Digital Backup Protection

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Power-system digital back-up protectionF.C. Chan, B.Sc, Ph.D., D.I.C., C.Eng., M.I.E.E., and B.J. Cory, D.Sc. (Eng.), A.C.G.I., C.Eng., F.I.E.E.,Sen. Mem. I.E.E.E.

Indexing terms: Power system protection, Digital control, Circuit breakers and circuit breaking, FaultlocationAbstract: The purpose of power-system back-up protection is to disconnect faulty primary equipment notisolated by the main protection and associated circuit breakers. Conventional back-up techniques are difficultto apply in a large power system and heavy reliance on main protection is required. In this paper, both sub-station-based and centrally co-ordinated back-up protection methods, using microprocessors and minicom-puters, are described. In a power system with a reliable communication network between substations, afault-directing method is proposed to perform centralised digital back-up protection. If the communicationnetwork is not available, a substation-based back-up method is suggested. Both methods can reduce back-upfault-clearance time with correct discrimination, high reliability and cost effectiveness.

1 IntroductionIn power systems, there are many different types of protectiverelay designed to satisfy various requirements for fault identifi-cation. As complete reliability is not possible, failures of theprotection systems, or the associated circuit breakers whichcontrol the faulty apparatus or circuits, must be considered.To safeguard against these failures, some form of back-upscheme is required to supplement the normally faster op-eration of the main protection system. Conventional localback-up techniques can cater for either main protectionfailure or circuit breaker failure, but not both. Althoughthese two kinds of function can be incorporated into a remoteback-up scheme, a minimum of circuit isolation is generallydifficult to achieve. For exam ple, a basic IDMTL relay, apartfrom its own role in primary protection, can perform localback-up for the primary relay and remote back-up for therelay and circuit breaker of the adjacent substation. However,an IDMTL relay has difficulty in providing proper discrimi-nation under all infeed conditions. Similarly, zones 2 and 3of a distance relay can provide local back-up and remoteback-up, yet the latter is affected by reach variations. With therecent advances in digital computer technology, it is nowfeasible to achieve a complete digital back-up protectionscheme with moderate cost, reliability and satisfactory dis-crimination properties. At present, digital techniques availablefor power system protection can be broadly classified intothree areas:(a) The central computer approach: in which one com-puter is used to perform protection for a whole power system.In a 1970 CIGRE paper [1 ], Ungrad and Glavitsch proposedcentrally co-ordinated back-up protection using a centralcomputer, to obtain reactive-power-flow information fromvarious points in the power system, and to compute fromthis information on the location of the fault. Later, Edgleyalso investigated the use of a central computer for back-upprotection [2] using fault-current directional flow datafrom all circuit terminations to identify the faulty circuitby a table look-up m etho d. For both cases, a powerful centralprocessor unit is required.(b) The dedicated approach: in which a single processoror computer is used to perform a specific protection functionfor a circuit or equipment [3]. Much modern digital relayingdevelopment has been devoted to transmission-line protectionPaper 2188C (Pll, P9), first received 18th February and in revisedform 12th July 1982Dr. Cory is with the Department of Electrical Engineering, ImperialCollege of Science & Technology, Exhibition Road; London. SW7 2BTyEngland. Dr. Chan was on study leave from the'C hin a Light & PowerCo. Ltd., Hong Kong, where he is now em ployed

and, in particular, to improving the speed of response to faultson high-voltage long transmission lines.(c) The integrated approach: in which a single computeror system in a substation is used to perform multiple pro tec-tion tasks [3]. The application of digital computers to theprotective relaying field was first suggested by Rockefellerin 1968, to perform multiple relaying functions [4]. Later,various multiple tasks structures and systems were also sug-gested. The difficulty, perhaps, is related to the speed responseand the the required reliability.Owing to delays in communication and general lack of sec-urity, the centralised approach is really only suited to areaback-up applications. On the other hand, the dedicated pro-cessor approach aims at achieving fast fault detection andis most suited to main protection. The integrated, approach,however, is more appropriate for the economic performance ofmultiple tasks, although the fault response time is somewhatslower than for dedicated relays. This method can be usedwithin a substation for back-up protection, where a slowerresponse than the main protection is required for discrimi-nation; although, for security, it is recommended thatduplicate main and back-up protection is required.This paper develops novel schemes using the integratedcomputer approach for local back-up in a substation, andcentralised computer methods for remote back-up purposes.2 Basic considerationsAn ideal protection system should arrange for the circuitbreaker(s) nearest to the fault to operate first for isolation.Only if a stuck breaker condition arises should adjacentbreakers be tripped to remove the fault from the system.

However, if the primary or main protection relays donot operate during a fault, the nearest circuit breaker must betripped by o ther m eans. This is the role of the local protectio nfail system, but the extent of this system will depend upon theconsequence of the loss of supply. In high-voltage systems,such consequences are serious enough to warrant the in-stallation of both local and remote back-up schemes.To minimise primary protection relay failure, duplicationtechniques are employed in which protection schemes aredesigned based on different principles and/or supplied bydifferent manufacturers, so that unusual faults which maycause failure of one scheme will be less likely to affect theother. However, by duplicating the primary protectionsystem, the probability of a failure to operate is reduced,but there is an increase in the possibility of false tripping.Because of th e difficulty in designing, bo th techn icallyand economically, an analogue relay to perform suitableprimary relay back-up protection, heavy reliance on main

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protec tion has been the practice in the past. At present, because fault clearance can be made more discriminativethe provision of primary relay back-up protection to cater by tripping th e circuit breaker closest to the fault, evenfor relay failure on a substation basis by microprocessor or following a primary relay failure. An integrated corn-minicom puter is feasible and can be economically achieved. puterised scheme can be devised to achieve this desired con-This form of local back-up protection is highly desirable, d it io n [ 5 ,6 ].

substationsubstation 1

fault' CB2" substation 2

next iacceptable p

Fig. 1 Digital back-up protection methodsDR = directional relay PRBP = primary-relay back-up protectionPR = primary relayCB = circuit breakerRBU = remote back-up

CBBP = circuit-breaker back-up protectionF c = fault clearanceIEEPROC, Vol. 129, Pt. C, No. 6, NOVEMBER 1982 307


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As an alternative, a centralised computer can be utilised,provid ed th at a reliable comm unica tion n etwork is available.Coded transmission can reduce the signal error. As inter-ference is a function of transmission time, the interferenceeffect can be minimised by an appropriate filtering networkallowing for a time delay (say 10ms). This small time delaywill not degrade significantly this fast back-up protection.After information has been sent back to the central computer,fault location can be performed. If the fault location can bequickly determined, a trip signal can be sent to the ap-propriate circuit breaker closest to the fault. A 'fault-directingmethod' is detailed in this paper for fast fault location [7,8 ] -Fig. 1 illustrates the various forms of back-up protectionon a flow chart. Both primary-relay back-up protection andcentralised back-up protection aim to achieve ideal faultclearance, and shorter back-up fault clearance, in general,is achieved. One important aspect of centralised back-upprotection is its ability to isolate the fault, in case of acatastrophic failure of a complete substation, e.g. owing toan earthquake.

3 Sub station digita l back-up prote ction systemThe major difference between primary and back-up protectionis the time necessary for discrimination before the back-upprotection can operate. With this time available, a back-upprotection system can be designed to look after a whole sub-station, not just a single circuit, thus producing economyof equipment. Fig. 2 shows how the proposed digital back-uppro tection system can be arranged to isolate the faulty circuitsthat have not been disconnected by the primary protectionand the associated circuit breaker. The system consists oftwo parts: the circuit-breaker back-up protection and theprimary-relay back-up protection.

double busbarsubstation

analogueinputs digitalinputs

dataacquisitioncircuit breakerback-upprotection

database

primary relayb a c k - u pprotectionbackgroundand Monitoringprogram

circuitbreakercontrolto othersubstation

3.1 System structureImplementation of the substation digital back-up protectionsystem, on a laboratory scale, was carried out with a 16-bitminicomputer (NOVA-1210) and a microprocessor (NationalSemiconductor IMP-16C) having memories and cycle timesof 12K , 1.35 /is and 2.5 K, 7.5 JUS, respectively. The data-acquisition system could handle up to 16 analogue channelsand 4 x 16-bit word digital channels. These enabled a double-busbar substation con figuration to be sim ulated. A samplingrate of 8 samples per cycle was chosen to give simplicity inrunning the back-up protection algorithms [ 9 , 1 0 ] .HV network

voltagecurrent

! ;

switchinginterface

primary Irelay _J

CB statusISstatus PR status dataacquisitioninterfaceIMP-16C >

I data collection |systeminformation

data base

CBBP ! PRBPU-l Ieventloggerstripcontrol

NOVA-1210

j

eventprintoutbackupmonitoringalarmsignalbackgroundprogram J

Fig. 2 Digital back-up protection system

Fig. 3 Program configurationFig. 3 shows the basic program structure developed for theback-up protection system. The major protection functionswere performed by the circuit-breaker back-up protection(CBBP) program in the IMP-16C microprocessor and theprimary-relay back-up protection (PRBP) program in theNova-1210 minicomputer. Two levels of program executionwere adopted in the IMP-16C, as shown in Fig. 4. The data-collection program was started on interrupts from the real-time clock at the beginning of the sampling interval. Afterthe CBBP program, the microprocessor returned to its back-ground program. In the Nova-1210 minicomputer, threelevels of program execution were used. The first level con-sisted of the basic data-communication program, the faultsensor, the event loggers and the trip program. The secondlevel was the heart of the PRBP program and the third levelconsisted of the real-time operating system and the eventprintout program.

3.2 Circuit-breaker back-up protectionA conventional back-up scheme was adopted for digitalimplementation. The breaker back-up timer was initiated uponthe receipt of: (a) the primary-relay trip signal, (b) a currentlevel detected by a current sensor and (c) the circuit breaker inthe closed position. The condition (c ) can be considered as anoption al safegard measure. After a predetermined time, if(a) and (b ) conditions were still present, back-up breakertripping (as determined by the associated isolator positions)was performed on appropriate adjacent circuit breakers.To suit operating requirements, the CBBP program wasdivided into four parts: (i) data grouping, (ii) timer andfault detector, (iii) zone determination and (iv) trip initiation

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programs. Software t imers of the parallel type were requiredto enhance the system reliably and simply [ 6 , 8 ] .

samplinginterval time

IMP-16CI clock| interrupt

NOVA-1210

Fig. 4 Multilevel structure in processors3.3 Primary-re lay back-up protection (PRBP)PRBP is the most important function performed by theproposed digital protection system. The operating time ofthe PRBP system should be in the range of 50 to 60 ms, sothat the total fault-clearance time, when the primary relayfails, is well below the remo te back-up time setting of 200 ms,or even lower if possible.A substation normally consists of transformers, switchgear,busbar and outgoing feeders. For plant situated locally, inwhich the protected zone is well defined by CTs positioned atboth ends, unit protection using differential principles wasemployed. For those outgoing feeders or feeder transformersconnected to remote substations, where single end-faultdetection was required, an impedance measurement method(distance protection) was adopted. In the PRBP scheme,back-up coverage for every piece of apparatus was not pro-vided and the back-up function concentrated on major prim-ary main protection. Thus for basbar, transformer or generatorprotection, a general differential scheme was chosen withindividual features incorporated (e.g. inrush magnetisingcurrent restraint function for transformer pro tectio n). Thesetechniques greatly simplified the system structure, as the pro-tection algorithm and data-acquisition system had only to dealwith differential and distance protection [5].As the PRBP has the same function as the primary relays,it is impo rtant to provide a high degree of security. Thus, toallow tripping to occur, three conditions needed to befulfilled:(i) the starter had operated(ii) the directional data logic summation method or itsextension had identified the fault [5, 8](iii) the d ifferential curren t had exceeded the setting or theimpedance value had fallen below the zone setting.For the busbar protection to initiate tripping, all three con-ditions must be satisfied. For transformer or feeder pro-tection tripping, if condition (ii) cannot be fulfilled, thencond ition (iii) has to be satisfied twice in succession.In this integrated multipurpose protection approach, thedigital system needs a starting device. After the fault starteroperates, the next step is to locate the faulty part among the

various protected circuits in the substation. A fault calculationalgorithm is then applied to identify the fault and to make atrip decision. The following Subsections discuss the funda-mental requirements for each stage of this development.3.3.1 Starting: Overcurrent detection was employed forstarting the primary-relay back-up protection program. Theinstantaneous value of the current in each phase was comparedwith a preset value during data acquisition, following analogueto digital conversion. A magnitude bit was generated tosignify and identify an overcurrent condition on those circuits.The overcurrent setting was not critical in affecting thestarting of the PRBP, but the setting should be greater thanthe maximum load current and less than the minimum faultcurrent. Should the minimum available fault current bebelow the maximum load current, an undervoltage detectorcan be incorporated. As two primary cycles of analogue datawere sorted in the data base, it was easy to detect an under-voltage condition by comparing the newly collected voltagedata to that collected one cycle previously.3.3.2 Fault detection system: Basically, the PRBP programhas two forms of fault identification:(i) a busbar fault, where the input quantities need a largenumber of current inputs(ii) a circuit fault, where the input quantities are either3-phase voltage and currents (distance) or two 3-phase currents(differential).Busbar fault detection is arranged to have a higher prioritythan a circuit fault, so after starting the PRBP program, thebusbar fault condition was first examined, followed bychecking of each circuit in a logical sequence. In case anadditional busbar fault occurred during circuit checking,the PRBP executed its busbar protection algorithm to initiatetripping action and return to circuit checking afterwards.The circuits to be checked can be reduced by the directionaldata logic summation extension method [5,8] and the power-flow-direction determination. Furthermore, the total numberof circuits the system can handle depends on the availableback-up time and the program-execution speed. In general,faster processing speed gives sho rter prot ecti on ca lculationtimes; hence, more circuits can be accommodated in thePRBP program.3.3.3 Protection algorithm: In digital relaying there arevarious kinds of protection algorithms having differentcharacteristics of program-execution time, fault-detection timeand degree of accuracy. The sampling rate, the data windowand the arithmetic complexity of the program are factorswhich govern the response characteristics of the protectionprogram.From the data processing point of view, digital relayingalorithms can be divided into two categories:(i) those working with a data window of a whole funda-mental cycle(ii) those working with only a portion of the fundamentalcycle.For the one-cycle data window method, a change in operatingconditions takes more time to reach a new value than witha short data window. However, this latter method needs moretime to settle down to its correct value. In considering theback-up protection application, fault detection is not requiredwithin 20 ms after fault inception. Therefore, the moreaccurate algorithm using whole cycle detection is preferredand a Fourier method is chosen for detection. (The inability

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of a Fourier method to handle large X/R ratios providinghigh DC offsets can be improved by using linear couplers.)For busbar pr otection , the d irectional data logic sum mationhas to be satisfied, together with the summated currentgreater than the fault setting IFIX- For distance protection,the impedance was calculated by extracting the fundamentalcomponent (Fourier coefficient) as:V = A sin co t + B cos co tI = E sin cot + Fcos cotZ = - = R + jX

so that R=(AE + BF)/\E 2 + F2) and X = {BE-AF)\(E 2 + F2).In this system, calculation of all six phase (3 phase-to-earth and 3 phase-to-phase) impedances was performed and aquadrilateral zone characteristic was identified.For transformer or generator protection, the ratio of thetwo end current transformer measurements was scaled beforeA/D co nversion; thu s direct sum mation was possible afterdata acquisition. Both load bias and harmonic bias couldbe provided to suit any stability requirement.3.4 Experimental resultsThe PRBP program was set up in a Nova-1210 minicom-pu ter; th e utilisation of its 12 K me mory is indicated in Table1. The CBBP program consisted of 1 K memory in the IMP-16C, the remaining 1.5 K being used for data-base storage.Both programs were tested in a laboratory double-busbarsubstation model with transmission lines represented bydiscrete elements of resistance and reactance.

Various faults were simulated including stuck circuitbreaker, generator fault, transformer fault, transmission-linefault, busbar fault and simultaneous faults. Both programsreacted satisfactorily and no maloperations were recorded.The respective protection -algo rithm execution time is shownTable 1 : Program location in Nov a-1210

Table 2: Data transfer and protection algorithm operating times, ms

Address

000000- 000377000400- 001777

002000- 005777

007000- 011777

012000- 014777

015000- 016777017000- 027210

Function

common data storage(1st level programs)data transferevent loggerfault-data collectiondata collectiontrip programfault sensorfault-data collection for(i) distance protection(i i) unit protection(i i i) BBZ protection(2nd level programs)protection algorithmsfault-data collec tion pre-arrangement programsdata transferfault- identif ication tableevent loggingzone checking

graphic data storage(3rd level program)background operatingsystem and event printoutprogram

Percentage o ftotal 12 Kmemory, %2.130.530.350.350.351.071.074.264.172.138.53

14.838.604.20

12.800.534.204.203.8712.808.53

36.27

ProtectionalgorithmsDistanceprotectionDifferentialprotectionBusbarprotection(per phase)

Prearrangementor fault-datacollec tion address0.200.45

0.70

Data transferand pre-calculation0.700.60

0.45

Protectioncalculation

6.201.40

0.10

Total

7.102.45

1.25

310

in Table 2. With these computational speeds a fault-detectiontime for back-up protection of 50 to 6 0 ms was achievedfor a double-busbar substation with up to 15 circuits.

4 Centralised back-up protectionThe centralised back-up protection method was first proposedin 1970 by Ungard and Glavitsch, using reactive-power-flow information from various substations to perform faultlocation [1 ]. L ater, a table look-up metho d by Edgley [2]and the discriminative logic approach by Matsuoka andTsuboi [11] were also developed. All these schemes use acentral computer and, in particular, a reliable communicationsystem is required for the back-up protection scheme.With advances in modern power-system-control techniques,it is possible to incorporate the back-up protection schemeinto the computer hierarchical communication arrangement.With the application of the 'fault-directing method' in thecentral computer, back-up protection for an area of power-system network can be achieved.4.1 System structureFig. 5 A shows a typical power-system control computerhierarchical communication structure; Fig. 5B is the cor-responding area separation of the power-system network.In this structure, the central computer can provide bothback-up protection for the tie lines interconnecting thevarious areas and for system splitting or islanding in the' case of a perm anent uncleared fault in an area. In each area,the area computer performs centralised back-up protectionfor its own power-system network. The area computer willnormally be a minicomputer or microprocessor depending onthe size of the power system network.To implement the 'fault-directing method', each circuit-breaker relay measuring point is equipped with a directionalovercurrent relay pointing outwards from the substation.Operation of the relay is communicated to the area computerand interrogated by it for determination of the fault location.As the fault-directing method (FDM) is a fast-fault-locationmethod, it divides the power-system network into two basicinterconnecting elements, the 'centre' and the 'link'. For FDMused in an area computer, the centre can be a busbar, or aportion of a busbar (having at least two connecting terminals);the link can be a transmission line or a teed circuit (havinga circuit breaker at each line end). For FDM used in thecentral computer, the centre will be the area and the link willbe the tie line. Fig. 6 shows these various possible con-figurations.4.2 Fault-directing method4.2.1 Fault location program : The fault-directing programin the area or central computer consists of a number of 'centre'routines and a number of 'link' routines corresponding to theactual power-system structure. The program excution allowsa 'jump' to be made from a 'centre' routine to a 'link' routine

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and back again, in such a way that a fault can be rapidlylocated.A jump from 'centre' routine to 'link' routine is madewhen the directional relay in the centre pointing towards thelink has operated. A jump from a 'link' routine to a 'centre'routine is made when no fault is found in the link. For a linkfault, the fault will be identified in the 'link' routine, whereas,for a centre fault, the number of entries to the 'centre ' routineis used for fault determination. This produces a straight-forward method without any use of the cumbersome tablelook-up techniqu e.A flow chart for FDM is shown in Fig. 7. The programoperation depends on the number of faults simultaneouslypresented, the number of directional relays operated and thetype of faults locate d. Typical fault-identification time is90jus for a Nova minicomputer (1.35jus cycle time) on asystem size as shown in Fig. 8 .

S=substationFig. 5A Power-system-control computer hierarchical communicationstructure

power \e- I system !network /

-0-directional relay

Fig. 5B Power-system-network area separationIEEPRO C, Vol. 129, Pt. C, No. 6, NOVEMBER 1982

4.2.2 Program illustration: The power-system network,shown in Fig. 8, consists of 24 lines interconnected between11 substations, where a total of 64 directional relays aresituated. Suppose there are three simultaneous faults onlines L and X and on busbar S8, and the directional relayshave operated as shown in Fig. 8 by the symbols. The programstarts, first, with an orderly checking of the operated direc-tional relays (in this case, a numerical order has been arranged).The checking is done in groups corresponding to all thedirectional relays in a substation. Hence, a centre routine ofsubstation S2 was started because the operated directionalrelay 8 was found and a jum p to 'link ' / was ma de. Sinceno fault was found in 'link' J, a jump to 'centre' substationS10 routine was then followed. After the directional relay58 was found, the 'link' L routine was called and a fault wasthus located, because both ends of the link showed relayoperation in opposite directions. After locating the first fault,the program was restarted by group checking of directionalrelays. Next the 'centre' routine S3 was started becausedirectional relay 14 was found, and so the 'link' V routine,'centre' S4 routine and finally 'link' X routine were enteredin sequence and the second fault was located. To locate thefault on the teed circuit X, directional relays 16, 19 and 21where all interrogated and found to have operated.Similarly, the directional relay 30 led to the substation57 and the program restarted after no fault in 'link' H wasfound. Similar processes took place on the 'links' /, F andG. As the 'centre' program had been entered four timeswithout any outgoing fault being located, a fault in substation58 was determined. The program also includes flagging of thedirectional relays which have operated for fault-directingand reset purposes. For a flagged relay signal, no furthertest is made and it is treated as if the relay had not operated.The program stops when all operated directional relays havebeen flagged.4.3 Trip decision algorithmTo provide a secure and reliable centralised back-up pro-tection, failure of the directional relay or communication

I -e-i -x -I -X -

v.two-terminalcentretwo-term inal link

, i-*-H

three-termin al l inkmult i terminalcentre

l ink faul tcentre faul t

-e-directional relayFig. 6 Basic elements in the fault-directing methoda Elements, b Faults


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;;initialisetestingigroup checkingof substationdirectional relaynumerical order

Iflowchartrepeatsas before

Fig. 7 Flow chart for the fault-directing method

T--directional relayoperated

Fig. 8 Fault-directing method for fault loca tion in a typical power-system network312 IEEPROC, Vol. 129, Pt. C, No. 6, NOVEMBER 1982


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Casenumber123456789

101112131415161718192021222 3242 5

Directional relayEnd End1 211110 *1 #01 #010001111110*11110

1110 *101 #01 #010011111010 *0000

End3

111101100 *000

FD MresultF

NFNFNFNFNFNFNFNFNFNFF

NFNF

N FN FN F

Table 3Main protectionEnd End1 20100110100010010011100101

0011001010001001000010000

: Trip decision tableEnd3

000100001000

TotalresultF

FFNFNFNFNFNFNFF

N FN FN F

Trip sendEnd End1 2XXX

X

XXX

XX

XX

XX

XXXXXXX

End3

XXXXXXX

Remarks

pure FDM identif icationtransfer trip'single DR failure# substation entry nonzero,channel failure at other substation# substation entry equal to zero,channel failure at # substationsingle DR maloperatepossible w rong signalpure FDM identif icationtransfer triptransfer trip

single DR failuresingle channel failuresingle DR maloperatepossible w rong signal

F = fau lt 1 = operateNF = nonfault 0 = nonoperateX = trip sendFDM = fault directing methodDR = directional relay

CBzcircui t breakerMPzmain protectionDR=directional relay

fault occurrencedirect iona l relaypower flow DIRsendmain protect ionoperatetransfer tr ip fromcentral computer

back-up act ionBU CB identif iedBU CB tr ipp ing

back-up fau l tclearence

substationdataCB MP DR

i

" - -

t ime,ms CB20

6080

100 ,120140160180

200220

,. -

CBoperTC B o p e n

da tareceivedMP

-.

-m

-

DR

centralcomputerFDM transferprogram trip 1

DR operate d Q t a

DR resetDR reset

data^.

3

- -

J-

f **

_

3Uimer

backgroundprogram

stopno faultinit iateBU t imer

ini t iateFDM p rogramstop#normalfault\Iclearence /

fur thertimerinitiation

Fig. 9 Timing control and fault-clearance chart for centralised back-up protectionCB = circuit breakerMP = main protectionDR = directional relayIEEPROC, Vol. 129, Pt. C, No. 6, NOVEMBER 1982 31 3


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channel has to be tolerated in the back-up scheme. Redundantinformation is therefore required to provide such faulttolerance, and for this purpose it is proposed that the mainprotection signaKis sent back to the central computer overa separate communication channel. With this primary-relayinfor matio n, together with the FDM fault identification,a single directional relay failure or single communication-channel failure will not affect the back-up protection func-tion . This is illustrated in Table 3 (cases 4 to 9, 20 to 23)where, in the case of wrong directional relay or main pro-tection inform ation, no tripping action is initiated.The transfer trip facilities in this centralised back-upscheme can also be utilised for distance protection signalling.For a single circuit condition, a permissive underreachingtransfer scheme can be ob tained , as illustrated in cases 2and 3 in Table 3. Furthermore, the scheme is also capableof providing teed-circuit protection (cases 1519), and itcan be considered as an improvement over the conventionalmultiended protection signalling scheme.4.4 Timing charac teristicFig. 9 illustrates the timing control and corresponding faultclearance excution necessary [7]. The function of this centra-lised back-up scheme can be listed as follows:4.4.1 Main-protection back-up: For a breaker operatingtime of 40 ms, main-pro tection fault-clearance time is equalto 8 0 ms. In the case of main- protec tion failure, a total fault-clearance time of 100 ms can be achieved. If this back-upscheme is used with two main protections as a two-out-of-three scheme, an increase in both security and reliabilitycan be achieved. Furthermore, the PRBP program can bemodified to allow an external start initiation for faster faultmeasurement.4.4.2 Circuit-breaker protection back-up: The centralisedback-up protection utilises local breaker back-up facilitiesand in case of both circuit-breaker and back-up protectionfailure, a fault-clearance time of 220ms can still be achieved.5 ConclusionsThis paper has described two newly developed digital tech-niques for power-system back-up protection. Both of theseschemes can give a faster back-up fault-clearance time notachievable by conventional techniques. In the substation-based system, primary-relay failure can be readily handled.On the other hand, the centralised back-up protection offersa much larger coverage of the power-system network, ascomplete substation failure can still be detected. The costof the centralised back-up protection scheme is expensive,dependent on the cost of the communication system; butthis can be reduced by sharing it with other power-system-control and data-acquisition functions. On the other hand,the provision of a reliable communication system is becomingmore important for fast power control and the centralisedapproach would be applicable to this philosophy.

In terms of power-system equipment, the substation-based system can give good back-up protection performanceof local power plant such as busbars, transformers and circuitbreakers, whereas the centralised back-up scheme is bestapplied to transmission-line back-up protection. Applicationof these two back-up schemes simultaneously, each with theirindividual merits, would form in combination a highly reliableback-up protection system.Laboratory experimental results of these two schemeshave shown the advantages offered by each. The treatmentof CBBP in the substation-based system was found to beeffective, particularly when parallel software timer techniqueswere applied. The PRBP scheme offers a new way of providingback-up protection. In centralised back-up protection,tolerance of single communication channel or directionalrelay failure has been provided, and the fault-identificationprocess has not shown to be affected. To cater for computerfailure, a standby computer will be needed, as with othercomputer-based schemes.Digital back-up protection techniques can offer a sub-stantial improvement over conventional techniques, withthe possibility of providing a better protection performanceto the future power-system network.6 AcknowledgmentsThis work was undertaken at the Imperial College with SERCfunds for the purchase of computer equipment. F.C. Chanwould like to thank the Commonwealth Scholarship Com-mission in the UK for the financial support and the ChinaLight and Power Co. Ltd., Hong Kong, for granting himstudy leave.7 References

1 UNGRAD, H., and GLAVITSCH, H.: 'Centrally co-ordinatedback-up protection and system security monitoring as constituentsof an integrated system for the automation of power transmission'.CIGRE, Paper 34-03, 19702 EDGLEY, R .K.: 'A central computer for power-system back-upprotection', Electr. Rev., 14th March 1975, pp. 321-3243 CORY, B.J., DROMEY, G., and MURRAY, B.E.: 'Digital systemsfor protection'. CIGRE, Paper 34-08, August 19764 ROCKEFELLER, G.D.: 'Fault protection with a digital computer',IEEE Trans., 1969, PAS-88, pp. 438 -46 45 CHAN, F .C , and CO RY, B.J.: 'Su bstation digital back-up pro-tection system', IE E Conf. Publ. 185, 1980, pp. 102-1066 CHAN, F .C : 'A power system back-up protectio n and its imple-mentation'. 14th UPEC, Loughborough, 19797 CHAN, F.C: 'Fault-directing method for power system centralizedback-up protectio n'. IE E Conf. Publ. 185, 1980, pp. 107-1118 CHAN, F .C : 'Power system digital back-up protection '. Ph D.Thesis, Imperial College, London, 19799 CHAN, F .C : 'Protec tion of substations using microprocessors -design considerations'. 13th UPEC, Edinburgh, 197810 CHAN, F. C : 'Digital back-up protec tion, system hardware andsoftware'. Power system report, Imperial College, 197911 MATSUOKA, T ., and TSUBOI, A.: 'The discriminative logic offault elements for a centralized back-up protection in power cir-cuits'. IFAC Symposium, Melbourne, Feb. 1977

314 IEEPROC, Vol. 129, Pt. C, No. 6, NOVEMBER 1982

Digital Backup Protection

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