ALSTOM T&D. Network Protection And Automation Guide (NPAG)

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Chapter 21  Relay Testing and Commissioning

21-7

Screened room

Support/analysis

equipment

Ante-chamber

E.U.T.

Impedance network

E.U.T. = Equipment under test

E.U.T.

Antenna

Earth Plane

10m

(b) Radiated Emissions test arrangement on an OATS

(a) Conducted EMC emissions test arrangement

Figure 21.6: EMC test arrangements

When performing these two tests, the relay is in a quiescent

condition, that is not tripped, with currents and voltages

applied at 90% of the setting values. This is because for the

majority of its life, the relay will be in the quiescent state and

the emission of electromagnetic interference when the relay is

tripped is considered to be of no significance. Tests are

conducted in accordance with IEC 60255-25 and EN 55022,

and are detailed in Table 21.3.

Frequency Range Specified Limits Test Limits

Radiated 30 - 230MHz

30dB(

V/m) at 30m 40dB(

V/m) at 10m

Radiated 230 - 1000MHz

37dB(

V/m) at 30m 47dB(

V/m) at 10m

Conducted 0.15 - 0.5MHz

79dB(

V) quasi-peak

66dB(

V) average

79dB(

V) quasi-peak

66dB(

V) average

Conducted 0.5 - 30MHz

73dB(

V) quasi-peak

60dB(

V) average

73dB(

V) quasi-peak

60dB(

V) average

Table 21.3: Test criteria for Conducted and Radiated Emissions tests

21.3.10 Conducted and Radiated Immunity Tests

These tests are designed to ensure that the equipment is

immune to levels of interference that it may be subjected to.

The two tests, conducted and radiated, arise from the fact that

for a conductor to be an efficient antenna, it must have a

length of at least ¼ of the wavelength of the electromagnetic

wave it is required to conduct.

If a relay were to be subjected to radiated interference at

150kHz, then a conductor length of at least



m500

410150

10300

would be needed to conduct the interference. Even with all

the cabling attached and with the longest PCB track length

taken into account, it would be highly unlikely that the relay

would be able to conduct radiation of this frequency, and the

test therefore, would have no effect. The interference has to be

physically introduced by conduction, hence the conducted

immunity test. However, at the radiated immunity lower

frequency limit of 80MHz, a conductor length of approximately

1.0m is required. At this frequency, radiated immunity tests

can be performed with the confidence that the relay will

conduct this interference, through a combination of the

attached cabling and the PCB tracks.

Although the test standards state that all 6 faces of the

equipment should be subjected to the interference, in practice

this is not carried out. Applying interference to the sides and

top and bottom of the relay would have little effect as the

circuitry inside is effectively screened by the earthed metal

case. However, the front and rear of the relay are not

completely enclosed by metal and are therefore not at all well

screened, and can be regarded as an EMC hole.

Electromagnetic interference when directed at the front and

back of the relay can enter freely onto the PCBs inside.

When performing these two tests, the relay is in a quiescent

condition, that is not tripped, with currents and voltages

applied at 90% of the setting values. This is because for the

majority of its life, the relay will be in the quiescent state and

the coincidence of an electromagnetic disturbance and a fault

is considered to be unlikely.

However, spot checks are performed at selected frequencies

when the main protection and control functions of the relay

are exercised, to ensure that it will operate as expected, should

it be required to do so.

The frequencies for the spot checks are in general selected to

coincide with the radio frequency broadcast bands, and in

particular, the frequencies of mobile communications

equipment used by personnel working in the substation. This

is to ensure that when working in the vicinity of a relay, the

personnel should be able to operate their radios/mobile phones

without fear of relay maloperation.

IEC 60255-22-3 specifies the radiated immunity tests to be

conducted (ANSI/IEEE C37.90.2 is used for equipment

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compliant with North American standards), with signal levels

of:

x IEC: Class III, 10V/m, 80MHz -2700MHz

x ANSI/IEEE: 35V/m 25MHz - 1000MHz with no

modulation, and again with 100% pulse modulation

IEC 60255-22-6 is used for the conducted immunity test,

with

a test level of:

x 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 is

immune to magnetic interference. The three tests, steady

state, pulsed and damped oscillatory magnetic field, arise from

the fact that for different site conditions the level and

waveshape is altered.

21.3.11.1 Steady state magnetic field tests

These tests simulate the magnetic field that would be

experienced by a device located within close proximity of the

power system. Testing is carried out by subjecting the relay to

a magnetic field generated by two induction coils. The relay is

rotated such that in each axis it is subjected to the full

magnetic field strength. IEC 61000-4-8 is the relevant

standard, using a signal level of:

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

duration

The test arrangement is shown in Figure 21.7.

Figure 21.7: Power frequency magnetic field test arrangement

To pass the steady-state test, the relay shall not maloperate,

and shall still perform its main functions within the claimed

tolerance. During the application of the short duration test,

the main protection function shall be exercised and verified

that the operating characteristics of the relay are unaffected.

21.3.11.2 Pulsed magnetic field

These tests simulate the magnetic field that would be

experienced by a device located within close proximity of the

power system during a transient fault condition. According to

IEC 61000-4-9, the generator for the induction coils shall

produce a 6.4/16Ps waveshape with test level 5, 100A/m with

the equipment configured as for the steady state magnetic field

test. The relay shall not maloperate, and shall still perform its

main functions within the claimed tolerance during the test.

21.3.11.3 Damped oscillatory magnetic field

These tests simulate the magnetic field that would be

experienced by a device located within close proximity of the

power system during a transient fault condition. IEC 61000-

4-10 specifies that the generator for the coil shall produce an

oscillatory waveshape with a frequency of 0.1MHz and 1MHz,

to give a signal level in accordance with Level 5 of 100A/m,

and the equipment shall be configured as in Figure 21.7.

21.4 PRODUCT SAFETY TYPE TESTS

A number of tests are carried out to demonstrate that the

product is safe when used for its intended application. The

essential requirements are that the relay is safe and will not

cause an electric shock or fire hazard under normal conditions

and in the presence of a single fault. A number of specific tests

to prove this may be carried out, as follows.

21.4.1 Dielectric Voltage Withstand

Dielectric Voltage Withstand testing is carried out as a routine

test i.e. on every unit prior to despatch. The purpose of this

test is to ensure that the product build is as intended by design.

This is done by verifying the clearance in air, thus ensuring that

the product is safe to operate under normal use conditions.

The following tests are conducted unless otherwise specified in

the product documentation:

x 2.0kV r.m.s., 50/60Hz for 1 minute between all

terminals and case earth and also between independent

circuits, in accordance with IEC 60255-27. Some

communication circuits are excluded from this test, or

have modified test requirements e.g.

those using D-type

connectors

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

open contacts intended for connection to tripping

circuits, in accordance with ANSI/IEEE C37.90

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Chapter 21  Relay Testing and Commissioning

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x 1.0kV r.m.s., 50/60Hz for 1 minute across the normally

open contacts of watchdog or changeover output

relays, in accordance with IEC 60255-27

The routine dielectric voltage withstand test time may be

shorter than for the 1 minute type test time, to allow a

reasonable production throughput, e.g. for a minimum of 1

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 type test

is to ensure that circuits and their components will withstand

overvoltages on the power system caused by lightning. Three

positive and three negative high voltage impulses, 5kV peak,

are applied between all circuits and the case earth and also

between the terminals of independent circuits (but not across

normally open contacts). As before, different requirements

apply in the case of circuits using D-type connectors.

The test generator characteristics are as specified in IEC

60255-27 and are shown in Figure 21.8. No disruptive

discharge (i.e. flashover or puncture) is allowed.

If it is necessary to repeat either the Dielectric Voltage or High

Voltage Impulse Withstand tests these should be carried out at

75% of the specified level, in accordance with IEC 60255-27,

to avoid overstressing insulation and components.

5kV peak

Duration (50 %) = 50 s

Figure 21.8: Test generator characteristics for insulation withstand

test

21.4.3 Single Fault Condition Assessment

An assessment is made of whether a single fault condition

such as an overload, or an open or short circuit, applied to the

product may cause an electric shock or fire hazard. In the case

of doubt, type testing is carried out to ensure that the product

is safe.

21.4.4 Earth Bonding Impedance

Class 1 products that rely on a protective earth connection for

safety are subjected to an earth bonding impedance (EBI) type

test. This ensures that the earth path between the protective

earth connection and any accessible earthed part is sufficiently

low to avoid damage in the event of a single fault occurring.

The test is conducted using a test voltage of 12V maximum

and a test current of twice the recommended maximum

protective fuse rating. After 1 minute with the current flowing

in the circuit under test, the EBI shall not exceed 0.1:.

21.4.5 CE Marking

A CE mark on the product, or its packaging, shows that

compliance is claimed against relevant EU directives e.g. Low

Voltage Directive 2006/95/EC and Electromagnetic

Compatibility (EMC) Directive 2004/108/EC.

21.4.6 Other Regional or Industry Norms

Many products intended for application in North America will

additionally need certification to the requirements of UL

(Underwriter's Laboratory), or CUL for Canada. Products for

application in mining or explosive environments will often be

required to demonstrate an ATEX claim

21.5 ENVIRONMENTAL TYPE TESTS

Various tests have to be conducted to prove that a relay can

withstand the effects of the environment in which it is

expected to work. They consist of: the following tests:

x temperature

x humidity

x enclosure protection

x mechanical

These tests are described in the following sections.

21.5.1 Temperature Test

Temperature tests are performed to ensure that a product can

withstand extremes in temperatures, both hot and cold, during

transit, storage and operating conditions. Storage and transit

conditions are defined as a temperature range of –25

C to

+70

C and operating as –25

C to +55

C. Many products now

claim operating temperatures of +70°C or even higher.

Dry heat withstand tests are performed at 70°C for 96 hours

with the relay de-energised. Cold withstand tests are

performed at –40°C for 96 hours with the relay de-energised.

Operating range tests are carried out with the product

energised, checking all main functions operate within tolerance

over the specified working temperature range –25°C to +55°C.

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21.5.2 Humidity Test

The humidity test is performed to ensure that the product will

withstand and operate correctly when subjected to 93% relative

humidity at a constant temperature of 40°C for 56 days. Tests

are performed to ensure that the product functions correctly

within specification after 21 and 56 days. After the test, visual

inspections are made for any signs of unacceptable corrosion

and mould growth.

21.5.3 Cyclic Temperature/Humidity Test

This is a short-term test that stresses the relay by subjecting it

to temperature cycling in conjunction with high humidity.

The test does not replace the 56 day humidity test, but is used

for testing extension to ranges or minor modifications to prove

that the design is unaffected.

The applicable standard is IEC60068-2-30 and test conditions

of:

x +25°C r3°C and 95% relative humidity/+55°C r

2°C

x 95% relative humidity

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

12h±0.5h

±0.5h

Figure 21.9: Cyclic temperature/humidity test profile

For these tests the relay is placed in a humidity cabinet, and

energised with normal in-service quantities for the complete

duration of the tests. In practical terms this usually means

energising the relay with currents and voltages such that it is

10% from the threshold for operation. Throughout the duration

of the test the relay is monitored to ensure that no unwanted

operations occur.

Once the relay is removed from the humidity cabinet, its

insulation resistance is measured to ensure that it has not

deteriorated to below the claimed level. The relay is then

functionally tested again, and finally dismantled to check for

signs of component corrosion and growth.

The acceptance criterion is that no unwanted operations shall

occur including transient operation of indicating devices. After

the test the relay’s insulation resistance should not have

significantly reduced, and it should perform all of its main

protection and communications functions within the claimed

tolerance. The relay should also suffer no significant corrosion

or growth, and photographs are usually taken of each PCB and

the case as a record of this.

21.5.4 Enclosure Protection Test

Enclosure protection tests prove that the casing system and

connectors on the product protect against the ingress of dust,

moisture, water droplets (striking the case at pre-defined

angles) and other pollutants. An ‘acceptable’ level of dust or

water may penetrate the case during testing, but must not

impair normal product operation, safety or cause tracking

across insulated parts of connectors.

21.5.5 Mechanical Tests

Mechanical tests simulate a number of different mechanical

conditions that the product may have to endure during its

lifetime. These fall into two categories

x response to disturbances while energised

x response to disturbances during transportation (de-

energised state)

Tests in the first category are concerned with the response to

vibration, shock and seismic disturbance. The tests are

designed to simulate normal in-service conditions for the

product, for example earthquakes. These tests are performed

in all three ax

es, with the product energised in its normal

(quiescent) state. During the test, all output contacts are

continually monitored for change using contact follower

circuits. Vibration levels of 1g, over a 10Hz-150Hz frequency

sweep are used. Seismic tests use excitation in a single axis,

using a test frequency of 35Hz and peak displacements of

7.5mm and 3.5mm in the

and

axes respectively below the

crossover frequency and peak accelerations of 2.0gn and 1.0gn

in these axes above the crossover frequency.

The second category consists of vibration endurance, shock

withstand and bump tests. They are designed to simulate the

longer-term affects of shock and vibration that could occur

during transportation. These tests are performed with the

product de-energised. After these tests, the product must still

operate within its specification and show no signs of

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Chapter 21  Relay Testing and Commissioning

21-11

permanent mechanical damage. Equipment undergoing a

seismic type test is shown in Figure 21.10, while the waveform

for the shock/bump test is shown in Figure 21.11

Figure 21.10: Relay undergoing seismic test

Pulse shape (half sine)

0.8A

1.2A

-0.2A

+0.2A

0.4D

2.5D 2.5D

6D = T

D D

2.4D = T

D - duration of nominal pulse

A - peak acceleration of nominal pulse

- as T

when a vibration generator is used

Figure 21.11: Shock/Bump Impulse waveform

The test levels for shock and bump tests are:

Shock response (energised):

x 3 pulses, each 10g, 11ms duration

Shock withstand (de-energised):

x 3 pulses, 15g, 11ms duration

Bump (de-energised):

x 1000 pulses, 10g, 16ms duration

21.6 SOFTWARE TYPE TESTS

Digital and numerical relays contain software to implement the

protection and measurement functions of a relay. This

software must be thoroughly tested, to ensure that the relay

complies with all specifications and that disturbances of

various kinds do not result in unexpected results. Software is

tested in various stages:

x unit testing

x integration testing

x functional qualification testing

The purpose of unit testing is to determine if an individual

function or procedure implemented using software, or small

group of closely related functions, is free of data, logic, or

standards errors. It is much easier to detect these types of

errors in individual units or small groups of units than it is in

an integrated software architecture and/or system. Unit

testing is typically performed against the software detailed

design and by the developer of the unit(s).

Integration testing typically focuses on these interfaces and

also issues such as performance, timings and synchronisation

that are not applicable in unit testing. Integration testing also

focuses on ‘stressing’ the software and related interfaces.

Integration testing is ‘black box’ in nature, i.e. it does not take

into account the structure of individual units. It is typically

performed against the software architectural and detailed

design. The unit developer normally performs the tests, unless

high-integrity and/or safety critical software is involved, when

the tester must be independent of the developer. The specified

software requirements would typically also be used as a source

for some of the test 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, precision

tracking, initialisation checking, value tracking, strong type

checking, macro analysis etc. While static unit testing can be

performed manually, it is a laborious and error prone process

and is best performed using a proprietary automated static unit

analysis tool. It is important to ensure that any such tool is

configured correctly and used consistently during development.

21.6.2 Dynamic Testing

Dynamic Testing is concerned with the runtime behaviour of

the unit(s) being tested and so therefore, the unit(s) must be

executed. Dynamic unit testing can be sub-divided into ‘

black

box’

testing and ‘

white box’

testing. ‘

Black box

’ testing verifies

the implementation of the requirement(s) allocated to the

unit(s). It takes no account of the internal structure of the

unit(s) being tested. It is only concerned with providing known

inputs and determining if the outputs from the unit(s) are

correct for those inputs. ‘

White box

’ testing is concerned with

testing the internal structure of the unit(s) and measuring the

test coverage, i.e. how much of the code within the unit(s) has

been executed during the tests. The objective of the unit

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testing may, for example, be to achieve 100% statement

coverage, in which every line of the code is executed at least

once, or to execute every possible path through the unit(s) at

least once.

21.6.3 Unit Testing Environment

Both Dynamic and Static Unit Testing are performed in the

host environment rather than the target environment.

Dynamic Unit Testing uses a

test harness

to execute the

unit(s) concerned. The test harness is designed such that it

simulates the interfaces of the unit(s) being tested - both

software-software interfaces and software-hardware interfaces

- using what are known as

stubs

. The test harness provides

the test data to those units being tested and outputs the test

results in a form understandable to a developer. There are

many commercially available testing tools to automate test

harness production and the execution of tests.

21.6.4 Software/Software Integration Testing

Software/Software Integration Testing is performed in the host

environment. It uses a test harness to simulate inputs 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 in the

target environment, i.e. it uses the actual target hardware,

operating system, drivers etc. It is usually performed after

Software/Software Integration Testing. Testing the interfaces

to the hardware is an important feature of Software/Hardware

Integration Testing.

Test cases for Integration Testing are typically based on those

defined for Validation Testing. However the emphasis should

be on finding errors and problems. Performing a dry run of the

validation testing often completes Integration Testing.

21.6.6 Validation Testing

The purpose of Validation Testing (also known as Software

Acceptance Testing) is to verify that the software meets its

specified functional requirements. Validation Testing is

performed against the software requirements specification,

using the target environment. In ideal circumstances,

someone independent of the software development performs

the tests. In the case of high-integrity and/or safety critical

software, this independence is vital. Validation Testing is

‘black box’ in nature, i.e. it does not take into account the

internal structure of the software. For relays, the non-

protection functions included in the software are considered to

be as important as the protection functions, and hence tested

in the same manner.

Each validation test should have predefined evaluation criteria,

to be used to decide if the test has passed or failed. The

evaluation criteria should be explicit with no room for

interpretation or ambiguity.

21.6.7 Traceability of Validation Tests

Traceability of validation tests to software requirements is vital.

Each software requirement documented in the software

requirements specification should have at least one validation

test, and it is important to be able to prove this.

21.6.8 Software Modifications - Regression Testing

Regression Testing is not a type test in its’ own right. It is the

overall name given to the testing performed when an existing

software product is changed. The purpose of Regression

Testing is to show that unintended changes to the functionality

(i.e. errors and defects) have not been introduced.

Each change to an existing software product must be

considered in its’ own right. It is impossible to specify a

standard set of regression tests that can be applied as a

‘catch-all’ for introduced errors and defects. Each change to

the software must be analysed to determine what risk there

might be of unintentional changes to the functionality being

introduced. Those areas of highest risk will need to be

regression tested. The ultimate regression test is to perform

the complete Validation Testing programme again, updated to

take account of the changes made.

Regression Testing is extremely important. If it is not

performed, there is a high risk of errors being found in the field.

Performing it will not reduce to zero the chance of an error or

defect remaining in the software, but it will reduce it.

Determining the Regression Testing that is required is made

much easier if there is traceability from properly documented

software requirements through design (again properly

documented and up to date), coding and testing.

21.7 DYNAMIC VALIDATION TYPE TESTING

There are two possible methods of dynamically proving the

satisfactory performance of protection relays or schemes; the

first method is by actually applying faults on the power system

and the second is to carry out comprehensive testing on a

power system simulator.

The former method is extremely unlikely to be used – lead

times are lengthy and the risk of damage occurring makes the

tests very expensive. It is therefore only used on a very limited

basis and the faults applied are restricted in number and type.

Because of this, a proving period for new protection equipment

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Chapter 21  Relay Testing and Commissioning

21-13

under service conditions has usually been required. As faults

may occur on the power system at infrequent intervals, it can

take a number of years before any possible shortcomings are

discovered, during which time further installations may have

occurred.

Power system simulators can be divided into two types:

x those which use analogue models of a power system

x those which model the power system mathematically

using digital simulation techniques

21.7.1 Use of Power System Analogue Models

For many years, relays have been tested on analogue models

of power systems such as artificial transmission lines, or test

plant capable of supplying significant amounts of current.

However, these approaches have significant limitations in the

current and voltage waveforms that can be generated, and are

not suitable for automated, unattended, testing programmes.

While still used on a limited basis for testing electromechanical

and static relays, a radically different approach is required for

dynamic testing of numerical relays.

21.7.2 Use of Microprocessor Based Simulation

Equipment

The complexity of numerical relays, reliant on software for

implementation of the functions included, dictates some kind

of automated test equipment. The functions of even a simple

numerical overcurrent relay (including all auxiliary functions)

can take several months of automated, 24 hours/day testing to

test completely. If such test equipment was able to apply

realistic current and voltage waveforms that closely match

those found on power systems during fault conditions, the

equipment can be used either for type testing of individual

relay designs or of a complete protection scheme designed for

a specific application. In recognition of this, a new generation

of power system simulators has been developed, which is

capable of providing a far more accurate simulation of power

system conditions than has been possible in the past. The

simulator enables relays to be tested under a wide range of

system conditions, representing the equivalent of many years

of site experience.

21.7.2.1 Simulation hardware

Equipment is now available to provide high-speed, highly

accurate modelling of a power system. The equipment is

based on distributed digital hardware under the control of real-

time software models, and is shown in Figure 21.12. The

modules have outputs linked to current and voltage sources

that have a similar transient capability and have suitable

output levels for direct connection to the inputs of relays –i.e.

110V for voltage and 1A/5A for current. Inputs are also

provided to monitor the response of relays under test (contact

closures for tripping, etc.) and these inputs can be used as part

of the model of the power system. The software is also

capable of modelling the dynamic response of CTs and VTs

accurately.

The digital simulator can also be connected digitally to the

relay(s) under test using IEC61850-8-1 for station bus, and

IEC 61850-9-2 for process bus signals.

Figure 21.12: Digital power system simulator for relay/protection

scheme testing

This equipment shows many advantages over traditional test

equipment:

x the power system model is capable of reproducing high

frequency transients such as travelling waves

x tests involving very long time constants can be carried

out

x it is not affected by the harmonic content, noise and

frequency variations in the a.c. supply

x it is capable of representing t

he variation in

the current

associated with generator faults and power swings

x saturation effects in CTs and VTs can be modelled

x a set of test routines can be specified in software and

then left to run unattended (or with only

occasional

monitoring) to completion, with a detailed record of test

results being available on completion

x the IEC61850 interface capabilities allow relays

intended for applications in digital substations to be

tested.

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A block schematic of the equipment is shown in Figure 21.13.

It is based around a computer which calculates and stores the

digital data representing the system voltages and currents.

The computer controls conversion of the digital data into

analogue signals, and it monitors and controls the relays being

tested.

21.7.2.2 Simulation software

Unlike most traditional software used for power systems

analysis, the software used is suitable for the modelling the

fast transients that occur in the first few milliseconds after

fault inception. Two very accurate simulation programs are

used, one based on time domain and the other on frequency

domain techniques. In both programs, single and double

circuit transmission lines are represented by fully distributed

parameter models. The line parameters are calculated from

the physical construction of the line (symmetrical,

asymmetrical, transposed or non-transposed), taking into

account the effect of conductor geometry, conductor internal

impedance and the earth return path. It also includes, where

appropriate, the frequency dependence of the line parameters

in the frequency domain program. The frequency dependent

variable effects are calculated using Fast Fourier Transforms

and the results are converted to the time domain.

Conventional current transformers and capacitor voltage

transformers can be simulated.

The fault can be applied at any one point in the system and

can be any combination of phase to phase or phase to earth,

resistive, or non-linear phase to earth arcing faults. For series

compensated lines, flashover across a series capacitor

following a short circuit fault can be simulated.

The frequency domain model is not suitable for developing

faults and switching sequences, therefore the widely used

Electromagnetic Transient Program (EMTP), working in the

time domain, is employed in such cases.

In addition to these two programs, a simulation program

based on lumped resistance and inductance parameters is

used. This simulation is used to represent systems with long

time constants and slow system changes due, for example, to

power swings.

21.7.2.3 Simulator applications

The simulator is used for checking the accuracy of calibration

and performing type tests on a wide range of protection relays

during their development. It has the following advantages

over existing test methods:

x 'state of the art' power system modelling data can be

used to test relays

x freedom from frequency variations and noise or

harmonic content of the laboratory's ow

n domestic

supply

x the relay under test does not burden the power system

simulation

model

amplifier

Current

circuits

interpolation

conversion

D/A

Voltage

amplifier

model

CVT

Contact

monitor

status

Sub-

I/O

system

test

under

Equipment

Signalling

Simulation

Channel

Computer

Keyboard

VDU

Keyboard

VDU

Key :

CVT

VDU - Visual display unit

- Capacitor voltage transformer

- Current transformer

Communications

RTDS

To second RTDS(When required)

circuits

interpolation

Linear

conversion

D/A

Storage

Figure 2

: Block diagram of rea

time digital relay test syste

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Chapter 21  Relay Testing and Commissioning

21-15

x all tests are accurately repeatable

x wide bandwidth signals can be produced

x a wide range of frequencies can be reproduced

x selected harmonics may be superimposed on the power

frequency

x the use of direct coupled current amplifiers allows time

constants of any length

x capable of simulating slow system changes

x reproduces fault currents whose peak amplitude varies

with time

x transducer models can be included

x automatic testing removes the likelihood of

measurement and setting errors

x two such equipments can be linked together

to simulate

a system model with two relaying points

The simulator is also used for the production testing of relays,

in which most of the advantages listed above apply. As the

tests and measurements

are made automatically, the quality of

testing is also greatly enhanced. Further, in cases of suspected

malfunction of a relay in the field under known fault

conditions, the simulator can be used to replicate the power

system and fault conditions, and conduct a detailed

investigation into the performance of the relay. Finally,

complex protection schemes can be modelled, using both the

relays intended for use and software models of them as

appropriate, to check the suitability of the proposed scheme

under a wide variety of conditions.

Figure 21.14(a) shows a section of a particular power system

modelled. The waveforms of Figure 21.14(b) show the three

phase voltages and currents at the primaries of VT1 and CT1

for the fault condition indicated in Figure 21.14(a).

Figure 21.14: Example of application study

21.8 PRODUCTION TESTING

Production testing of protection relays is becoming far more

demanding as the accuracy and complexity of the products

increase. Electronic power amplifiers are used to supply

accurate voltages and currents of high stability to the relay

under test. The inclusion of a computer in the test system

allows more complex testing to be performed at an economical

cost, with the advantage of speed and repeatability of tests

from one relay to another.

Figure 21.15 shows a modern computer-controlled test bench.

The hardware is mounted in a special rack. Each unit of the

test system is connected to the computer via an interface bus.

Individual test programs for each type of relay are required, but

the interface used is standard for all relay types. Control of

input waveforms and analogue measurements, the monitoring

of output signals and the analysis of test data are performed by

the computer. A printout of the test results can also be

produced if required.

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Network Protection & Automation Guide

21-16

Figure 21.15: Modern computer-controlled test bench

Because software is extensively tested at the type-testing

stage, there is normally no need to check the correct

functioning of the software. Checks are limited to determining

that the analogue and digital I/O is functioning correctly. This

is achieved for inputs by applying known voltage and current

inputs to the relay under test and checking that the software

has captured the correct values. Similarly, digital outputs are

exercised by using test software to actuate each output and

checking that the correct output is energised. Provided that

appropriate procedures are in place to ensure that only type-

tested software is downloaded, there is no need to check the

correct functioning of the software in the relay. The final step

is to download the software appropriate to the relay and store

it in the relay's non-volatile memory.

21.9 COMMISSIONING TESTS

Installation of a protection scheme at site creates a number of

possibilities for errors in the implementation of the scheme to

occur. Even if the scheme has been thoroughly tested in the

factory, wiring to the CTs and VTs on site may be incorrectly

carried out, or the CTs/VTs may have been incorrectly installed.

The impact of such errors may range from simply being a

nuisance (tripping occurs repeatedly on energisation, requiring

investigation to locate and correct the error(s)) through to

failure to trip under fault conditions, leading to major

equipment damage, disruption to supplies and potential

hazards to personnel. The strategies available to remove these

risks are many, but all involve some kind of testing at site.

Commissioning tests at site are therefore invariably performed

before protection equipment is set to work. The aims of

commissioning tests are:

x to ensure that the equipment has not been damaged

during transit or installation

x to ensure that the installation work has been carried out

correctly

x to prove the correct functioning of the protection

scheme as a whole

The tests carried out will normally vary according to the

protection scheme involved, the relay techno

logy used, and the

policy of the client. In many cases, the tests actually

conducted are determined at the time of commissioning by

mutual agreement between the client’s representative and the

commissioning team. Hence, it is not possible to provide a

definitive list of tests that are required during commissioning.

This section therefore describes the tests commonly carried out

during commissioning.

The following tests are invariably carried out, since the

protection scheme will not function correctly if faults exist.

x wiring diagram check, using circuit diagrams showing

all the reference numbers of the interconnecting wiring

x general inspection of the equipment, checking all

connections, wires on relays terminals, labels on

terminal boards, etc.

x insulation resistance measurement of all circuits

x perform relay self-test procedure and external

communications checks on digital/numerical relays

x test main current transformers

x test main voltage transformers

x check that protection relay alarm/trip settings have

been entered co

rrectly

x tripping and alarm circuit checks to prove correct

functioning

In addition,

the following checks may be carried out,

depending on the factors noted earlier.

x secondary injection test on each relay to prove

operation at one or more setting values

x primary injection tests on ea

ch relay to prove stability

for external faults and to determine the effective current

setting for internal faults (essential for some types of

electromechanical relays)

x testing of protection scheme logic

This section de

tails the tests required to cover the above items.

Secondary injection test equipment is covered in Section

21.10. Section 21.11 details the secondary injection that may

be carried out. Section 21.12 covers primary injection testing

and Section 21.13 details the checks required on any logic

involved in the protection scheme. Finally section 21.14 details

the tests required on alarm/tripping circuits tripping/alarm

circuits.

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