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

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Chapter 20  System Integrity Protection Schemes

20-15

components and harmonic components. But unlike filters such

as the Discrete Fourier Transform (DFT), a Kalman Filter can

provide signal components at spectral frequencies other than

the fundamental and its harmonics. Kalman Filters are well

suited to the application of protecting compensated lines, since

they can be used to detect, for example, the negative sequence

component at fundamental as well as resonances that may

transiently appear when a line is opened at both ends while its

shunt reactors are connected. The outline of a Kalman filter is

shown in Figure 20.27, and the state diagram is shown in

Figure 20.28.

klk

1klk

Figure 20.27: Kalman Filter block diagram

Figure 20.28: Kalman Filter state diagram.

The pre-processing unit takes the fundamental frequency

phasor components of the inputs, the derived sequence and

spectral components, and uses them to determine a number of

decisions about the system based on quantities that reflect the

‘openness’ or ‘closedness’ of the system such as DeltaP, active

power, reactive power, etc.. These decisions are used by the

Fuzzy logic to produce a conclusion as to the state of the line

(open or closed).

20.4.2.3 Fuzzy Logic

Conventional, or Boolean, logic is commonly used in power

system protection. It is based on signals being in one state or

the other (i.e. ON or OFF). Fuzzy logic is based on signals

being in more than one state and where there may be overlap

between states. Consider Figure 20.29, which shows a

number of states for the active power.

Membership

(Crisp Input)

Active Power

Nil

Low Normal

1.0 pu

Figure 20.29: Fuzzy Logic Sets

The value 1.0pu is resident in both the “Low” state as well as

the “Normal” state. It is not resident in the “Nil” state.

Its membership of the “Nil” state is 0, its membership of the

“Normal” state is 1, and its membership of the “Low” state is

somewhere between 0 and 1. This process is known as

“fuzzification”. Although called “fuzzy”, it is, in fact, a

repeatable process to which rules can be applied.

A fuzzy logic approach can resolve conflicts that may arise in

doubtful cases such as during line energisation at no-load, or

where capacitive voltage transformers induce electro-magnetic

transients suggesting that the active power is not zero (line

closed) while the line is actually open. Fuzzy logic is ideally

suited to resolving such conflicts since the decision is based on

several criteria with adaptable weighting factors.

The fuzzy decision system consists of three main steps:

x Fuzzification of the selected decisions features from the

pre-processor

x Fuzzy logic inference on these features, using rules or

criteria

x Crisp decision sent to the output

Fuzzification is a necessary step prior to a fuzzy-logic based

reasoning system. It consists in transforming the crisp

variables provided by the pre-processor into categories easily

described by common language

terms such as "Normal",

"Small", "Large", "Very Large", etc.

0.5

Active Power

Line closed with

full certainty

0.5

Delta P

Line open with

full certainty

“SMALL” Active power “LARGE” DeltaP

Figure 20.30: Fuzzification of typical decision features

Figure 20.30 illustrates this fuzzification process for two typical

variables: the Active Power and the DeltaP. The first feature is

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Network Protection & Automation Guide

20-16

defined as "SMALL" when its crisp value is below a given

threshold (

) while the DeltaP feature is defined as "LARGE"

for crisp values higher than (

A set of rules is applied to these fuzzy variables according to

basic principles. "Reliability Rules" are used specifically to

assess the open-line condition and "Security Rules" target

closed-line situations.

An example for each type of rule is given below:

x Security: if (ActivePower is not SMALL) LINE is

CLOSED

x Reliability: if (DeltaP is LARGE) LINE is OPEN

The fuzzy logic engine contains a number of these rules for

detecting the open-line status under various circumstances

(reliability rules), and a number of rules for detecting the

closed-line status (security rules). In each category, the rules

can be further classified in terms of transient and steady-state

rules. For instance, the reliability rule above is a transient one

while the security rule is a steady state.

The last stage of the process is called defuzzification. This

consists of converting the evaluation results of the criteria,

which are applied by the rules, into a single decision. Each

criterion states that the line is open or closed with a given

probability and defuzzification reconciles all partial decisions

using an aggregation procedure into a single decision.

The output from the open line detector is the flag that is

asserted when an open line is detected.

20.4.2.4 Open Line Detection Summary

In the context of interconnected and heavily loaded power

systems, fast detection of topology change is considered an

efficient means to instigate remedial actions for the defence of

a power system against severe events. Open Line Detection,

based on intelligent devices operating using locally-measured

power system quantities can provide such detection for a

typical topology change that could impact system stability.

20.4.3 System Synchronism Stability

In general, occurrence of extreme contingencies in a power

system can lead to voltage, frequency or angular instabilities.

Angular or frequency instabilities are related more to the

incapacity of the system to provide appropriate real power

flows between generation areas, or between generation and

load areas to maintain system equilibrium. Such instabilities

may lead to either under/overfrequency situations or loss-of-

synchronism conditions.

In the first case, the frequency will be the same for the whole

system but out of the normal range, whilst in the second case,

area or sub-system frequencies will differ and induce loss-of-

synchronism conditions characterised by periodic zero voltage,

or virtual faults. Anticipation of such loss of synchronism and

subsequent fast remedial action, may allow system stability to

be maintained.

Prediction of an impending loss of synchronism, or 'out-of-

step' condition, using signals from just a single point in the

system can be achieved using conventional or novel

algorithms.

Unlike conventional out-of-step relaying introduced in the

earlier chapter on distance protection, the novel algorithms

provide a system integrity protection scheme designed to

predict

out-of-step conditions on a transmission system. This

prediction can be used to initiate actions to preserve power

system stability such as engaging surge arrestors in order to

protect primary equipment against over-voltages before they

exceed the insulation rating, or islanding the system in

advance into areas of equilibrium resulting in the minimum

loss of load or generation.

20.4.3.1 Conventional Out-of-Step Protection

Blinder unit

Figure 20.31: Impedance loci during power swings

Detection of power swings has been briefly introduced in

chapters 11 and 17.

Classical approaches for transmission line applications make

use of auxiliary elements in the R, X plane in combination with

timers. Figure 20.31 illustrates this principle with two mho

relays (outer and inner zones) and a blinder unit. Z

initial

is the

pre-disturbance apparent impedance seen by a relay located at

one end of a transmission line. Whilst a fault would cause an

instantaneous movement of apparent impedance Z

app

from the

load area to the line impedance Z

, a power swing will cause

app

to move at a progressive rate and will be detected if the

sequential pickup of the outer and inner mho units takes more

than a predetermined time. It may be considered unstable if

the blinder line is crossed, and stable if Z

app

returns to the load

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Chapter 20  System Integrity Protection Schemes

20-17

area. Typically, this scheme will trip upon unstable swings and

block for stable ones.

When tripping is required, excessive breaker stresses will be

reduced if the two systems to be separated do not move close

to anti-phase. It is therefore highly desirable either to

anticipate the unstable swing or to delay the trip signal until

after the pole slip. Anticipating the instability can rarely be

done with such a scheme however, because the Z

app

trajectory

is hard to predict for all conditions prevailing in a real system

and tripping on a stable swing is not desirable. On the other

hand, with a delayed trip signal, the magnitude and duration

of voltage excursions will be more severe and will decrease the

chances of island survival.

These difficulties can be overcome with more sophisticated

schemes for predicting the dynamic behaviour of power

swings.

20.4.3.2 Predictive Out-of-Step

Rather than waiting for the out-of-step condition to occur, the

novel predictive out-of-step protection provides early warning

of impending system instability.

Predictive out-of-step protection can be achieved with a

special algorithm called an RPS algorithm. The

implementation of this algorithm is similar to that of the

detection of open lines as already described. Kalman

filters

and fuzzy logic engines are used to apply multiple criteria that

are indicative of impending loss of synchronism.

A specially designed Kalman filter extracts the voltage and

current positive-sequence phasors used as the primary

variables in these applications.

A transmission line model is used to compute values for the

remote voltage and current phasors V

and I

, given the local

phasors V

and I

These sets are then used to derive decisional

variables such as line angle and frequency shifts, local and

remote

Vcos and derivative quantities.

The RPS variables are used to track and anticipate the stable

or unstable evolution of power swings across the transmission

line. Three angular variables, one concavity index, two voltage

and six

Vcos variables are derived and processed to prepare

them for the fuzzy-logic algorithm.

Any one of these variables individually can provide an

indication of system stability, but given the wide range of

operating conditions found in large power systems, the

performance of single-variable-based algorithms is limited

because of the margin for error in any one of them. Fuzzy logic

enables multi-criteria algorithms to take advantage of the

strength of all the variables by considering them all together to

assess the stability condition. The construction of the fuzzy

logic to implement the RPS algorithm is similar to that already

described, with fuzzification giving linguistic categories to the

signals and the fuzzy logic applying weighted rules to

determine the RPS output, indicating whether the system is

predicted to go out-of-step or not.

In practical applications, the out-of-step prediction is raised

almost half a second before the instability, giving time to

initiate remedial actions to prevent the event and protect the

system.

20.4.4 Acknowledgements

Alstom Grid gratefully acknowledges the research,

development and expert assistance provided by Hydro Québec

during the joint development of the DLO (MiCOM P846 Open

Line Detector and RPS (MiCOM P848 Predictive Out-Of-Step)

products.

20.5 REFERENCES

[20.1] L. H. Fink and K. Carlsen, “Operating Under Stress and

Strain,”

IEEE Spectrum

, vol. 15, pp. 48–53, March

1978.

[20.2] CIGRE Task Force 38.02.19: “System Protection

Schemes in Power Networks”, Final Draft v 5.0, 2000.

[20.3] R Grondin, S Richards, et al, “Loss of Synchronism

Detection. A Strategic Function for Power System

Protection”, CIGRE B5-205, 2006.

copyright notice but may not be copied or displayed for commercial purposes without the prior written permission of Alstom Grid.

Alstom Grid 21-1

Chapter 21

Relay Testing and Commissioning

21.1 Introduction

21.2 Electrical Type Tests

21.3 Electromagnetic Compatibility Tests

21.4 Product Safety Type Tests

21.5 Environmental Type Tests

21.6 Software Type Tests

21.7 Dynamic Validation Type Testing

21.8 Production Testing

21.9 Commissioning Tests

21.10Secondary Injection Test Equipment

21.11Secondary Injection Testing

21.12Primary Injection Tests

21.13Testing of Protection Scheme Logic

21.14Tripping and Alarm Annunciation Tests

21.15Periodic Maintenance Tests

21.16Protection Scheme Design for Maintenance

21.1 INTRODUCTION

The testing of protection equipment schemes presents a

number of problems. This is because the main function of

protection equipment is solely concerned with operation under

system fault conditions, and cannot readily be tested under

normal system operating conditions. This situation is

aggravated by the increasing complexity of protection

schemes, the use of relays with extensive functionality

implemented in software, and the trend towards peer-to-peer

logic implemented via Ethernet (as opposed to more traditional

hardwired approaches).

The testing of protection equipment may be divided into four

stages:

x type tests

x routine factory production tests

x commissioning tests

x periodic maintenance tests

21.1.1 Type Tests

Type tests are required to prove that a relay meets the

published specification and complies with all relevant

standards. Since the principal function of a protection relay is

to operate correctly under abnormal power conditions, it is

essential that the performance be assessed under such

conditions. Comprehensive type tests simulating the

operational conditions are therefore conducted at the

manufacturer's works during the development and certification

of the equipment.

The standards that cover most aspects of relay performance

are IEC 60255 and IEEE C37.90. However compliance may

also involve consideration of the requirements of IEC 61000,

60068 and 60529, while products intended for use in the EU

also have to comply with the requirements of Directives (see

Appendix C). Since type testing of a digital or numerical relay

involves testing of software as well as hardware, the type

testing process is very complicated and more involved than a

static or electromechanical relay.

21.1.2 Routine Factory Production Tests

These are conducted to prove that relays are free from defects

during manufacture. Testing will take place at several stages

during manufacture, to ensure problems are discovered at the

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Network Protection & Automation Guide

21-2

earliest possible time and hence minimise remedial work. The

extent of testing will be determined by the complexity of the

relay and past manufacturing experience.

21.1.3 Commissioning Tests

These tests are designed to prove that a particular protection

scheme has been installed correctly prior to setting to work.

All aspects of the scheme are thoroughly checked, from

installation of the correct equipment through wiring checks

and operation checks of the individual items of equipment,

finishing with testing of the complete scheme.

21.1.4 Periodic Maintenance Checks

These are required to identify equipment failures and

degradation in service, so that corrective action can be taken.

Because a protection scheme only operates under fault

conditions, defects may not be revealed for a significant period

of time, until a fault occurs. Regular testing assists in

detecting faults that would otherwise remain undetected until

a fault occurs.

21.2 ELECTRICAL TYPE TESTS

Various electrical type tests must be performed, as follows:

21.2.1 Functional Tests

The functional tests consist of applying the appropriate inputs

to the relay under test and measuring the performance to

determine if it meets the specification. They are usually carried

out under controlled environmental conditions. The testing

may be extensive, even where only a simple relay function is

being tested., as can be realised by considering the simple

overcurrent relay element of Table 21.1.

To determine compliance with the specification, the tests listed

in Table 21.2 are required to be carried out. This is a time

consuming task, involving many engineers and technicians.

Hence it is expensive.

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

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 0.005

Element Range Step Size

IEEE IDMT Time Delay

IEEE Moderately Inverse

IEEE Very Inverse

IEEE Extremely Inverse

US-C08 Inverse

US-C02 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 IDMT Reset Time

IEEE Moderately Inverse

IEEE Very Inverse

IEEE Extremely Inverse

US-C08 Inverse

US-C02 Short Time Inverse

Table 21.1: Overcurrent relay element specification

Test no. Description

Test 1

Three phase non-directional pick up and drop off accuracy over complete current

setting range for both stages

Test 2

Three phase directional pick up and drop off accuracy over complete RCA setting

range in the forward direction, current angle sweep

Test 3

Three phase directional pick up and drop off accuracy over complete RCA setting

range in the reverse direction, current angle sweep

Test 4

Three phase directional pick up and drop off accuracy over complete RCA setting

range in the forward direction, voltage angle sweep

Test 5

Three phase directional pick up and drop off accuracy over complete RCA setting

range in the reverse direction, voltage angle sweep

Test 6 Three phase polarising voltage threshold test

Test 7 Accuracy of DT timer over complete setting range

Test 8 Accuracy of IDMT curves over claimed accuracy range

Test 9 Accuracy of IDMT TMS/TD

Test 10 Effect of changing fault current on IDMT operating times

Test 11 Minimum Pick-Up of Starts and Trips for IDMT curves

Test 12 Accuracy of reset timers

Test 13 Effect of any blocking signals, opto inputs, VTS, Autoreclose

Test 14 Voltage polarisation memory

Table 21.2: Overcurrent relay element functional type tests

When a modern numerical relay with many functions is

considered, each of which has to be type-tested, the functional

type-testing involved is a major issue. In the case of a recent

relay development project, it was calculated that if one person

had to do all the work, it would take 4 years to write the

functional type-test specifications, 30 years to perform the

tests and several years to write the test reports that result.

Automated techniques/ equipment are clearly required, and

are covered in Section 21.7.2.

21.2.2 Rating Tests

Rating type tests are conducted to ensure that components are

used within their specified ratings and that there are no fire or

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Chapter 21  Relay Testing and Commissioning

21-3

electric shock hazards under a normal load or fault condition of

the power system. This is in addition to checking that the

product complies with its technical specification. The following

are amongst the rating type tests conducted on protection

relays, the specified parameters are normally to IEC 60255-1.

21.2.3 Thermal Withstand

The thermal withstand of VTs, CTs and output contact circuits

is determined to ensure compliance with the specified

continuous and short-term overload conditions. In addition to

functional verification, the pass criterion is that there is no

detrimental effect on the relay assembly, or circuit

components, when the product is subjected to overload

conditions that may be expected in service. Thermal withstand

is assessed over a time period of 1s for CTs and 10s for VTs.

21.2.4 Relay Burden

The burdens of the auxiliary supply, optically isolated inputs,

VTs and CTs are measured to check that the product complies

with its specification. The burden of products with a high

number of input/output circuits is application specific i.e. it

increases according to the number of optically isolated input

and output contact ports which are energised under normal

power system load conditions. It is usually envisaged that not

more than 50% of such ports will be energised concurrently in

any application.

21.2.5 Relay Inputs

Relay inputs are tested over the specified ranges. Inputs

include those for auxiliary voltage, VT, CT, frequency, optically

isolated digital inputs and communication circuits.

21.2.6 Relay Output Contacts

Protection relay output contacts are type tested to ensure that

they comply with the product specification. Particular

withstand and endurance type tests have to be carried out

using d.c., since the normal supply is via a station battery.

21.2.7 Insulation Resistance

The insulation resistance test is carried out according to IEC

60255-27, i.e. 500V d.c.

r 10%, for a minimum of 5 seconds.

This is carried out between all circuits and case earth, between

all independent circuits and across normally open contacts.

The acceptance criterion for a product in new condition is a

minimum of 100M:. After a damp heat test the pass

criterion is a minimum of 10M:.

21.2.8 Auxiliary Supplies

Digital and numerical protection relays normally require an

auxiliary supply to provide power to the on-board

microprocessor circuitry and the interfacing opto-isolated input

circuits and output relays. The auxiliary supply can be either

a.c. or d.c., supplied from a number of sources or safe supplies

- i.e. batteries, UPSs, etc., all of which may be subject to

voltage dips, short interruptions and voltage variations. Relays

are designed to ensure that operation is maintained and no

damage occurs during a disturbance of the auxiliary supply.

Tests are carried out for both a.c. and d.c. auxiliary supplies

and include mains variation both above and below the nominal

rating, supply interruptions derived by open circuit and short

circuit, supply dips as a percentage of the nominal supply,

repetitive starts. The duration of the interruptions and supply

dips range from 2ms to 60s intervals. A short supply

interruption or dip up to 20ms, possibly longer, should not

cause any malfunction of the relay. Malfunctions include the

operation of output relays and watchdog contacts, the reset of

microprocessors, alarm or trip indication, acceptance of

corrupted data over the communication link and the corruption

of stored data or settings. For a longer supply interruption, or

dip in excess of 50ms, the relay self recovers without the loss

of any function, data, settings or corruption of data. No

operator intervention is required to restore operation after an

interruption or dip in the supply.

In addition to the above, the relay is subjected a number of

repetitive starts or a sequence of supply interruptions. Again

the relay is tested to ensure that no damage or data corruption

has occurred during the repetitive tests.

Specific tests carried out on d.c. auxiliary supplies include

reverse polarity, a.c. waveform superimposed on the d.c.

supply and the effect of a rising and decaying auxiliary voltage.

All tests are carried out at various levels of loading of the relay

auxiliary supply.

21.3 ELECTROMAGNETIC COMPATIBILITY

TESTS

There are numerous tests that are carried out to determine the

ability of relays to withstand the electrical environment in

which they are installed. The substation environment is a very

severe environment in terms of the electrical and

electromagnetic interference that can arise. There are many

sources of interference within a substation, some originating

internally, others being conducted along the overhead lines or

cables into the substation from external disturbances. The

most common sources are:

x switching operations

x system faults

x lightning strikes

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Network Protection & Automation Guide

21-4

x conductor flashover

x telecommunication operations e.g. mobile phones

A whole suite of tests are performed to simulate these types of

interference, and they fall under the broad

umbrella of what is

known as EMC, or Electromagnetic Compatibility tests.

Broadly speaking, EMC can be defined as:

‘The ability of different equipment to co-exist in the same

electromagnetic environment’

It is not a new subject and has been tested for by the military

ever since the advent of electronic equipment. EMC can cause

real and serious problems, and does need to be taken into

account when designing electronic equipment.

EMC tests determine the impact on the relay under test of

high-frequency electrical disturbances of various kinds. Relays

manufactured or intended for use in the EU have to comply

with Directive 2004/108/EC in this respect. To achieve this, in

addition to designing for statutory compliance to this Directive,

the following range of tests are carried out:

x d.c. interrupt test

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

x d.c. ramp test

x high frequency disturbance test

x fast transient test

x surge immunity test

x power frequency interference test

x electrostatic discharge test

x conducted and radiated emissions tests

x conducted and radiated immunity tests

x 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 the auxiliary supply

without de-energising, e.g. switching off, and that when this

time is exceeded and it does transiently switch off, that no

maloperation occurs.

It simulates the effect of a loose fuse in the battery circuit, or a

short circuit in the common d.c. supply, interrupted by a fuse.

Another source of d.c. interruption is if there is a power system

fault and the battery is supplying both the relay and the circuit

breaker trip coils. When the battery energises the coils to

initiate the circuit breaker trip, the voltage may fall below the

required level for operation of the relay and hence a d.c.

interrupt occurs. The test is specified in IEC 60255-11 and

comprises interruptions of 10, 20, 30, 50, 100, 200, 300, 500,

1000, and 5000ms. For interruptions lasting up to and

including 20ms, the relay must not de-energise of maloperate,

while for longer interruptions it must not maloperate. Many

modern products are capable of remaining energised for

interruptions up to 50ms

The relay is powered from a battery supply, and both short

circuit and open circuit interruptions are carried out. Each

interruption is applied 10 times, and for auxiliary power

supplies with a large operating range, the tests are performed

at minimum, maximum, and other voltages across this range,

to ensure compliance over the complete range.

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

This test (IEC 60255-11) determines that the relay is able to

operate correctly with a superimposed a.c. voltage on the d.c.

supply. This is caused by the station battery being charged by

the battery charger, and the relevant waveform is shown in

Figure 21.1. It consists of a 15% peak-to-peak ripple

superimposed on the d.c. supply voltage.

1393

1306

1219

1132

1045

958

871

784

697

610

523

436

349

262

175

Voltage (V)

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

For auxiliary power supplies with a large operating range, the

tests are performed at minimum, maximum, and other

voltages across this range, to ensure compliance for the

complete range. The interference is applied using a full wave

rectifier network, connected in parallel with the battery supply.

The relay must continue to operate without malfunction during

the test.

21.3.3 D.C. Ramp Down/Ramp Up

This test simulates a failed station battery charger, which

would result in the auxiliary voltage to the relay slowly ramping

down. The ramp up part simulates the battery being

recharged after discharging. The relay must power up cleanly

when the voltage is applied and not maloperate.

There is no international standard for this test, so individual

manufacturers can decide if they wish to conduct such a test

and what the test specification shall be.

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Chapter 21  Relay Testing and Commissioning

21-5

21.3.4 High Frequency Disturbance Test

The High Frequency Disturbance Test simulates high voltage

transients that result from power system faults and plant

switching operations. It consists of a 1MHz decaying

sinusoidal waveform, as shown in Figure 21.2. The

interference is applied across each independent circuit

(differential mode) and between each independent circuit and

earth (common mode) via an external coupling and switching

network. The product is energised in both normal (quiescent)

and tripped modes for this test, and must not maloperate

when the interference is applied for a 2 second duration.

Voltage

Time

Figure 21.2: High Frequency Disturbance Test waveform

21.3.5 Fast Transient Test

The Fast Transient Test simulates the HV interference caused

by disconnector operations in GIS substations or breakdown of

the SF

insulation between conductors and the earthed

enclosure. This interference can either be inductively coupled

onto relay circuits or can be directly introduced via the CT or

VT inputs. It consists of a series of 15ms duration bursts at

300ms intervals, each burst consisting of a train of 50ns wide

pulses with very fast (5ns typical) rise times (Figure 21.3),

with a peak voltage magnitude of 4kV.

Figure 21.3 Fast Transient Test waveform

The product is energised in both normal (quiescent) and

tripped modes for this test. It must not maloperate when the

interference is applied in common mode via the integral

coupling network to each circuit in turn, for 60 seconds.

Interference is coupled onto communications circuits, if

required, using an external capacitive coupling clamp.

21.3.6 Surge Immunity Test

The Surge Immunity Test simulates interference caused by

major power system disturbances such as capacitor bank

switching and lightning strikes on overhead lines within 5km

of the substation. The test waveform has an open circuit

voltage of 4kV for common mode surges and 2kV for

differential mode surges. The waveshape consists on open

circuit of a 1.2/50Ps rise/fall time and a short circuit current of

8/20Ps rise/fall time. The generator is capable of providing a

short circuit test current of up to 2kA, making this test

potentially destructive. The surges are applied sequentially

under software control via dedicated coupling networks in both

differential and common modes with the product energised in

its normal (quiescent) state. The product shall not maloperate

during the test, shall still operate within specification after the

test sequence and shall not incur any permanent damage.

21.3.7 Power Frequency Interference

This test simulates the type of interference that is caused when

there is a power system fault and very high levels of fault

current flow in the primary conductors or the earth grid. This

causes 50 or 60Hz interference to be induced onto control and

communications circuits.

There is no international standard for this test, but one used by

some Utilities is:

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

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

applied to circuits for which power system inputs are not

connected.

Tests are carried out on eac

h circuit, with the relay in the

following modes of operation:

x current and voltage applied at 90% of setting, (relay not

tripped)

x current and voltage applied at 110% of setting, (relay

tripped)

x main protection and communications functions are

tested to determine the effect of the interference

The relay shall not maloperate during the

test, and shall still

perform its main functions within the claimed tolerance.

21.3.8 Electrostatic Discharge Test

This test simulates the type of high voltage interference that

occurs when an operator touches the relay’s front panel after

being charged to a high potential. This is exactly the same

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Network Protection & Automation Guide

21-6

phenomenon as getting an electric shock when stepping out of

a car or after walking on a synthetic fibre carpet.

In this case the discharge is only ever applied to the front panel

of the relay, with the cover both on and off. Two types of

discharges are applied, air discharge and contact discharge.

Air discharges are used on surfaces that are normally

insulators, and contact discharges are used on surfaces that

are normally conducting. IEC 60255-22-2 is the relevant

standard this test, for which the test parameters are:

x cover on: Class 4, 8kV contact discharge, 15kV air

discharge

x cover off: Class 3, 6kV contact discharge, 8kV air

discharge

In both cases above, all the lower test levels are also tested.

The discharge current waveform is shown in Figure 21.4.

Current, % of Peak

Time, ns

Rise Time = 0.7 to 1.0 ns.

Current specified for 30 ns and 60 ns

100

0 102030405060708090

Figure 21.4: ESD Current Waveform

The test is performed with single discharges repeated on each

test point 10 times with positive polarity and 10 times with

negative polarity at each test level. The time interval between

successive discharges is greater than 1 second. Tests are

carried out at each level, with the relay in the following modes

of operation:

x current and voltage applied at 90% of setting, (relay not

tripped)

x current and voltage applied at 110% of setting, (relay

tripped)

x main protection and communications functions are

tested to determine the effect of the discharge

To pass, the re

lay shall not maloperate, and shall still perform

its main functions within the claimed tolerance.

21.3.9 Conducted and Radiated Emissions Tests

These tests arise primarily from the essential protection

requirements of the EU directive on EMC. These require

manufacturers to ensure that any equipment to be sold in the

countries comprising the European Union must not interfere

with other equipment. To achieve this it is necessary to

measure the emissions from the equipment and ensure that

they are below the specified limits.

Conducted emissions are measured only from the equipment’s

power supply ports and are to ensure that when connected to

a mains network, the equipment does not inject interference

back into the network which could adversely affect the other

equipment connected to the network.

Radiated emissions measurements are to ensure that the

interference radiated from the equipment is not at a level that

could cause interference to other equipment. This test is

normally carried out on an Open Area Test Site (OATS) where

there are no reflecting structures or sources of radiation, and

therefore the measurements obtained are a true indication of

the emission spectrum of the relay. An example of a plot

obtained during conducted emissions tests is shown in Figure

21.5.

Emissions Level, dBuV

Frequency, MHz

100

0.1 1 10 100

Quasi-peak limits

Average limits

Typical trace

Figure 21.5: EMC Conducted Emissions test - example test plot

The test arrangements for the conducted and radiated

emissions tests are shown in Figure 21.6.

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

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