11 OUT OF STEP PROTECTIONIN MODERNPOWER NETWORKS D Paunescu,F Lazar Transelectrica,Romanis B PavlovJ ZakonjSek NEK - NDC, BulgariaABB Power Technologies,Sweden Significant changes of loadas wellas faults andtheir clearanceinpowersystemsoftenresultin electromechanical oscillations, whichgenerally donot affect the system stability. Major disturbances might on thcotherhandcausebiggeroscillations,whichcan influenceunwantedoperationofdifferentprotection functionsandi nthemostseverecaseslossof synchronism(polesliporsocalledout-of-step conditions)betweengeneratorgroupslocatedin different subsystems. .In the paper, we concentrate on so called asynchronous operationof powersystems, whichmaycharacterize theoperation of%dividualgenerating units as wellas their groups (power plants)connectedin parallel wilh thepowersystemoreventheoperationoftwo interconnected power systems. The asynchronous mode of operation is characterized by deep acliveandreactive poweroscillations withtime limited constant excitation of synchronous machines. It producesimportant disturbancestothepowersystem operatingparameters,tothequalityparametersof deliveredelectricalpower,causessevereelectricand mechanicstress of primaryequipment, etc. It tendsto extend very fast to the remaining parts of power system. Disconnectionofsubsystemsinpointsclosesttothe electrical center of oscillation proved to prevent the best waycollapseofacompletesystem.Developmentof modemtechnologymadeitpossibletointegratethe functionalityofonceveryexpensiveandfromthe system pointof viewinsufficiently selective generator poleslipprotectiondevicesintolineprotection terminals. It is at the same time adjusted to the specific systemrequirementslike:operationduringpoleopen conditions, oscillation development caused by the faults and during their clearance, etc. Additional arguments justifying the introduction of pole slipprotectioninmodempowersystemscouldbe specifiedasfollows.1)UCTErecommendation regardingthepreventionofdisturbanceexport between interconnected systems by installationof pole slipprotectiononimportant tielines.2 ) Experiences gainedduringtheanalysisofmajordisturbances.3) Possibilitiesofmodemmicroprocessorbased technology used in state of the art numerical devices. IntheviewoffurtherUCTEextensiontowardsthe South ~ East Europe it has been decided by NEK-EAD inBulgaria as wellas by TranslectricainRomania to installpoleslipprotectionontheirallimportanttie lines.Thepaperdescribes,togetherwithsome theoreticalbackground,alsoapracticalexampleof applying pole slip protectionon a 400 kVtransmission linebetweenS/STantareni (RO)andKozloduy(BG) from two different pointsof view:system studies and selection of protection setting parameters. THEORETICAL BACKGROUND Atwo-machinemodelaspresentedinFigureIcan alwaysbeusedtopresentoscillatingpartsofpower system regardless theamountofoscillatingmachines, which oscillate relative to each other. Figure I : Basic model of a two-machine system Power flow in a loss-less two machine system depends on voltage difference between left and rightside EMFs itandE, respectively and total system impedance as defined by (1). Only corresponding reactance applies for loss-less conditions. ,(1) . .. .z, =z, +ZL +z, ActivePand reactiveQ powerflows in relaypoint REL are functions of6 angular difference between two EMFs as presented by the following equations. E. E,Q =-j.cos( 6) t ., Bulgaria representedbyNEK - EADandRomania representedbyTranselectricaareregularUCTE members from 08 May 2003. 0 2004 The Institution of Electrical Engineers. Printed and published by the IEE, Michael Faraday House, SixHills Way, Stevenage. SG1 2AY Here isk=lb?Rl/l&I , Diagram in Figure 2presents a typicalexampledftransmittedactiveandreactive power within the two-machine system asa function of angledifference6.Itisinterestingthatactiveand reactive powerdo notfollow the same dependence on rotor angle difference. The reactive power flow is at the same time dependent very much on difference between themagnitudesofbothEMFs.Thereisnoreactive power flow when theyareequal bymagnitude andin phase.Maximumreactivepowerappearswhenboth EMFs have opposite direction. Active power isinthis case zero. Thefirst partof active power dependence inFigure 2 for6 =(0 - 180) deg is known as a power-angle curve andwelldescribedinbasicliterature.Thesocalled equalareacriteriaisalsoawellknownapproach towards the estimation of dynamic stability for a power system and for this reason not addressed more in details within this paper. , 1,-Figure 2: Typical example of active and reactive power flow as a function of6 angle between two EMFs IMPEDANCEMEASUREDIN RELAY POINT Swingingofactiveandreactivepowerbetween differentpartsofasynchronouslyrunpowersystem reflectsalsoindynamicperformanceofmeasured impedances in different relay points. Measured current in relay point REL in Figure 1 follows the expression: TherelayvoltageasmeasuredinrelaypointRELis equal to: Impedanceasmeasuredbyimpedancerelaysinrelay point REL is this way equal to: If we place the relay point REL in coordinate center of an impedance plane, than it is possible to show that the impedance trajectories asseen bytheimpedance relay follows the equation of circles, as presented in Figure 3. The impedance trajectory passes in at least one point the system impedance line between impedance points-Zs,. and(Z, +Z,). This point is called "electrical center of oscillation",ECO. The diameter ofthecircledepends on value of factork=E, / E, and becomes infinite for the case whenk=1 . Impedance trajectory becomes in thiscaseastraight-lineperpendicularonthebefore mentioned system impedance line. It is evident that such performance of measured impedance may influence the securityofdistancerelaysonpowerlinesaswellas otherimpedancemeasuringelementsinthepower systems. Figure3:ImpedancetrajectoriesinrelaypointREL during pole slip conditions in a two-machine system 1 1 3Figure 4: Phase voltage and current in relay point at the beginning of pole slip and three consecutive slips Oscillationsarecharacteristicforphasecurrentsand voltagesaswellandpresentedforatypicalcasein Figure 4.Here we can observe typical increase of slip frequencyforconsecutiveoscillations,whichrequire adaptiveapproachalsoinoscillationdetection measuring elements, independent on basicprinciples of their operation. Unwantedoperation of overcurrent and undervoltage basedprotectionfunctionsinrelaypoint during system oscillations must be prevented by correct selectionoftheirsettingparameters.Operationof impedance measuringelementsmustbeprevented by specialoscillationdetectionelements,implemented in distance protectiondevices. OSCILLATIONDETECTIONBASEDON IMPEDANCEMEASUREMENT Differentoscillationdetectionmethodsareinuse worldwide.Twomostspreadbetweenthemareso calledM/AtandU . cosqmethods. The first one is implementedmainlyinlinedistancerelaysandthe second one in pole slip protectionused closed to bigger generating units. Within this paper we concentrate more onthefirstmethodbecauseitisimplementedalsoin protectionterminalsinstalledontielinesbetween Bulgaria and Romania. TheoperationofasocalledAZ/ At oscillation detectionprincipleisbasedonafact .that impedance measuredinrelaypointchangesduringoscillations relativelyslow in comparisonwith nearlyinstantaneous change fromloadtofaultimpedance duringdifferent faults in primaty system. Figure 5presents this principle by simplified schematic. ,-., -+Figure 5:=/ At principle based oscillation detection Oscillation OSC is detected if the impedance measured in relay point needs to enter the internal boundary INT with time delay longer than the time delay t set on time measuringelementafterithasenteredtheexternal boundary EXT. It should be noted that the time element needsanadaptivetimedelay,whichmustbefor consecutive slips shorter thanthe initial time delay for the first slip.This is of outmost importance in order to distinguish betweenfastconsecutiveslipsandslowly developing faults, whichappear forexampleinseries compensatednetworksorduringslowlydeveloping earthfaults (e.g.whenthefaultdevelops overanice coat around the phase conductor). Powersystemoscillationsareingeneralnotathree phasephenomena.Theycanbeinitiatedbydifferent faultsinpowersystemandmaydevelopfurtheron duringthefaultclearingprocessorduringthetime whenthepowersystemisstillinemergentstate. Typical example of such performance is development of oscillation over a power line, whichoperates only in a two-phasemodeduringthedead-timeofsingle-pole autoreclosing cycle. OUT-OF-STEPPROTECTIONONTIELINE BETWEEN BULGARIAAND ROMANIA Installation of out-of-stepprotectionon 400 kV tie lines between Bulgaria and Romania is justified by different reasons,asithasbeenalreadymentionedandalso proved on different practical examples. Specialistsfrompowerutilitiesinbothcountries performed a comprehensive study of systemconditions underwhichout-of-stepmodeofoperationbetween Bulgarianand Romanianpowersystems could appear withECO on or close to the 400 kV tie line Tantareni (RO)- Kozloduy(BG).Additionaltaskswerealso: determining the expected slip frequency and settings for the pole slip protections, whichare of the same design at both line ends. The results of studies confirmed that double circuit 400 kV line between SIS Tantareni and Kozloduy represents a very stable system connection. It is nearlyimpossible thatout-of-stepconditionswouldappearoverthis circuitundernormaloperatingconditions.The asynchronousoperationof generatingunitslocatedat theendsofthislinecouldstartonlyunderextreme system conditions.Oneof themconsiders 400kVtie linesSofiaWest(BG) ~ Nis(YU)andBlagoevgrad (BG) ~ Thessaloniki (GR)out of operation, export of 1000 MW from the BG power system and a three-phase shortcircuitat400kVKozloduyNPPbus-bar eliminated with0.5s time delay. Figure6 presentsthe impedance trajectory as seen by the pole slip protection installed in Tantareni (RO) end of the tie line. Measuredimpedance changes veryfastfrom pre-fault load conditions (-120 +j25) ohmto impedance marking theendof protectedline(Kozloduy).Faultclearance alsopresentsfastchangeinmeasuredimpedanceto approximately(0+j25)ohm.Herestartsslower movement of impedance in anticlockwise direction and exitstheimpedancemeasuringareacompletely.The oscillation wouldcontinue further on and even change itsdirectiontotheclockwiseone,ifitwouldbe permittedtocontinuewithoutanyactionfromthe installedpoleslipprotection,whichoperating characteristic is also presented in Figure 6. Measuredimpedancetrajectotyenterstheexternal impedance boundary again at approximately (-17+j60) ohmand withtime delay longer than 40 ms (set value) also the inner impedance bonndaty at (-17 +j40) ohm. SO 60 40 E d20 ?i n -20 4 0Figure 6:Impedance trajectory as seen by pole slip protection in Tantareni Si S after a three-phase fault at Kozloduy S/S Thisissufficientconditionforprotectiontodeclare system oscillation. Big generation units behind line ends do not permit long oscillations with more than one slip. The pole slip protection is for this reason set to trip the corresponding circuit breaker even before the complete slipreallyappears,inthiscasewhenthemeasured impedancereachestheleAtrippinglineat approximately (-5 +j28) ohm. Theoperating characteristic of the poleslip protection appliedprovidesapossibilitytocontrolthetripping conditionswithrespecttothephaseanglebetween EMFsofpowergeneratorsinbothsubsystems.This preventsadditionalstressesofprimaryequipment.It alsoprovidesremotehack-uppossibilitiesforswings with their ECO behind the remote ends of the protected line (see the area around the Kozloduy Si S in Figure 6) .CONCLUSIONS Asynchronous operating conditions are a fact in modem powersystemsandshouldheeliminatedasfastas possible in order to keep theirstronger parts in normal operation.Introductionofpoleslipprotectionontie linesprovedtobeoneofthehestsolutions.This requires comprehensive stability studies under different operatingconditionsandin manycases alsoexcellent cooperation of experts from different utilities, and even different countries. Operating characteristics of applied pole slip protection devicesmustsecurecorrectoperationunderdifferent systemoperatingconditions.Theymustdetect oscillations caused by faults and similar big changes in powersystemaswellasoscillations,whichappear during emergent system operating conditions. References 1.YGPainthankar,2000,TransmissionNetwork Protection - Theoly and Practice 2. ABB Manual IMRK 506 074,06-2001