CHAPTER 10 HIGH VOLTAGE TESTING OF ELECTRICAL APPARATUS - - PowerPoint PPT Presentation

1.

Introduction

2.

Classification of High Voltage Tests

3.

Test Voltages

4.

High Voltage Testing of Electrical Apparatus

CHAPTER 10 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

• n several electrical apparatus. Normally, high

voltage (HV) testing is to investigate the insulation performance.

 International/national specifications for testing are

• utlined (details of test, specific equipment,

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

2.1 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

• f 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

• n the nature
• f 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

• n 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.

Atmospheric Correction Factors

 The electrical characteristics of the insulators and

• ther 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

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

18

Absolute 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
• ut 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

• vervoltage tests,
• Type test

Temperature rise and mechanical tests

• The most important test
• n 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

• bject 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

• f 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

• f negative polarity for 30 min)
• VI. Impulse withstand voltage tests, (withstand five impulse of

prescribed magnitude without any damage

• VII. partial discharge tests,