Wednesday, March 14, 2012

Solid core insulators : Testing

The standard specifications followed are IS 2544 and IEC 60168

Tests are categorised in three parts

  • Routine test
  • Acceptance test
  • Type tests
  • Routine test
  • Visual examination

Bending load test at 50% of the load for 1 minute in all 4 directions

  • Acceptance test
  • Visual examination
  • Dimensional verification
  • Temperature cycle test
  • Mechanical Failing load test
  • Porosity test
  • Galvanization test

Like in disc and Pin insulators, here too acceptance tests are carried out on sampled pieces. IS/IEC standards specify the sampling plan. The sample as per the standard for a lot of below 500 Nos of insulators is not specifically mentioned. Instead this is to be decided between the buyer and the supplier. Normally 1% of the total number of offered quantity is selected for tests

Again this samples are divided into two parts, if the sampled lot is more than one number. Sampled pieces are subjected to above tests

Visual and dimensional checks and temperature cycle tests are carried out on the full quantity of the sample. Then it is divided into two and one part is used for testing Mechanical load failing test and the other part is used for galvanization and porosity test

Mechanical failing load test

Every solid core insulator is subjected to different kinds of stresses at site. Depending on the equipment size and the site conditions, the load on the support insulators may vary. So, according to the maximum load that may occur on the insulators, the failing load requirement shall be fixed. This load is mentioned in Kilo Newton.

The solid core insulators are fixed on a platform with nuts and bolts. A load equal to the maximum cantilever load in KN as specified in the purchase order is applied on the top metal part radially. This is a cantilever load. This load is kept on the insulator for 1 minute. During the test no damage shall occur to the insulator, that means, the applied load shall not come down drastically. This test is done on one part of the sampled lot. Solid core insulators, according to the voltages in which they are used are used in two to four stacks. For a 145 KV system generally one unit is used where as for a 245 KV system two units in a stack shall e used. While cantilever load test is done, the test is to be done on the full set bolted together.

Galvanization and porosity tests are done on the other part of the sample. tests are carried out as described in the earlier write up on Disc and Pin insulators

Type tests :

  1. Lightning Impulse test
  2. Wet power frequency voltage  withstand test (for outdoor units)
  3. Dry power frequency voltage withstand voltage test (for indoor units)
  4. Lightning Impulse test : This has been explained earlier

Wet Power frequency test :

For an out door unit, the voltage withstand capacity shall be higher since the insulators are exposed to various climate conditions including rain. Hence, this test is conducted simulating rain and the insulator is kept in the rain. A power frequency high voltage is applied accross the insulator. There shall not be any collapse in the applied voltage. Values of voltage are mentioned in the standards

Dry Power frequency test

This test is carried out on the insulators that are used for indoor purposes. Values of voltages that is to be applied across the insulators are mentioned in the IS/IEC standars. This test is done on the insulators as mentioned elsewhere in this writeup

Electrical Type test reports are valid for 1 years from the date of test. No need to repat the type tests on the insulators that are manufactured with the same design with which the type tested insulator has been made

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