Percentage Impedance of Power Transformer

"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 percentage 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.
The impedance of a transformer can be measured by means of a short-circuit test.
The secondary of the transformer whose percentage impedance is to be measured is shorted.  The voltage on the primary is gradually increased from zero till the secondary current reaches the transformer's rated value. 
The percentage impedance of the transformer is calculated as
Z%= (Impedance Voltage/Rated Voltage)*100
Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.
The percentage impedance of a transformer a crucial parameter when operating transformers in parallel. It also determines the fault level of a system during faults. 


Importance of Percentage Impedance (%Z):

1. It decides suitability of transformer to the system
2. It limits Short Circuit Current (Inversely)
3. It causes voltage drop (Directly)

For transformers up to 2 MVA it is around 5%, Above 2 MVA it varies up to 25% also.

Note: %Z is depends on Resistance value (%Z = V R^2 + X^2). Resistance value may affect with temperature, so, all resistance and Impedance values must convert in to standard temperature of 75%.