NCEES #513

[jcbabb]

Has anybody else worked #513 without using per-unit analysis?

I've searched the forum for examples without any luck. The per-unit solution in the NCEES book does not help me at all.

I worked it without the PU conversion and came up with an answer of 418A (the answer given is 415A). My solution took a total of 2 lines and seemed much simpler than the PU analysis.

Because the transformer impedance is based on the percentage of voltage drop internal to the transformer I have:

V= 92.5% (or 100%-7.5%) of 12kV = 11.1kV

I=(V/sqrt3)/R where V=11.1kV and R=15.312ohms

Anything wrong with my assumptions? Did I get the right answer by dumb luck?

 

[pbo064]

Your assumptions are reasonable but you may run into issues on more complicated problems or if NCEES puts multiple choice answers that are more closely grouped together. I would highly reccomend the MVA method, covered in the link and also in the Camara Reference book. It is ideal for any problem that asks for short circuit current that would otherwise require per unit.

http://www.arcadvisor.com/pdf/ShortCircuitABC.pdf

 

[Flyer_PE]

Using PU analysis for this problem is discussed here. The MVA method gets the exact same answer.