Testing of insulator sets concerns the electric characteristics. Requirements on the performance under power frequency voltage as well as lightning and switching surge voltages are verified using testing.

The design of the insulation has to guarantee that the required data corresponding to the selected insulation levels. Insulation levels on the other hand are taken from the Insulation Coordination study done on the transmission line system.

Testing of insulators might be done to verify these characteristics. But these are not mandatory, as per IEC 60 383-2.

During testing, insulators must be clean and dry. They should be stored on test laboratory ambient temperature for sufficiently long period so that temperature equalizing is reached before conducting the test. This is done to prevent condensation on the insulator surfaces.

Below are some of tests conducted on insulator sets, used in transmission lines.

Power Frequency Voltage Test
Power frequency voltage test is made under rain conditions and is foreseen only for insulators sets up to 245 kV. The power frequency withstand voltage applied is the specified value after its correction. It is applied for 1 minute and no flashover or puncture may occur.

The flashover voltage can be determined only as additional information or even as the result of the previous agreement, by means of increasing the applied 75% value of the withstand voltage by 2 steps up to the point when flashover occurs. The flashover voltage is determined as the arithmetic mean of the last five consecutive reading.

Fast Front and Slow Front Overvoltage Test
The Fast Front and Slow Front Overvoltage Test dry, simulating lightning surges, is required for every voltage level. While the Slow Front Overvoltage Test, wet simulating switching surge, is a standardized for insulators sets only for highest voltage equipment above 245 kV.

Either a standard 1, 2/50  microsecond lightning surge or a 250/2500  microsecond switching surge is applied with both positive and negative polarities. No damage may occur to the insulators, although slight traces on the surface or crumbling of cementing material may be allowed.

Radio Interference Strength Test
Radio interference  levels caused by corona at high voltage lines are important only at voltage above 245 kV. Because of this, the Radio Interference test is only an optional test.

Corona Onset or Extinction Voltage Test
As a guideline, the extinction voltage should at least 1.05 Us sqrt 3. Test are usually performed on a complete insulator sets.

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