Single Phase TX Configured in Three Phase

[BigCity78]

If I have 3-single phase 75kVA transformer configured into a 3-phase bank does the individual fault current of each 75kVA transformer add up for the total fault contribution on the three phase system? (i.e. each 75kVA contributes 5kA, then 3-phase system would have 15kA)

 

[tem1234]

No, you must consider this like a 3 phase transformer of 3x75kVA=225kVA. Then with the impedance of the transformer (if the impedance of each transformer isn't the same, you can take the worst case depending on your application) you can calcute the short circuit current. It'll be less the 3x5kA.

 

[trevorm]

The single phase fault current will remain the same. The phase to phase fault and three phase fault currents will depend on positive, negative and zero sequence network connections.

 

[BigCity78]

Ahh, yes I see why I was so confused I was determining the impedance of the single phase transformers by mistakenly treating them as three phase instead of single phase, thus when I tried to use that impedance on the three phase system I was coming out much higher than I felt it should. Always the small things that trip you up. That and the drawings were calling out just the single phase fault contribution instead of the three phase.