% Impedance

Dera Sir,

The transformer % impedance of 250 kVA, 11kV/0.415kV can be defined by testing in manufacturer shop test. The manufacturer itself shall follow the standard for a certain kVA rating.

Refer to Tabel 1 of IEC 60076-5, Impedance voltage at rated current, given as percentage of the rated voltage of the winding to which the voltage is applied.

So,the above transformer (250 kVA)shall follow transformer kVA range rating "Up to 630 kVA" and the impedance voltage is 4%".

The manufaturing tolerance on the above transformer percent impedance voltage is ± 10% for two winding transformers.

For short-circuit caculation we can use this value if there is no other information of transformer impedance available.

Regards,

If you are using the ANSI standard, you could use 5.2%.. You could solve it if you have the allowable maximum short circuit current you expect the transformer to hold. But always see to it that it falls in the range given by the standards you are using.

According to me if you want to calculate the % impedance of 250KVA then u have to give us details of short circuit test like value of load loss at ambient temperature, applied rated current , ambient temperature and wheather transformer winding is of copper or alluminium. According to IS Standard:2026 rated % impedence is 4.5% +/-10% IS Tolerence.