5} Operators' Role : Operatorsmustbe able to recognize voltage stability-relatedsymptomsandtake requiredremedial actions to preventvoltage collapse.On-line monitoringandanalysis to identifypotentialvoltage stabilty problemsandappropriate remedialmeasuresare extremelyhelpful. 6} Coordinationof protections/controls: Adequate coordinationshouldbe ensuredbetweenequipmentprotections/controlsbasedondynamic simulation studies.Tripping of equipmenttoavoid an overloadedconditionshouldbe the last alternative.Controlledsystemseperationandadaptive orintelligent control could also be used. 7} Control of network voltage and generatorreactive output : Severalutilities in the world suchas EDF (France),ENEL(Italy) are developingspecial schemesforcontrol of networkvoltagesandreactive power.
- 2. 8} GeneticAlgorithm: Optimal reactive powerdispatch problemis one of the difficult optimization problemsin the power system.Reactive powerdispatchprobleminclude the maximumuse of efficiencyof existinggenerator busvoltage magnitude,transformertapsettingandoutputof reactive powersourcestoreduce the lossesand maintain the voltage stability. 9} Saddle node bifurcation : Ian Dobsongivesa methodforcalculation of closetsaddle node bifurcationpoint of the system.New iterative and direct methodis usedforthe calculation of load powerat which bifurcation occurs.The nature of the load cannot be estimatedhence worstcase load powermargin is foundout forthe preventionfrom voltage collapse. 10} Synchrophasor Method : Early detectionof voltage instability in the systemis alwayspreferable thanthe actual occurrence voltage instability state.The area detectionof voltage instability is obtainedfromthe synchronized phasormeasurementmethod.Initially,setof algebraic equationsare solvedandsensitivityis foundfor the identification of combination of load powers.Time domain simulation is done on Nordic 32 test system,withand withoutmeasurementof noise.The computationof voltage instability is foundby the sensitivityof the systembefore lowvoltage occursand the effectof noise is also takeninto consideration.The effectivenessof the systemisobservedbasedonavailability of Phasor Measurement Unit (PMU) configurationof the systemtogive the exactstatusof voltage instability. Finally impedance matching criteria is providedbutit showspoorsignificance in case of large-disturbance scenario.It gives the early detectionof impendingvoltage instability. 11} FACTS Devices: To increase the voltage stability margin and securityof the system, FACTSdevicesare usedwhichcan regulate the active and reactive powerof the systemdue to theirflexibility and quick operation. Introductionof FACTS devicesin suitable position improvesthe voltage stability of the system.Genetic algorithm is usedforthe allocation of FACTSdevicesconsideringthe cost of devicesand powersystem losses.The effectivenessof the systemistestedonIEEE 30 bus system.The enhancementinvoltage stability is done byusing three FACTSdevicesTCSC,SVCand UPFC.The application of geneticalgorithm methodandlocation of multi FACTS devicesis appliedfor enhancementinvoltage stability. The effectof SVCon IEEE 14 bussystemis studiedforinvestigating the effectof voltage stability in the system. 12} Artificial Neural Network : Analysisof voltage stability in the multi-buspowersystemis done byusing artificial neural networkand linear voltage stability indicator in . Linear voltage stability indicator is usedfor the predictionof voltage collapse of the systemwhile its combinationwith the ANN givesan estimation of voltage security of the system.ANN isusedfor the online monitoring of voltage stability of the system. 13} Fuzzy Logic : The applicability of this methodis studiedon the basis of sample 5 bus systemandIEEE 14 bus system.Thismethodgivesproximityof voltage collapse before systembecome unstable.
- 3. 14} Operation of Relay: Relaysare usedfor the protectionpurpose in the system, butits undesirable zone 3 distance relay operationleadsto the collapse of the system. Anadaptive algorithm is proposedwhichhelpsto avoid the undesirable distance of the relays.Thisalgorithm preventsundesirable relayoperationandmal tripping of relay.It helpsto maintain voltage stability of the systemandpreventsitfrom voltage collapse. 15} Excitation Control : Whenthe load on the supplysystemchanges,the terminalvoltage of the alternatoralso varies due to the changesin voltage dropin the synchronousreactance of the armature.Since the alternatorshave to be run at a constantspeed,the inducedemfs,therefore,cannotbe controlledbyadjustmentof speed. The voltage of the alternator can be keptconstantbychanging the fieldcurrentof the alternator in accordance withthe load.This is knownas EXCITATION CONTROLMETHOD.The excitationcontrol methodis satisfactoryonly forrelatively shorttransmission lines. 16} Booster Transformer : Sometimes,itis desiredtocontrol the voltage of a transmissionline at a point far away fromthe main transformer.Thiscanbe convenientlyachievedbythe use of a boostertransformer.The secondaryof the boostertransformeris connectedinserieswith the line whose voltage is to be controlled.The primary of this transformerissuppliedfrom a regulating transformerfittedwithon-loadtap-changing gear.The boostertransformerisconnectedin sucha waythat its secondaryinjectsa voltage in phase with the line. 17} Phase ShiftingTransformer : This is basedonthe conceptthat modificationof voltage magnitudesand/ or their phase can be achievedbyadding a control voltage.A special form of a 3-phase–regulatingtransformerisrealizedby combining a transformerthatis connectedinserieswith a line to a voltage transformerequippedwitha tap changer.The windings of the voltage transformerare so connectedthatonits secondaryside, phase-quadrature voltagesare generatedandfedintothe secondarywindingsof the seriestransformer. Thus the addition of small, phase-quadrature voltage componentstothe phase voltagesof the line createsphase-shiftedoutputvoltageswithoutanyappreciable change in magnitude.A phase-shifting transformeris therefore able tointroduce a phase shiftin a line. 18} Secondary Voltage Regulation: Automaticvoltage regulationof certain load buses,usuallyreferredtoas pilot buses,thatcoordinately controls the total reactive powercapability of the reactive powersourcesin pilot buses’areas,has proventobe an effective waytoimprove voltage stability. These are basically hierarchical controls that directly vary the voltage setpoints of generatorsandstatic compensatorsona pilot bus’ control area;so that all controllable reactive powersourcesare coordinatedto adequatelymanage the reactive power capability in the area, keepingsome of these sourcesfromreachingtheirlimits at relatively low load levels.
- 4. 19} SeriesCapacitors : Seriescapacitors are usefulin effectivelyshorteninglonglines,thus,decreasingthe netreactive loss.In addition, the line can delivermore reactive powerfroma strongsystemat one endto one experiencing a reactive shortage at the otherend. 20} Must Run Generation: Operate uneconomicgeneratorstochange powerflowsorprovide voltage supportduringemergencies or whennewlinesor transformersare delayed.
- 5. References 01 } ShodhgangaChapter-02,Voltage Stability. 02,03,04,05,06,07} D P Kothari, I J Nagrath,“PowerSystem Engineering”,secondedition,(TataMcGraw Hill publication). 08 } ArunaJeyanthy,P.,Dr. Devaraj,D.,“Optimal Reactive PowerDispatch forVoltageStability EnhancementUsingRealCodedGeneticAlgorithm”,InternationalJournalof ComputerandElectrical Engineering,Vol.2, No. 4, pp: 1793-8163, August,2010. 09 } Ian Dobson,Liming Lu, “NewMethodsForComputingA ClosetSaddle Node BifurcationAnd Worst Case Load PowerMargin For Voltage Collapse”,IEEE transaction on PowerSystem, Vol.8,no-3, pp:905- 913, August1993. 10 } MevludinGlavic, ThierryVan Cutsem, “Wide-AreaDetectionof Voltage Instabilityfrom SynchronizedPhasorMeasurementsPartII:SimulationResults”,IEEE Transactionson PowerSystems, vol. 24, no.3, pp: 1417-1425, August2009. 11 } Baghaee,H.R.,Jannati, M.,Vahidi,B., “Improvementof Voltage Stabilityand Reduce PowerSystem LossesbyOptimal GA basedAllocation of Multi-type FACTSDevices”,11th InternationalConference on Optimization of Electrical andElectronic Equipment, June 2008. 12 } BhavikSuthar,Balasubramanian,R.,“A NovelANN BasedMethodforOnline Voltage Stability Assessment”,14thInternationalConference onIntelligentSystemApplicationstoPowerSystems,ISAP Kaohsiung,Taiwan,November4– 8, 2007. 13 } Senthil Kumar,S.,AjayVimal Raj,P.,“FuzzyLogic basedStability Index PowerSystemVoltage Stability Enhancement”,InternationalJournalof ComputerandElectrical Engineering,Vol.2, no.1, February,2010 14 } Mattias Jonsson,JaapDaalder,E., “An Adaptive Scheme toPreventUndesirable Distance Protection OperationDuring Voltage Instability”,IEEE Transactions onPowerDelivery,Vol.18, no.4,pp: 1174-1180, October2003. 15 } V.K.MehtaandR.Mehta.Principlesof powersystem, Multicolorillustrative Edition, (S Chand Publishers,2009) 16 } V.K.MehtaandR.Mehta.Principlesof powersystem, Multicolorillustrative Edition, (S Chand Publishers,2009) 17 } R. M Mathurand R. K. VarmaThyristor-BasedFactsControllersforElectrical Transmission Systems,A JohnWiley & SonsInc.Publication (IEEE Pressand Wiley Interscience,New-York,2002) 18,19,20 } ShodhgangaChapter-02,Voltage Stability.