Answer / raul

percentage or per unit impedance = (Acutal impedance of the
transformer) divided by Base impedance .
1) The percentage impedance basically defines the short
circuit capability of the transformer bcoz short circuit
current is inversely proportional to the p.u impedance of
the system.
2) Secondly, it defines the voltage regulation of the
transformer bcoz max. voltage regulation is directly
proportional to the p.u impedance of the transformer.

So a high p.u impedance will reduce the short circuit level
and on the other it will result in poor voltage regualtion.
And a low p.u impedance will result in higher short circuit
level and good voltage regulation.

So the transformer are usually designed with a moderate p.u
impedance of say 5.75% to 8.75% to achieve both the low
short circuit level and good voltage regulation.

Answer / shahid bhutta

The impedance of a transformer is defined as the percentage of the drop in voltage to the at full load to the rated voltage of the transformer. This drop in voltage is due to the winding resistance and leakage reactance.
Alternatively, the percentage of a transformer can be described as the percentage of the nominal voltage in the primary that is required to circulate the rated current in the secondary.

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