Method C IEC Short Circuit Calculations- 20 Hz?

[rcw retired EE]

A co-worker stumped me with an IEC question.

ETAP software provides three methods for calculating short circuit currents. Method A gives approximate results, Method B provides more detailed results and requires more information. Method C appears to calculate the impedances at 20 Hz.

Our client is claiming that we should use Method C instead of B.

Is anyone familiar with Method C? Can someone shed light on the advantages of running a short circuit study calculation at 20 Hz?

Or do I not understand what Method C is since I am ANSI and not IEC?

Thanks,
R C Wilson

 

[dpc]

R C,

If you have Conrad St. Pierre's book, A Practical Guide to Short Circuit Calculations, he has a pretty good description of the various IEC methods. He also proposes an alternative approach that he feels is acceptable under IEC-60909.

Method C does use 40% of the actual reactance to compute the X/R ratio, but it also uses the base frequency X/R and uses the two together to come up with an adjusted X/R. This is supposedly a more accurate representation of real-world situation. But I'm not burdened by a lot of knowledge of IEC calculations, so I could be wrong.

If you're doing any IEC calcs and come from the ANSI world, I'd highly recommend getting his book.

 

[ijl]

Any of the methods A, B or C can be used to calculate the peak short circuit current, that is, the maximum value of the sum of the AC and the DC components. As dpc writes, the 20Hz calculation (or 24Hz in 60Hz networks) is needed only to get the X/R ratio, not the short circuit current itself. If the client prefers method C, let them have it. It seems that the standard considers the methods A, B and C equally acceptable and does not prefer any of them over the others.

The initial symmetrical current (the "proper" short circuit current), the breaking current and the steady state current are calculated at 50Hz or 60Hz.