Short-Circuit Withstand Rating for Motors

[recs]

According to NEMA MG 1-2011 section 32.13, generators can withstand a maximum short-circuit current equal to the three phase fault current at its terminals for at least 30 seconds (unless otherwise specified by the manufacturer) when operated at rated kVA and power factor and with a maximum of 5% over voltage and fixed excitation.

The question is the following:

When the generator has a three-phase fault current at its terminal it will provide a variable three-phase short-circuit current, starting at a high value (X") then diminishing to a lower value (X') and finally reaching a low steady state value. This low steady state value is reached way before the 30 seconds and is a couple of times the FLA of the generator.

The point is that the withstand rating of 30 seconds of the short-circuit current (I assume is the X') is not real. The generator will never see this current. Why are we testing for this unreal situation?

I am missing a piece of this concept. Anyone can explain this question?

Thanks so much.

RECS

 

[7anoter4]

You are right, recs. However, the standard does not detail how to calculate the short-circuit current and in my opinion it is considered the change of the reactance in time. At the short-circuit start the generator has to be loaded at rated apparent power and power factor, an overvoltage of 5% and it has to keep constant excitation.
I think it is referred to a test-if any possible-and not to the calculation.