Transformer Impedence

The % Z of a transformer define as The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reluctance expressed as a percentage of the rated voltage.

It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions.

The impedance is measured by means of a short circuit test. With one winding shorted, a voltage at the rated frequency is applied to the other winding sufficient to circulate full load current - see below:

The percentage impedance can then be calculated as follows:

Z% = Impedance Voltage x 100
Rated Voltage

The most economical arrangement of core and windings leads to a 'natural' value of impedance determined by the leakage flux. The leakage flux is a function of winding ampere turns and the area and length of the leakage flux path. These can be varied at the design stage by changing the volts per turn and the geometric relationship of the windings.

Thanks for your reply, now pl. one more thing what is impedence vltage & how we can calculate

impedance voltage is the applied voltage at rated frequince, please see the circuite, it is self explanatory.

For a transformer rated 20/25MVA ONAN/ONAF - percentage impedance is 12.5% - What is the base considered for specifying the percentage impedance ?? the ONAN rating or the ONAF rating ??

I think it is 25 MVA as the lower rating is a temp rise constrain and transformer can be loaded upto 25 MVA by using ONAF.