SC Current on Primary

[GMIEE]

If I am given a maximum fault current at a transformers secondary from the utility and the transformers impedance, how do I get the fault current that is seen at the primary side of the transformer (in order to get clearing times with the transformers protective device).

Xmfr: 3 phase, 150Kva, 12.47->120/208, Fault Current: 32,784 Amps, Impedance = 1.9%

Thanks.

 

[davidbeach]

If I'm punching the buttons on my calculator correctly you don't have 32,784 Amps of fault current on the secondary of the transformer, the most you have is 21,930 Amps based on an infinite bus connected to the high side of the transformer.

But for what ever fault current, the primary 3-phase fault current would be 208/12470 times the secondary fault current.

 

[tinfoil]

You didn't state if this is for a 3ph fault, or for some other condition. The phase-group of the xmfr may be important.

For example, a delta-wye-gnd transformer will have two primary phases contributing to a secondary fault, each at 1/sqrt(3) magnitude w.r.t the line-to-line turns ratio * sec current.