3. What is Impedance Voltage in Transformer?
Answer Posted / manoj kumar tiwari
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.
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