Infinite Bus Method

A very simplified method known by many as the “infinite bus” calculation method is a good way to approximate short circuit calculations.

This article takes you thought the steps with an example of the infinite bus calculation process.

Three-phase transformers contain some valuable data on their nameplate such as the kVA rating, primary and secondary voltage, and percent impedance. With this minimal data, you can calculate the worst case short circuit current through a transformer. The calculation will provide the three-phase rms symmetrical short circuit current on the secondary bus of the transformer. This process only has two simple calculation steps:

Step 1 – Calculate the secondary full load current rating of the transformer:
FLAsecondary = ( kVA3phase ) / kVL-L x Sqrt (3)

Step 2 – Calculate the short circuit current on the transformer secondary bus:
SCAsecondary = x ( FLAsecondary x 100 ) / % Z

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