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IEEE Transactionson Power Systems, Vol. 10, No. 3 , August 1995 1467Load Shedding on an Isolated System

Charles Concordia Lester H. Fink George PoullikkasBoza AvramovicConsultant ECC, Inc. Electricity Authority of CyprusAbstractUnderfrequenc load shedding OJFLS), in previous genera-tions shunned gy the industry as an admission of poor plan-ning and operation, has become a enerally accepted prac-tice. There is by now a considerab7e body of literature onthe subect, dating back to midcentury. That literature, how-ever, deals with the subject in the context of large intercon-nected systems. Smaller, isolated systems, which are morevulnerable to serious disturbances, have o erating characteris-tics that require somewhat different gui&lines, due to theirlower inertia, limited reserves, and lack of access to off-system assistance.This paper reviews the objectives and principles of UFLS,and reports their application, to a small island power system.IntroductionUnderfrequency load shedding FFLS) is a practice usedthroughout the power industry. ower systems are designedto withstand the effects of an array of credibleworst distur-bances. The function of UFLS is to serve as back-up protec-tion for the system in cases which might occur that were notcovered by the design process.Fortunately, even in the emergency state, the power systemis usually robust enough to enable carefully designed heuris-tic load shedding strategies to be effective. It is importanthowever, that these strategies be designed on the basis of ma-ture understanding of the charactenstics of the system in-volved, including system topology and dynamic characteris-tics of its generation and its load. A poorly designed loadshedding program may be ineffective, or worse may exacer-bate stresses on the transmission network leading to its cas-cading disruption. Over the years, however, utility ex-perience and extensive studies on a number of systems haveresulted in dependable guidelines for the design of effectiveload shedding programs. When plans designed on the basisof these guidelines are validated by carefully modeled simula-tions, they provide reliable tools for preservation of the sys-tem under severe conditions.In the following section, rinciples and guidelines for loadshedding are reviewed. &e succeeding section deals withap lication of these guidelines in development and testing ofa 8FLS plan for a small island system.Principles an d GuidelinesLoad SheddingUnderfrequency load shedding is a coarse tool for use in anextreme situation. As a last-resort ex edient, it should besimple, rapid, and decisive. To be erfective, it should beautomatic, be distributed uniformly across the system at eachste to avoid aggravating line overloading, be locally control-tem splitting, and should not be &pendent on communicationle P in response to local frequenc to be independent of sys-95 WM 140-4 PWRS A paper recommended and approvedby the IEEE Power System Engineering Committee ofthe IEEE Power Engineering Society for presentationat the 1995 IEEEfPES Winter Meeting, January 29, toFebruary 2, 1995, New York, NY. Manuscript submittedJuly 14, 1994; made available for printingJanuary 5 , 1995.

links. The sheddin schedule should not require frequentchanges, as a well-designed and well distributed scheme willbe sufficiently robust to accommodate such changes.The major parameters that must be considered in devising anUFLS pro ram are the level and distribution of spinningreserve and the specific load sheddin arameters: the fre-quency thresholds, the total amount of Lad to be shed, theMW step sizes, and intentional time delays. An importantcharacteristic of any schedule is the amount of undersheddingand/or overshedding in which it is like1 to result over therange of cases that mi ht be encountere$. The response ofterized by an average value within a band of actual values per-taining to individual machines and determined by the inter-differences across the net-achine oscillations.work can result in randomness o load shed.Amount of load to be shed. Since underfrequency load shed-ding is a last resort that must work if the system is to besaved from colla se, it does not pay to be timid. If the s stem goes down &cause a particular load was not shed, tza;load oes down anyway, carrying the rest of the system withit. 8or an isolated (non-interconnected) system, maximumload subject to and scheduled for shedding should, in prin-ciple, include all load. As a practical matter, perhaps 80%should be included, some with appreciable time delay.Frequency threshold. The first step threshold should not betoo close to normal frequency, to avoid tripping on severebut non-emergency frequency swings. However, frequencythresholds must be coordinated with machine protective relay-ing (and vice versa).Step size & number of steps. A significant consideration indetermining step sizes is the potential for excess load shed-ding. Frequenc steps must be far enough apart to avoidoverlap of sheding due to (intentional or inherent) timedelays. Althou h average excess load shed might theoreti-cally be reducefby increasin the number (and reducing theMW size) of steps, the n d or rapid, decisive action ismore important. Moreover, there are other important con-siderations weighing against more than a minimal number ofsteps. Step frequency thresholds too close together may inthemselves cause overshedding: a second step at one bus maybe triggered before the first step at another, due to the tran-sient differences in bus frequencies. Again, a second stepbefore system inertia responds to the previ-, since load shed at each ste should be dis-

modifications to fo ow the continual load and system

system frequency to a Poad shedding schedule will be charac-Frequencr

e entire system, many smaP steps requires

sheddin?autious y, for many of the reasons just cited for separation

very many more relays.Time delay. Some delay is needed in order both to ride outshort time transient frequenc excursions, and to accom-modate the res onse time of txe system to each step of loaddwe ver , time delays should be introducedof frequency thresholds.Priorities and Distribution. There are two basic rioritiesfor planning the shedding of articular blocks of road: theimportance of that load, and t benefit of the action to thesystem. The importance of each load block establishespriorities, but these must be subject to the requirement thateach step must be distributed as equally as ossible across thesystem, since the location and extent of a Sisturbance cannotbe known in advance.

0885-8950/95/$04.000 1995 IEEE

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1468Other considerations. It might seem that, to be optimally ef-fective, UFLS should be based on an accurate estimate of theamount and location of the generation that caused the systemupset, and control means should shed that amount of load inthat area. Both of these objectives are unattainable. Whilesystem frequency in the steady-state is a sensitive indicator ofany small mismatch between load and generation within aclosed system, it is not an unambiguous immediate indicatorof the magnitude of a sudden change in the loadgenerationbalance, and no indicator at all of the location. Even. the ipi-tial rate of change of fr uency is ambi uous, since in-dividual generators react i3ividua lly accor%in to their ownand from other machines. A third problem, related to thefirst, is inability to determine, in the available time frame,the exact topology and d namic state of the s stem. Finallthe amount of load a c d y shed by relays wib de end on t kactual load demand at the time of shedding, wk ch is notlikely to be equal to the potential (peak connected) load onthose feeders.Rate of change of frequency, while it is in princi le an ear-balance than absolute frequency, is even more subject to dis-tortion b local dynamics than is the latter. This is anotheras ect wherein, because of the emergency conditions underwgich UFLS is invoked, simplici and robustness are to beeral, a reliable UFLS schedule can be developed wi outresorting to rate of change conditioning. If for some reasonit seems desirable to resort to the rate of change, measure-ments must be regressed in order to avoid erratic perfor-mance, which may mtroduce unacceptable delays.In effect, UFLS supplements spinning reserve. For smallload-generation mismatch and without load shedding, maxi-mum deviation from normal frequenc increases sharply asthe magnitude of the mismatch approacies the amount of spin-ning reserve. Conversely, as the amount of spinning reseryeapproaches several times the disturbance magmtude, the maxi-mum deviation a proaches the associated steady state fre-quenc offset. {pinning reserve also serves to reduce ex-pFt.dexcess shedding for any given schedule, but the effectdimnishes as size of disturbance increases [1,4]. (It shouldbe noted that the ma itude of a frequency swing following adisturbance is ve %pendent on the size of the disturbance((amount of actuaYgeneration, not generatin ca acilost)) relative to the concurrent level of the foaB) e thatffec-stiveness of spinning reserve is strong1 affected by its dis-tribution over the equivalent electricalrarea of the network;and the number oI and its distribution among, responsivegenerators [4],Load RestorationSecond in importance to a reliable load sheddin program isa correspondmg program for safe restoration of interruptedload as rapidly as may be feasible. This is a desirable ele-ment in load shedding plans for most systems, and a neces-sary element for many. A robust UFLS, in order to be effec-tive, may often have to shed load in excess of generation thathas been lost, and (slower) s inning reserve will usually beavailable to pick up some or ay1 of that load. It is usually thecase that load shedding is effected at unattended sites, andconsequently local manual restoration will be time consumingand protracted.The same basic priorities affect the restoration of load as dothe sheddin of load: the importance of the load, andathebenefit of ti e action to the system. However, the implica-tions of these two considerations are not the same for shed-din and restoring circumstances. For planning the sheddingof foad, importance must be subject to the r uirement thateach step must be distributed as equally as p o s s l e across thesystem. For restoring load, however, the condition of thesystem should be known, and in this case the sequence of res-toration may hew more closely to the optimal sequence estab-

inertia, and to their electrical distance from t%e disturbance

lier indicator of an instantaneous change in loadP eneration

=r-referred to more refined, carefulT tuned designs.

lished by load priorities, sub'ect to the operator's jud ementthat no security constramts (e.g. line loadings) will /% vio-lated. rinci le it might seem that thehigher priority loads shoulf be sted last and restored first,t h s guideline is subject to differing constraints in sheddingand restoration.The arameters to be considered for an automatic restorationsch2 le are the frequency threshold for restoration, the sizeof the steps, and time delays.

Thus, although in

Integration into SCADA/EMSFor maximum effectiveness, UFLS should function in fullcoordination with SCADMEMS facilities. Basic capabilitiesto be considered include monitonng of relay status, andmonitoring and archiving of load shedding actions. Beyondthis, three modes of coordination may be considered: (i) sup-lemental manual load shedding, (ii) reclosure of selectedIreakers ("restoration") following either manual or automaticsheddm , and (iii) remote setting of.UFLS relays. The firsttwo of gese are conventional functions of modem SCADAs stems- are conventionally known as "LoadZurtai1ient!Reh e third function is unconventional, and todate does not appear to have been implemented on any sys-tem. Each of these modes will be discussed m urn.Supplemntal Load Shedding. Ca ability should be providedfor manual shedding of selected bgck s of load by the systemoperator under circumstances other than those that triggerautomatic UFLS. Such circumstances will includenot be limited to) individual line or circuit overloa s, unac-ceptably low bus volta es s stem-wide cajacity shorta es,and any situations in w k l h Jequency decline is arrestefbyautomatic UFLS, but fr uency does not fully recover to itsnormal level. Operator%rected shedding should be limitedto steps no reater than 5% of total s stem load. The loca-sari1 will be fictated by the incidence of those conditions.She dkg for s stem-wide capacity shortages should followfor%FAwhich may be achieved by utilizing the. firststep(s) of the UFLS schedule. However, thi? policy will re-quire modification for rotating "brownouts, if such are re-quired.Reclosure of Breakers. Following UFLS, a capability for per-missive automatic load restoration (where "permissiveautomatic" means automatic followin operator approval andacknowledgement) is highly desireabk. Reclosure on loads,subject to securi constraints, should be in order of priorityAccordingly, once frequency has recovered, and availablegeneration exceeds that required for necessary spinningreserve, the o erator should be provided with the reverse or-der list of loa& to be restored, m blocks not to exceed 5% oftotal system load prior to the initiation of shedding. Each as-sociated breaker should be reclosed only given a satisfactorylevel of bus voltage and following operator acknow-ledgement.Remote Setting of UFLS Relays. Utmost caution is advisablebefore implementation of any scheme for remote setting andenabling of relays. Properly designed load shedding, as al-ready emphasized, is calculated to respond to unforeseen dis-turbances in a modulated but sufficiently drastic manner tosave the system when design criteria have been exceeded.Attempts at elaborate fme-tunin , and frequent arming and

Put ay

tion of shedi n for line overloads or Y w bus voltages neces-the rinci le o ydistribution across the system recommended

(higher priority P ads first) subject to security constralnts.

disarming of UFLS relays, coulB eave the system vulnerable

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1469MW as turbine units, and in 1997 a new Station C will beaddecfcontaining three 60 MW steam units.EAC transmission includes 390 kilometers of 132kV circuits.and 850 km of 66kV circuits. The system incorporates someprotection a ainst underfrequency conditions which shed loadat pr ed ef mJ levels between 49.2Hz and 48.4Hz. N o inten-tional time dela s are included in the settings. Followingtripping, manuaYrestoration of load is required.In 1991, a trip of one of the two 132kV circuits out of Sta-tion B while the other circuit was on maintenanceprecipitated a splitting of the system and a total systein shut-down. This and other considerations resulted in EAC com-missioning a thourough study of its protective system.ModelingDevelopment and validation was done in three stages, usingsuccessively more detailed models: Stage 1, a screening stageto explore a wide variety of UFLS schedules; Stage 2 ,testing of one or a few selected schedules; Stage3,redibilfityfm verification of the recommended schedule. Stage 1used a lumped model - a single equivalent ienerator, and auivalent load. Sta e 2 representate the individualof a network model. Stage 3 used a full transient-stabilitymodel.The Sta e 2 model consisted of dynamic (AGC level) modelsof all pfants, cou led with a load flow re resentation of thetervals, and plant outputs and line flows recorded. The sys-tem was sub ected to a series of increasingly severe genera-tion losses, deginning with one unit, and extending to entireloss of each of the plants (two plants fo r 1993 and 1997,three plants for 2000).The Stage 3 model was used for a final test of systemdynamic behavior during the course of the load sheddingschedule. This test was mtended for very limited purposes,since the bus schedule that was assumed was somewhat ar-bitra . The Stage 3 test, accordingly, was intended only as(i) a%al verification of the validity of the Stage 1 and Stage2 models b comparison with a recognized, fully detailedtransient stagility program, and (ii) to indicate the stability ofthe system during the course of the load shedding.Development of a Load Shedding PlanCriteria. In order to provide a screening (Stage 1) ofplausible UFLS plans, three load and eneration levels on theexisting 1993 s stem were chosen as stown below. For eachcandidate U F d lan, the system was subjected to the loss ofone, two, three, four, and in the case of the medium and highload levels, five 60 MW units.

Generating Lost Generator SpinningLoad Level Capacity % Loading ReserveLight 200 330 60 130 MWMedium 360 410 100 50 MWHeavy 503 530 100 27 MWEarly decisions, in accordance with the objectives andciples, were i) to make a total of 80% of the system loacf$$b-ka me , (ii) to separate the steps %y at least 0.2 Az.In order to provide an objective analysis that would reflectthe relative performance of the alternative schedules, fourstatistics were calculated for each schedule:

(i) "RMS Hz. the root-mean-square (rms) value of th emaximum deviation of frequency over the entirescenario set (i.e. the cases that were simulated),

units, in7 vidual bus loads, a connected to appropriate busessystem. System Synamics were integrateCQ at one-second in -

ect to load s6edding, dependin on the severit of the distur-

at the time when drastic UFLS is most needed. As an in-stance, the 1977 New York blackout was due in part to a par-tial disabling of load shedding activation.Development of a System Specific UFLS PlanSmall Isolated System ConsiderationsGeneral principles. In applying general principles to small,isolated systems, the distinguishmg charactenstics of suchs stems should be kept in view. Small isolated systems lacktie supporting inertia of an embedding interconnection; mother words, they lack the sup ort of fr uency provided byinertial power flows on ties. hence, U% schedules mustaccommodate a much wider band of normal frequenc devia-tions, but at the same time, more drastic action must ge takento avoid unnacceptable frequency excursions. There issmaller margin for error. Fewer and larger steps will be re-uired than for larger interconnected system, time dela s%odd be minimized, and a greater percentage of load sux-ject to shedding will be required.Reserves. For systems embedded in large interconnections,spinning reserve is intended to replace lost generation withinseveral minutes; immediate covera e for a local load-eneration deficiency is provided by be practical1 infinite-f u s inertial reserve of the interconnection (within ti pick-upcapacity of the tie lines). This is not true for small, isolatedsystems, and the effective inertial reserve ("responsivereserve'' [ 2 ] )must be considered [4].This means that it is even more important on small isolatedsystems than in general, that reserve margins be spreadacross as many units as feasible, rather than assi ed to a fewlarger, or to smaller inexpensive, units. InertiaYresponse ofall units not at their limits will decrease frequency excursions[%and thus minimize exposure of the system to activation ofU LS, s well as to the amount of load shedding that will beincurred. Obviously, the effective amount of inertial reseryeof a unit is much less than its unloaded margin (its nominals inning reserve) [2]. These considerations will influencetie setting of the frequency threshold for the first step.Ln view of the possibility of system splitting durin a distur-bance, reserves (active and reactive), as well as Boad shed-ding sites, should be spread as uniformly as possible acrossthe system: active reserves on all units, other reactivereserves as close as possible to loads.Restoration. For restoration on an isolated system, activa-tion should be well above normal frequency. Ste size(s) ineneral should be smaller than those used in loalsheddmg.h e y should be well within s inning reserve capability of theunits on line; stated conversi , spuming reserve dunng res-toration should be kept well atove the magnitude of restora-tion step sizes. Time delays should be provided to avoid reac-tion to short duration frequency excursions (such as mightresult from overshedding), and to accommodate the responsecapability of spinning reserve generation.The EA C SystemThe Electricity Authority of Cyprus (EAC) is responsible forthe generation, trimsmssion, and distnbution of electricpower throughout the island of C IUS (although the Turkishoccu ied ortion of the island (?&!OA) is inaccesible to theEAC!staf$. With the exce tion of 25 MW of eneration lo-cated within the TOA, FAC provides all o[ the island'sener y requirements. The resent system peak is in excess of5 0 0 k W , and is expectecf to reach 880 MW by the year2000.The EAC system includes two oil-fired power stations, Sta-tion A having six 30 MW steam units and two 37.5 MW gasturbine units, and Station B havin six 60 MW steam umts.Station A will be expanded in 1955 by two additional 37.5

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~

1470(ii)

(iii)

I

"StD. Hz . " the sample standard deviation of the mini-mum frequency,"S.S. .Hz. the sample standard deviation of the post-sheddin stead state frequency, and"Max. kc. Jz." the maximum frequency excursion(i.e. the minimum of the minimum frequencies).

Of these four statistics, the first provides an indicationof theperformance of the schedule in minimizing the frequency ex-cursion over the range of test cases; the second provides anindication of the consistency of performance over thescenario set, the third provides an indication of the perfor-mance relative to overlunder-sheddin and the fourthUltimately, sixteen candidate UFLS schedules were tested ina process involving several hundred tests on an evolving se-quence of schedules developed to minimize the selected statis-tical measures.Results. Improvements in performance were achieved byretaining the gradual separation of thresholds, increasing theamount of load shed for the first three steps, and increasingthe cumulative amount shed over the first three steps (whilekee in the total load shed to 80%). In this wa , the stan -dari feviation of minimum frequency was rd ced from0.823 to 0.684, while the sample standard deviation of ost-shedding steady-state frequency was held to 0.189, xownfrom 0.476 for the existing schedule. This latter statistic isdeemed less critical than the former, since it can, and alwayswill, be corrected by either automatic or manual generationcontrol. The worst frequency excursion for the preferredschedule was 3.18 Hz., down from 3.58 for the existingschedule.Review of the results indicated that one schedule providedthe best over-all performance:

E3 Shed, % 15 20 25 10 10Delay, sec.Cum. % Shed 15 35 60 70 80Several experiments were made with extended time delays forlater stages of shedding. No overall improvement in erfor-mance was achieved b this means. For instance, in tge caseof the preferred Schdule, there were two cases in which allfive steps were shed: light loading with a loss of 240 MW,and medium loading with a loss of 30 0 MW. In the formercase, the effect of a 1 second time delay on the last ste wasto avoid the shedding of the last ste , resulting in 4 d W ofundershedding, rather than 16 M d o overshedding; therewas no effect on the minimum frequency. In the latter case,however, shedding of the last step was not avoided, and theeffect of the 1 second time dela was to depress the minimumfrequency an additional 0.41 dz. On balance, the latter ef-fect seems more detrimental than the slight overshedding inthe former case.Evaluation of the Load Shedding PlanOnce the screening process had generated a preferred plan,attention was turned to rigorous evaluation of that plan. Thefirst sta e of this evaluation, still utilizing the Stage1 model,tested t i e plan against the circumstances that led to the 1991system blackout, and against both existing and forecast sys-tem conditions. The next sta e was evaluationon the Sta e 2model, which required speciication of bus specific shedi ngplans. The final sta e was varification of representativeresults using the Stage 8 transient stability model.In what follows, we will refer to a division of the system intothree, somewhat arbitrary, areas by two boundaries at whichthe system might conceivably split under certain extreme con-ditions. Two of these areas contained one of the two existingpower lants, the third could remain with one or the otherplant, Bepending on which of the two boundaries opened.

provides a worst-case performance over tke scenario set.

Th.Hz. 49.0 48.8 48.4 47.8 47.00.20 0.20 0.35 0.35 0.35

0 O.P.40.60.8 1 1.2l.41.61.8 2 2.22.42.62.83Time (sec)

A: With 1991 UFLS Schedule51.0 -

- _ .........~ ..... .

0 O.P.40.W.8 1 1.21.41.61.82 222.42.62.8 3l ime (sec)B: With 1993 UFLS Schedule

0 0.a3.40.60.8 1.21.41.61.8 2 222.42.62.83Time (sec)C: With Preferred UFLS Schedule

Simulation of Incident of November 1991Figure 1

November 1991 Incident Simulation. The Sta e 1 model,which had been used to develop the UFLS Sckdule, wasused further to study the res onse of the system, under threeschedules, to the event of November 1991, when area 3was split from the remainder of the system. The event wassimulated with three UFLS schedules: the one in effect at thetime of the incident, the one currently in effect, and the onethat is now proposed. The assumptions, matching systemconditions at the time of the event, were that area 3 includedfour full loaded units (200 MW) and 22% of the total sys-tem l o d For the first two cases, ste s 1, 2, and 3 of theUFLS schedule consisted onl of area f; load, which was al-ready lost in the 22%. In SUCI a case, from the standpoint ofthe remaining portion of the system, 20 0 MW of generationwould have been lost alon with 22% of the system !oad; theremanun steps would be b l l y effective. For the thud case,with loafshedding spread across the system, only the area 3ortion (22%)of the first three steps would have been lost,gut available load shedding on the remaining portion of thesystem would be discounted at each step to 78% of itsnominal value. The results are shown in Figures 1A , B, and

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1471C. This example shows the importance of dktributin eachstep of the load shedding across the system, and injicatesclearly that, had the proposed UFLS schedule been in use,areas 1 and 2 of the system, with an unshed 2 4% of the sys-tem load, would not have been lost.Bus-s ecijic schedules. The next step was to provide bus-s eci& schedules for load shedding that satisfied reasonablytle recommended schedule. No attempt was made at thlssta e to provide a practical schedule meeting all the multi-was made to schedule load sheddmg at the proper level f&each step within each of the three areas alluded to above, andfor the system as a whole. Minimum adjustments were madein the schedule for the two later ears to accommodate sys-tem rowth. The resulting schedure for the year 1997, show-ing &e percent load in each of the three areas, he percent ofarea load to be shed at each step in area, and the resulting per-cent of system load to be shed at each step in the system as awhole, is shown as typical:

1997 System: Load Sh edding Schedule by Area

tuc f nous constraints of the actual system. Instead, an attem t

Area: 1 2 3 System39.2 41.3 19.5 1 0 0% Load Cumu lative percent shed at eac h step1 13.5 18.0 10.0 14.72 34.2 32.6 43 .5 35 .43 61 .0 57 .1 62.2 59.64 73 .0 66 .4 73 .4 70 .45 82 .5 74 .9 81 .3 79 .1

Stage 2 model evaluation. The UFLS plan was evaluatedagmst all combinations of unit losses for the system modelsprovided for the years 1993, 1997, and 2000. Fi re 2presents the results of simulation of the loss of six 6 r M Wunits (two of which had been loaded at 45 MW), with a s s-tem load of 50 0 MW, and a capacity on line of 530 M kFour of the five sta es of shedding were invoked, for a totaiof 355 MW shed. %he minimum frequency was 48 Hz, hemaximum was 51 Hz; fter ten seconds it settled at 50.4 Hz.Figure 3 resents the results of the loss of six 60 MW units(one of whch had been overloaded to 70 MW) with a systemload of 718 MW and a capacity on line of 70 8 MW.In this case, three of the five stages of shedding were in-voked, for a total of 431 MW shed. The minimum frequencywas 48 .2 Hz, he maximum was 51.1 Hz, and after tenseconds, it settled at 50.6 Hz.Figure 4 presents the results of the loss of nine 60 MW unitsat two stations, with a s stem load of 88 2 MW and a capacityon line of 891 MW. h his case, four of the five steps ofshedding were invoked, for a total of 62 6 MW shed. Fre-quency dipped to 47.6 Hz, vershot to 51 .8 Hz, nd settledout after ten seconds at 50.8 Hz.52 1

46 4 I0 5 10 15 20Time (sec)

Loss of 360 M W on year 1993 SystemFigure 2

525150

NI49484746

525150

NI49484746

Loss of 360MW on Year 1997 SystemFigure 3

. - . . - . . . - . -_._ - . . . . . . . . .

- . - - . . . . . . .- . - . - . . - .

0 5 10 15 20Time (sec)Loss of 540M W on Year 2000 System

Figure 4Sta e 3 model evaluation. The Stage 3 model was used for afin$ evaluation of system d namic behavior during thecourse of the load shedding scledule. These studies, as ex-lained, were intended for a very limited purpose. Oneknitation was encountered; the stability program that wasused has a limitation in the number of underfrequency relayssuch that only a few relays at level 3 could be represented.Accordingly, attention was limited to the most severe of thecases where only 2 levels of shedding were encountered. Itwas ossible to represent a few step 3 relays, to determinewhetger or not step 3 would have been initiated.Figures 5A , B, and C present the results of one scenario forthe 2000 system, the loss of a new Station C , using the Sta eload was 88 2 MW, and 180 M f i e r e lost out of availablegeneration of 89 1 MW. Two of the five stages of sheddingwere involved, for a total of 309 MW shed. The minimumfr uencies indicated b the three models were 48.77 Hz,ten seconds were 50.45 Hz, .40 Hz and 50 .39 Hz respec-tively.It should be noted that, due to their being produced b different pro rams, the plots in the three cases have di&eren;scales. d s t mportantly, the time scales have a duration of3 seconds in Figure 5A , a duration of 20 seconds in Figure5B, nd a duration of 10 seconds in Figure 5C .E ec t of S innin Reserve Level. It will be noted that onlytP first oF Lhe t ee scenarios rovided for testing, that ofthe light load level, met the f a r est sin le unit rule-of-thumb for spinning reserve; and %is is tie only case thatnever resulted in an load sheddin The medium load levelroad level scenario provided only 27 . A clear indication ofthe benefits [4] of increased levels of spinning reserve is

1 , Stage 2 , and Stage 3 models res tively. In this case, tEe4 8 3 0 Hz, nd 48.75 d , res ectively; the frequencies after

rovided 50 of the d sirable 60 M b eserve, while the heavy

I

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1472

4847

shown by comparin three cases with 0, 60 MW and 120MW of reserve. RJ uc tion s in the RMS of the minimum fre-quency excursion were 12% and 20% for the two levels, andin the maximum frequency excursion were 15% and 25%respectively. These reductions are significant.

..............................................................................................................................................

..48.0 ........... ........ "47.0 ......................46.0

0 0.23.40.60.8 1.21.41.61.8 2 2.22.42.62.8 3Time (sec)

Vertical divisions represent 1 Hz.Horizontal scale w e n d s to 20 secondsB: Stage 2 Model

IO . 30IO. 20

-n. IO C

I

Vertical divisions represent 0.5 Hz.Horizontal scale w e n d s to 10 secondsC: Stage 3 ModelLoss of 180 M W at Station C, 2000 System

Figure 5

Table: Effects of Spinning Reserve LevelM a x i -mumReserve Exc. Hz. Exc. Hz.

spinning RMS

None 2.49+ 60MW 2.19+ 120 MW 1.98 4.203.583.14A significant factor, illustrated b the entire range of casesstudied, is that, in general, the efktiveness of load sheddingincreases with increasing load. This is, of course, becausewith increasin load, a reater amount of load is shed at eachthe load and, under the su gested schdule, such loss wouldrequire the sheddm of at feast two steps, whereas at a levelof 500 MW, 60 hfW re resent only 12% of the load an dsuch loss would r uire tfe shedding of only one step. This

erformance at the light load level to the severe loss of 144!hW with 60 MW and 130 MW of reserve. It may also beseen, more realistically, b comparing the erformance of theUFLS at medium and hi g i load levels wit: either 60 M W or120 MW reserve: load sheddmg is more effective (requiresfewer ste s to be shed) at the higher load level for each num-ber of 68MW units lost. This effect is fortunate because i tis easier to provide a larger margin of spinning reserve atlight loads than at heavy loads.ConclusionsAn effective, robust UFLS program is a necessa contribu-tion to the reliability of eveimportant for small, isolat3 s stems. Properly designed,implemented, and maintained, &LS can enable such systemsto survive many disturbances that otherwise would result. intotal blackouts. This paper has developed and a pliedcriteria for UFLS lans that may differ in some aegreeconnected systems.The authors are not unaware of work on optimal load shed-ding rograms [e. . 31, but remain convinced that such lansmay fail to be ekectwe when most needed, especial$ onsmaller, more vulnerable systems.References1. R.M.Maliszewski et al.: Fr uency Actuated LoadShedding and Restoration; IEEg Trans. v.PAS-90 n.4,2. R.J.Kafka: Load and Generation Coordination;presented at ECC Seminar on Power System Restora-tion, A ril 1992, Arlington, V ir inia, and October1992, dmt ri ch t, Netherlands, E C E Inc.

D.Kottick, M.Blau, Y.Halevi: Evaluation of an Under-Frequencx Load Shedding Optimization Algorithm;Proc. 11 PSCC, Avignon, France, Aug./Sept. 1993,4. C.Concordia: Performance of hterconnected S stemsFollowing Disturbances; IEEE Spectrum, v.2 n . l June

step: at a le v i of 200 hW, 60 MW re resents 30 percent ofeffect can be seenb c o m p m g the dramatic difference in

power system, an7 t is most(although not in kinB from those appropriate to large, inter-

1971, pp. 1452-59.

3 .pp.437-41.

1965, pp.68-80.BiographiesCharles Concordia, D.Sc., LF, independent consultant.Lester H. ink, LF, Exec. V . P . R&D Mgr., ECC, Inc.Boza Avramovic, Ph.D., SM, Lead Consultant ECC, Inc.GeorgePoullikkas, Asst. T&D Mgr., EAC.