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TG 2
Short Circuit
Jim Phillips, P.E.
Have you wondered is there a simpler way to calculate short circuit currents without a computer program? A very simplified method known by many as the“infinte bus” calculation method is a good way to approximate short circuit calculations. Three-phase transformers contain some valuable data on their nameplate such as the kVA rating, primary and secondaryvoltage, 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 shortcircuit current on the secondary bus of the transformer. help you fill the other 90 percent.

jim phillip

technical training group

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Short Circuit Calculations – Infinite Bus Method
Step 1 – Calculate the secondary full load current rating of the transformer: FLAsecondary = 1500 kVA/ 0.48 kVL-L x Sqrt (3) FLAsecondary = 1804 Amps Step 2 – Calculate the short circuit current on the transformer secondary bus. SCAsecondary = 1804 Amps x 100 / 5.75% SCAsecondary = 31,374 Amps Allof the variables listed above are: FLAsecondary kVL-L kVA3phase Sqrt (3) %Z SCAsecondary = Secondary Full Load Amps = Secondary Voltage in kV = Transformer Three Phase kVA = Square Root of Three (1.73)= Percent Impedance of Transformer = Short Circuit Amps at Secondary Bus

This process only has two simple calculation steps: Step 1 – Calculate the secondary full load current rating of thetransformer: 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 Here is an exampleof the calculation: The existing transformer is rated 1500 kVA with a secondary voltage of 480Y/277V and an impedance of 5.75%.

A few words of caution: The impedance of a transformer must be the...
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