CHAPTER 10 HIGH VOLTAGE TESTING OF ELECTRICAL APPARATUS 1. Introduction 2. Classification of High Voltage Tests 3. Test Voltages 4. 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 powerfrequency alternating voltages (AC) Direct voltages (DC) impulse voltages. TEST VOLTAGES
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.
Atmospheric Correction Factors 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 : 20 o C Pressure : 1013 millibars (or 760 torr) Absolute humidity : 11 gm/m 3 Since it is not always possible to do tests under these reference conditions, correction factors have to be applied.
Atmospheric Correction Factors V 0 = voltage under actual test conditions, V s = 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, o C.
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 Absolute humidity = 18g/m 3 18
Example 1 Dry Temp. = 25 C Wet Temp. = 22 C 1 atm = 1013.25 milibar h=0.92 0.92
4.0 HIGH VOLTAGE TESTING OF ELECTRICAL APPARATUS Transformer Circuit breaker Insulator Cable
Impulse testing on transformer 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 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 Failure detection (Insulation failure) Short Circuit test
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, Temperature rise and mechanical tests Type test 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) lowvoltage 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 Power Frequency Test Dry and Wet flashover test Dry and Wet Withstand test increased at a uniform rate of about 2 % per second of 75% of the estimated test voltage 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. Test on Insulator Impulse Tests Impulse Withstand Voltage Test Impulse Flashover Test Both positive and negative polarities Insulator passed the test if five consecutive waves do not cause flashover or puncture 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, nonuniform 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. III. IV. resistance and corrosion tests, dielectric power factor tests, 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,