Impedance voltage of a transformer is the voltage required
short circuit the secondary winding terminals of the
transformer and apply voltage gradually from zero at
primary winding.increase the voltage gradullay till the
full load current flows in the secondary.The voltage at
which full load current flows that voltage is the impedance
voltage of the transformer.This is required for short
circuit calculations.You can find on the transformer name
plate as Impedance voltage or percentage impedance.
This is the voltage required to drive full load current in a
winding of a transformer when the other winding is short
for example if percentage impedance of a 33/11kv transformer
is 6%. it means that 33x6/100=1.98KV would be required to
drive full load current in 33kv winding when 11kv winding is
The percentage impedance of a transformer is the volt drop
on full load due to the winding resistance and leakage
reactance expressed as a percentage of the rated voltage."
"It is also the percentage of the normal terminal voltage at
on side required to circulate full-load current under short
circuit conditions on other side."
The impedance of a transformer has a major effect on system
fault levels. It determines the maximum value of current
that will flow under fault conditions.
It is easy to calculate the maximum current that a
transformer can deliver under symmetrical fault conditions.
By way of example, consider a 2 MVA transformer with an
impedance of 5%. The maximum fault level available on the
secondary side is:
2 MVA x 100/5 = 40 MVA
and from this figure the equivalent primary and secondary
fault currents can be calculated.
A transformer with a lower impedance will lead to a higher
fault level (and vice versa).
The figure calculated above is a maximum. In practice, the
actual fault level will be reduced by the source impedance,
the impedance of cables and overhead lines between the
transformer and the fault, and the fault impedance itself.
Impedance voltage is calculated with short-circuit
test.which is very low compared to rated voltage. Impedance
voltage is the voltage which circulates the rated currents
when one of the two windings is shorted.