Fault Current Calculation For the Transformer

This formula can be used to determine the secondary-fault interrupting rating required for the substation transformer protective device, to properly protect a particular size and impedance of transformer:I = (57.8P) / [(%Z)E]The formula assumes an infinite (zero-impedance) source.P- power in KVA%Z Percentage ImpedanceE Secondary voltage

Where:I=Inherent secondary-fault current, amperesP=Transformer self-cooled three-phase base rating, kVAE=System phase-to-phase voltage, kV%Z=Percent transformer primary-to-secondary impedance, referredA protective device is appropriate for the application if its secondary-fault interrupting rating is equal to or greater than the value for I calculated above.EXAMPLE:The inherent secondary-fault current for a 20/40/50-MVA, 115-kV, 8%-impedance transformer would be:I = [(57.8) (20 000 kVA)] / [(8) (115 kV)] = 1257 A