new ncees #530

[cabby]

I can come up with the 480v Base Isc of 1203A. However, I do not understand how they are obtaining the p.u. Isc? 1MVA is given, transformer has a 4% impedence, I assume that is the .04. But where are they getting the .025?

thanks,

 

[Flyer_PE]

Since you are given the fault capability of the system in MVA the impedance is 1p.u. on a 40 MVA base. The value needed is the utility impedance on the transformer base (1MVA).

Zpu-tx = Zpu-utility*MVAtx/MVAutility

Zpu-tx = 1*1MVA/40MVA = 0.025

Iscpu = Vpu/(Ztx-pu+Zutility) = 1/(.04+.025) = 15.4 pu

 

[cabby]

Thank you. I got it now.

Since you are given the fault capability of the system in MVA the impedance is 1p.u. on a 40 MVA base. The value needed is the utility impedance on the transformer base (1MVA).
Zpu-tx = Zpu-utility*MVAtx/MVAutility

Zpu-tx = 1*1MVA/40MVA = 0.025

Iscpu = Vpu/(Ztx-pu+Zutility) = 1/(.04+.025) = 15.4 pu