improve reliability using phasor measurement units for smart grid_sajal jain

8/8/2019 Improve Reliability Using Phasor Measurement Units for Smart Grid_Sajal Jain

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ImproveReliabilityUsingPhasorMeasurement

UnitsforSmartGridSajalJain,9028654127,USCViterbiSchoolofEngineering

EE444

Power

System

Technology

Abstract

For the transmission and distribution system to become more affordable, reliable andsustainable the grid needs to become smarter. During the past few years a considerablenumberofactivitieshavebeencarriedout inUnitedStates toachieveasmartpowergrid.SmartGrid isenvisionedtouseallpresent technologies intransforminggrid intelligently inbettersituationalawarenessandoperationfriendliness.Atthesametimeblackouts inpastandreliabilityofgridaremajorissuesforsystemengineers.Thispaperdescribesasystemtoimprove the security of grid against false faults and load swing. This system called,thePhasorMeasurementUnit(PMU)isanimportantandpromisingtechnologyinmakingthefuturegridsmarter.

1. INTRODUCTIONThe societydependson reliableelectricityasanessential source for security,healthand

welfare;communication,finance,transportation,food,watersupply,heating,cooling, leisure,

computers,entertainment,education,andalmosteveryaspectoflifewecouldthinkof.People

always expect that electricitywill be availablewhen they flick on the switch. But providing

electricity with high reliability is a tough challenge with so many components and unseen

conditions. After generation of electricity, it requires a reliable, efficient and affordable

transmissionsystemtodeliverpowerfromutilitytocustomer.

Theelectricityisgeneratedatlowervoltagesofabout10KVto25KVfromvarioussourcesof

generationandthensteppeduptohighvoltageof230KVandabovefortransmissiontoreduce

losses, transmit bulk power reliably and economically over long distances. The transmission


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lose of power, loss of communication, etc. This situation is known as blackout or rolling

blackout.

2.BLACKOUTS

Thecascadingfailuresinapowersystemwhichresultinthelossofpoweroveraregionfor

considerabledurationoftimearereferredasblackout.Thesehaveamajordirectandindirect

impactontheeconomyandnationalsecurity.Althoughlargecascadingblackoutsarerelatively

rare, their impact on the system results into such a high risk that it becomes necessary to

mitigateandavoidtheminthebestpossibleway.

TherehasbeennumberofblackoutsinUnitedStatesoverthetimewhichresultedinnew

transmission policies for reliability improvement every time. Themajor blackouts and their

causesandimpactwere:

2.a. November9,1965Oneof the five230KV lineoperating fromBeckplant inOntario to theTorontoarea

wentdownduetooperationofabackupprotectionrelay.Whiledistributionofpoweronthe

fourremaining lines, theystartedtrippingevery2.5secondsresulting intohugepowerswing

and cascading outage. It blacked out almost entire northeast affecting around 30 million

peopleslife.Ittookabout13hourstobringbackthepowerfortheentireregion.

2.b. July13,1977ThisblackoutthoughconcentratedtotheNewYorkCityonlybuthasmajor impacton

theeconomyandsecurityofthepeople. Ittriggeredfromatotalcollapseoftwo345KVlines

onacommontowerstruckby lightingandtrippingoff.Theutilitydispatchertriedtosavethe

systemoverthenexthour,butthesystemcollapsedandresultedintoblackoutwhichaffected

9millionNewYorkCityresidentsandlastedforabout26hrs.

2.c. July2,1996Inthesummerof1996intheWesternNorthAmericaalinetogroundfaultduetoatree

resultedintoflashoverofa345KVline.Theprotectivesystemsdetectedthefaultandoperated

todeenergizetheline,butduetofaultyoperationofprotectiverelayonaparalleltransmission

line itdeenergizeda second line.The lossof2 lines from the system reduced theabilityof


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systemtocarrythepowerfromgeneratingstationtoloadcausingshutdownoftwooutoffour

generating units at the plant. This resulted into unbalance between load and supply and

frequency begin to decline. The system became unstable, automatic protection systems

initiatedandoutagesoccurredforfewcustomerstosavetheentiresystem.Itaffectedaround

2millioncustomersand tookfromfewminutes toseveralhours forcompletelyreenergizing

thesystem.

2.d. August10,1996Inthesamesummerof1996anotherblackoutoccurred.Thistimethesystemtriggered

from random transmission lineoutages, resulting into system instability causing fourelectric

islands inwestern interconnection. It isbelieved thatbefore theevent the lineswereheavily

loadeddue toextremedemandcausedbyhotweatherday throughoutmostof thewestern

regionandalsobecauseofhighelectrictransfersfromCanada intonorthwesttoCaliforniaas

thehydroelectricplantswereworking inexcellentcondition.Againdue to trees touching the

linesmultiple short circuitsoccurredon 500kv lines resulting into cascading outagesdue to

overloads. Itwasalsodiscovered theoperatorshaddone theadequateoperating studies. It

affectedaround7.5millioncustomersandlastedforabout9hrs.

2.e. July25,1998Theblackouttriggeredfromalightningstrikinga345KVlineinMinnesotaandinitiating

systemprotectiontodeenergizetheline.Thisresultedintooverloadingofthelowvoltagelines

intheregion,butthelightningstruckanother345KVlineandprotectivesystemdeenergizedit

too. This resulted into extreme overloading of lower voltage transmission lines and system

protectionbeganremoving them fromservice.Thecascading removalof linescontinuedand

entirenorthernMAPPregionformedthreeislands,resultingintoblackoutofthenorthwestern

OntarioHydrosystem.Itaffectedaround152thousandpeopleandlastedforabout19hrs.

2.f.

August

14,

2003

ThebiggestblackoutinthehistoryofNorthernAmericaresultedintoshuttingdownof508

generatingunitsin265powerplantsacrossthenortheast.50millionpeoplenotonlyinUSbut

Canadatoowereleftwithoutelectricity.Itisbelievedthatduetoveryhighelectricdemanda

generatingunitinEastlake,OhiowentofflineputtingstrainonHVtransmissionlines.Butagain


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short circuits occurred due to overgrown trees, resulting into cascading effects ultimately

forcing to shutdownmany power plants. The various direct causes and contributing factors

included:

o Failuretomaintainadequatereactivepowersupporto Failuretoensureoperationwithinsecurelimitso Inadequatevegetationmanagemento Inadequateoperatortrainingo Failure to identifyemergency conditionsandcommunicate that status toneighboring

systems

o Inadequateregionalscalevisibilityoverthebulkpowersystem.

Thevariousreportsonblackoutshaveshownthattheseblackoutswerepreventable.Most

of the instances due to inefficiency of the system and operator to respond in such a short

periodformultiplecascadingeffectshavebeenthereason.Thoughafter1965blackoutseveral

regulationsandrecommendationsweremadetopreventsimilarsituationtoariseinfuture,still

August2003blackouthasmanysimilaritiestotheearlierones.

For improvementofthereliabilityofthesystemwerequirecomprehensivemonitoringof

thesystem,trainingandenforcementofstandardswithinthesystem,sothatitcouldresponsetocriticalsituation,earliestpossibleoveralargerareaintheshortestpossibletimeperiodwith

minimumhumaninterferencerequired.

3. SMARTGRIDAccordingtoTheSmartGrid:An IntroductionpublicationbyUSDepartmentofEnergys

OfficeofElectricityDeliveryandEnergyreliability,Asmartergridappliestechnologies,toolsand

techniquesavailablenowtobringknowledgetopower knowledgecapableofmakingthegrid

workfarmoreefficiently

Ensuringitsreliabilitytodegreeneverbeforepossible Maintainingitsaffordability Reinforcingourglobalcompetitiveness


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Fullyaccommodatingrenewableandtraditionalenergysources Potentialreducingourcarbonfootprint Introducingadvancementsandefficienciesyettobeenvisioned

It is a technology thatwill force theUtilities and suppliers to redesign the electric grid and

rethink itsoperations. Itwillenabletodeliverelectricityto theconsumerfromsupplierusing

the complete power system with ability to save energy, improve efficiency, quality and

reliabilityandreduceconsumercost. Itwillenabledistributedgenerationgridconnection,grid

energy storage for distributed generation load balancing and containing failures due to

widespreadpowergridcascadingfailuresbyusingadvancedsensing, information technology,

networks,communicationtechniques,control,transportanddistributeelectricity.

Fig3BasicSmartGridIngredients


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4. PHASORMEASUREMENTUNITS

4.1 INTRODUCTIONAphasorisacomplexnumberusedtodeterminemagnitudeandrelativeangleforavoltage

andcurrentwaveform.Aphasormeasurementunit (PMU)alsoknownasSynchrophasor, isa

devicewhich inadditiontovoltageandcurrentphasorscanalsomeasuresimultaneouslyand

synchronizetheassociatedfrequencyandelectricpower.Themeasurementsaresynchronized

throughGlobalPositioningSatellite(GPS)usingonepulsepersecond(1pps)asthereference.

Fig4Phasorrepresentationofasinusoidalsignal(a)Sinusoidalsignal(b)Phasorrepresentation

Thesinusoidalwaveformisrepresentedas

x(t)=Xmcos(t+ )

Thenthephasorrepresentationofthesignalisgivenby

X=Xm/sqrt(2)*ej=Xm/sqrt(2)*(cos +jsin)

It canbenoted that signal frequency isnot states in thephasor representation.Thus the

phasorimpliesastationarysinusoidalwaveform.PMUsuseadatawindowwhichisoneperiod

of fundamental frequency of input signal. If the system frequency deviates PMUs uses a

frequencytrackingandseparatethefundamentalfrequencyanditsphasorrepresentation.

Asynchrophasorinfrastructureconsistsof3layers:

1. MeasurementLayer,madeupofPMUsconnectedattransmissionvoltagelevel


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2. DataCollectionLayer,madeupofphasordataconcentratorstocollectandsynchronizedata

3. ApplicationLayer,madeoftoolsofPMUdataforgridoperatorsandofflineanalysis.

Fig5 ARepresentativeSynchrophasorsInfrastructure

4.2 APPLICATIONS

a. PowerSystemMonitoringPresentlysystem ismonitoredusingstateestimatorsoftwarebasedonmodelsanddatafrom

SupervisoryControlandDataAcquisition (SCADA) to findoutvoltagemagnitudeandangles.

Thesearemeasuredinintervalsofseveralseconds.ButwithPMUsinsteadofestimatingactual

measuringofsystemstatecouldbedone.Therealtimemonitoringand timesynchronization

will continuouslyanalyzeoperatingconditionsand inform theoperatorsabout stressedgrid.

Dynamic systemmodels couldbe improvedbydetecting and analyzing interareaoscillation

modesandcanfurtherbeusedtofinetuneandoptimizeexistingsystemstabilizers.


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b. PowerSystemStateEstimationPriortoPMUsstatecouldnotbemeasuredbutjustinferredusingunsynchronizedpowerflow

measurements. But now state estimation algorithms use measurements of line flows and

injections, both real and reactive power, to estimate all bus voltages and magnitude. It

improvestheaccuracyandrobustnessofbaddata,fastersolutionstolinearsystemproblems,

and availability of data on external network. Also PMU derived state estimation provides

possibility for 3phase or 3sequence state estimator to monitor phase unbalance due to

groundingorequipmentdegradation.

c. PowerSystemEventAnalysisPMUs provide high resolution data for dynamic event analysis. Earlier data recorders and

loggersdidnthavetimesynchronization,makingthejobofunderstandingandreconstructinga

timeline of what happened very difficult and time consuming. But with PMUs and GPS

coordination troubleshooting time can cut down from few hours to few seconds. NERC

reported that data recorded by PMUs during 2008 Florida event helped the event analysis

tremendously.

d. LineParameterCalculationPMUdatafromendsoflinecanbeusedtocalculateactuallineparameters.Ithelpsinverifying

designdatabasedon linegeometry.Lineparametermonitoringandcalculationcontinuously

canhelpinmodelingthechangeslineparameterwithexternalfactorslineisexposedto.

e. RealTimeCongestionManagementItisdonetomaintainrealtimeflowacrosstransmissionlinesandpathswithinreliabletransfer

capabilities. It is an important function tomanagedemand in an economicmannerwithout

challengingtransmissionlimits.Forcongestionmanagementactualflowonaline iscompared

tonominal transfer capability (NTC)of the linewhich isprecalculated.Thisprecalculations

havelimitationofthermalfactors,voltageandstabilityconditions.TheassumptionsforNTCare


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conservativeandcan result inexcessivemargins incongestionmanagement.ButwithPMUs

highly accuratemeterdata in real time conditions, calculations forpath limit andpath flow

improves highly. The high speed realtime algorithms provide Real Time Transfer Capability

(RTC)limitswithcriticalstabilityandvoltagepaths.

f. PowerSystemProtectionDistancerelayszone3orbackupprotectionsmanytimetripsduetoloadencroachmentduring

powersystemdisturbances.Thisforcesprotectionengineerstoremovebackupzone,essential

fordownstreamprotectionincaseoffailureofprotectionsystemtoremoveafault.ButPMUs

avoid false failure by back up zone supervision. PMUs uses wide area measurements for

restrainingbackuprelaystooperateincaseofpowerorloadswingbutcoordinatingwithother

PMUsdataanddiscriminatingloadswingandfault

g. AdaptiveProtectionConventionalprotectionsystemrespondstofaults inapredeterminedmanner irrespectiveof

the prevailing system conditions. Adaptive relaying assumes that system characteristics and

protection parameters should be coordinated according to prevailing conditions. PMUs

application for adaptive protection with out of step relays and line relays provide better

securityanddependability.PMUshavemuchaccuratemeasureof line impedance foractual

faultlocation.PMUusesdatafrombothendsofthetransmissionlineforfaultcalculation.

h. PowersystemControlPrior to introduction of PMUs the system controlwas set up locally.Many subsystems like

machinesonly had local control signals. Butwith the adventof Synchrophasors the remotecontrol based uponmeasured quantities has taken place.A time tag is associatedwith the

phasor data so that control of the system based on past conditions can be calculated. The

frequencyofmeasurementsevery1560Hz i.e.14timespersecondhandlesthecontroltask

efficiently.


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5. CONCLUSIONIt is believed that smart grid compared to present transmission grid is something like

supercomputersaretoabacus.Itismorethanjustapowergrid,withtwowaycommunication

and information technologybeingan integralpartof it. Itoffershigherchallenges to system

protectionengineers forenhanced reliabilityof thegridwithnew standardsand regulations.

Phasormeasurementunitshavebeenastateofart tool thathasproven itsworth insolving

existing problems and better understanding of the power system. For implementation of

phasor measurement technology by utilities, identification and selection of applications

suitableforthebenefitoftheindividualsystemandtheinterconnectedgridisamust.

ItisbelievedPMUtechnologyislikelytobeimplementedinitiallyfor:

a)validationofsystemmodels,and

b)accuratepostmortemanalysis.

Then with experience in PMU data and real time operation, much more complex state

estimationalgorithmsforenergyandsystemmanagementcouldbedeveloped.Ultimately,the

goal is to improve protection and control functions and eliminate catastrophic failures or

reduceseverityofsuchfailuresfromthefutureofpowersystem.


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6. REFERENCES1. Grid2030:ANationalVision forElectricitys Second100Years, July2003byUnited

StatesDepartmentofEnergyofficeofTransmissionandDistribution

2. TheSmartGrid:AnIntroductionpreparedfortheU.S.DepartmentofEnergybyLitosStrategicCommunication

3. NationalTransmissionGridStudy,May2002byU.S.DepartmentofEnergy4. Examplesofmajorbulkelectricsystempoweroutages,NERCwebsitedocuments5. "FinalReportontheAugust14,2003BlackoutintheUnitedStatesandCanada: Causes

andRecommendations,"U.S.CanadaPowerSystemOutageTaskForce,April5,2004

6. Farhangi,H,Thepathof the smartgrid IEEEPowerandEnergyMagazineVolume8Issue1,JanuaryFebruary2010

7. DeLaRee,J. Centeno,V. Thorp,J.S. Phadke,A.G.,SynchronizedPhasorMeasurementApplicationsinPowerSystemsIEEETransactionsonSmartGridVolume1Issue1June

2010

8. Bhatt, N.B., Role of Synchrophasors Technology in Development of a SmarterTransmissionGrid IEEEPowerandEnergySocietyGeneralMeeting2010,2529 July

2010

9. Skok,S. Ivankovic,I. Cerina,Z.,ApplicationsBasedonPMUTechnologyforImprovedPowerSystemUtilizationIEEEPowerEngineeringSocietyGeneralMeeting2007,2428

June2007

10.Tholomier, D.; Kang,H.; Cvorovic, B., PhasorMeasurementUnits: Functionality andApplications,PowerSystemsConference,2009.PSC'09,1013March2009

improve reliability using phasor measurement units for smart grid_sajal jain

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