ieee-c. taylor - improving grid behaviour

8/3/2019 IEEE-C. Taylor - Improving Grid Behaviour

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Improwing( AKSONonneville'TAYLORower behavior

Administration

The inglorious sum me r-of-I99 6 blackouts taug ht the West to improve emergencycontrol and protection and t o sharpen simulation techniques

HE ELECTRIC POWER INDUSTRY is be maintained in th e more competitive environnientundergoing rapid institutional and tec hno - char was em ergin g. In what to l lows, a brief descr ip~logical chang c Utilities are splittin g up into tion of the summer-of-1996 failures precedes an outline'Ienrr atm n, tranbmissinn, and distribution of th e most important lessons learned and the actionscompanirs, and m rrnbers o l the first group especially taken to reduce th e risk of similar events.arc bccuming more cnrnpelitive,with more new inde-p r n d e n t p o w e r pruduce r s en te r ing the f r ay . The rootsof power lossTcchn


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Explainingreactive powern an ac circuit, the current generallyleads or lags the voltage. Thus, theI urrent consists of an active compo-

nent inphase wi th the voltage. and anout-of-phase, or reactive, component.The activecomponent, measured nwa rn (vo ltage imes in-phase current),i s the part that powers equipment.The reactive power, measured in volt-amperes reactive (VAr), represents theenergy stored inelectric or magneticfields. (These relations ohe n are de-picted as a right-angle triangle, whereapparent power is the hypotenuseand active and reactive power are theother twosides.)

Reactive power is consumed or ab-sorbed in the magnetic field of in-ductive equipment; it is produced orgenerated in capacitive equipment.Induction motors consume it becauseof their magnetizing current require-ments (consumption even at no load)and the inductive reactance in theirwindings. Shunt capacitor banks pro-

either produce or absorb it.t o depress transmissionvoltage,whileitsproduction or injection ends to sup-por t voltage. Transmission lines bothconsume it (because of their seriesinductance) and produce it (from heirshunt capacitance).

Because transmission line voltage i sheld relatively constant the productionof reactive power s nearly constant. Itsconsumption, however, is low at lightload and high at heavy load.

The variable net reactivepower re-quirements of a transmission ine giverise to a vo ltage control problem. Gen-erators and reactive-power mmpensa-tion equipment must absorb reactivepower during light load, and produceit during heavy load.

Ina general emergency, when thereare outages and high l oading on re-maining transmission lines, those linesconsume reactive power tha t must besupplied by nearby generators andshunt capacitorbanks. (Reaaive powercan be transmitted only over relativelyshort distances.)If reactive power cannot be suppliedpromptly enough in an area of decay-ing voltage, voltage may in effect col-lapse. Insufficient voltage support mayin addition contribute to synchronousinstability. -C.W.T

IA i y i t r m l mhcrcnt risk (11 laiiurr ilrprnd,

on t i i c i m s wch a5 tlw magnitudcs nt p o pi i lat ion an d loads in r e l a t i m i ro generationIP~OUICCS .Highly meshed transniission n c r ~work5 Witli eveii ly distnhuted oad and ge i i~erat io i i ar c i i i o rc rcliablc t h a n inctworksarranged lonpirudinally. n loops, or radially(a s on pcninsular, for instai>cc:i.

Chamcrcnsticof rhcwcstcrn NorthArncr~ica i i intcrconncctcd power sysrcm arc longdistances bctwc cn gcncrarion an d load arcas[Fig I] Geographic tcaturcr likc large u n ~pupulatcd dcrcrt a m i m a n hat thr trans^m i w u n nctwurk i s wrakly rne\hrd, withmajorlmrs l u r ming a loop nrtwur k aroundarras in I l b h arid Nrvada Th, 'KIII' OLCXl,lockr wpport Irom t h e w m tIn ipring a n d wi i i n i c i . the i m i n powertransfers flow from hydroelectric resources111 rhe Pacific Nol-thwesr (including Briti5hColumbia) to Calitornia Power also t lowsf rom coalLfircdgcncrarion far inland to tlicload cciitcrs on thc Pacificcoast. The majorpowcr transfcr path from northcrn BritishColumbia to Paci f ic coast load centers a p ~proniniatcs a longitudinai powcr sysrcm.Load and gcncrarion sires along this pathincludc Vancowcr . S e a t t l c . Por t l and . SanFrancisco Lor .Angelcr. a n d San Dicgu.The 2 July failureOn Ttir.uIay, 1 u l \ ~ .c i n p c i a t u i r ~ erearound 3R '( 1 i n wii t l i r r r i Idaho and Utah.an d loads were verv high. :Ah>,ower c x ~ports tnxn thr Pacitic Northwest ro Calktornia o n t l ie Pacif ic hC and DC intcrticsw r r e hlgh 8,43uo tMW? and ?nu0 5IW.rcspcctivcly). Thc Pacific AC liitcrtic rat-iiig at tlic California Orcgnn border s 4RWh1W an d thc I'acitic HV I IC II>ICI.~ICating


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[Fig . 21 Fur the r ca scad ing sepa ra ted th epower system into tive electrical islands.The next day the same ini t ia l eventsoccurred. Cond i t ions wer r s l igh t ly ICESs t r e s sed , however , and Idaho Power C o .operators were able to trip hOO hlW of loadwhen voltage started to collapre. T h i q actioncon ta ined th e d i s tu rbance , p reven t ing arepeat of the 2 July wide-area failure.The 10August failureTh e Saturday afternoon of 10August wasunusually warm in the Pacific Northwest-38 "C i n Po r t l and , O rc . Reco rd r a infa llmeant hydroelectnc c onditions were excel-lent In late summer, in any case, generationcapability is highest in British Columbia,and powrr transfers trom Canada throughthe s la tes oi Oregon and California were

near ra tcd values . But thc ra in and hotweather between them had made trees growfaster than usual, and r ight-of-way main-tcnancr had not kept up.So tree faults, even betore the m ain out-age , had pu t th ree 500-kV l ine sect ionsf r om t h e l o w e r Co l u m b i a R i v er to t h ePor t l and , Sa lem, and Eugene loadcen te r sin Oregon ou t of service. Wh ile these lineswere lightly loaded , their capacitance pro-v id e d m u c h ~ n e e d e d eact ive power sup^p o r t f o r t h e h i g h C a n a d a - t o - Ca l i f o r n i apower transfers.T h e serious trouble started with an out-age of thc Keeler-Allston line, whe n an ele-men t s agged in to a t r ee , s eve r ing the500-kV path between Seattle and Portlandtu th r west of the Cascade Mountains . Thel ine loading was over 1300 MW T h e

Canad a-to-California power increased ont h e l incs cast of t hc Cascade M oun ta ins ,causing a voltage dcprcssion in t he lowerColumbia River area. Irnpropervoltagecon-trol at thrre pow cr plants then contributedto the problem, with voltagrs sagging fromaround 540 kV to 504-510 kV.T he transmission line outages overloadedparallel lower-vo ltage lines in the Portlandarea. About five minutes later, a relay failuretr ipped a Il5~kVine , and a 23O-kV lineragged into a tree, also tripping. About thesame time generators at the McN aly hydro-electr ic p lant s tar ted t r ipping because offaulty relays. I n the course of 80 seconds, al l1 3 units tripped, further reducing voltagesupport an d adding to system stress.Increasing oscillations soon causcd syn -chronous instabi li ty T he ensuing cascade

-__500 -h

s400 -- 500-kV voltages300 -

200 1 " " I " " i " " I " "0 10 20 30 40 50

Time, seconds

I

I

I

I21 Portable power system monlton WSMs)pmduced the data Ilef tl on voltage sags andresponses at the Garrison and Malin MO-kVsubstations during he 2 July 1996 outages.Garrison, in western Montana, yieldedthe best overview of switching actions anddynamic performance n the ares eastof th eCascades. Mafin.on he Oregon-Californiaborder. is a key Pacific AC intertiestation.reactionsto specific events. Onea m p l e sthetrlpplngof IMXIMWatthelimBtidgerJenerating natlon in wthwestern Wyoming.hnother ir the trippingof the crwlal Ampsline between western M O ~ Mnd Idaho.

This last went prompted the p l u m nvoltageat Malin. Wimout the A m p line rip,sparitor banksat Malinwould YK)n havenjectedthe inewith up to 4M)MVAr,possiblyitabilizingw b e tabove500 MI.

An expansionof thatdata [helow] darifies

>xg 530-sMaiin shunt capacitor banks(2 - 200 MVAr were setto insert a t 530 kV with 5 second and 8 second delay.The time delays are now much shorter.

5 0 0 1 I I I I I , , , , , , I I , , , ,0 10 20 30 40

Time, seconds0

T \Y L O H ~~ C H I U S ON THC LO


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tnppmg u t transmission line5 hmke the interLunncLtion into four e~ectrica ~slands withhat the 1996 ma swr load and generation loss [Table I ]

Ueacting to the power failures committeesof he Western Systems Coordinating Counal(WSCC)prepared reports recommending 144actions The WSCC i s one of the 10 regionalreliabilitv councils of the North AmrricanElectric Rcliability Council (NERC)Lessons learned, actions taken

American Electric An important umclusion of the WSCCreports was that powr r system conditions o n10August had not been adequately studiedearher and that operatorr hadunknow ingly opera t ed thr system In a condition in which outageut the Keeler-Allrtnn l ine could led to cascadinn outaner C ascadme outapes resulting rom

with' wnan - for nonc&plm&Tne Plann ng Standards empha-

size the desjgn and operation ofcontro l and protection, system mon-:tor,ng, and power plant resting orvalidating simulat;on models anddata. One example isa requirementthat synchronous generators con-ne@ to the transmission networkoperate tn a voltagecontrol mode.

Overall, the NERC Operating Stan-dards specifyben eng.neeringvac-tice in many areas affecting thedynamic performance and reliabil-ity of power systems.

A


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sively high power schedules to California.The water spill requirement i s ongoing,hutunits a t each plant have been modified.Some unirs can now be operated as unloadedsynchronous motors providingvoltage sup-port by excitation control. Compressed airi s used to "unwatei' the units so that the NI-bine i s spinning in air.As additional reactive power support, two55O-kY 460-MVAr shunt capacitor bankswcre installed n the lower-to-midColumbiaRiver area. Probably the worlds largest, hebanks assure thc ncarby generators of anincrcascdreserve of continuously controlledreactive power.

While control centers typically moni-tor transmission voltage magnitudes, thereactive power rescwesat power plants area more sensitive indicator of voltage secu-rity. If generators are near their reactivepower limits, they can supply on ly limitedsupport fordisturbances, wen if voltages areinitia lly near normal. In 1997, he BonnwillePower Administration madethis aspect of agenerator's output more visible to its oper-ators by implementing a reactive powermonitor. If he reactive power reserve at a

Better simulation modelingInvestigation found many data problemsin simulation programs, ncluding problems

concerning the reactive power capability ofkey power plants. To remedy matters, theWSCC has made many mprovements nsim-dation methods. A key requirement s vali-dation of steady-state and dynamic simula-tion data by power plant testing. Dynamicsimulation methods are more detailed andinclude modeling of slower-acting equipmentsuch as generator overexcitation limiters

Inter-area simultaneous transfer capa-bi li ti rs are determined season by season.Simulation procedures are more rigorous,andreliabilitycriteria for planningandoper-ation have been strengthened, especially forvoltage support. Operation outside the con-ditions studied is not allowed.

Theseasonalsimulationstudiesneedalotof manpower. Actual operating conditionsare inevitably different from the conditionsstudied. I f there is a forced outage of, say, a1000-MVA, 500/230-kV transformer, powertransfers may have to be reduced until anengineercanmodifya previousdataset,andsimulateand analyze the new situation.certain number of units slips below certain

limit,, alarms wi ll alert operators to takewr ie cr iv r actions, perhaps b y reducingpower schedules.St i l l other measures are being taken atpower plants to improve voltage supportcapability. For example, automatically con-trolling the transmission networkvoltage ismore effective than contro lling generatorterminal voltage.

Real-time, on-line transfer capability andsecurity assessment are as yet just a goal.The technology i s essentially available, butimplementation s no trivial task. On-l inesecurity assessment i s basedon a static stateestimation nvolving housands of measure-ments for even one region of an intercon-nected power system. Network state esri-mation i s working on a regional basis, butfurther data exchange and developmenti s reouired for WSCC-wide state estima-Power oscillation dam pingTh e mechanism underlying the 10August

instability was growing electromechanicaloscillations negativedamping)due not onlyto highpowrr transten from BritishColum-bia to Califumiabutalso to the impairmentof the Lower Columbia area. By and large,negative damping i s causedby phase ag sand high gain in a generatois automaric volt-age control. Usually damping i s added byequippingageneratorvoltage regulatorwitha supplcmentary contro l calleda power sys-tem stabilizer.

That August day, though, the powersys-

tion. State estimation and the resultingon-line power flow model ar e the startingpoints for evaluating transfer capability,a s constrained by reliabilitv criteria. forpotential outages.Implications of the failures

The power failures of the summer of 1996show that on ly arrention to detail and theapplication of best engineeringpracticeswillreduce the likelihood of large power fa i l -ures.But the cost of reliability has to be bal-anced against the cost of failure.The com-mercial structure of the electric power

tem stabilifers a t a larghnuclear plant inSouthern Calitornia were out of sewice.(Powrr yrtrm stabilization a t this locationIS capecially effective because it s near oneend of the north-south intertie oscillationmode 11 Oth er stabilizers also were out o fsewice, OT neffective hecauie of noisy f re -quency transducers. Nuclear p lant stabi-lizers are now in service, and other PSSimprovements are under way.

Other means of improving damping areunder evaluation Especially promising sswitching between maximumand minimumoutput (bang-bangswitching) of a thyris-tor-cunrrullrd series capacitorI A I L U K (,Kll>i ON I H t LO

industly will continue to evolve, with unre ~lent ing competi tion and pressure to reducecosts. Merge n and consolidation of gen-eration and transmission companies wil l notgo away. Load will increase and genera-tion will be added.

Few transmission lines wil l he added,however. During the transition from cost-based regulated monopolies to market-basedcompetition, many companies are deferringinvestment in transmission until the poten-rial return i s better defined. Even wi th f inamcia1 incentive, the lines are difficult to buildbecause of environmental concerns and thenot-in-my-backyardattitude.

How o maintain power system reliabil-it yi n this envimnmenti Technological nno-vation will be vital. While the emphasis herehas been on gas turbines, technology suchas high-voltage power electronics andvar-ious forms of smaller distributed generation(such as microturbines and fuel cells) wil lalso play a role.

Perhaps the starring pam wil l be played byInformationAge technology Just as an over-night courier sewice may spend more forcomputers than for trucks, the future trans-mission company may invest more in com-puter control and communications than intran5missionlinci Transmission companiesare currently adding housands of kilometenof fiber-optic communications network, butfew kilometen o f transmission network.

Blackouts n the hture can he minimizedby technologies such as on-line securityassessmentandwide-area control. Butcon-cern with detail so that a protective relayinstalled to detect a short circuit does notgo into action during an overload emergencyremains vital. Since multiple failures arealways possible, emergency controls suchas load shedding and controlled separationprovide defense in depth +

To probe furtherThe reports on the power disturbances of thesummer of 1996 may be downloaded fromwww.wscc.com. while the North AmericanElectric ReliabilityCouncil planning and oper-ating standards may be downloaded fromw.nerc.com.The January and October 1997 issues of IEEEComputer Applications in Power magazinecontain articles related o the power failures."Model Validation for the August 10, 1996WSCC System Outage." by Dmitry Korterev,Carran W. Taylor, and William A. Mittelrtadt,and "Design and Implementation of AC Vol-tage Dependent Current Order Limiter a tPacific HVDC Intertie" by Richard Bunch andKosterev are to appear in EEE TransactionsonPower Systems."Information. Reliability, and Control in theNew Power System" by John F Hauer and C.W.Taylor appears in the Proceedingsof the 1998American Control Conference.

About the authorCarson W. Taylor (F) is a principal engineer a tBonneville Power Administration and chairs theIEEE Power System Stability Controls Subrom-minee. The author of Power System VoltageStability(McGraw~Hill,994). he established n1986 Carson Taylor Seminars. a company spe-cializing in electric power system education.

Spectrum editor: William Sweet4 5
http://www.wscc.com/http://w.nerc.com/http://w.nerc.com/http://www.wscc.com/

ieee-c. taylor - improving grid behaviour

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