The percent impedance (%Z) is the percent of the rated load impedance possessed by a transformer. The percent impedance is important in that it allows us to:
- Calculate available fault currents (both individual and bank).
- Determine whether two transformers are suitable for paralleling.
In the last chapter, we learned how to determine the short-circuit voltage of a transformer. We now can use that voltage to determine the percent impedance:
Erated/Eshort circuit = %Z
To calculate the fault current available from a transformer if a dead short occurs across the secondary terminals, use the formula:
Irated (Secondary)/%Z = Fault current
Remember to use the percentage as a decimal not the full number. For example, 2.5% is actually .025.
How to calculate fault current using %Z:
What is the available fault current of a step-down transformer rated at 50 kVA, 1200 V – 120 V and 2.75% percent impedance?
I(full load) =
I(full load) = 417 Amps
I(short circuit) =
I(short circuit) = 15,151 Amps
A transformer is rated 20KVA, 4800 V – 240 V. With the secondary short-circuited, it takes 96 volts to reach the rated primary current (4.2 amps). Determine the %Z and the available fault current.
%Z = 2
I(rated secondary) =
I(rated secondary) = 83 Amps
I(Short circuit) =
I(Short circuit) = 4150 Amps