Short Circuit Calculations and Generator Pre-Fault Loading 1

[nealger]

I am hoping that someone out there can explain something that has been bugging me.

In my power system analysis class I learned that pre-fault loading of a synchronous generator results in an increase in the generator's "voltage behind the reactance", which in turn results in an increase in the generator's fault current contribution. Every text on power system analysis that I have seen backs this up. However, as far as I can tell, the ANSI/IEEE method for calculating short circuit currents completely ignores this effect. Can anybody explain why? I am assuming that this is not a fundamental flaw in the ANSI/IEEE method because it is an accepted standardized method that has been around for a while. I just want to understand the rationale for ignoring pre-fault loading effects.

Perhaps this is explained in ANSI C37.010, but unfortunately I don't have access to a copy and I am not inclined to spend $86 to get a copy unless I know for certain that it will answer my question. I do have access to both the IEEE Red Book and Buff Book, but neither has been any help on this point.

The only good explanation I have been able to come up with is that perhaps the test current for an ANSI rated circuit breaker is equal to the breaker's rated load current plus its rated short circuit current. If this is the case, then the effects of pre-fault load current would be built-in to the breaker's fault current rating and there would be no need to include pre-fault current effects in the calculations.

Thanks in advance for any help.

 

[bacon4life]

This is from IEEE Std C37.010-1999 Annex A
"This annex shows the derivations that permit the use of a semi-rigorous procedure for short-circuit current calculations. It avoids the intricate procedure required in the rigorous procedure while providing a degree of accuracy that is within the practical limits of short-circuit calculation considering the accuracy of constants that are usually available for such computations."

After quick glance through the standard, I did not see anything that directly answers your question. It does however mention that special care is needed when evaluating faults near generation, as the X/R ratio is high, and it changes as the reactance goes from subtransient to transient to steady state.

Typically this site is for work rather than school, so the thread may get red flagged.

 

[nealger]

bacon4life:

Thanks for the info on C37.010. The concern about the X/R near a generator being high makes sense (high X/R = high dc offset and slow decay), just not how the loading effects can be ignored without the calculated short circuit current being too low.

And for clarification, I have not been a student for a few years now. I am just trying to figure out why the way I learned to do short circuit calculations in school does not agree with the way they are actually done in practice. For the most part I have just been accepting the ANSI/IEEE method and getting on with my life, but I hate not totally understanding stuff like this.

 

[itsmoked]

That's the spirit!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com