impulse withstand and power frequency of a transformer

Your question is not detailed because there are two types of impulse teats and there are two types of power frequency tests. (1) Impulse withstand test are designed to test performance of electrical cables/switchgear/transformers/motors etc. with lightning impulse 1.2/50 and/or switching impulse followed by before and after readings of tandelta, insulation resistance, conductor resistance etc. (2) Power frequency tests are designed to test performance of electrical cables/switchgear/transformers/motors etc. with power frequency (50 or 60Hz) high voltage under dry or wet conditions. I think above answers your question.

thank you for your valuable answer

but in the case of transformer testing, both the tests are done. the impulse withstand test and power frequency test. what is the difference between these two tests???

Could I make a suggestion, do your own research!

During the regular operation of a transformer, it could be subjected to Power Frequency Over-voltages - due to, say, a High Voltage Line falling on to a Medium Voltage Line, etc. - and under such a condition, the transformer windings will be subjected to higher voltages, but with the same frequency as that of its nominal rated voltage. To ensure that the transformer winding/insulation/components would not fail under such conditions, a power freqyuency test is done on a transformer.

Also, the transformer could be subjected to Impulse Voltages - either a Lightning Impulse or a Switching Impulse and to ensure that the transformer winding/insulation/components would not fail under such conditions, an impulse withstand test is done.

But, I think the impulse withstand test is a Type Test and the Power Frequency Withstand Test is a Routine Test.

One thing to add to the above - the voltages used will likely be very different. Impulse tests (1.2/50) are usually carried out at 2, 3 or even 5 times the voltage of power frequency tests. See the relevant IEC standards for values to be used.

Lightning frequency Impulse tests are based on BIL. Usually, 6.35/11KV rated equipments are subjected to 10 shots of +/- 75KV impulses. Similarly, 11/11 KV rated equipements are subjected to 10 shots of +/- 95KV impulses.

High voltage test is a part of routine test as well as type test. Usually, routine tests are non-destructive and subjected for less time. Example 6.35/11 KV rated equipments are subjected to high voltage test (routine tests) with 17KV for 5 minutes. In type tests, high voltage test is destructive type test which is at 3 times U0 for 4 hours so 6.35/11KV rated equipments undergoes 20KV for 4 hours. Technically, life of test object is zero after type test. I think it helps to have some more information.

neatly answered sir.

may i know what are the difference between earth and non earth the body in both the power freq and impulse test ? also switch in what position required - open or close while doing this testing?