Percentage Impedance (Z%)
The impedance of a transformer is marked on most nameplates - but what is it and what does the Z% figure mean?
It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions
The percentage impedance can then be calculated as follows:
Z% = Impedance Voltage
Changing the Impedance Value
The Effect of Higher and Lower Impedances
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.
As well as fault level considerations, the impedance value also:
Sequence Impedance (Z1
As with all passive plant, the positive and negative sequence impedances (Z1 and Z2) of a transformer are identical.
However, the zero sequence impedance is dependent upon the path available for the flow of zero sequence current and the balancing ampere turns available within the transformer. Generally, zero sequence current requires a delta winding, or a star connection with the star point earthed. Any impedance in the connection between the star point and earth increases the overall zero sequence impedance. This has the effect of reducing the zero sequence current and is a feature that is frequently put to practical use in a distribution network to control the magnitude of current that will flow under earth fault conditions.