Transformer Short Circuit Rating at Terminals

[sykimk]

Hello Folks,

I am wondering if there are any requirements on short circuit ratings of transformer's terminals.

I have 63kA rated 4.16kV switchgear considering parallel operation of two 13.8kV/4.16kV, 15MVA transformers with Z=6.0%.

Of course, 63kA rating is far exceeding SC curent feeding from one transformer, but do I need to consider maximum 63kA at transformer's terminal or transformer internal due to parallel operation?

Please advise what the basis is used to design XFMR's SC withstand capability.

Thanks in advance.

 

[dpc]

Actually, for a through-fault with the transformers in parallel, neither transformer will ever see the total fault current - only its own contribution. If the fault is inside the transformer, you don't have to worry about withstand capability.

(If I understand your question correctly.)

 

[jghrist]

Transformers have to withstand full fault current, limited only by their impedance. A 15 MVA 4.16 kV transformer with 6% impedance has a short circuit withstand rating of 34.7 kA.