Power Frequency Tests of Insulators

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Power Frequency Tests of Insulators

  1. Dry Flashover Test
  2. Wet Flashover Test
  3. Wet and Dry Withstand Test

1. Dry Flashover Test

  • The insulator is in completely dry and clean state. It is connected to the metallic structure or tower as shown in Figure A.
    Dry Flashover Test Setup
    Figure A

    The power frequency source is connected across the insulator string (between cross arm and conductor). The power frequency source applies the test voltage of standard frequency to the insulator string for one minute. Then voltage is increased gradually till flashover of insulators occurs. The procedure is repeated for 4-5 times, the flashover voltages are noted. The mean of flashover voltages is taken. This mean of flashover voltage should not be less than specified power frequency voltage.

  • The observations are tabulated as below :

Standard test voltage =…. volt at … Hz for 1 minute.

Sr.
No.

Flashover
Volatge

Mean
Flashover Volatge

1.

 

 

2.

 

3.

 

4.

 

5.

 

2. Wet Flashover Test

Specified conditions for test :
Rate of waterfall = 3 + 10% (mm/min)
Direction of waterfall = 45° vertical
Water temperature = Ambient 15°C
Wet Power Frequency withstand test
Figure B
  • The test setup is shown in Figure B. Artificial rain arrangement is made maintaining the specified conditions of test. Power frequency source applies standard test voltage to insulator string (between cross arm and conductor) for one minute.
  • Then voltage is gradually increased till flashover occurs. After first flashover, the flashover voltage is noted and same flashover voltage is applied after 10 sec. Four to five observations are taken and mean of these flashover voltage is taken.
  • The observations are noted as below :

Time (Sec.) →

10

20

30

40

50

Flashover Volatge (kV)

….

….

….

….

….

Mean
Flashover Volatge = ….(Kv)

Rate of waterfall = …. mm/min.

Direction of waterfall adjusted 45°vertical.
Water temperature = ….°C
Conductivity of water = Microsimens
Standard test voltage = …volt at … Hz for 1 minute
3. Wet and Dry Withstand Test
  • The test is performed in wet and dry conditions separately. Before commencement of the test, the insulator is exposed to artificial rain produced as per IS 2071 (Part 1) – 1974 for at least one minute before application of voltage and there after throughout the test. The setup in exactly similar to dry and wet flashover test
  • The test voltage applied to the insulator is the specified value of the wet power frequency withstand voltage adjusted for atmospheric condition at the time of test.
  • The voltage of about 75% of the test voltage is applied and then increased gradually to reach the test voltage in a time not less than five seconds.
  • The test voltage is maintained at this value for one minute as per I.S. 2071 (Part II) – 1974. Two flashover at positive polarity and one flashover at negative polarity is permitted.

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