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TESTING OF BUSHINGS

POWER FREQUENCY TESTS

POWER FACTOR–VOLTAGE TEST:

  • In this test, bushing is set up as in service or immersed in oil.
  • It is connected such that the line conductor goes to the high-voltage side and the tank or earth portion goes to the detector side of the high-voltage Schering-bridge.
  • Voltage is applied up to the line value varied in steps and then reduced.
  • The capacitance and power factor (or tan δ) are recorded at each step and their characteristics are drawn.
  • This is a normal routine test but sometimes may be conducted on percentage basis.

INTERNAL OR PARTIAL DISCHARGE TEST

This test is used to find the deterioration or failure due to internal discharges caused in the composite insulation of the bushing.

  • The voltage versus discharge magnitude as well as the quadratic rate, gives an excellent record of the performance of the bushing in service.
  • This is a routine test for high-voltage bushings.

MOMENTARY WITHSTAND TEST AT POWER FREQUENCY:

This is done as per the Indian Standard Specifications, IS: 2099, applied to bushings.

  • The bushing has to withstand without flashover or puncture for a minimum time( ~ 30 s) to measure the voltage.
  • Presently this test is replaced by the impulse withstand test.

ONE MINUTE WET WITHSTAND TEST AT POWER FREQUENCY

Common and routine tests used for all electrical apparatuses are the one-minute wet, and dry voltage withstand tests.

  • In wet test, voltage specified is applied to the bushing mounted as in service with the rain arrangement.
  • A properly designed bushing has to withstand the voltage without flashover for one minute.

VISIBLE DISCHARGE TEST AT POWER FREQUENCY:

This test is used for determining whether the bushing is likely to give radio interference in service, when the voltage specified in IS: 2009 is applied.

  • No discharge other than that from the arcing horns or grading rings should be visible to the observers in a dark room.
  • The test arrangement is the same as that of the withstand test, but the test is conducted in a dark room.

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