Percentage Impedance (Z%)
The impedance of a transformer is marked on most nameplates - but what is it and what does the Z% figure mean?
Definition
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 required to circulate full-load current under short circuit conditions
Measuring Impedance
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
Changing the Impedance Value
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.
The Effect of Higher and Lower Impedances
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.
As well as fault level considerations, the impedance value also:
 | determines the volt drop that occurs under load - known as 'regulation' |
| affects load sharing when two or more transformers operate in parallel |
Sequence Impedance (Z1
Z2
Z0)
The calculation above
deals with a balanced 3-phase fault.
Non-symmetrical faults (phase-earth, phase-phase etc) lead to more complex
calculations requiring the application symmetrical component theory. This
in turn involves the use of positive, negative and zero sequence impedances (Z1,
Z2 and Z0
respectively).
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.
End