Porcelain Insulators - String and Rigid type: testing
Before explaining about testing or inspection of insulators, we have to know about the classification of insulators based on its electrical strength. There are two types of insulators that has been classified for the purpose of identifying the tests to be carried out. They are type A and Type B. All tests need not be done on all the insulators
Type A insulators are those insulators in which the length of the shortest puncture path thro the solid insulating material is equal to the half the length of the shortest flash over path outside the insulator ..
Type B insulators are those insulators in which the length of the shortest puncture path thro the solid insulating material is equal to less than the half the length of the shortest flash over path outside the insulator ..
Tests are classified based on the types. As per the standards, Class A insulators are subjected to only mechanical tests during the routine or acceptance tests and all the Electrical tests are prescribed as type tests on them
Class B insulators are subjected to both electrical and mechanical routine and acceptance tests
Tests on string (disc) insulators and Pin (Rigid) insulators :
For Disc Insulators and Pin Insulators : The relevant specification for these insulators is IS 731. As per this,
Routine tests are :
Mechanical routine tests (for string insulators)
Electrical routine test ( for string and Pin insulators)
Routine tests are carried out by the manufacturer's quality control department and is expected to carryout all the above tests on each insulator
Visual examination is carried out mainly to ensure that the surface finish is good and the the firing is proper, Any kind of surface defects can be detected by visual examination.
Mechanical tests : This test, as a routine test is carried out on every disc insulator. Disc insulators are subject to a load equal to 60% or 40% depending on the type of the insulators. type A insulators are applied with 60% Load for 1 minute where as type B is applied with 40% load for 10 seconds. The ones that can not withstand these loads are considered failed the test
Electrical routine tests :
Here, a voltage that can produce continuous flashover on the insulators is applied for 5 minutes. During this period, no insulator shall fail. A failure occurs when the the insulator conducts the electricity through it. Once it fails, there will not be any flash overs observed. These insulators are rejected
Acceptance test :
These tests are carried out on randomly selected insulators in presence of the Buyer's representative. Sampling plan according to the number of insulators is recommended in the IS/IEC standards.
Tests are :
3)Temperature cycle test
4) Mech performance test ( instead of Twenty four hours mechanical strength test )
5) Power frequency Flashover test
6) Puncture test
7) Mechanical failing Load test (Type A)
8) electromechanical failing load test (type B)
9) Porosity test
All tests are not done on all the sampled insulators. The sampled insulators are divided into two parts and certain tests are carried out on the first part and other tests on the second part
Visual examination, Dimensional verification, Temperature cycle tests and the Power frequency flashover tests are carried out on all the samples
Dimensional verification : Dimensions of an insulator shall be verified with the help of the drawing. The major parameters of dimensions to be checked are a) The total creep age distance b) Shed diameter , c) Total length of the insulator, d) Dimensions of the metal parts. Creepage distance is the total length of the insulator over which a potential difference has to travel to make a flashover to the nearest metal part. This is measured using a glued tape that is stuck all along the surface from one point to other even touching the inner sides of each sheds. This tape, then removed from the insulator and measured the length with a tape scale
Temperature cycle test : Insulators are subject to sudden temperature variations at site as and when there is a sudden climatic change. Rain may occur on a hot dry day suddenly and due to this the surface of the insulator may be suddenly cooled down from a higher temperature to a very low temperature. Since ceramic insulators are brittle, this sudden deviation may cause breakage.
Construction of insulators shall be in such a way that sudden change in temperature does not cause any damages to the insulators. This test is carried out by immersing the sampled insulators in the hot and cold water alternately after a regular intervals. For string and Pin insulators, they are immersed in a hot water first for 15 Minutes and then immediately transferred to cold water. It is kept there for 15Minutes and then transferred to hot water. This cycle is continue thrice in hot and thrice in cold. The difference between the hot water and the cold water shall be maintained at 70 or above degree Celsius. After this test, insulators may be visually examined for any cracks on the surface. There shall be no cracks on the surface
Power frequency flashover test :
This test is carried out using a voltage at power frequency. This test is post test of the temperature cycle test. So, in case the insulator is cracked in the temperature cycle test, during the flashover test, it may fail.
All the sampled insulators are subjected to a high voltage that will produce continuous flashover on the surface of the insulators. This voltage is kept for 1 minute. (There is no specific value of voltage mentioned in the standards) During this test no flashover shall occur through the core of the insulators. A failure occurs when the the insulator conducts the electricity through it. Once it fails, there will not be any flash overs observed. If any failure occurs, double sampling shall be selected and tested as mentioned in the IS
Twenty four hours mechanical strength test
Each insulator has a maximum failing load capacity. Insulators are designed to carry a certain load depending upon the site conditions and the conductors that are drawn through them. Normally string insulators are designated with maximum failing load in KN which is the maximum pulling load that can withstand. Whereas in a pin insulator, the failing load is a canti lever load that may occur on the insulator due to conductors drawn over them.
This test is done on Class B string insulators only. A tensile load equal to 2/3 of the specified failing load is applied on the string of insulators and maintained this load for 24 Hours. During this test, no insulator shall fail to carry the load. After this test all insulators will be subjected to five minutes power frequency flash over test.
However, this test is carried out only if the Buyer specifies
Mechanical Failing load test
This test is done on Class A disc insulators. A tensile load equal to its specified failing load is applied on the insulators. During the test no insulator shall fail to carry the load. This test is carried out on one part of the sample as mentioned above. Duration of the test is momentary .
Electromechanical Load test
This test is done on the class B type of string insulators. Here , as the name of the test indicates, Mechanical and electrical stresses are applied simultaneously. While the rated tensile load is applied, simultaneously a potential difference that is around 60% of the wet power frequency high voltage test value is applied. The applied potential shall not collapse during the test. It is tested instantaneously. This test is done on one part of the sample
Puncture test :
This test is done on the Class B of the String insulators only. The other part of the sample is subjected to a high voltage. The value of which is equal to 3 times the voltage applied on the insulators to create flashover (refer Power frequency flash over test above) . To avoid external flashovers or discharges, this test is done after immersing the insulator in an insulated medium. Generally transformer oil is used for this. The string insulator of class B is subjected to this test. High voltage is applied on one metal part and the other part is earthed. The insulator shall withstand this voltage for a moment. The insulator is considered failed if the voltage collapses abruptly
Metal parts after this test is removed for Galvanization test.
Galvanization test :
This test is done to ensure the uniformity of the zinc coating on the metal parts. Normally the metal parts are zinc coated in order to prevent rusting. The material used to make socket and pins in the insulators are made of cast iron.
Copper sulphate solution is the major agent in this test. copper sulphate is dissolved in water and made a solution with a specific gravity of 1.198. This solution is maintained at a temperature of +1 or - 1 .186 degree Celsius. Metal parts are dipped in this solution for 4 times for one minute duration each. After each dip, the metal parts are taken out and washed and again dipped. After the completion of the 4 dips, there shall not be any copper deposits on the metal parts. In case iron get in contact with CuSo4, then Fe reacts wit CuSo4 and Cu is formed. This is deposited on Fe. This copped deposit small not be removable with the finger nails or the blunt side of a knife > >
Insulators made out of clay may have pores, if the process in every stage is not proper. Especially, firing in the kiln shall be the best. To ensure that there are no pores in any part of the insulator, a few pieces broken from one part of the sample insulators are taken and immerse in a dye called Fuschin solution. This solution with the insulator pieces are subjected to a pressure, the product of the total duration and the applied pressure in Kpascal is equal to 1880. Normally, as a thumb rule, a pressure of 600 Kg/sqcm is applied for 3.25 Hours which is equivalent to 1880.
Tests on Pin insulators.
- Visual Examination
- Dimensional check
- Temperature cycle test
- Power frequency flash over test
- Puncture test
- Mechanical failing Load test
- Porosity test
Like in String insulators, sampling plan is applicable for pin insulators too. The sampled quantity is divided into two parts as mentioned in the standards and tests are divided among the two parts
Visual, Dimensional check, Temperature cycle test and power frequency flashover test are carried out on all the samples. Puncture test is conducted on one part and mechanical load is done on another part. Porosity is conducted on the insulators that has been subjected to mechanical failing load test
Mechanical Failing Load test :
For a pin insulator, the nature of load occurring on it is mainly of bending load. So, while the Maximum failing load is mentioned, it is the maximum cantilever load that can be applied on the insulator. Bending load is applied using a hydraulic loading machine that has been connected through a load cell. A load cell is a transducer that converts the force into electrical pulses and this indicates the applied load in KN or Kg
All other tests are carried out as mentioned in the case of String insulators Type tests applicable for Disc and Pin insulators The most common type tests conducted on string and pin insulators are: Lightning Impulse test Visible discharge test
Lightning impulse test
This test simulates a lightning that may strike an electrical system in which insulators are a part. A lightning impulse of 1.2/50 wave shape is applied on one end of the metal part and the other metal part is earthed. During this test, the insulator shall not puncture.
Visible discharge test
This test is conducted in a dark room. Power frequency voltage equal to its Flashover voltage shall be applied on the insulators. The insulator under test is visually observed during the test for any visible discharges from the insulator to the earth points. There shall not be any discharges