Computerised /b-s/ft/testing of feeder protectionsA.C. Webb, B.Sc.

Indexing terms: Transmission and distribution plant, Power system protection, Relays, Computer appli-cations

Abstract: A description is given of the general arrangement of the overall feeder protection systems used bythe CEGB. It is shown how this arrangement is only designed to accommodate protection relay failure whenthis failure leads to nonoperation under power-system fault conditions. The increasing importance of ensuringrelay failures do not cause incorrect tripping is highlighted and existing methods of preventing this arediscussed. A new approach to overcome this problem is described; that of computerised maintenance testing.A computerised protection test set specificaly developed for this purpose is described. Details are given ofthe hardware, software and mode of operation of the equipment, and test results produced by it are used todemonstrate the tests which the equipment can perform.

List of symbols

VbVr,Vy/0r,0y,

Vr,Ir

Zs

ZjA (K)

= 3-phase injected voltages= single-phase injected current.

0b,0l- = phase angles of injected voltages and cur-rents

= faulted phase relay voltage and current= source impedance= line impedance (relay reach setting)= 96-element injection array K — 0 . . . 95= constant determined by required frequency

and number of samples per cycle

1 Introduction

The safe and secure operation of a modern high-voltage trans-mission system is highly dependent on both the speed anddiscriminative ability of its protection systems. This is especiallytrue if the major load centres are remote from the generationor if economic pressures entail the operation of the powersystem with reduced security standards, as for example duringsystem outages.

To achieve the high performance required by the powersystem, modern feeder protections have, of necessity, becomecomplex electronic systems using large numbers of diversetypes of electronic components. Inevitably this can lead to adecrease in the protection's reliability as the frequency ofcomponent failure must increase as the number of componentsused is increased, in spite of considerable efforts by protectionrelay manufacturers to ensure that only the highest qualitycomponents are used in protection relays.

The possible decline in the high standards of reliabilityexpected of protection systems raises the need for improvedmanagement of protection systems installed on power systems.This paper outlines the systems and philosophy of feeder pro-tection used in this country by the UK Central ElectricityGenerating Board. Methods of improving the reliability offeeder protections are outlined, and a novel development, acomputerised protection test set, is described which aims toimprove the reliability of protection systems by enablingimproved test techniques to be employed.

2 Requirements of power-system protection

The main requirements for power system protection are:(a) high speed of operation for internal faults(b) high level of discriminating ability between internal

and external faults.

Paper 2326C (P7), first received 2nd August and in revised form 28thOctober 1982Mr. Webb is with the Regional Engineering Department, CEGB SouthWestern Region, Bedminster Down, Bristol BS13 8AN, England

The ability of a protection system to fulfil these requirementsis severely degraded by component failures within it, andfailure to meet these requirements under power system faultconditions can pose major problems to the security of thepower system.

In recent practice the avoidance of slow or nonoperation ofprotection for internal faults is considered to be of primeimportance, and in its application of protection the CEGBreflects this by duplicating main protections to give diversityof type and manufacturer, with both main protection relaystripping independently. The preferred arrangement is shown inFig. 1. This configuration also covers the case where one pro-tection is either nonoperative due to an inherent inability torecognise certain types of power-system fault or when afault in the protection equipment itself precludes its oper-ation.

Although the need for discriminative ability has alwaysbeen recognised as important, less than adequate safeguardshave been built into feeder protection systems to monitorfailures degrading their discriminative ability. As reliance isplaced on a protections's inherent discriminative ability toguard against maloperation, the use of two independentlytripping protections must increase the probility of incorrectcircuit tripping due to component failures within the pro-tection.

This approach — that of ensuring the protection systemwill always clear internal faults, although accepting that asmall number of maloperations on through faults may occur —is acceptable if the power system is operated in such a mannerthat it can tolerate the loss of more than one circuit for asingle fault. If system operating requirements become more

busbar

drcuit \jreaker /

*

—

transducer

transducer

tripcoil 1

K v

feeder

tripcoil 2

L_

tripsupply

tripsupply

1st mainprotection

2nd mainprotection

Fig. 1 General arrangement of feeder protection showing independenceof the two protections

IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983 0143-7046/83/010051 +09 $01.50/0 51

onerous due to economic pressures or maintenance require-ments, protection relay maloperations become less acceptable.

3 Methods for improving protection system reliability

The reliability of the protection systems fitted to a feedercan be improved by using redundancy techniques; indeed,present practice uses this to guard against nonoperation. Tobuild in redundancy in the overall system to guard againstmaloperation would however be both prohibitively expensiveand difficult to achieve. For example, a third protection con-nected to the first and second main protections in a two-out-of-three tripping arrangement would reduce the risk of in-correct circuit tripping due to protection faults, but mayhowever increase the risk of nonoperation for faults which canremain undetected by some relays under certain conditions.To maintain the present level of redundancy for operation andimprove the level of integrity to prevent maloperation wouldrequire the application of at least four protections using threedifferent fault detection principles.

The alternative to the redundant system approach is toimprove the reliability of the individual protection relays.This can be achieved by designing the relay with as few compo-nents as possible, and hence reduce the probability of compo-nent failure; however, the overiding need for both high speedof operation and a high level of discriminating ability necessarilyincreases the complexity of the relay, and, depending on theextent of use of integrated circuits, may increase the numberof components used. This is especially true of relays usinganalogue techniques, as the level of component integration isless with analogue than with digital devices.

Although the need for reliable and correct operation ofprotection is essential during power-system fault conditions,a certain level of relay faults is acceptable provided they canbe found and rectified before a power-system fault occurs.If this approach is followed, a number of alternative faultfinding methods are available to enable the improvement ofthe overall performance of the protection.

3.1 Continuous monitoringContinuous monitoring of protection is a technique which hasbeen employed by relay manufacturers for a number of years[1]. It involves building into the relay, at its developmentstage, equipment (usually microprocessor-based) which con-tinuously monitors various signals within the relay. By com-paring these signals with one another and with power-systemconditions, an assessment can be made of the integrity of therelay and its ability to perform correctly under power-systemfault conditions. Alarms are given if the protection appearsto be at fault.

There are two main drawbacks with this solution:(i) It cannot be applied retrospectively to relays installed

on a power system, it must be built in as an integral part of therelay.

(ii) It can only detect failed components which have acatastrophic effect on the equipment; not calibration faults ordeteriorating components.

3.2 Automatic testing using power-system signalsThis technique of testing involves automatically taking theprotection out of service at predetermined time intervals, and,by modifying the characteristics of the relay, its response topower system voltages and currents is measured. From thesemeasurements an attempt is made to predict the relays re-sponse to internal or external faults when it is operating withits normal characteristics, and thus give an indication as towhether the relay is healthy. Also logic test signals can bepresented to the relay, thus proving its tripping and signallinglogic. This method too has the disadvantage that it must be

built into the relay at the design stage. As with continuousmonitoring this inclusion of test equipment in the relay mustbe carefully integrated if reliability is not to be furtherimpaired. The test equipment must also be provided withfacilities such that it can detect power-system faults and putthe relay back into service with its characteristics restored tonormal if a fault occurs while testing is in progress. A furtherdisadvantage is the possibility that the load levels on thecircuit to which the relay is fitted may only rarely be suitablefor performing the automatic test.

3.3 Manual testingThe more often a protection relay is comprehensively testedconfidence in its ability to perform correctly under systemfault conditions increases. But there is a limit to both thefrequency and depth to which testing can be done. Automatictesting as outlined above allows a high frequency of testing,but, because of the indeterminacy of the test signals, it cannotbe arranged to be in great depth. Alternatively, manual testingallows in-depth testing, but the frequency has to be low, asmanual testing of a modern complex feeder protection cantake many hours. Access restrictions and manpower require-ments preclude this as a realistic means of regular use to im-proving confidence in the protections ability to performcorrectly.

3.4 Computerised maintenance testingThis method consists of using automatic computer-basedequipment to test the protection. This enables comprehensivetesting to be carried out in a short time, thus reducing accessrequirements and allowing the frequency of testing to beincreased.

The high speed of testing is achieved by the test equipmentbeing 'plugged into' test points available on the protection,and injecting simulated power system signals into the CT andVT inputs of the protection. The various test points and out-puts of the relay are then monitored by the test equipmentas it changes the injected currents and voltages. This enablesthe test equipment to ascertain and then display the character-istics of the relay. All of this is done automatically by the testequipment after the protection has been made available fortesting by manually taking it out of service.

Operational procedures used by the CEGB allow theoperation of a circuit for a maximum of 8 h per year withone protection out of service. This allows 2h per protectionrelay per year for routine testing (each circuit has four mainprotection relays associated with it). If testing is to take placeat 3-month intervals a maximum testing time of 30min canbe allowed. The amount of manual testing which can takeplace in this time is very limited, and experience in the CEGBhas shown that it is inadequate to prove the correct functioningof a modern feeder protection. For this reason it was decidedto develop a computerised protection test set which wouldbe capable of testing all types of modern feeder protectionsused on the CEGB system, the main criteria of design beingthat the equipment should be capable of performing com-prehensive testing in a minimum of time. The remainder ofthis paper describes this development.

4 Automatic protection test set

Previous authors [2, 3] have described computerised testingas an aid to the development of new designs of protectionrelay, this form of testing being ideal for the simulation ofcomplex power-system signals which are difficult to producewith classical or synthetic test benches. It is believed, however,that the test equipment described here is a novel development,in that it is designed for in-situ routine testing to check that a

52 IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983

particular protection relay is performing satisfactoraly withinits known capabilities.

4.1 Mode of operationThe basic requirement for any protection test set is that thesimulated power system signals, be they current, voltages ora combination of both, are continually changed until therequired relay response is observed. In normal test sets thesignals are typically derived from three-phase mains inputs,using variable transformers etc., which are manually adjusteduntil the relay operates. In this test set the test signals areproduced mathematically within a computer, and are outputas a series of numbers, corresponding to instantaneous valuesof sine waves, to digital-to-analogue convertors where they arethen reconstituted into low-level sine waves. These low-levelsignals are then used to drive power amplifiers which producethe necessary levels of voltage and current with the requiredphase relationship to test the relay. At the same time theoutputs of the relay are monitored by the computer so thatthe effect of the input signals can be noted. Suitable strategiesare then used to change the input signals until the requiredoutput from the relay is obtained. This production of testsignals by a combination of mathematical and electronicmeans allows very complex combinations of current, voltagesand phase angle to be obtained.

Fig. 2 Equipment in use testing distance protection

When testing a protection scheme it is necessary to knowwith a given level of accuracy the magnitude and phase of thetest signals to ensure that the relay is measuring correctly.With existing manual test sets these parameters are determinedby the use of measuring instruments. In a computerised testset two methods are available to ensure that the accuracy ofthe test signals is maintained:

(i) by using the computer to measure the magnitude of theoutput signals and applying this information to make anynecessary adjustments to the output of the equipment

(ii) by calibrating the injection equipment and specifyinga range of relay burdens over which the required accuracy isachieved.

Of these two methods the latter was chosen as this con-siderably reduces the hardware requirements of the equipmentand simplifies the software.

4.2 Hardware descriptionFig. 2 shows the complete equipment in use when testing adistance-type protection relay. Fig. 3 shows a block diagramof the equipment. From both Figures it is apparent that theequipment is physically and functionally in three main parts.

4.2.1 Computer: The computer used is a readily available16-bit word-length machine. It was chosen as it has the requiredprocessing power and readily available peripheral devices. It isequiped with 32kwords of random-access memory, and, asthis memory is volatile, the application software is stored ona magnetic tape cartridge and is automaticaly loaded into themachine's memory when the computer is initially turned on.By using tape as the program storage medium, the ability tostore test results on tape is also given. The analogue testsignals are produced by a 12-bit digital-to-analogue converter,while digital input to and output from the computer ishandled by a 16-bit parallel input/output interface card. Con-trol of the test set by the operator is achieved by the use ofpush buttons and thumbwheel switches which communicatewith the computer via the digital input interface. The pushbuttons give two commands, yes and no, whereas the thumb-wheel switches in conjunction with an enter pushbutton allow20 options to be selected. All of the software is written sothat these simple input commands and options are all thatare necessary to select different test routines and to control

feedback

Y 3-phase* currentB outputN

3-phasevoltageoutput

protectionmonitoringpoint

Fig. 3 Block diagram of automatic protection test equipment

IEEPROC, Vol. 130, Pt. C, No. I, JANUARY 1983 53

the equipment. The results of a protection test sequenceare displayed on a CRT monitor, using either an alphanumericor graphical presentation depending on the type of results tobe presented. Connection of a printer allows the production ofa permanent record of the results.

Portability of the test equipment is improved by housingit in two separate cases. Filtering and surge suppresion devicesare fitted to all connections as they enter the cases. This isto guard against both damage and functional failure of theequipment being caused by the severe electrical noise environ-ment which can exist in high-voltage substations.

4.2.2 Injection equipment: The function of the injectionequipment is to convert the low-level outputs of the digital-to-analogue convertors to the required levels of voltage and/orcurrent suitable for injecting protection relays. It is designedaround commercially available power amplifiers which havebuilt-in thermal and overload protection. Four channels areprovided, as this is the minimum required for the testing ofboth distance and phase comparison types of protection. Forphase comparison relay testing three-phase current is required,whereas for distance relays a three-phase voltage source and asingle-phase current source are necessary. The switching ofthree amplifiers between current and voltage sources and thephase selection of the current produced by the fourth amplifieris achieved by relays which are driven by digital outputs fromthe computer.

As mentioned previously, the injection equipment iscalibrated so that it produces a known output into a rangeof burdens for a given input from the digital-to-analogueconvertors. This is achieved by the use of feedback around thepower amplifiers such that the output is maintained constantfor different relay burdens. The arrangement of this feedbackis shown in Fig. 4 for one of the current/voltage sources. Themode of operation is changed from that of a current to avoltage source by the operation of the relay contact. Threesources are configured in this manner, the fourth source hasonly to supply current, its output circuitry is shown in Fig. 5,and is such that its output can be switched between protectionrelay phases using the relay contacts as shown according towhich type of power-system fault is to be simulated.

The injection equipment is arranged such that it is capableof producing the outputs shown in Table 1; these outputsbeing adequate to prove that a protection relay is performingcorrectly. The outputs of the digital-to-analogue convertorsconsist of a series of DC voltage levels which correspond tothe instantaneous values of the sinusoidal waveform at 15°intervals. A typical output is shown in the top trace of Fig. 6.To obtain a waveform suitable for relay testing this waveformmust be filtered to remove the high-frequency componentsbefore amplification. This is achieved by the use of lowpassfilters. The filtered and amplified output of a voltage sourceis shown in the lower trace of Fig. 6. As can be seen the filterintroduces a phase error between the signal produced by the

Table 1: Range of outputs obtainable from equipment

Source

Voltageorcurrentcurrent

Maximumoutput

65 V

2.1 A2 A

Minimumoutput

0.27 V

10mA8.2 mA

Resolution

0.032 V

1.02 mA0.97 mA

Maximumburden

65 V A

10VA20 VA

Regulation

1%

1.4%1%

yellow

blue

neutral

Fig. 5 Current source with phase selection

Injection

R - EY - EB - ER - YY - BB - R

C = closed

ContactA

COOCOO

O = open

B

OCOOCO

C

OOCOOC

D

OOOCOO

E

OOOOCO

F

OOOOOC

G

CCCOOO

computer and the actual output of the equipment. This errorhas to be accounted for by the software when relay timingtests are being performed. Fig. 7 shows a spectral analysis ofthe output of a voltage source, and Fig. 8 shows a typicaloutput of the equipment when testing a distance-type relay. Inthis case a load condition is initally injected and is thenchanged to a red earth fault condition.

4.2.3 Protection interface: The purpose of the protectioninterface is to allow the computer to monitor signals withinthe relay and find the relay's response to the simulated powersystem signals it is injecting into the relay.

As the available relay signals and connection arrangementsvary between different types of relay, a different interface hasto be used for each type of protection. Each interface isphysically designed to enable rapid and simple connection tothe relay under test.

Fig. 4 Voltage/current source

54

Fig. 6 Top trace: D/A convertor output; bottom trace: voltage sourceoutput

Horizontal: 5 ms/div

IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983

Both input and output signals of the protection are elec-trically isolated from the test equipment in the interface toensure no damage can be caused to the relay circuitry if highlevels of electrical interference or circulating currents arecreated during the testing period by either switching operationsor by the occurrence of system faults. The isolation is achievedby the use of optical isolators and relays.

Or

-060

III0.0 Hz 1000.0

Fig. 7 Spectral analysis of voltage source output

Vertical: lOdB/divHorizontal: lOOHz/div

4.3 Software

4.3.1 Choice of software system: From the initial stages ofthe development of the test equipment it was recognised thatthe software should ideally be written in a high-level languagebecause the level of testing required of the equipmentnecessitates the writing of different programs to test differentprotection relays, even if they are of the same basic type ofdifferent manufacture. The software sys'sm adopted uses afully compiled high-level language, and so is capable of a highspeed of operation. Its overall design is very flexible, and thishas enabled the inclusion of special commands for this particularapplication. The hardware and software systems used were

F ig. 8 Injected wa veforms for R -E fault

a Protection outputb Red phase current: 0.2 A prefault; 2 A 75° lag during faultc Red phase voltage: 63.5 V prefault; 8 V during faultd Yellow phase voltagee Blue phase voltage

chosen as a combined package. At first sight the processingpower available may seem to be extravagant, however, it isfelt that if the total life-cycle costs of the equipment are con-sidered the use of a 16-bit machine combined with a high-levellanguage is fully justified by the ease of software productionand maintenance.

4.3.2 Production of injection quantities: The whole conceptof the automatic test set is based on the ability to produceand change the AC signals which, after amplification, are tobe injected into the relay by means of software routines. Theroutine which produces the injected AC signals is shown inflow chart form in Fig. 9. The inner loop of this routine istime critical in that it determines the frequency of the wave-forms produced. For normal injection purposes 24 samples percycle are used, and this allows the production of four 50 Hzsignals.

As can be seen, each pass of the loop results in consecutivevalues of array A being output by the digital-to-analogueconvertors. The loop is repeated 24 times to produce onecycle, and after each cycle a counter is incremented andcompared with the desired number of cycles to be injected.The loop is only left if the desired number of cycles has beenproduced or if the digital input, which is monitored on everypass, changes state. This digital input is connected via theinterface to a relay signal so that operation of the relay can bedetected. The operate time of the relay can be found from thevalues of CYCLE and K when the routine is left, due allowancebeing made for the phase delay discussed earlier.

The array A, which holds the instantaneous values of thewaveforms at 15° intervals, has to be calculated before the

stort

output to D/Aconvertors

Vr"Vy "vh

I

: A: A

= A= A

(K)(K

( K .

(K.

• 1 )

2)

3)

operate

K=K.4

incrementoperationcounter

Fig. 9 Flow diagram of injection routine

IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983 55

injection routine is entered. It is formed using the equations:

A (K) = Vy sin | — + 0 y | + 2048

A(K 1) = Vy sin

A (K + 2) = Vb sin

— + 0y I + 2048

0J +2048

A (K + 3) = / sin I— + 0,-) + 2048

where co is determined by the required frequency and samplerate of the waveform to be produced. The equations are shownfor the production of three-phase voltages and a single-phasecurrent, as used for distance relay injection, and are calculatedfor A" = 0,4, 8 , . . . , 92 .

The magnitude of the D/A outputs is determined by thevalues used for Vr, Vy, Vb and/ , 2048 giving the maximumoutput. The relative phase is determined by 0r, 0y , 0b , and 0,-.The D/A convertors are 12-bit devices scaled to use offsetbinary; hence the addition of 2048 in the equations.

si —,__,.. max

a, a,•4m

step A B C D E

JB-CI

FT

1 2 3 U 5 6 7 8 9 10 11 12 13 K 15injections

Fig. 10A Diagramatic representation of strategy for finding relaysetting

set up injectionequipment torelement to test

decreaseimpedence

by currentstep size

Fig. 10B Flow diagram for finding distance relay setting

56

4.3.3 Test programs: To test a protection relay the injectionroutine is used in conjunction with routines which vary theoutput of the equipment until the desired response of therelay is found. For finding the reach setting of a distance relaythe injected waveforms are calculated so that the effectiveimpedance presented to the relay is reduced until operation isfound. The strategy used to achieve this is demonstrated inFig. 10A. Initially the faulted phase voltage and current aresuch that the impedance presented is at a maximum. If nooperation occurs the subsequent injection uses an impedancereduced by the step size A. This is repeated until an operationis found. The next injection presents an impedance of B lessthan the last injection which did not cause operation. Injectionscontinue using step size B until a second operation is found.Injection then starts before the second operation and con-tinues using reductions of C on subsequent injections. Thisprocess is repeated using decreasing step sizes, thus enablingthe operate level of the relay to be found with a high resolutionin as few injections as possible. For testing distance-type relaysit has been found that the use of five step sizes, each stepsize being 25% of that preceding, gives the best compromisebetween speed and resolution.

The reduction in impedance presented to the relay can beachieved by two different methods:

(i) by maintaining the injected current constant andreducing the faulted phase voltage

(ii) by changing both the injected current and the faultedphase voltage.

In using method (ii) a source impedance has to be assumed inthe calculation of the voltage and current. This is shown inFig. 11, where Zj is the impedance to be presented to therelay. The use of this method is limited by the maximumcurrent which the equipment can produce (2 A). Hence aminimum source-to-line impedance ratio occurs for a givenrelay reach below 'vhich this method can not be used. How-ever, the main use of this method is in ensuring the correctoperation of healthy phase crosspolarisation circuits, whichcan be most easily tested with low values of faulted phasevoltage which occur for high source-to-line impedance ratios.

In both methods the healthy phase voltages are maintainedat their nominal magnitude and phase relationship. A flowdiagram demonstrating the testing of a distance-type relayis shown in Fig. 10B. This program uses method (i) for deter-mining the voltage and current to be injected and, as canbe seen, will find the relay reach at different phase anglesbetween the voltage and current. This allows the polar charac-teristic of the relay to be found.

The test routine used for finding the sequence currentsettings of phase-comparison-type protection relays uses asimilar strategy to that shown in Fig. 10A, but in this casethree-phase current is the injected quantity, the injectionstarting at a low level of current with subsequent injectionsat increasing current levels. With these relays, because of thesmaller range of settings available, the use of three different

Fig. 11 Calculation of faulted phase voltage and current for un-balanced faults

Impedance presented to relay = Vr/Ir = Zj, where Vr = VsZi\{Z\ + Zs)

IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983

Fig. H 2 Results of reach test for R-E elements of distance relay

—) S1 ' h .

Fig. 14 Results ofzone-4 and directional earth fault tests

CP- r^> ~ ;• r~n i>r\ c^- r^Titvi '••-' i^z. ^ ti=J u >i Q ~ l y j

s

/ " "

1

•

Mi

•

/• /

/

I'i i

a o

/ \

Fig. 13 Results of timer tests and zone-1 operations F ig. 15 Polar diagram for balanced fault

step sizes, each a tenth of the size of the previous step, hasbeen found to give optimum speed of testing and resolution.

5 Tests performed by equipment

Section 4.3.3 outlined the tests the equipment performs tofind the basic setting levels of a relay. The flexibility andautomatic nature of the equipment, however, allows muchmore specific tests to take place, some of which are difficultto achieve using manual testing techniques, even if adequatetesting time is available. This Section illustrates this, usingactual test results produced by the equipment.

5.1 Distance protectionThe automatic test sequence for a distance protection initiallyfinds the reach setting of each element at its characteristicangle in the forward and reverse directions. The results displayfor the red-earth elements is shown in Fig. 12. Similar displaysare given for the other five groups of elements. Fig. 13 showsthe results of the next part of the test, in which the operatetime of the timer elements is found. The results are shown fora blocked-mode-type relay, hence the zone-four results(reverse looking block control element) and the permissivetrip timer (PT). The zone-three time test is performed byinjecting a zone-three fault, midway between the zone-two andzone-three settings found in the reach tests, and timing a zone-

IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983

three comparator. The same injection is then repeated, butthis time the trip output of the relay is timed, the differencebetween the two times being displayed as the zone-three timersetting.

A blocked-mode distance relay will operate after thepermissive trip time for a zone-two fault if no block signal isreceived. So the setting of the zone two and PT timers can befound by measuring the operate time of the relay both withand without a block receive signal. The block receive signal ispresented to the relay by a computer digital output via theprotection interface. The last two timers to be tested are thosewhich control the trip-on-close feature of the relay.

The zone-one timing tests are done using injection wave-forms which change from load to fault conditions as shown inFig. 8. The signalling logic is also checked to ensure that noblock signals are being incorrectly sent. If the relay beingtested is part of an acceleration scheme, a check is made toensure that an acceleration signal is sent for a zone oneoperation.

Fig. 14 shows the test results for the zone-four and direc-tional earth fault elements. This test is only performed for ablock-mode relay. Again the zone-four tests are achieved byinjecting a load condition and changing it to a fault condition.The signalling and tripping logic is also monitored, and therequisite messages are given. The final part of the automatictest is to time the operation of the relay either for a nonblocked

57

Fig. 16 Polar diagram for unbalanced fault

RESULTS SUMMARYP.P.S. STARTERS

SETTING = 30.3 V. 1_ . SSETTING = 14.3 V.

N.P.S. STARTERSSETTING - 9.S X L.SSETTING = 6.6 X

DP. QK

DP. QK

NDN-IMPULSE STARTERSSETTING = 30.6 Y. L.S. DP. DKSETTING - 19.7 V.

PRESS YES FDR NEXT PAGE

p . p . '::;.. ., T u f~- I t: R -;:• R H T E T I M E : •••• i /•-.»•> M

R r .:• r T a . »••'..IHH'I. L T I M E ~ ^̂ -";;>»a ?-1 •" r:

• TIME ~ 1*5 . M ••!PE.ET O.K.DWELL TIME ~ fo^a."5 M : \:

?4.P.~. STARTERS'ATE TIME = 1 "5-0 M

KL.ET Q.K.DUtLL TIME ~ 54-6.7' MCtOPEPATE TIME = 1 £ . *5 H

PF:Z.ET a. K -JDUEl-l_ TIME •- 7 0 9 , a H^E

E3 TOP NEM f PrtGE

Fig. 18 Results of starter element timing tests

i>••RG IHHI... *.jUH

GriHi. rap 9HRPIER FDR

FOR "=?-

Fig. 17 Results of phase comparison protection setting test Fig. 19 Results of trip angle and marginal guard tests

zone-two fault or an accelerated zone-two fault, depending onthe relay's mode of operation.

The automatic test as used at present takes approximately12min to complete for a blocked mode and lOmin for anaccelerated mode relay.

Polar diagrams produced by the equipment are shown inFigs. 15 and 16 for balanced and unbalanced faults, respectively.The unbalanced fault characteristic including the effect ofsource impedance. Fig. 15 is produced by finding the reach at15° intervals, whereas Fig. 16 uses intervals of 5°. The pro-duction of these diagrams is time consumming, comparedwith the automatic test. The advantage of producing polardiagrams, however, is that the polarising voltage circuits canbe proved to be healthy. It is proposed that the automatictest will be enhanced by the inclusion of a routine to find theresistive reach of each element, at a source-to-line impedanceratio such that the healthy phase crosspolarising signal pre-dominates. The results of this test will then be displayed alongwith a calculated reach obtained using the known healthyphase polarising characteristic used by the particular relay.

5.2 Phase comparison protectionFor a phase comparison scheme the correct sequence andtiming between starter operations at the two ends of thefeeder is essential for stability to be given on external faults.The philosophy of the automatic test is to test the two ends of

the scheme independently, but in enough depth to ensureeach end will behave correctly under system fault conditions.As long as the timing and setting levels at both ends are foundto be within tolerance then stability should be ensured.

Fig. 17 shows the results of the setting injection tests fora typical relay. The settings are found using three-phasenegative or positive sequence current injections. To ensurethe correct order of operation of the low- and high-set starters,at the high-set setting the low-set is monitored to ensure itis operating. At the low-set setting the high-set is monitored toensure it is not operating. If these conditions are not meterror messages are displayed.

The timing tests the equipment performs are shown inFig. 18. The operate, reset and dwell times of each starterare measured for current injections of three times the settingpreviously found. Fig. 19 shows the results obtained by theequipment in evaluating the trip angle and the performance ofthe marginal guard feature. The equipment then tests thecarrier part of the relay by ensuring that low-set operationresults in the transmission and reception of continuous carriersignals. Delays between the transmitter starting to transmitand the receiver detecting carrier are monitored, and if theseare excessive an error message is given. Delays can be causedby oscillator frequency drift or centre frequency drift in thereceive filter. The modulator threshold is then determined bycurrent injection, and the phase delay in the modulator is

58 IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983

F ig. 20 Results of modulator and carrier tests

checked. Typical results are shown in Fig. 20.The total time to carry out these automatic tests is approxi-

mately 7min. After completion, the equipment, in conjunctionwith the operator, performs receiver sensitivity tests, bringingthe total test time to lOmin.

It should be noted that for all test sequences the testequipment only makes qualititive judgements on the relay'scondition when the decision is a simple yes or no. Incorrectsetting levels or drifts in the relay's performance have to berecognised and acted on by the engineer using the equipment.The equipment's purpose is to gather as much information onthe relay's performance as possible in a minimum of time,thus enabling a trained protection engineer to make a validjudgement on the state of the relay.

6 Conclusions

This paper has set out the problems which are posed to pro-tection and power-system engineers by reliability problemswith protection relays. Various means of mitigating these

problems have been discussed, and a new development hasbeen presented aimed at improving the reliability of protectionsalready installed on a power system, by providing rapid andcomprehensive insitu testing.

The test equipment described has been thoroughly tested,both in the laboratory and in 400 kV substations where ithas performed satisfactoraly. As a result of this, ten of theseequipments are to be manufactured and used by site engineersfor routine testing of protection relays in the South WesternRegion of the CEGB.

At the time of writing five of these equipments havebeen completed, and these are now being used on site to testone type of distance protection and one type of phase com-parison protection. Work is well in hand on designing furtherinterfaces and software such that the equipment can be usedon a further two types of phase comparison relay and twotypes of distance relay. It is believed that the equipment,because of its flexibility, will also be capable of being pro-grammed for the testing of other existing types of relay andnew types of feeder protection relays when they are availableand used on the CEGB system.

7 Acknowledgments

The author is grateful to the UK CEGB South Western Regionfor permission to publish this paper, and would like to expressthanks to colleagues in the SWR for helpful suggestions andencouragement given during the development of the equip-ment described in this paper.

8 References

relay185,

1 USHER, B., and ALDERSON, W.E.: 'A distanceautomatic self checking system'. IEE Conf. Publ.1980, pp. 201-208

2 REDFERN, M.A., ELKATEB, M.M., and WALKER, E.P.:'An investigation into the effects of travelling wave phenom-ena on the performance of a distance relay'. Ibid., pp. 269—273

3 COISH, R.G., ROIK, J.N., LEHN, W.H., and SWIFT, G.W.:'Minicomputer based performance evaluation of protectiverelays'. Ibid., pp. 264-268

Book reviewsPutnam's power from the wind — 2nd edn. Gerald W. KoepplVan Nostrand Reinhold, 1982, 470 pp., £23.40ISBN: 0-44223-299-3Wind energy for the eighties. N.H. Lipmann, P.J. Musgrove andG.W.W. PontinPeter Peregrinus Ltd., 1982, 372pp., £19.50ISBN: 0-906048-73-7

At the beginning of 1982 the US utility, Pacific Gas andElectric contracted to purchase electricity frofn wind turbinestotalling 350MWe, to be built, owned and operated by Wind-farms Ltd. of San Francisco. The turbines are to be installed inthree phases, and the first phase, with 51 turbines providing92MWe, is due for completion by the end of 1985. This isnow the only contract of its type, and there is also someinterest in direct ownership of wind turbines by utilities, but itis a measure of the present credibility of the wind turbine as asource of commercial electric power.

A major part of the recent improvement in prospects forwind power is due to the extensive development work by theUS Federal Wind Energy Program on large horizontal-axiswind turbines. Its present achievement is the MOD 2 producedby Boeing with a quoted performance of 5 cents per kW hr in

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IEEPROC, Vol. 130, Pt. C, No. 1, JANUARY 1983 59