1. Introduction

2. Classification of High Voltage Tests

3. Test Voltages

4. High Voltage Testing of Electrical Apparatus

HIGH VOLTAGE TESTING OF

ELECTRICAL APPARATUS

1. INTRODUCTION

Purpose of the testing: To ensure that the electrical

equipments are capable of withstanding the overvoltages that

are met with in service.

Covers basic requirements procedures for testing on several

electrical apparatus. Normally, high voltage (HV) testing is to

investigate the insulation performance.

International/national specifications for testing are outlined

(details of test, specific equipment, procedure and acceptable

limits) to meet the users’ and manufacturers’ requirements.

2. CLASSIFICATION OF HIGH VOLTAGE

TESTS

Destructive Test

Normally the equipment underwent destructive

test cannot be used in the service.

Test voltage is higher than its normal working

voltage.

Breakdown test.

Con’t

Non-Destructive Test

Mainly done to assess the electrical properties,

eg. Resistivity, dielectric constant and loss

factor.

The apparatus is not destroyed during the test

and can be used again.

2.2 TYPES OF TESTS

1. Routine Tests

Made by the manufacturer on every finished piece of product.

To fulfills the specifications.

2. Type Tests

Performed on each type of equipment before their supply on a

general commercial scale – demonstrate performance

characteristics.

No need to repeat the test unless changes are made in the

design of the product.

Con’t

3. Maintenance Tests

Usually carried out after maintenance/repair of

the equipment.

Conducted according to schedule provided.

Purpose of the test : To ensure the equipment

lifetime is achieved.

Types of tests

The range of high voltage tests depends on the nature of the equipment being tested

3.0 TEST VOLTAGES

TEST VOLTAGES

impulse voltages.

power-frequency alternating

voltages (AC)

Direct voltages

(DC)

Con’t

Test with Direct Voltage (DC).

Mainly to test equipment used in HVDC

transmission systems.

Insulation testing, fundamental investigations in

discharge physics and dielectric behavior.

Rate of voltage rise above 75% of its estimated

final value should be about 2% per second.

Con’t

Test with Alternating Voltage (AC).

Frequency range : 40-60 Hz, sinusoidal shape.

Dry withstand test : Most common routine test

for all types of electrical equipment especially

insulators, bushing, rod gaps etc.

Applied voltage between two to three times of

the normal working voltage.

Con’t

Test with Alternating Voltage (AC) – cont.

Wet withstand test : To simulate the effect of natural rain on

external insulation.

Recommended for tests on apparatus which are designed for outdoor

used.

Use artificial rain.

Applied for 30-60 seconds.

Con’t

Test with Impulse Voltage.

Is designed to investigate the

insulation performance due to the

lightning stroke or Lightning

impulse chopped on the front

switching operation.

3 types of impulse voltages, ie;

1) Full wave

2) Chopped wave

3) Switching wave

BS 923: Part 2: 1980

Con’t

BS 923: Part 2: 1980

Lightning impulse chopped

on the tail

Full switching impulse

Con’t

Rated impulse withstand test

For test on non-self-restoring insulation, 3 impulses are applied.

For withstand tests on self-restoring insulation, 2 procedures

are used:

1) 15 impulses (rated withstand voltage) with the specified shape and

polarity are applied

2) Test procedure for determining 50% disruptive discharge voltage is

applied

The method used for determining the levels of applied voltage

is up-and-down methods.

The electrical characteristics of the insulators and other apparatus are normally referred to the reference atmospheric conditions.

According to the British Standard Specifications, they are:

Temperature : 20oC

Pressure : 1013 millibars (or 760 torr)

Absolute humidity : 11 gm/m3

Since it is not always possible to do tests under these reference conditions, correction factors have to be applied.

Atmospheric Correction Factors

Atmospheric Correction Factors

V0 = voltage under actual test conditions,

Vs = voltage under reference atmospheric conditions,

h = humidity correction factor, and

d = air density correction factor.

The air density correction factor is given by,

where, b = atmospheric pressure in millibars, and

T= atmospheric temperature, oC.

Humidity correction factor h is obtained from the temperatures of

a wet and dry bulb thermometer, by obtaining the absolute

humidity and then computing h from the absolute humidity.

Example 1

Sarzaminor conducted an AC flashover test at power

frequency on a cap and pin insulator for his FYP project.

From the test, it was found that flashover occurred at 80 kV.

If the dry and wet temperatures during the test were 25˚C

and 22˚C respectively, and atmospheric pressure was at 1

atm, determine the flashover voltage under the reference

atmospheric condition.

Example 1

Dry Temp. = 25˚C

Wet Temp. = 22˚C

18Absolute humidity =

18g/m3

Example 1

Dry Temp. = 25˚C

Wet Temp. = 22˚C

1 atm = 1013.25 milibar

0.92

h=0.92

4.0 HIGH VOLTAGE TESTING OF

ELECTRICAL APPARATUS

Transformer

Circuit breaker

Insulator

Cable

Impulse testing on transformer

• A change in the waveshape of the voltage and current both before and after the chopped waves have been applied.

• The existing of acoustic noise.• Visual signs of flashover

• According to BS 171: Part 3. Carried out at room temperature with the transformer not energized.

• Used standard impulse waveshapes. Full and chopped waves. Impulse Test

Temperature Rise Test

Short Circuit test

Failure detection (Insulation

failure)

Testing of Circuit Breaker

(BS 5227: Part 2, IEC 56)

• Consist of overvoltage withstand tests of power frequency, lightning and switching impulse voltages.

• Circuit breaker in both the open and closed positions

Dielectric tests or overvoltage tests,

• Type test

Temperature rise and mechanical tests

• The most important test on circuit breaker since these tests assess the primary performance of these devices, i.e. their ability to safely interrupt the fault currents

Short Circuit Tests

Short Circuit Test Methods of conducting short circuit tests are

(I) Direct Tests

(a) using a short circuit generator as the source

(b) using the power utility system or network as the source.

(II) Synthetic Tests

Consists of two separate sources : one power source acting as a current source supplying the required short-circuit current at a (relative) low-voltage level and a second source working as a voltage source supplying the required voltage.

Based on the fact that for certain (short circuit) tests the test object is stressed by a high current and by a high voltage at different time periods.

Tests on Insulator

Test on Insulator

Power Frequency Test

Dry and Wet flashover test

increased at a uniform rate of about 2 % per second of 75% of the estimated test voltage

Dry and Wet Withstand test

Voltage applied under dry or wet conditions for a period of 1 minute with an insulator mounted as in

service conditions. The test piece should withstand the specified voltage.

Impulse Tests

Impulse Withstand Voltage Test

Both positive and negative polarities

Insulator passed the test if five consecutive waves do not cause flashover or puncture

Impulse Flashover Test

Pollution Testing

Due to outdoor electrical insulation and consequent problems of the maintenance of

electrical power systems. Eg. Dust, industrial pollution (smoke & petroleum vapor), desert

pollution, snow

Pollution causes corrosion, non-

uniform gradients, deterioration of the

material, partial discharges and radio

interference.

Routine and Type test on cables According to BS 923: Part 2, IEC 60-2, IEC 55-1, IEC 230 and BS6480.

Different tests on cables may be classified into

I. mechanical tests like bending test, dripping and drainage test, and fire

II. resistance and corrosion tests,

III. dielectric power factor tests,

IV. power frequency withstand voltage tests, (a.c. voltage of 2.5 limes the

rated value for 10 min.)

V. Dc withstand voltage tests, (1.8 times the rated d.c. voltage of negative

polarity for 30 min)

VI. Impulse withstand voltage tests, (withstand five impulse of prescribed

magnitude without any damage

VII. partial discharge tests,