21 relay testing and commissioning - fecime.org · relay is to operate correctly under abnormal...

Introduction 21.1

Electrical type tests 21.2

Electromagnetic compatibility tests 21.3

Product safety type tests 21.4

Environmental type tests 21.5

Software type tests 21.6

Dynamic validation type testing 21.7

Production testing 21.8

Commissioning tests 21.9

Secondary injection test equipment 21.10

Secondary injection testing 21.11

Primary injection testing 21.12

Testing of protection scheme logic 21.13

Tripping and alarm annunciation tests 21.14

Periodic maintenance tests 21.15

Protection scheme design for maintenance 21.16

References 21.17

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21.1 INTRODUCTION

The testing of protection equipment schemes presents anumber of problems. This is because the main functionof protection equipment is solely concerned withoperation under system fault conditions, and cannotreadily be tested under normal system operatingconditions. This situation is aggravated by theincreasing complexity of protection schemes and use ofrelays containing software.

The testing of protection equipment may be divided intofour stages:

i. type tests

ii. routine factory production tests

iii. commissioning tests

iv. periodic maintenance tests

21.1.1 Type Tests

Type tests are required to prove that a relay meets thepublished specification and complies with all relevantstandards. Since the principal function of a protectionrelay is to operate correctly under abnormal powerconditions, it is essential that the performance beassessed under such conditions. Comprehensive typetests simulating the operational conditions are thereforeconducted at the manufacturer's works during thedevelopment and certification of the equipment.

The standards that cover most aspects of relayperformance are IEC 60255 and ANSI C37.90. Howevercompliance may also involve consideration of therequirements of IEC 61000, 60068 and 60529, whileproducts intended for use in the EEC also have to complywith the requirements of Directives 89/336/EEC and73/23/EEC. Since type testing of a digital or numericalrelay involves testing of software as well as hardware,the type testing process is very complicated and moreinvolved than a static or electromechanical relay.

• 21 • Relay Test ingand Commiss ioning

N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e • 3 7 1 •

21.1.2 Routine Factory Production Tests

These are conducted to prove that relays are free fromdefects during manufacture. Testing will take place atseveral stages during manufacture, to ensure problemsare discovered at the earliest possible time and henceminimise remedial work. The extent of testing will bedetermined by the complexity of the relay and pastmanufacturing experience.

21.1.3 Commissioning Tests

These tests are designed to prove that a particularprotection scheme has been installed correctly prior tosetting to work. All aspects of the scheme arethoroughly checked, from installation of the correctequipment through wiring checks and operation checksof the individual items of equipment, finishing withtesting of the complete scheme.

21.1.4 Periodic Maintenance Checks

These are required to identify equipment failures anddegradation in service, so that corrective action can betaken. Because a protection scheme only operates underfault conditions, defects may not be revealed for asignificant period of time, until a fault occurs. Regulartesting assists in detecting faults that would otherwiseremain undetected until a fault occurs.

21.2 ELECTRICAL TYPE TESTS

Various electrical type tests must be performed, asfollows:

21.2.1 Functional Tests

The functional tests consist of applying the appropriateinputs to the relay under test and measuring theperformance to determine if it meets the specification.They are usually carried out under controlledenvironmental conditions. The testing may be extensive,even where only a simple relay function is being tested.,as can be realised by considering the simple overcurrentrelay element of Table 21.1.

To determine compliance with the specification, the testslisted in Table 21.2 are required to be carried out. This isa time consuming task, involving many engineers andtechnicians. Hence it is expensive.

When a modern numerical relay with many functions isconsidered, each of which has to be type-tested, thefunctional type-testing involved is a major issue. In thecase of a recent relay development project, it wascalculated that if one person had to do all the work, it

would take 4 years to write the functional type-testspecifications, 30 years to perform the tests and severalyears to write the test reports that result. Automatedtechniques/ equipment are clearly required, and arecovered in Section 21.7.2.

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Element Range Step Size

I>1 0.08 - 4.00In 0.01In

I>2 0.08 - 32In 0.01In

Directionality Forward/Reverse/Non-directional

RCA -95° to +95° 1°

Characteristic DT/IDMT

Definite Time Delay 0 - 100s 0.01s

IEC IDMT Time Delay

IEC Standard Inverse

IEC Very Inverse

IEC Extremely Inverse

UK Long Time Inverse

Time Multiplier Setting (TMS) 0.025 - 1.2 0.025

IEEE Moderately Inverse

IEEE Very Inverse

IEEE IDMT Time Delay IEEE Extremely Inverse

US-CO8 Inverse

US-CO2 Short Time Inverse

Time Dial (TD) 0.5 - 15 0.1

IEC Reset Time (DT only) 0 - 100s 0.01s

IEEE Reset Time IDMT/DT

IEEE DT Reset Time 0 - 100s 0.01s

IEEE Moderately Inverse

IEEE Very Inverse

IEEE IDMT Reset Time IEEE Extremely Inverse

US-CO8 Inverse

US-CO2 Short Time Inverse

Table 21.1: Overcurrent relay element specification

Table 21.2: Overcurrent relay element functional type tests

Test 1

Test 2

Test 3

Test 4

Test 5

Test 6

Test 7

Test 8

Test 9

Test 10

Test 11

Test 12

Test 13

Test 14

Three phase non-directional pick up and drop off accuracyover complete current setting range for both stages

Three phase directional pick up and drop off accuracyover complete RCA setting range in the forward direction,

current angle sweep

Three phase directional pick up and drop off accuracyover complete RCA setting range in the reverse direction,

current angle sweep

Three phase directional pick up and drop off accuracyover complete RCA setting range in the forward direction,

voltage angle sweep

Three phase directional pick up and drop off accuracyover complete RCA setting range in the reverse direction,

voltage angle sweep

Three phase polarising voltage threshold test

Accuracy of DT timer over complete setting range

Accuracy of IDMT curves over claimed accuracy range

Accuracy of IDMT TMS/TD

Effect of changing fault current on IDMT operating times

Minimum Pick-Up of Starts and Trips for IDMT curves

Accuracy of reset timers

Effect of any blocking signals, opto inputs, VTS, Autoreclose

Voltage polarisation memory

21.2.2 Rating Tests

Rating type tests are conducted to ensure thatcomponents are used within their specified ratings andthat there are no fire or electric shock hazards under anormal load or fault condition of the power system. Thisis in addition to checking that the product complies withits technical specification. The following are amongstthe rating type tests conducted on protection relays, thespecified parameters are normally to IEC 60255-6.

21.2.3 Thermal Withstand

The thermal withstand of VT’s, CT’s and output contactcircuits is determined to ensure compliance with thespecified continuous and short-term overload conditions.In addition to functional verification, the pass criterion isthat there is no detrimental effect on the relay assembly,or circuit components, when the product is subjected tooverload conditions that may be expected in service.Thermal withstand is assessed over a time period of 1sfor CT’s and 10s for VT’s.

21.2.4 Relay Burden

The burdens of the auxiliary supply, optically isolatedinputs, VT’s and CT’s are measured to check that theproduct complies with its specification. The burden ofproducts with a high number of input/output circuits isapplication specific i.e. it increases according to thenumber of optically isolated input and output contactports which are energised under normal power systemload conditions. It is usually envisaged that not morethan 50% of such ports will be energised in anyapplication.

21.2.5 Relay Inputs

Relay inputs are tested over the specified ranges. Inputsinclude those for auxiliary voltage, VT, CT, frequency,optically isolated digital inputs and communicationcircuits.

21.2.6 Relay Output Contacts

Protection relay output contacts are type tested toensure that they comply with the product specification.Particular withstand and endurance type tests have to becarried out using d.c., since the normal supply is via astation battery.

21.2.7 Insulation Resistance

The insulation resistance test is carried out according toIEC 60255-5, i.e. 500V d.c. ±10%, for a minimum of 5

seconds. This is carried out between all circuits and caseearth, between all independent circuits and acrossnormally open contacts. The acceptance criterion for aproduct in new condition is a minimum of 100MΩ. Aftera damp heat test the pass criterion is a minimum of10MΩ.

21.2.7 Auxiliary Supplies

Digital and numerical protection relays normally requirean auxiliary supply to provide power to the on-boardmicroprocessor circuitry and the interfacing opto-isolated input circuits and output relays. The auxiliarysupply can be either a.c. or d.c., supplied from a numberof sources or safe supplies - i.e. batteries, UPS’,generators, etc., all of which may be subject to voltagedips, short interruptions and voltage variations. Relaysare designed to ensure that operation is maintained andno damage occurs during a disturbance of the auxiliarysupply.

Tests are carried out for both a.c. and d.c. auxiliarysupplies and include mains variation both above andbelow the nominal rating, supply interruptions derived byopen circuit and short circuit, supply dips as apercentage of the nominal supply, repetitive starts. Theduration of the interruptions and supply dips range from2ms to 60s intervals. A short supply interruption or dipup to 20ms, possibly longer, should not cause anymalfunction of the relay. Malfunctions include theoperation of output relays and watchdog contacts, thereset of microprocessors, alarm or trip indication,acceptance of corrupted data over the communicationlink and the corruption of stored data or settings. For alonger supply interruption, or dip in excess of 20ms, therelay self recovers without the loss of any function, data,settings or corruption of data. No operator interventionis required to restore operation after an interruption ordip in the supply. Many relays have a specification thatexceeds this requirement, tolerating dips of up to 50mswithout operation being affected.

In addition to the above, the relay is subjected to a numberof repetitive starts or a sequence of supply interruptions.Again the relay is tested to ensure that no damage or datacorruption has occurred during the repetitive tests.

Specific tests carried out on d.c. auxiliary suppliesinclude reverse polarity, a.c. waveform superimposed onthe d.c. supply and the effect of a rising and decayingauxiliary voltage. All tests are carried out at variouslevels of loading of the relay auxiliary supply.

21.3 ELECTROMAGNETIC COMPATIBILITY TESTS

There are numerous tests that are carried out todetermine the ability of relays to withstand the electrical

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environment in which they are installed. The substationenvironment is a very severe environment in terms of theelectrical and electromagnetic interference that canarise. There are many sources of interference within asubstation, some originating internally, others beingconducted along the overhead lines or cables into thesubstation from external disturbances. The mostcommon sources are:

a. switching operations

b. system faults

c. lightning strikes

d. conductor flashover

e. telecommunication operations e.g. mobile phones

A whole suite of tests are performed to simulate thesetypes of interference, and they fall under the broadumbrella of what is known as EMC, or ElectromagneticCompatibility tests.

Broadly speaking, EMC can be defined as:

‘The ability of equipment to co-exist in the sameelectromagnetic environment’

It is not a new subject and has been tested for by themilitary ever since the advent of electronic equipment. EMCcan cause real and serious problems, and does need to betaken into account when designing electronic equipment.

EMC tests determine the impact on the relay under testof high-frequency electrical disturbances of variouskinds. Relays manufactured or intended for use in theEEC have to comply with EEC Directive 89/336/EEC inthis respect. To achieve this, in addition to designing forstatutory compliance to this Directive, the followingrange of tests are carried out:

a. d.c. interrupt test

b. a.c. ripple on d.c. supply test

c. d.c. ramp test

d. high frequency disturbance test

e. fast transient test

f. surge immunity test

g. power frequency interference test

h. electrostatic discharge test

i. conducted and radiated emissions tests

j. conducted and radiated immunity tests

k. power frequency magnetic field tests

21.3.1 D.C Interrupt Test

This is a test to determine the maximum length of time

that the relay can withstand an interruption in theauxiliary supply without de-energising, e.g. switchingoff, and that when this time is exceeded and it doestransiently switch off, that no maloperation occurs.

It simulates the effect of a loose fuse in the batterycircuit, or a short circuit in the common d.c. supply,interrupted by a fuse. Another source of d.c. interruptionis if there is a power system fault and the battery issupplying both the relay and the circuit breaker trip coils.When the battery energises the coils to initiate thecircuit breaker trip, the voltage may fall below therequired level for operation of the relay and hence a d.c.interrupt occurs. The test is specified in IEC 60255-11and comprises a interruptions of 2, 5, 10, 20, 50, 100 and200ms. For interruptions lasting up to and including20ms, the relay must not de-energise of maloperate,while for longer interruptions it must not maloperate.

The relay is powered from a battery supply, and bothshort circuit and open circuit interruptions are carriedout. Each interruption is applied 10 times, and forauxiliary power supplies with a large operating range,the tests are performed at minimum, maximum, andother voltages across this range, to ensure complianceover the complete range.

21.3.2 A.C. Ripple on D.C. Supply

This test (IEC 60255-11) determines that the relay is ableto operate correctly with a superimposed a.c. voltage onthe d.c. supply. This is caused by the station battery beingcharged by the battery charger, and the relevant waveformis shown in Figure 21.1. It consists of a 12% peak-to-peakripple superimposed on the d.c. supply voltage.

For auxiliary power supplies with a large operating range,the tests are performed at minimum, maximum, andother voltages across this range, to ensure compliancefor the complete range. The interference is applied usinga full wave rectifier network, connected in parallel withthe battery supply. The relay must continue to operatewithout malfunction during the test.

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436

349

262

175881

0.00

10.00

20.00

30.00

40.00

50.00

60.00

Volta

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Time (ms)

Figure 21.1: A.C. ripple superimposed on d.c.supply test

21.3.3 D.C. Ramp Down/Ramp Up

This test simulates a failed station battery charger, whichwould result in the auxiliary voltage to the relay slowlyramping down. The ramp up part simulates the batterybeing recharged after discharging. The relay must powerup cleanly when the voltage is applied and notmaloperate.

There is no international standard for this test, so individualmanufacturers can decide if they wish to conduct such atest and what the test specification shall be.

21.3.4 High Frequency Disturbance Test

The High Frequency Disturbance Test simulates highvoltage transients that result from power system faultsand plant switching operations. It consists of a 1MHzdecaying sinusoidal waveform, as shown in Figure 21.2.The interference is applied across each independentcircuit (differential mode) and between eachindependent circuit and earth (common mode) via anexternal coupling and switching network. The product isenergised in both normal (quiescent) and tripped modesfor this test, and must not maloperate when theinterference is applied for a 2 second duration.

21.3.5 Fast Transient Test

The Fast Transient Test simulates the HV interferencecaused by disconnector operations in GIS substations orbreakdown of the SF6 insulation between conductorsand the earthed enclosure. This interference can eitherbe inductively coupled onto relay circuits or can bedirectly introduced via the CT or VT inputs. It consists ofa series of 15ms duration bursts at 300ms intervals, eachburst consisting of a train of 50ns wide pulses with veryfast (5ns typical) rise times (Figure 21.3), with a peakvoltage magnitude of 4kV.

The product is energised in both normal (quiescent) andtripped modes for this test. It must not maloperate whenthe interference is applied in common mode via theintegral coupling network to each circuit in turn, for 60seconds. Interference is coupled onto communicationscircuits, if required, using an external capacitive couplingclamp.

21.3.6 Surge Immunity Test

The Surge Immunity Test simulates interference causedby major power system disturbances such as capacitorbank switching and lightning strikes on overhead lineswithin 5km of the substation. The test waveform has anopen circuit voltage of 4kV for common mode surges and2kV for differential mode surges. The test waveshapeconsists on open circuit of a 1.2/50ms rise/fall time anda short circuit current of 8/20ms rise/fall time. Thegenerator is capable of providing a short circuit testcurrent of up to 2kA, making this test potentiallydestructive. The surges are applied sequentially undersoftware control via dedicated coupling networks in bothdifferential and common modes with the productenergised in its normal (quiescent) state. The productshall not maloperate during the test, shall still operatewithin specification after the test sequence and shall notincur any permanent damage.

21.3.7 Power Frequency Interference

This test simulates the type of interference that is causedwhen there is a power system fault and very high levelsof fault current flow in the primary conductors or theearth grid. This causes 50 or 60Hz interference to beinduced onto control and communications circuits.

There is no international standard for this test, but oneused by some Utilities is:

a. 500V r.m.s., common mode

b. 250V r.m.s., differential mode

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Repetition period t

V

V

t

Burst duration (1/15 ms)

Burst period, 300 ms

5 ns rise time, 50 ns pulse width

Figure 21.3: Fast Transient Test waveform

Volta

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0Time

Figure 21.2: High Frequency DisturbanceTest waveform

applied to circuits for which power system inputs are notconnected.

Tests are carried out on each circuit, with the relay in thefollowing modes of operation:

1. current and voltage applied at 90% of setting,(relay not tripped)

2. current and voltage applied at 110% of setting,(relay tripped)

3. main protection and communications functionsare tested to determine the effect of theinterference

The relay shall not maloperate during the test, and shall stillperform its main functions within the claimed tolerance.

21.3.8 Electrostatic Discharge Test

This test simulates the type of high voltage interferencethat occurs when an operator touches the relay’s frontpanel after being charged to a high potential. This is exactlythe same phenomenon as getting an electric shock whenstepping out of a car or after walking on a synthetic fibrecarpet.

In this case the discharge is only ever applied to the frontpanel of the relay, with the cover both on and off. Twotypes of discharges are applied, air discharge and contactdischarge. Air discharges are used on surfaces that arenormally insulators, and contact discharges are used onsurfaces that are normally conducting. IEC 60255-22-2is the relevant standard this test, for which the testparameters are:

a. cover on: Class 4, 8kV contact discharge, 15kV airdischarge

b. cover off: Class 3, 6kV contact discharge, 8kV airdischarge

In both cases above, all the lower test levels are alsotested.The discharge current waveform is shown in Figure 21.4.

The test is performed with single discharges repeated oneach test point 10 times with positive polarity and 10times with negative polarity at each test level. The timeinterval between successive discharges is greater than 1second. Tests are carried out at each level, with the relayin the following modes of operation:

1. current and voltage applied at 90% of setting,(relay not tripped)

2. current and voltage applied at 110% of setting,(relay tripped)

3. main protection and communications functionsare tested to determine the effect of the discharge

To pass, the relay shall not maloperate, and shall stillperform its main functions within the claimed tolerance.

21.3.9 Conducted and Radiated Emissions Tests

These tests arise primarily from the essential protectionrequirements of the European Community (EU) directiveon EMC. These require manufacturers to ensure that anyequipment to be sold in the countries comprising theEuropean Union must not interfere with otherequipment. To achieve this it is necessary to measure theemissions from the equipment and ensure that they arebelow the specified limits.

Conducted emissions are measured only from theequipment’s power supply ports and are to ensure that whenconnected to a mains network, the equipment does not injectinterference back into the network which could adverselyaffect the other equipment connected to the network.

Radiated emissions measurements are to ensure that theinterference radiated from the equipment is not at alevel that could cause interference to other equipment.This test is normally carried out on an Open Area TestSite (OATS) where there are no reflecting structures orsources of radiation, and therefore the measurementsobtained are a true indication of the emission spectrumof the relay. An example of a plot obtained duringconducted emissions tests is shown in Figure 21.5.

The test arrangements for the conducted and radiatedemissions tests are shown in Figure 21.6.

When performing these two tests, the relay is in aquiescent condition, that is not tripped, with currentsand voltages applied at 90% of the setting values. Thisis because for the majority of its life, the relay will be inthe quiescent state and the emission of electromagneticinterference when the relay is tripped is considered to beof no significance. Tests are conducted in accordancewith IEC 60255-25 and EN 50081-2, and are detailed inTable 21.3.

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0

Curr

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% o

f Pea

k

Time, ns

Rise Time = 0.7 to 1.0 ns.Current specified for 30 ns and 60 ns

102030405060708090

100

0 10 20 30 40 50 60 70 80 90

Figure 21.4: ESD Current Waveform

Table 21.3: Test criteria for Conducted andRadiated Emissions tests

Frequency Range Specified Limits Test Limits

30 - 230MHz30dB(µV/m) 40dB(µV/m)

at 30m at 10m

230 - 1000MHz37dB(µV/m) 47dB(µV/m)

at 30m at 10m

0.15 - 0.5MHz79dB(µV) 79dB(µV)

quasi-peak quasi-peak66dB(µV) average 66dB(µV) average

0.5 - 30MHz73dB(µV) 73dB(µV)

quasi-peak quasi-peak60dB(µV) average 60dB(µV) average

Radiated

Conducted

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dBuV

Frequency, MHz

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100

Quasi-peak limits

Average limits

Typical trace

Figure 21.5: Conducted Emissions Test Plot

Screened room

Support/analysisequipment

Access panel

Ante-chamber

E.U.T.Impedance network

E.U.T. - Equipment under test

E.U.T.

Antenna

Turntable

Earth Plane

10m

(b) Radiated Emissions test arrangement on an OATS

(a) Conducted EMC emissions test arrangement

Figure 21.6: EMC test arrangements

21.3.10 Conducted and Radiated Immunity Tests

These tests are designed to ensure that the equipment isimmune to levels of interference that it may be subjectedto. The two tests, conducted and radiated, arise from thefact that for a conductor to be an efficient antenna, itmust have a length of at least 1/4 of the wavelength ofthe electromagnetic wave it is required to conduct.

If a relay were to be subjected to radiated interference at150kHz, then a conductor length of at least

λ = 300 x106/(150 x 103 x 4)

= 500 m

would be needed to conduct the interference. Even withall the cabling attached and with the longest PCB tracklength taken into account, it would be highly unlikelythat the relay would be able to conduct radiation of thisfrequency, and the test therefore, would have no effect.The interference has to be physically introduced byconduction, hence the conducted immunity test.However, at the radiated immunity lower frequency limitof 80MHz, a conductor length of approximately 1.0m isrequired. At this frequency, radiated immunity tests canbe performed with the confidence that the relay willconduct this interference, through a combination of theattached cabling and the PCB tracks.

Although the test standards state that all 6 faces of theequipment should be subjected to the interference, inpractice this is not carried out. Applying interference tothe sides and top and bottom of the relay would havelittle effect as the circuitry inside is effectively screenedby the earthed metal case. However, the front and rearof the relay are not completely enclosed by metal and aretherefore not at all well screened, and can be regarded asan EMC hole. Electromagnetic interference whendirected at the front and back of the relay can enterfreely onto the PCB’s inside.

When performing these two tests, the relay is in aquiescent condition, that is not tripped, with currentsand voltages applied at 90% of the setting values. Thisis because for the majority of its life, the relay will be inthe quiescent state and the coincidence of anelectromagnetic disturbance and a fault is considered tobe unlikely.

However, spot checks are performed at selectedfrequencies when the main protection and controlfunctions of the relay are exercised, to ensure that it willoperate as expected, should it be required to do so.

The frequencies for the spot checks are in generalselected to coincide with the radio frequency broadcastbands, and in particular, the frequencies of mobilecommunications equipment used by personnel workingin the substation. This is to ensure that when working inthe vicinity of a relay, the personnel should be able to

operate their radios/mobile phones without fear of relaymaloperation.

IEC 60255-22-3 specifies the radiated immunity tests tobe conducted (ANSI/IEEE C37.90.2 is used for equipmentbuilt to US standards), with signal levels of:

1. IEC: Class III, 10V/m, 80MHz -1000MHz

2. ANSI/IEEE: 35V/m 25MHz - 1000MHz with nomodulation, and again with 100% pulsemodulation

IEC 60255-22-6 is used for the conducted immunity test,with a test level of:

Class III, 10V r.m.s., 150kHz - 80MHz.

21.3.11 Power Frequency Magnetic Field Tests

These tests are designed to ensure that the equipment isimmune to magnetic interference. The three tests,steady state, pulsed and damped oscillatory magneticfield, arise from the fact that for different site conditionsthe level and waveshape is altered.

23.3.11.1 Steady state magnetic field tests

These tests simulate the magnetic field that would beexperienced by a device located within close proximity ofthe power system. Testing is carried out by subjectingthe relay to a magnetic field generated by two inductioncoils. The relay is rotated such that in each axis it issubjected to the full magnetic field strength. IEC 61000-4-6 is the relevant standard, using a signal level of:

Level 5: 300A/m continuous and 1000A/m short duration

The test arrangement is shown in Figure 21.7.

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E.U.T.

Induction coil

Induction coil

Ground plane

E.U.T. - Equipment under test

Figure 21.7: Power frequency magnetic field set-up

To pass the steady-state test, the relay shall notmaloperate, and shall still perform its main functionswithin the claimed tolerance. During the application ofthe short duration test, the main protection functionshall be exercised and verified that the operatingcharacteristics of the relay are unaffected.

21.3.11.2 Pulsed magnetic field

These tests simulate the magnetic field that would beexperienced by a device located within close proximity ofthe power system during a transient fault condition.According to IEC 61000-4-9, the generator for theinduction coils shall produce a 6.4/16µs waveshape withtest level 5, 100A/m with the equipment configured asfor the steady state magnetic field test. The relay shallnot maloperate, and shall still perform its main functionswithin the claimed tolerance during the test.

21.3.11.3 Damped oscillatory magnetic field

These tests simulate the magnetic field that would beexperienced by a device located within close proximity ofthe power system during a transient fault condition. IEC61000-4-10 specifies that the generator for the coil shallproduce an oscillatory waveshape with a frequency of0.1MHz and 1MHz, to give a signal level in accordancewith Level 5 of 100A/m, and the equipment shall beconfigured as in Figure 21.7.

21.4 PRODUCT SAFETY TYPE TESTS

A number of tests are carried out to demonstrate thatthe product is safe when used for its intendedapplication. The essential requirements are that therelay is safe and will not cause an electric shock or firehazard under normal conditions and in the presence of asingle fault. A number of specific tests to prove this maybe carried out, as follows.

21.4.1 Dielectric Voltage Withstand

Dielectric Voltage Withstand testing is carried out as aroutine test i.e. on every unit prior to despatch. Thepurpose of this test is to ensure that the product build isas intended by design. This is done by verifying theclearance in air, thus ensuring that the product is safe tooperate under normal use conditions. The following testsare conducted unless otherwise specified in the productdocumentation:

1. 2.0kV r.m.s., 50/60Hz for 1 minute between allterminals and case earth and also betweenindependent circuits, in accordance with IEC60255-5. Some communication circuits areexcluded from this test, or have modified testrequirements e.g. those using D-type connectors

2. 1.5kV r.m.s., 50/60Hz for 1 minute across normally

open contacts intended for connection to trippingcircuits, in accordance with ANSI/IEEE C37.90

3. 1.0kV r.m.s., 50/60Hz for 1 minute across thenormally open contacts of watchdog orchangeover output relays, in accordance with IEC60255-5

The routine dielectric voltage withstand test time may beshorter than for the 1 minute type test time, to allow areasonable production throughput, e.g. for a minimum of1 second at 110% of the voltage specified for 1 minute.

21.4.2 Insulation Withstand for Overvoltages

The purpose of the High Voltage Impulse Withstand typetest is to ensure that circuits and their components willwithstand overvoltages on the power system caused bylightning. Three positive and three negative high voltageimpulses, 5kV peak, are applied between all circuits andthe case earth and also between the terminals ofindependent circuits (but not across normally opencontacts). As before, different requirements apply in thecase of circuits using D-type connectors.

The test generator characteristics are as specified in IEC60255-5 and are shown in Figure 21.8. No disruptivedischarge (i.e. flashover or puncture) is allowed.

If it is necessary to repeat either the Dielectric Voltage orHigh Voltage Impulse Withstand tests these should becarried out at 75% of the specified level, in accordancewith IEC 60255-5, to avoid overstressing insulation andcomponents.

21.4.3 Single Fault Condition Assessment

An assessment is made of whether a single faultcondition such as an overload, or an open or short circuit,applied to the product may cause an electric shock or fire

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5kV peakRise time (10 % to 90 %) = 1.2 sDuration (50 %) = 50 s

Volta

ge

Time

Figure 21.8: Test generator characteristics for insulation withstand test

hazard. In the case of doubt, type testing is carried outto ensure that the product is safe.

21.4.4 Earth Bonding Impedance

Class 1 products that rely on a protective earthconnection for safety are subjected to an earth bondingimpedance (EBI) type test. This ensures that the earthpath between the protective earth connection and anyaccessible earthed part is sufficiently low to avoiddamage in the event of a single fault occurring. The testis conducted using a test voltage of 12V maximum and atest current of twice the recommended maximumprotective fuse rating. After 1 minute with the currentflowing in the circuit under test, the EBI shall not exceed0.1Ω.

21.4.5 CE Marking

A CE mark on the product, or its packaging, shows thatcompliance is claimed against relevant EuropeanCommunity directives e.g. Low Voltage Directive73/23/EEC and Electromagnetic Compatibility (EMC)Directive 89/336/EEC.

21.5 ENVIRONMENTAL TYPE TESTS

Various tests have to be conducted to prove that a relaycan withstand the effects of the environment in which itis expected to work. They consist of: the following tests:

1. temperature

2. humidity

3. enclosure protection

4. mechanical

These tests are described in the following sections.

21.5.1 Temperature Test

Temperature tests are performed to ensure that aproduct can withstand extremes in temperatures, bothhot and cold, during transit, storage and operatingconditions. Storage and transit conditions are defined asa temperature range of –25°C to +70°C and operating as–25°C to +55°C.

Dry heat withstand tests are performed at 70°C for 96hours with the relay de-energised. Cold withstand testsare performed at –40°C for 96 hours with the relay de-energised. Operating range tests are carried out with theproduct energised, checking all main functions operatewithin tolerance over the specified working temperaturerange –25°C to +55°C.

21.5.2 Humidity Test

The humidity test is performed to ensure that theproduct will withstand and operate correctly whensubjected to 93% relative humidity at a constanttemperature of 40°C for 56 days. Tests are performed toensure that the product functions correctly withinspecification after 21 and 56 days. After the test, visualinspections are made for any signs of unacceptablecorrosion and mould growth.

21.5.3 Cyclic Temperature/Humidity Test

This is a short-term test that stresses the relay bysubjecting it to temperature cycling in conjunction withhigh humidity.

The test does not replace the 56 day humidity test, but isused for testing extension to ranges or minormodifications to prove that the design is unaffected.

The applicable standard is IEC 60068-2-30 and testconditions of:

+25°C ±3°C and 95% relative humidity/+55°C ±2°C and95% relative humidity

are used, over the 24 hour cycle shown in Figure 21.9.

For these tests the relay is placed in a humidity cabinet,and energised with normal in-service quantities for thecomplete duration of the tests. In practical terms thisusually means energising the relay with currents andvoltages such that it is 10% from the threshold foroperation. Throughout the duration of the test the relay ismonitored to ensure that no unwanted operations occur.

Once the relay is removed from the humidity cabinet, itsinsulation resistance is measured to ensure that it hasnot deteriorated to below the claimed level. The relay isthen functionally tested again, and finally dismantled to

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708090

100

Rela

tive

hum

idity

%Am

bien

t Te

mpe

ratu

re °

C

+55°C

+25°C+28°C

+22°C

Time

95%

80%90%

95%96%

End of temperaturerise

temperature

15min

3h

12h±0.5h 6h

24h

3h

±0.5h

Time

Figure 21.9: Cyclic temperature/humiditytest profile

check for signs of component corrosion and growth.

The acceptance criterion is that no unwanted operationsshall occur including transient operation of indicatingdevices. After the test the relay’s insulation resistanceshould not have significantly reduced, and it shouldperform all of its main protection and communicationsfunctions within the claimed tolerance. The relay shouldalso suffer no significant corrosion or growth, andphotographs are usually taken of each PCB and the caseas a record of this.

21.5.4 Enclosure Protection Test

Enclosure protection tests prove that the casing systemand connectors on the product protect against the ingressof dust, moisture, water droplets (striking the case at pre-defined angles) and other pollutants. An ‘acceptable’ levelof dust or water may penetrate the case during testing,but must not impair normal product operation, safety orcause tracking across insulated parts of connectors.

21.5.5 Mechanical Tests

Mechanical tests simulate a number of differentmechanical conditions that the product may have toendure during its lifetime. These fall into two categories

a. response to disturbances while energised

b. response to disturbances during transportation(de-energised state)

Tests in the first category are concerned with theresponse to vibration, shock and seismic disturbance.The tests are designed to simulate normal in-serviceconditions for the product, for example earthquakes.These tests are performed in all three axes, with theproduct energised in its normal (quiescent) state. Duringthe test, all output contacts are continually monitoredfor change using contact follower circuits. Vibrationlevels of 1gn, over a 10Hz-150Hz frequency sweep areused. Seismic tests use excitation in a single axis, usinga test frequency of 35Hz and peak displacements of7.5mm and 3.5mm in the x and y axes respectively belowthe crossover frequency and peak accelerations of 2.0gnand 1.0gn in these axes above the crossover frequency.

The second category consists of vibration endurance,shock withstand and bump tests. They are designed tosimulate the longer-term affects of shock and vibrationthat could occur during transportation. These tests areperformed with the product de-energised. After thesetests, the product must still operate within itsspecification and show no signs of permanentmechanical damage. Equipment undergoing a seismictype test is shown in Figure 21.10, while the waveformfor the shock/bump test is shown in Figure 21.11

The test levels for shock and bump tests are:

Shock response (energised):

3 pulses, each 10g, 11ms duration

Shock withstand (de-energised):

3 pulses, 15g, 11ms duration

Bump (de-energised):

1000 pulses, 10g, 16ms duration

21.6 SOFTWARE TYPE TESTS

Digital and numerical relays contain software toimplement the protection and measurement functions ofa relay. This software must be thoroughly tested, toensure that the relay complies with all specifications andthat disturbances of various kinds do not result inunexpected results. Software is tested in various stages:

a. unit testing

b. integration testing

c. functional qualification testing

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Pulse shape (half sine)

0.8A

1.2AA

-0.2A0

+0.2A

0.4D

2.5D 2.5D

6D = T2

D D

2.4D = T1

D - duration of nominal pulseA - peak acceleration of nominal pulseT1- minimum time for monitoring of pulse when conventional shock/bump machine is usedT2 - as T1 when a vibration generator is used

Figure 21.11: Shock/Bump Impulse waveform

Figure 21.10: Relay undergoing seismic test

The purpose of unit testing is to determine if anindividual function or procedure implemented usingsoftware, or small group of closely related functions, isfree of data, logic, or standards errors. It is much easierto detect these types of errors in individual units or smallgroups of units than it is in an integrated softwarearchitecture and/or system. Unit testing is typicallyperformed against the software detailed design and bythe developer of the unit(s).

Integration testing typically focuses on these interfacesand also issues such as performance, timings andsynchronisation that are not applicable in unit testing.Integration testing also focuses on ‘stressing’ thesoftware and related interfaces.

Integration testing is ‘black box’ in nature, i.e. it does nottake into account the structure of individual units. It istypically performed against the software architecturaland detailed design. The specified software requirementswould typically also be used as a source for some of thetest cases.

21.6.1 Static Unit Testing

Static Unit Testing (or static analysis as it is often called)analyses the unit(s) source code for complexity, precisiontracking, initialisation checking, value tracking, strongtype checking, macro analysis etc. While Static UnitTesting can be performed manually, it is a laborious anderror prone process and is best performed using aproprietary automated static unit analysis tool. It isimportant to ensure that any such tool is configuredcorrectly and used consistently during development.

21.6.2 Dynamic Testing

Dynamic Testing is concerned with the runtimebehaviour of the unit(s) being tested and so therefore,the unit(s) must be executed. Dynamic unit testing canbe sub-divided into ‘black box’ testing and ‘white box’testing. ‘Black box’ testing verifies the implementationof the requirement(s) allocated to the unit(s). It takes noaccount of the internal structure of the unit(s) beingtested. It is only concerned with providing known inputsand determining if the outputs from the unit(s) arecorrect for those inputs. ‘White box’ testing is concernedwith testing the internal structure of the unit(s) andmeasuring the test coverage, i.e. how much of the codewithin the unit(s) has been executed during the tests.The objective of the unit testing may, for example, be toachieve 100% statement coverage, in which every line ofthe code is executed at least once, or to execute everypossible path through the unit(s) at least once.

21.6.3 Unit Testing Environment

Both Dynamic and Static Unit Testing are performed inthe host environment rather than the targetenvironment. Dynamic Unit Testing uses a test harnessto execute the unit(s) concerned. The test harness isdesigned such that it simulates the interfaces of theunit(s) being tested - both software-software interfacesand software-hardware interfaces - using what areknown as stubs. The test harness provides the test datato those units being tested and outputs the test resultsin a form understandable to a developer. There are manycommercially available testing tools to automate testharness production and the execution of tests.

21.6.4 Software/Software Integration Testing

Software/Software Integration Testing is performed inthe host environment. It uses a test harness to simulateinputs and outputs, hardware calls and system calls (e.g.the target environment operating system).

21.6.5 Software/Hardware Integration Testing

Software/Hardware Integration Testing is performed inthe target environment, i.e. it uses the actual targethardware, operating system, drivers etc. It is usuallyperformed after Software/Software Integration Testing.Testing the interfaces to the hardware is an importantfeature of Software/Hardware Integration Testing.Test cases for Integration Testing are typically based onthose defined for Validation Testing. However theemphasis should be on finding errors and problems.Performing a dry run of the validation testing oftencompletes Integration Testing.

21.6.6 Validation Testing

The purpose of Validation Testing (also known asSoftware Acceptance Testing) is to verify that thesoftware meets its specified functional requirements.Validation Testing is performed against the softwarerequirements specification, using the targetenvironment. In ideal circumstances, someoneindependent of the software development performs thetests. Validation Testing is ‘black box’ in nature, i.e. itdoes not take into account the internal structure of thesoftware. For relays, the non-protection functionsincluded in the software are considered to be asimportant as the protection functions, and hence testedin the same manner.Each validation test should have predefined evaluationcriteria, to be used to decide if the test has passed orfailed. The evaluation criteria should be explicit with noroom for interpretation or ambiguity.

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21.6.7 Traceability of Validation Tests

Traceability of validation tests to software requirementsis vital. Each software requirement documented in thesoftware requirements specification should have at leastone validation test, and it is important to be able toprove this.

21.6.8 Software Modifications - Regression Testing

Regression Testing is not a type test in its’ own right. Itis the overall name given to the testing performed whenan existing software product is changed. The purpose ofRegression Testing is to show that unintended changesto the functionality (i.e. errors and defects) have notbeen introduced.

Each change to an existing software product must beconsidered in its’ own right. It is impossible to specify astandard set of regression tests that can be applied as a‘catch-all’ for introduced errors and defects. Eachchange to the software must be analysed to determinewhat risk there might be of unintentional changes to thefunctionality being introduced. Those areas of highestrisk will need to be regression tested. The ultimateregression test is to perform the complete ValidationTesting programme again, updated to take account ofthe changes made.

Regression Testing is extremely important. If it is notperformed, there is a high risk of errors being found inthe field. Performing it will not reduce to zero thechance of an error or defect remaining in the software,but it will reduce it. Determining the Regression Testingthat is required is made much easier if there istraceability from properly documented softwarerequirements through design (again properlydocumented and up to date), coding and testing.

21.7 DYNAMIC VALIDATION TYPE TESTING

There are two possible methods of dynamically provingthe satisfactory performance of protection relays orschemes; the first method is by actually applying faultson the power system and the second is to carry outcomprehensive testing on a power system simulator.

The former method is extremely unlikely to be used –lead times are lengthy and the risk of damage occurringmakes the tests very expensive. It is therefore only usedon a very limited basis and the faults applied arerestricted in number and type. Because of this, a provingperiod for new protection equipment under serviceconditions has usually been required. As faults mayoccur on the power system at infrequent intervals, it cantake a number of years before any possible shortcomingsare discovered, during which time further installationsmay have occurred.

Power system simulators can be divided into two types:

a. those which use analogue models of a powersystem

b. those which model the power systemmathematically using digital simulation techniques

21.7.1 Use of Power System Analogue Models

For many years, relays have been tested on analogue modelsof power systems such as artificial transmission lines, or testplant capable of supplying significant amounts of current[21.1]. However, these approaches have significantlimitations in the current and voltage waveforms that canbe generated, and are not suitable for automated,unattended, testing programmes. While still used on alimited basis for testing electromechanical and static relays,a radically different approach is required for dynamictesting of numerical relays.

21.7.2 Use of Microprocessor Based SimulationEquipment

The complexity of numerical relays, reliant on softwarefor implementation of the functions included, dictatessome kind of automated test equipment. The functionsof even a simple numerical overcurrent relay (includingall auxiliary functions) can take several months ofautomated, 24 hours/day testing to test completely. Ifsuch test equipment was able to apply realistic currentand voltage waveforms that closely match those foundon power systems during fault conditions, the equipmentcan be used either for type testing of individual relaydesigns or of a complete protection scheme designed fora specific application. In recognition of this, a newgeneration of power system simulators has beendeveloped, which is capable of providing a far moreaccurate simulation of power system conditions than hasbeen possible in the past. The simulator enables relaysto be tested under a wide range of system conditions,representing the equivalent of many years of siteexperience.

21.7.2.1 Simulation hardware

Equipment is now available to provide high-speed, highlyaccurate modelling of a section of a power system. Theequipment is based on distributed microprocessor-basedhardware containing software models of the variouselements of a power system, and is shown in Figure 21.12.The modules have outputs linked to current and voltagesources that have a similar transient capability and havesuitable output levels for direct connection to the inputsof relays –i.e. 110V for voltage and 1A/5A for current.Inputs are also provided to monitor the response of relaysunder test (contact closures for tripping, etc.) and theseinputs can be used as part of the model of the power

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system. The software is also capable of modelling thedynamic response of CT’s and VT’s accurately.

Where it is desired to check the response of a relay orprotection scheme to an actual power system transient,the transient can be simulated using sophisticated powersystems analysis software and the results transferreddigitally to the simulator, or the event recorder recordingof the transient can be used, in either digital or analogueform as inputs to the simulator model. Output signalconversion involves circuits to eliminate the quantisationsteps normally found in conventional D/A conversion.Analogue models of the system transducercharacteristics can be interposed between the signalprocessors and the output amplifiers when required.

This equipment shows many advantages over traditionaltest equipment:

a. the power system model is capable of reproducinghigh frequency transients such as travelling waves

b. tests involving very long time constants can becarried out

c. it is not affected by the harmonic content, noiseand frequency variations in the a.c. supply

d. it is capable of representing the variation in thecurrent associated with generator faults and powerswings

e. saturation effects in CT’s and VT’s can be modelled

f. a set of test routines can be specified in software andthen left to run unattended (or with only occasionalmonitoring) to completion, with a detailed record of

test results being available on completion

A block schematic of the equipment is shown in Figure21.13, is based around a computer which calculates andstores the digital data representing the system voltagesand currents. The computer controls conversion of thedigital data into analogue signals, and it monitors andcontrols the relays being tested.

21.7.2.2 Simulation software

Unlike most traditional software used for power systemsanalysis, the software used is suitable for the modellingthe fast transients that occur in the first fewmilliseconds after fault inception. Two very accuratesimulation programs are used, one based on time domainand the other on frequency domain techniques. In bothprograms, single and double circuit transmission lines arerepresented by fully distributed parameter models. Theline parameters are calculated from the physicalconstruction of the line (symmetrical, asymmetrical,transposed or non-transposed), taking into account theeffect of conductor geometry, conductor internalimpedance and the earth return path. It also includes,where appropriate, the frequency dependence of the lineparameters in the frequency domain program. Thefrequency dependent variable effects are calculatedusing Fast Fourier Transforms and the results areconverted to the time domain. Conventional currenttransformers and capacitor voltage transformers can besimulated.

The fault can be applied at any one point in the system andcan be any combination of phase to phase or phase

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Figure 21.12: Digital power system simulatorfor relay/protection scheme testing

to earth, resistive, or non-linear phase to earth arcing faults.For series compensated lines, flashover across a seriescapacitor following a short circuit fault can be simulated.

The frequency domain model is not suitable fordeveloping faults and switching sequences, therefore thewidely used Electromagnetic Transient Program (EMTP),working in the time domain, is employed in such cases.

In addition to these two programs, a simulation programbased on lumped resistance and inductance parametersis used. This simulation is used to represent systems withlong time constants and slow system changes due, forexample, to power swings.

21.7.2.3 Simulator applications

The simulator is used for checking the accuracy ofcalibration and performing type tests on a wide range ofprotection relays during their development. It has thefollowing advantages over existing test methods:

a. 'state of the art' power system modelling data canbe used to test relays

b. freedom from frequency variations and noise orharmonic content of the a.c. supply

c. the relay under test does not burden the powersystem simulation

d. all tests are accurately repeatable

e. wide bandwidth signals can be produced

f. a wide range of frequencies can be reproduced

g. selected harmonics may be superimposed on the

power frequency

h. the use of direct coupled current amplifiers allowstime constants of any length

i. capable of simulating slow system changes

j. reproduces fault currents whose peak amplitudevaries with time

k. transducer models can be included

l. automatic testing removes the likelihood ofmeasurement and setting errors

m. two such equipments can be linked together tosimulate a system model with two relaying points

The simulator is also used for the production testing ofrelays, in which most of the advantages listed aboveapply. As the tests and measurements are madeautomatically, the quality of testing is also greatlyenhanced. Further, in cases of suspected malfunction ofa relay in the field under known fault conditions, thesimulator can be used to replicate the power system andfault conditions, and conduct a detailed investigationinto the performance of the relay. Finally, complexprotection schemes can be modelled, using both therelays intended for use and software models of them asappropriate, to check the suitability of the proposedscheme under a wide variety of conditions. To illustratethis, Figure 21.14(a) shows a section of a particular powersystem modelled. The waveforms of Figure 21.14(b) showthe three phase voltages and currents at the primaries ofVT1 and CT1 for the fault condition indicated in Figure21.14(a).

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modelCT

amplifierCurrent

IA

IB

IC

VA

VB

VC

circuitsinterpolationconversion

D/A Voltageamplifiermodel

Linear CVT

Contact

monitorstatus

Sub-I/O

system

testunder

Equipment

Signalling

SimulationChannel

Computer

Keyboard

VDU

Keyboard

VDU

Key :CTCVTVDU - Visual display unit

- Capacitor voltage transformer- Current transformer

link to secondCommunications

RTDS

To second RTDS(When required)

circuitsinterpolation

Linear

conversionD/A

Storage

Figure 21.13: Block diagram of microprocessor-based automated relay test system

21.8 PRODUCTION TESTING

Production testing of protection relays is becoming farmore demanding as the accuracy and complexity of theproducts increase. Electronic power amplifiers are usedto supply accurate voltages and currents of high stabilityto the relay under test. The inclusion of a computer in thetest system allows more complex testing to be performedat an economical cost, with the advantage of speed andrepeatability of tests from one relay to another.

Figure 21.15 shows a modern computer-controlled testbench. The hardware is mounted in a special rack. Eachunit of the test system is connected to the computer viaan interface bus. Individual test programs for each typeof relay are required, but the interface used is standardfor all relay types. Control of input waveforms andanalogue measurements, the monitoring of outputsignals and the analysis of test data are performed by thecomputer. A printout of the test results can also beproduced if required.

Because software is extensively tested at the type-testing stage, there is normally no need to check thecorrect functioning of the software. Checks are limitedto determining that the analogue and digital I/O isfunctioning correctly. This is achieved for inputs by

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load 1

4G CB3 CT3

LR3

F3F F4F

F1 F2F

Line 2

LR4

CT4 CB4

3G

8G

9G

CT2

LR1

Line 1

LR2

CB1 CT1

VT1 VT2

CB211G

Relay 1 Relay 2

load 2

load 3

N

L

Infinite bus

Figure 21.14: Example of application study

0 0.08

VaVV

VbVV

VcVV

IaI

IbII

IcII

0.16 0.24 0.32 0.4 0.560.48

Figure 21.15: Modern computer-controlledtest bench

(b) Voltages and currents at VT1/CT1

(a) Example power system

applying known voltage and current inputs to the relayunder test and checking that the software has capturedthe correct values. Similarly, digital outputs areexercised by using test software to actuate each outputand checking that the correct output is energised.Provided that appropriate procedures are in place toensure that only type-tested software is downloaded,there is no need to check the correct functioning of thesoftware in the relay. The final step is to download thesoftware appropriate to the relay and store it in theEPROM fitted in the relay.

21.9 COMMISSIONING TESTS

Installation of a protection scheme at site creates anumber of possibilities for errors in the implementationof the scheme to occur. Even if the scheme has beenthoroughly tested in the factory, wiring to the CT’s andVT’s on site may be incorrectly carried out, or theCT’s/VT’s may have been incorrectly installed. The impactof such errors may range from simply being a nuisance(tripping occurs repeatedly on energisation, requiringinvestigation to locate and correct the error(s)) throughto failure to trip under fault conditions, leading to majorequipment damage, disruption to supplies and potentialhazards to personnel. The strategies available to removethese risks are many, but all involve some kind of testingat site.

Commissioning tests at site are therefore invariablyperformed before protection equipment is set to work.The aims of commissioning tests are:

1. to ensure that the equipment has not beendamaged during transit or installation

2. to ensure that the installation work has beencarried out correctly

3. to prove the correct functioning of the protectionscheme as a whole

The tests carried out will normally vary according to theprotection scheme involved, the relay technology used,and the policy of the client. In many cases, the testsactually conducted are determined at the time ofcommissioning by mutual agreement between theclient’s representative and the commissioning team.Hence, it is not possible to provide a definitive list oftests that are required during commissioning. Thissection therefore describes the tests commonly carriedout during commissioning.

The following tests are invariably carried out, since theprotection scheme will not function correctly if faults exist.

a. wiring diagram check, using circuit diagramsshowing all the reference numbers of theinterconnecting wiring

b. general inspection of the equipment, checking allconnections, wires on relays terminals, labels onterminal boards, etc.

c. insulation resistance measurement of all circuits

d. perform relay self-test procedure and externalcommunications checks on digital/numerical relays

e. test main current transformers

f. test main voltage transformers

g. check that protection relay alarm/trip settingshave been entered correctly

h. tripping and alarm circuit checks to prove correctfunctioning

In addition, the following checks may be carried out,depending on the factors noted earlier.

i. secondary injection test on each relay to proveoperation at one or more setting values

j. primary injection tests on each relay to provestability for external faults and to determine theeffective current setting for internal faults (essentialfor some types of electromechanical relays)

k. testing of protection scheme logic

This section details the tests required to cover items(a)–(g) above. Secondary injection test equipment iscovered in Section 21.10 and Section 21.11 details thesecondary injection that may be carried out. Section21.12 covers primary injection testing, and Section 21.13details the checks required on any logic involved in theprotection scheme. Finally, Section 21.14 details the testsrequired on alarm/tripping circuits tripping/alarmcircuits.

21.9.1 Insulation Tests

All the deliberate earth connections on the wiring to betested should first be removed, for example earthinglinks on current transformers, voltage transformers andd.c. supplies. Some insulation testers generate impulseswith peak voltages exceeding 5kV. In these instancesany electronic equipment should be disconnected whilethe external wiring insulation is checked.

The insulation resistance should be measured to earthand between electrically separate circuits. The readingsare recorded and compared with subsequent routinetests to check for any deterioration of the insulation.

The insulation resistance measured depends on theamount of wiring involved, its grade, and the sitehumidity. Generally, if the test is restricted to onecubicle, a reading of several hundred megohms should beobtained. If long lengths of site wiring are involved, thereading could be only a few megohms.

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21.9.2 Relay Self-Test Procedure

Digital and numerical relays will have a self-testprocedure that is detailed in the appropriate relaymanual. These tests should be followed to determine ifthe relay is operating correctly. This will normally involvechecking of the relay watchdog circuit, exercising alldigital inputs and outputs and checking that the relayanalogue inputs are within calibration by applying a testcurrent or voltage. For these tests, the relay outputs arenormally disconnected from the remainder of theprotection scheme, as it is a test carried out to provecorrect relay, rather than scheme, operation.

Unit protection schemes involve relays that need tocommunicate with each other. This leads to additionaltesting requirements. The communications pathbetween the relays is tested using suitable equipment toensure that the path is complete and that the receivedsignal strength is within specification. Numerical relaysmay be fitted with loopback test facilities that enableeither part of or the entire communications link to betested from one end.

After completion of these tests, it is usual to enter therelay settings required. This can be done manually viathe relay front panel controls, or using a portable PC andsuitable software. Whichever method is used, a check bya second person that the correct settings have been usedis desirable, and the settings recorded. Programmablescheme logic that is required is also entered at this stage.

21.9.3 Current Transformer Tests

The following tests are normally carried out prior toenergisation of the main circuits.

21.9.3.1 Polarity check

Each current transformer should be individually tested toverify that the primary and secondary polarity markingsare correct; see Figure 21.16. The ammeter connected tothe secondary of the current transformer should be a

robust moving coil, permanent magnet, centre-zero type.A low voltage battery is used, via a single-pole push-button switch, to energise the primary winding. Onclosing the push-button, the d.c. ammeter, A, should givea positive flick and on opening, a negative flick.

21.9.3.2 Magnetisation Curve

Several points should be checked on each currenttransformer magnetisation curve. This can be done byenergising the secondary winding from the local mainssupply through a variable auto-transformer while theprimary circuit remains open; see Figure 21.17. Thecharacteristic is measured at suitable intervals of appliedvoltage, until the magnetising current is seen to rise veryrapidly for a small increase in voltage. This indicates theapproximate knee-point or saturation flux level of thecurrent transformer. The magnetising current shouldthen be recorded at similar voltage intervals as it isreduced to zero.

Care must be taken that the test equipment is suitablyrated. The short-time current rating must be in excess ofthe CT secondary current rating, to allow for themeasurement of the saturation current. This will be inexcess of the CT secondary current rating. As themagnetising current will not be sinusoidal, a moving ironor dynamometer type ammeter should be used.

It is often found that current transformers withsecondary ratings of 1A or less have a knee-point voltagehigher than the local mains supply. In these cases, astep-up interposing transformer must be used to obtainthe necessary voltage to check the magnetisation curve.

21.9.4 Voltage Transformer Tests

Voltage transformers require testing for polarity andphasing.

21.9.4.1 Polarity check

The voltage transformer polarity can be checked usingthe method for CT polarity tests. Care must be taken toconnect the battery supply to the primary winding, with

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A

_ +

_ +

P1P

S1S2

P2PP

Figure 21.16: Current transformerpolarity check

P1

P2 S2

S1

CBA

A

VTo relaycoils

250Va.c. supply

Step-up transformerif required

Variable transformer250V 8A

Test plug isolatingcurrent transformersfrom relay coils

Main circuitbreaker open

Figure 21.17: Testing current transformermagnetising curve

the polarity ammeter connected to the secondarywinding. If the voltage transformer is of the capacitortype, then the polarity of the transformer at the bottomof the capacitor stack should be checked.

21.9.4.2 Ratio check

This check can be carried out when the main circuit isfirst made live. The voltage transformer secondaryvoltage is compared with the secondary voltage shownon the nameplate.

21.9.4.3 Phasing check

The secondary connections for a three-phase voltagetransformer or a bank of three single-phase voltagetransformers must be carefully checked for phasing.With the main circuit alive, the phase rotation is checkedusing a phase rotation meter connected across the threephases, as shown in Figure 21.18. Provided an existingproven VT is available on the same primary system, andthat secondary earthing is employed, all that is nownecessary to prove correct phasing is a voltage checkbetween, say, both ‘A’ phase secondary outputs. Thereshould be nominally little or no voltage if the phasing iscorrect. However, this test does not detect if the phasesequence is correct, but the phases are displaced by 120°from their correct position, i.e. phase A occupies theposition of phase C or phase B in Figure 21.18. This canbe checked by removing the fuses from phases B and C(say) and measuring the phase-earth voltages on thesecondary of the VT. If the phasing is correct, only phaseA should be healthy, phases B and C should have only asmall residual voltage.

Correct phasing should be further substantiated whencarrying out ‘on load’ tests on any phase-angle sensitiverelays, at the relay terminals. Load current in a knownphase CT secondary should be compared with theassociated phase to neutral VT secondary voltage. Thephase angle between them should be measured, andshould relate to the power factor of the system load.

If the three-phase voltage transformer has a broken-delta tertiary winding, then a check should be made ofthe voltage across the two connections from the brokendelta VN and VL, as shown in Figure 21.18. With therated balanced three-phase supply voltage applied to thevoltage transformer primary windings, the broken-deltavoltage should be below 5V with the rated burdenconnected.

21.9.5 Protection Relay Setting Checks

At some point during commissioning, the alarm and tripsettings of the relay elements involved will require to beentered and/or checked. Where the complete scheme isengineered and supplied by a single contractor, thesettings may already have been entered prior to despatchfrom the factory, and hence this need not be repeated.The method of entering settings varies according to therelay technology used. For electromechanical and staticrelays, manual entry of the settings for each relayelement is required. This method can also be used fordigital/numerical relays. However, the amount of data tobe entered is much greater, and therefore it is usual touse appropriate software, normally supplied by themanufacturer, for this purpose. The software also makesthe essential task of making a record of the data enteredmuch easier.

Once the data has been entered, it should be checked forcompliance with the recommended settings ascalculated from the protection setting study. Whereappropriate software is used for data entry, the checkscan be considered complete if the data is checked priorto download of the settings to the relay. Otherwise, acheck may required subsequent to data entry byinspection and recording of the relay settings, or it maybe considered adequate to do this at the time of dataentry. The recorded settings form an essential part of thecommissioning documentation provided to the client.

21.10 SECONDARY INJECTION TEST EQUIPMENT

Secondary injection tests are always done prior toprimary injection tests. The purpose of secondaryinjection testing is to prove the correct operation of theprotection scheme that is downstream from the inputs tothe protection relay(s). Secondary injection tests arealways done prior to primary injection tests. This is

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A

B

C

V1

V1

V2

V2

VN

VL

V

A B CPhase rotation

meter

C B

A

Figure 21.18: Voltage transformerphasing check

because the risks during initial testing to the LV side ofthe equipment under test are minimised. The primary(HV) side of the equipment is disconnected, so that nodamage can occur. These tests and the equipmentnecessary to perform them are generally described in themanufacturer's manuals for the relays, but brief detailsare given below for the main types of protection relays.

21.10.1 Test Blocks/Plugs for SecondaryInjection Equipment

It is common practice to provide test blocks or testsockets in the relay circuits so that connections canreadily be made to the test equipment withoutdisturbing wiring. Test plugs of either multi-finger orsingle-finger design (for monitoring the current in oneCT secondary circuit) are used to connect test equipmentto the relay under test.

The top and bottom contact of each test plug finger isseparated by an insulating strip, so that the relay circuitscan be completely isolated from the switchgear wiringwhen the test plug is inserted. To avoid open-circuitingCT secondary terminals, it is therefore essential that CTshorting jumper links are fitted across all appropriate‘live side’ terminals of the test plug BEFORE it is inserted.With the test plug inserted in position, all the testcircuitry can now be connected to the isolated ‘relayside’ test plug terminals. Some modern test blocksincorporate the live-side jumper links within the blockand these can be set to the ‘closed’ or ‘open’ position asappropriate, either manually prior to removing the coverand inserting the test plug, or automatically uponremoval of the cover. Removal of the cover also exposesthe colour-coded face-plate of the block, clearlyindicating that the protection scheme is not in service,and may also disconnect any d.c. auxiliary supplies usedfor powering relay tripping outputs.

Withdrawing the test plug immediately restores theconnections to the main current transformers andvoltage transformers and removes the test connections.Replacement of the test block cover then removes theshort circuits that had been applied to the main CTsecondary circuits. Where several relays are used in aprotection scheme, one or more test blocks may be fittedon the relay panel enabling the whole scheme to betested, rather than just one relay at a time.

Test blocks usually offer facilities for the monitoring andsecondary injection testing of any power systemprotection scheme. The test block may be used eitherwith a multi-fingered test plug to allow isolation andmonitoring of all the selected conductor paths, or with asingle finger test plug that allows the currents onindividual conductors to be monitored. A modern testblock and test plugs are illustrated in Figure 21.19.

21.10.2 Secondary Injection Test Sets

The type of the relay to be tested determines the type ofequipment used to provide the secondary injectioncurrents and voltages. Many electromechanical relayshave a non-linear current coil impedance when the relayoperates and this can cause the test current waveform tobe distorted if the injection supply voltage is fed directlyto the coil. The presence of harmonics in the currentwaveform may affect the torque of electromechanicalrelays and give unreliable test results, so some injectiontest sets use an adjustable series reactance to control thecurrent. This keeps the power dissipation small and theequipment light and compact.

Many test sets are portable and include precisionammeters and voltmeters and timing equipment. Testsets may have both voltage and current outputs. Theformer are high-voltage, low current outputs for usewith relay elements that require signal inputs from a VTas well as a CT. The current outputs are high-current,low voltage to connect to relay CT inputs. It isimportant, however, to ensure that the test set currentoutputs are true current sources, and hence are notaffected by the load impedance of a relay elementcurrent coil. Use of a test set with a current output thatis essentially a voltage source can give rise to seriousproblems when testing electromechanical relays. Anysignificant impedance mismatch between the output ofthe test set and the relay current coil during relayoperation will give rise to a variation in current from thatdesired and possible error in the test results. The relayoperation time may be greater than expected (never lessthan expected) or relay ‘chatter’ may occur. It is quitecommon for such errors to only be found much later,after a fault has caused major damage to equipmentthrough failure of the primary protection to operate.Failure investigation then shows that the reason for theprimary protection to operate is an incorrectly set relay,due in turn to use of a test set with a current outputconsisting of a voltage-source when the relay was lasttested. Figure 21.20 shows typical waveforms resultingfrom use of test set current output that is a voltage

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Figure 21.19: Modern test block/plugs

source – the distorted relay coil current waveform givesrise to an extended operation time compared to theexpected value.

Modern test sets are computer based. They comprise aPC (usually a standard laptop PC with suitable software)and a power amplifier that takes the low-level outputsfrom the PC and amplifies them into voltage and currentsignals suitable for application to the VT and CT inputs ofthe relay. The phase angle between voltage and currentoutputs will be adjustable, as also will the phase anglesbetween the individual voltages or currents making up a

3-phase output set. Much greater precision in thesetting of the magnitudes and phase angles is possible,compared to traditional test sets. Digital signals toexercise the internal logic elements of the relays mayalso be provided. The alarm and trip outputs of the relayare connected to digital inputs on the PC so that correctoperation of the relay, including accuracy of the relaytripping characteristic can be monitored and displayedon-screen, saved for inclusion in reports generated later,or printed for an immediate record to present to theclient. Optional features may include GPS timesynchronising equipment and remote-located amplifiers

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Saturation level ofmagnetic circuit (current)limited only by D.C.resistance ofrelay coil

Relay with saturationof CDG magnetic circuit(phase shift from CDGinductive load shown).

V relay/source

a) Relay current coil waveform distorted due to use of voltage source

b) Undistorted relay current coil current distorted due to use of current source

Sinusoidal CURRENT whenchanging impedance of relayis swamped out by highsource impedance

Typical VOLTAGE waveformappearing across relaycurrent coils with sinusoidal Iabove the relay setting (10 x shown).

Time

Time

Figure 21.20: Relay current coil waveforms

to facilitate testing of unit protection schemes, anddigital I/O for exercising the programmable scheme logicof modern relays.

The software for modern test sets is capable of testingthe functionality of a wide variety of relays, andconducting a set of tests automatically. Such sets easethe task of the commissioning engineer. The softwarewill normally offer options for testing, ranging from atest carried out at a particular point on the characteristicto complete determination of the tripping characteristicautomatically. This feature can be helpful if there is anyreason to doubt that the relay is operating correctly withthe tripping characteristic specified. Figure 21.21illustrates a modern PC-based test set.

Traditional test sets use an arrangement of adjustabletransformers and reactors to provide control of currentand voltage without incurring high power dissipation.Some relays require adjustment of the phase betweenthe injected voltages and currents, and so phase shiftingtransformers may be used. Figure 21.22 shows thecircuit diagram of a traditional test set suitable forovercurrent relay resting, while Figure 21.23 shows thecircuit diagram for a test set for directional/distancerelays. Timers are included so that the response time ofthe relay can be measured.

21.11 SECONDARY INJECTION TESTING

The purpose of secondary injection testing is to checkthat the protection scheme from the relay inputterminals onwards is functioning correctly with thesettings specified. This is achieved by applying suitableinputs from a test set to the inputs of the relays andchecking if the appropriate alarm/trip signals occur atthe relay/control room/CB locations. The extent of

testing will be largely determined by the clientspecification and relay technology used, and may rangefrom a simple check of the relay characteristic at a singlepoint to a complete verification of the trippingcharacteristics of the scheme, including the response totransient waveforms and harmonics and checking ofrelay bias characteristics. This may be important whenthe protection scheme includes transformers and/orgenerators.

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A

Fine controlvariable

transformer

Coarsecontrolreactor

Backingtransformer10% control

K2 K1

>I

Stoptimer

Starttimer

Relaycoil

Mediumcontrolreactor

250Va.c. supply

Relay short-circuitingswitch

Injectiontransformer

I

Rangeadjusting CT

Relay current, I = Ammeter reading (A) K1 x K2

Figure 21.22: Circuit diagram of traditional test set for overcurrent relays

Figure 21.21: Modern PC-based secondaryinjection test set

The testing should include any scheme logic. If the logicis implemented using the programmable scheme logicfacilities available with most digital or numerical relays,appropriate digital inputs may need to be applied andoutputs monitored (see Section 21.13). It is clear that amodern test set can facilitate such tests, leading to areduced time required for testing.

21.11.1 Schemes using Digital or NumericalRelay Technology

The policy for secondary injection testing varies widely.In some cases, manufacturers recommend, and clientsaccept, that if a digital or numerical relay passes its’ self-test, it can be relied upon to operate at the settings usedand that testing can therefore be confined to those partsof the scheme external to the relay. In such cases,secondary injection testing is not required at all. Moreoften, it is required that one element of each relay(usually the simplest) is exercised, using a secondaryinjection test set, to check that relay operation occurs atthe conditions expected, based on the setting of therelay element concerned. Another alternative is for thecomplete functionality of each relay to be exercised. Thisis rarely required with a digital or numerical relay,probably only being carried out in the event of asuspected relay malfunction.

To illustrate the results that can be obtained, Figure21.24 shows the results obtained by a modern test setwhen determining the reach settings of a distance relayusing a search technique. Another example is the testingof the Power Swing blocking element of a distance relay.Figure 21.25 illustrates such a test, based on usingdiscrete impedance points. This kind of test may not beadequate in all cases, and test equipment may have theability to generate the waveforms simulating a powerswing and apply them to the relay (Figure 21.26).

21.11.2 Schemes usingElectromechanical/Static Relay Technology

Schemes using single function electromechanical orstatic relays will usually require each relay to beexercised. Thus a scheme with distance and back-upovercurrent elements will require a test on each of thesefunctions, thereby taking up more time than if a digitalor numerical relay is used. Similarly, it may be important

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Fault A-N

-10.0-10.0-15.0

-5.0

-7.5

-2.5

-5.0 0.0

15.0

2.5

0.0

5.0

7.5

10.0

12.5

22.5

17.5

20.0

X( )

5.0 10.0 15.0 R( )

Figure 21.24: Distance relay zone checkingusing search technique and tolerance bands

ABCN

Variabletransformer

control

440V3 phase4 wire supply

pp

>

Relay

PAA

voltage elementV

adjusting CT

Choke

Variable transformer for current control

PA

To other voltageelementsof relayunder test

y

(if required)

V VoltmeterA AmmeterPA Phase angle meter

440/110Vphaseshiftingp

transformerg

Supply switch

A

A

Figure 21.23: Circuit diagram for traditionaltest set for directional/distance relays

X

R

PSB-Zone

*

Zn

Figure 21.25: Testing of power swingblocking element – discrete points

Figure 21.26: Simulated power swing waveform

to check the relay characteristic over a range of inputcurrents to confirm parameters for an overcurrent relaysuch as:

i. the minimum current that gives operation at eachcurrent setting

ii. the maximum current at which resetting takesplace

iii. the operating time at suitable values of current

iv. the time/current curve at two or three points withthe time multiplier setting TMS at 1

v. the resetting time at zero current with the TMS at 1

Similar considerations apply to distance and unitprotection relays of these technologies.

21.11.3 Test Circuits for Secondary Injection Testing

The test circuits used will depend on the type of relayand test set being used. Unless the test circuits aresimple and obvious, the relay commissioning manual willgive details of the circuits to be used. Commonly usedtest circuits can also be found in Chapter 23 of reference[21.1]. When using the circuits in this reference, suitablesimplifications can easily be made if digital or numericalrelays are being tested, to allow for their built-inmeasurement capabilities – external ammeters andvoltmeters may not be required.

All results should be carefully noted and filed for recordpurposes. Departures from the expected results must bethoroughly investigated and the cause determined. Afterrectification of errors, all tests whose results may havebeen affected (even those that may have given correctresults) should be repeated to ensure that the protectionscheme has been implemented according tospecification.

21.12 PRIMARY INJECTION TESTS

This type of test involves the entire circuit; currenttransformer primary and secondary windings, relay coils,trip and alarm circuits, and all intervening wiring arechecked. There is no need to disturb wiring, whichobviates the hazard of open-circuiting currenttransformers, and there is generally no need for anyswitching in the current transformer or relay circuits.The drawback of such tests is that they are timeconsuming and expensive to organise. Increasingly,reliance is placed on all wiring and installation diagramsbeing correct and the installation being carried out asper drawings, and secondary injection testing beingcompleted satisfactorily. Under these circumstances, theprimary injection tests may be omitted. However, wiringerrors between VT’s/CT’s and relays, or incorrect polarity

of VT’s/CT’s may not then be discovered until eitherspurious tripping occurs in service, or more seriously,failure to trip on a fault. This hazard is much reducedwhere digital/numerical relays are used, since the currentand voltage measurement/display facilities that exist insuch relays enable checking of relay input values againstthose from other proven sources. Many connection/wiringerrors can be found in this way, and by isolatingtemporarily the relay trip outputs, unwanted trips can beavoided.

Primary injection testing is, however, the only way toprove correct installation and operation of the whole ofa protection scheme. As noted in the previous section,primary injection tests are always carried out aftersecondary injection tests, to ensure that problems arelimited to the VT’s and CT’s involved, plus associatedwiring, all other equipment in the protection schemehaving been proven satisfactory from the secondaryinjection tests.

21.12.1 Test Facilities

An alternator is the most useful source of power forproviding the heavy current necessary for primaryinjection. Unfortunately, it is rarely available, since itrequires not only a spare alternator, but also sparebusbars capable of being connected to the alternator andcircuit under test. Therefore, primary injection is usuallycarried out by means of a portable injection transformer(Figure 21.27), arranged to operate from the local mainssupply and having several low voltage, heavy currentwindings. These can be connected in series or parallelaccording to the current required and the resistance ofthe primary circuit. Outputs of 10V and 1000A can beobtained. Alternatively, modern PC-controlled test setshave power amplifiers capable of injecting currents up toabout 200A for a single unit, with higher current ratingsbeing possible by using multiple units in parallel.

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A

250V a.c.supply

Variable transformer40A

Injection transformer250/10 + 10 + 10 + 10V10kVA

Figure 21.27: Traditional primaryinjection test set

If the main current transformers are fitted with testwindings, these can be used for primary injection insteadof the primary winding. The current required for primaryinjection is then greatly reduced and can usually beobtained using secondary injection test equipment.Unfortunately, test windings are not often provided,because of space limitations in the main currenttransformer housings or the cost of the windings.

21.12.2 CT Ratio Check

Current is passed through the primary conductors andmeasured on the test set ammeter, A1 in Figure 21.28.The secondary current is measured on the ammeter A2 orrelay display, and the ratio of the value on A1 to that onA2 should closely approximate to the ratio marked onthe current transformer nameplate.

21.12.3 CT Polarity Check

If the equipment includes directional, differential orearth fault relays, the polarity of the main currenttransformers must be checked. It is not necessary toconduct the test if only overcurrent relays are used.

The circuit for checking the polarity with a single-phasetest set is shown in Figure 21.29. A short circuit is placedacross the phases of the primary circuit on one side ofthe current transformers while single-phase injection iscarried out on the other side. The ammeter connected in

the residual circuit, or relay display, will give a reading ofa few milliamperes with rated current injected if thecurrent transformers are of correct polarity. A readingproportional to twice the primary current will beobtained if they are of wrong polarity. Because of this, ahigh-range ammeter should be used initially, for exampleone giving full-scale deflection for twice the ratedsecondary current. If an electromechanical earth-faultrelay with a low setting is also connected in the residualcircuit, it is advisable to temporarily short-circuit itsoperating coil during the test, to prevent possibleoverheating. The single-phase injection should becarried out for each pair of phases.

21.12.4 Primary Injection Testing of Relay Elements

As with secondary injection testing, the tests to becarried out will be those specified by the client, and/orthose detailed in the relay commissioning manual.Digital and numerical relays usually require far fewertests to prove correct operation, and these may berestricted to observations of current and voltage on therelay display under normal load conditions.

21.13 TESTING OF PROTECTION SCHEME LOGIC

Protection schemes often involve the use of logic todetermine the conditions under which designated circuitbreakers should be tripped. Simple examples of suchlogic can be found in Chapters 9-14. Traditionally, thislogic was implemented by means of discrete relays,separate from the relays used for protection. Suchimplementations would occur where electromechanicalor static relay technology is used. However, digital andnumerical relays normally include programmable logic aspart of the software within the relay, together withassociated digital I/O. This facility (commonly referred toas Programmable Scheme Logic, or PSL) offers importantadvantages to the user, by saving space and permittingmodifications to the protection scheme logic throughsoftware if the protection scheme requirements changewith time. Changes to the logic are carried out using

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Primaryinjectiontest set

A

A

B

C

Temporarythree-phaseshort circuit

250V a.c.supply

Relay

Figure 21.29: Polarity check on main currenttransformers

CBA

Temporaryshort circuit

Test pluginsulationu

Relay or test blockcontact fingers

Primary injectiontest set

A1

P2PP

P1P

S2

S1

A

250Va.c. supply

Relay

Figure 21.28: Current transformer ratio check

software hosted on a PC (or similar computer) anddownloaded to the relay. Use of languages defined in IEC61131, such as ladder logic or Boolean algebra iscommon for such software, and is readily understood byProtection Engineers. Further, there are severalcommonly encountered protection functions thatmanufacturers may supply with relays as one or more‘default’ logic schemes.

Because software is used, it is essential to carefully testthe logic during commissioning to ensure correctoperation. The only exception to this may be if therelevant ‘default’ scheme is used. Such logic schemeswill have been proven during relay type testing, and sothere is no need for proving tests during commissioning.However, where a customer generates the scheme logic,it is necessary to ensure that the commissioning testsconducted are adequate to prove the functionality of thescheme in all respects. A specific test procedure shouldbe prepared, and this procedure should include:

a. checking of the scheme logic specification anddiagrams to ensure that the objectives of the logicare achieved

b. testing of the logic to ensure that the functionalityof the scheme is proven

c. testing of the logic, as required, to ensure that nooutput occurs for the relevant input signalcombinations

The degree of testing of the logic will largely depend onthe criticality of the application and complexity of thelogic. The responsibility for ensuring that a suitable testprocedure is produced for logic schemes other than the‘default’ one(s) supplied lies with the specifier of thelogic. Relay manufacturers cannot be expected to takeresponsibility for the correct operation of logic schemesthat they have not designed and supplied.

21.14 TRIPPING AND ALARM ANNUNCIATION TESTS

If primary and/or secondary injection tests are not carriedout, the tripping and alarm circuits will not have beenchecked. Even where such checks have been carried out,CB trip coils and/or Control Room alarm circuits may havebeen isolated. In such cases, it is essential that all of thetripping and alarm circuits are checked.

This is done by closing the protection relay contactsmanually and checking that:

1. the correct circuit breakers are tripped

2. the alarm circuits are energised

3. the correct flag indications are given

4. there is no maloperation of other apparatus thatmay be connected to the same master trip relay orcircuit breaker

Many designs of withdrawable circuit breaker can beoperated while in the maintenance position, so thatsubstation operation can continue unaffected except forthe circuit controlled by the circuit breaker involved. Inother cases, isolators can be used to avoid the need forbusbar de-energisation if the circuit involved is not readyfor energisation.

21.15 PERIODIC MAINTENANCE TESTS

Periodic testing is necessary to ensure that a protectionscheme continues to provide satisfactory performancefor many years after installation. All equipment issubject to gradual degradation with time, and regulartesting is intended to identify the equipment concernedso that remedial action can be taken before schememaloperation occurs. However, due care should be takenin this task, otherwise faults may be introduced as adirect result of the remedial work.

The clearance of a fault on the system is correct only ifthe number of circuit breakers opened is the minimumnecessary to remove the fault. A small proportion offaults are incorrectly cleared, the main reasons being:

a. limitations in protection scheme design

b. faulty relays

c. defects in the secondary wiring

d. incorrect connections

e. incorrect settings

f. known application shortcomings accepted asimprobable occurrences

g. pilot wire faults due to previous unrevealeddamage to a pilot cable

h. various other causes, such as switching errors,testing errors, and relay operation due tomechanical shock

The self-checking facilities of numerical relays assist inminimising failures due to faulty relays. Defects insecondary wiring and incorrect connections are virtuallyeliminated if proper commissioning after schemeinstallation/alteration is carried out. The possibility ofincorrect settings is minimised by regular reviews ofrelay settings. Network fault levels change over time,and hence setting calculations may need to be revised.Switching and testing errors are minimised by adequatetraining of personnel, use of proven software, and well-designed systematic working procedures. All of thesecan be said to be within the control of the user.

The remaining three causes are not controllable, whiletwo of these three are unavoidable – engineering is notscience and there will always be situations that aprotection relay cannot reasonably be expected to coverat an affordable cost.

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21.15.1 Frequency of Inspection and Testing

Although protection equipment should be in soundcondition when first put into service, problems candevelop unchecked and unrevealed because of itsinfrequent operation. With digital and numerical relays,the in-built self-testing routines can be expected toreveal and annunciate most faults, but this does notcover any other components that, together, comprise theprotection scheme. Regular inspection and testing of aprotection scheme is therefore required. In practice, thefrequency of testing may be limited by lack of staff or bythe operating conditions on the power system.

It is desirable to carry out maintenance on protectionequipment at times when the associated power apparatusis out of service. This is facilitated by co-operationbetween the maintenance staff concerned and thenetwork operations control centre. Maintenance testsmay sometimes have to be made when the protectedcircuit is on load. The particular equipment to be testedshould be taken out of commission and adequate back-upprotection provided for the duration of the tests. Suchback-up protection may not be fully discriminative, butshould be sufficient to clear any fault on the apparatuswhose main protection is temporarily out of service.

Maintenance is assisted by the displays of measuredquantities provided on digital and numerical relays. Incorrectdisplay of a quantity is a clear indication that something iswrong, either in the relay itself or the input circuits.

21.15.2 Maintenance Tests

Primary injection tests are normally only conducted outduring initial commissioning. If scheme maloperationhas occurred and the protection relays involved aresuspect, or alterations have been made involving thewiring to the relays from the VT’s/CT’s, the primaryinjection tests may have to be repeated.

Secondary injection tests may be carried out at suitableintervals to check relay performance, and, if possible, therelay should be allowed to trip the circuit breakersinvolved. The interval between tests will depend uponthe criticality of the circuit involved, the availability ofthe circuit for testing and the technology of the relaysused. Secondary injection testing is only necessary onthe selected relay setting and the results should bechecked against those obtained during the initialcommissioning of the equipment.

It is better not to interfere with relay contacts at allunless they are obviously corroded. The performance ofthe contacts is fully checked when the relay is actuated.

Insulation tests should also be carried out on the relaywiring to earth and between circuits, using a 1000Vtester. These tests are necessary to detect anydeterioration in the insulation resistance.

21.16 PROTECTION SCHEME DESIGNFOR MAINTENANCE

If the following principles are adhered to as far aspossible, the danger of back-feeds is lessened and faultinvestigation is made easier:

i. test blocks should be used, to enable a test plug tobe used, and a defective unit to be replaced quicklywithout interrupting service

ii. circuits should be kept as electrically separate aspossible, and the use of common wires should beavoided, except where these are essential to thecorrect functioning of the circuits

iii. each group of circuits which is electricallyseparate from other circuits should be earthedthrough an independent earth link

iv. where a common voltage transformer or d.c.supply is used for feeding several circuits, eachcircuit should be fed through separate links orfuses. Withdrawal of these should completelyisolate the circuit concerned

v. power supplies to protection schemes should besegregated from those supplying other equipment andprovided with fully discriminative circuit protection

vi. a single auxiliary switch should not be used forinterrupting or closing more than one circuit

vii. terminations in relay panels require good access,as these may have to be altered if extensions aremade. Modern panels are provided with specialtest facilities, so that no connections need bedisturbed during routine testing

viii. junction boxes should be of adequate size and, ifoutdoors, must be made waterproof

ix. all wiring should be ferruled for identification andphase-coloured

x. electromechanical relays should have highoperating and restraint torques and high contactpressures; jewel bearings should be shrouded toexclude dust and the use of very thin wire for coilsand connections should be avoided. Dust-tightcases with an efficient breather are essential onthese types of electromechanical element

xi. static, digital and numerical relays should have testfacilities accessible from the front to assist in faultfinding. The relay manual should clearly detail theexpected results at each test point when healthy

21.17 REFERENCES

21.1 Protective Relays Application Guide, 3rd edition.AREVA Transmission and Distribution, Protectionand Control, 1987.

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