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Fault Current Calculations in Electrical Systems

Transformer percent impedance is a measured value stamped on the nameplate is a voltage measurement. It's a tested value that the manufacturers do for power distribution transformers and is used in the . This is important for coordinating over-current protection devices (OCPD), short circuit analysis, harmonic analysis and arc flash studies.

The percent impedance is the percent of the rated voltage required to cause the rated current to flow when the secondary windings are short-circuited at the rated voltage tap and frequency.
Percentage impedance listed on a transformer nameplate.

Click image to see the nameplate.

Example of Percent Impedance

If a transformer has a 6.33% impedance, it would require 6.33% of the input primary voltage to cause 100% of the rated current on the secondary windings when a worst case fault occurs. In electrical distribution systems, the worst case fault is when a low impedance metal bar shorts the lines and is called a bolted fault.

Now if 100% of the voltage is applied to the primary input, then approximately 100/6.33 = 15.8x rated current would flow in the secondary winding under a worse case fault. This is the maximum short circuit current that we would have in your system.

Percent Impedance Testing

The worst case fault condition is tested with the secondary leads of a transformer bolted together, actually bolted with copper bars with an ammeter placed in series.

Very carefully, the voltage on the primary lines are stepped up until the secondary full load current is reached.

For example, this 2500 kVA, 12.47 kV to 600/347 V transformer:

FLA calculation example for electrical systems.

Once the secondary current reaches 2406A, a voltage reading is taken on the primary to see what input voltage is required to reach this full load ampacity on the secondary. In this case, the technician would read 789.35 V.

Doing a simple calculation:

Percent impedance calculation for a transformer.
Important note to electrical engineers - always read the measured % Impedance from the nameplate. On this unit in Edmonton, Alberta, the nameplate read 6.33 while the accompanying literature stated only 6%. It was a small difference but accuracy is important on electrical assessments.

Typical Percent Impedance Values

As electrical consulting engineers, these are the typical impedance levels we've seen on transformers.

What is a Transformer's Percent Impedance used for?

To rate and size our projection equipment properly, we need to know the maximum amount of fault current that our electrical distribution system can encounter. We are not going do this empirically for each transformer. That would be expensive and dangerous to the factory workers!

All transformers follow the laws of physics and we can use mathematics in electromagnetism to estimate the maximum fault current. Follow the math:

Using percent impedance to calculate the maximum fault current in a circuit.

The simple non-destructive percent impedance test on a transformer gives an accurate reading for fault calculations.