Testing of insulators

Testing of insulators

The insulators are tested according to the standards. The following tests are performed while testing an insulator :

1. Mechanical tests          2. Electrical insulation tests  3. Environmental tests  4. Temporary cycle tests 5. Corona and radio interference tests.

These tests are further grouped into three classes according to the standards which are,

1. Type tests : These tests are performed to judge the design and specified ratings of the insulator. These tests include mechanical tests and electrical flash over tests.

2. Sample tests : These are performed to judge the material used, its quality and mechanical properties. These are also called performance tests. These include some mechanical tests.

3. Routine tests : On every insulator some standard tests are performed called routine tests.

1. Mechanical Tests

The mechanical tests are performed to judge the ability of an insulator to withstand various mechanical stresses. The mechanical tests include :

1. Mechanical strength test 2. Compression test  3. Torsional test

4. Minimum bending test 5. Mechanical vibration test

In each of these tests, an insulator is subjected to a particular mechanical stress, for particular specified time. The insulator must be capable of sustaining the mechanical stress as per the specifications associated with it.

2. Electrical Insulation Tests

These tests are performed to judge the electrical properties of the insulation under test. The main test in this category is the flash over test.

Flash over tests are further classified as,

1. 50 Cycle Dry Flash-Over Test : In this test, the voltage is applied between the electrodes of the insulator and is gradually increased. The voltage at which the surrounding air breaks down is called the flash-over voltage. This voltage must be greater than the specified limit. Insulator must sustain the minimum voltage for one minute.

2. 50 Cycle Wet Test : The test is conducted similar to the dry test but in addition to the applied voltage, the water is sprayed over the surface at an angle of 45°, resembling the raining condition. The insulator must be capable of withstanding the minimum standard voltage for 30 seconds under wet conditions. The wet test is applicable only to the outdoor insulators.

Other electrical tests include,

1. Power frequency withstand test : Normal power frequency voltage is continuously applied to the insulator. This causes dust particles to align on the surface causing leakage currents. The flash-over is avoided. The voltage magnitude is twice the specified rated voltage. It is applied for 1 minute. There should not be flash-over or puncture. This is a type test as well as routine test.

2. Impulse voltage withstand test : In this test, standard impulse voltage surge is applied to the insulator. Such surges are caused due to the lightning in practice. The standard lightning impulse wave is of 1.2 µsec wave front and 50 psec wave tail hence called 1.2/50 impulse wave. For this test, the generator developing the lightning voltage surges is used.

The impulse voltage at very high frequency of several hundred thousands Hz is applied to the insulator and the spark over voltage is noted down. The impulse ratio is defined as,

Impulse ratio = Impulse spark over voltage / 50 cycles per sec spark over voltage

This ratio should be 1.4 for pin type while 1.3 for suspension type insulators.

3. Puncture voltage test : In this test, the insulator is suspended in the oil and certain minimum voltage is applied. This value in case of suspension insulator is 1.3 times the dry flash over voltage. The insulator should not puncture under this test.

3. Environment and Temporary Cycle Tests

In these tests the insulator is subjected to the alternate temperature cycles, sudden temperature changes, pollution and some other environmental stresses. Out of these tests, for extra high voltage insulators sudden temperature drop test, extremely low temperature test and pollution test are compulsory tests.

In temperature cycle test, the insulator is heated in water at 70 °C for one hour and is then immediately cooled in water at 7 °C for another hour. Such three cycles are repeated. Then the insulator is dried. After this test, the glaze of the insulator should not be damaged.

4. Corona and Radio Interference Test

When the voltage stress level increases beyond corona inspection level, corona discharge starts. The corona discharge means the violet glow, hissing noise and production of ozone gas around a line. This causes inductive interferences with neighbouring communication lines. By providing suitable voltage grading rings, smooth surface and higher size of conductors, corona discharge and radio interference can be eliminated for certain voltage range.

5. Other Important Tests

These tests include,

1. Porosity test : This is also called destructive test. The insulator under test is broken into pieces and immersed in a 1 % solution of dye in alcohol under a pressure of 140 kg/cm2 for about a day. Then the samples are removed and inspected. The porosity is indicated by the deep penetration of the dye into it.

2. Proof load test : All types of the insulators are assembled and a tensile load of 20 % in excess of specified load is applied, for about one minute.

3. Galvanising test : In this test, galvanised metal parts of the insulator are tested for the strength of galvanization.

4. Corrosion test : The insulator is tested against the corrosion. In this test, insulator is suspended in a copper sulphate solution at 15.6 °C for one minute. Then it is removed, dried, cleaned and again put it in the solution. The cycle is repeated for four times and there should not be any type of deposition on the metal. Out of these tests, proof load and corrosion are routine tests. For special type of insulators, some more tests may be performed as per the decision of supplier and the consumer.

Review Question

1. Explain the various tests conducted on insulators.