Generator fault current

[EC Dan]

It cancels in your math, but why are you using the genset kVA? Shouldn’t this be based on the alternator base rating?

I followed the Cummins white paper PT-6008 which says to convert the p.u. reactance from the alternator base to the genset base. One it's in the genset base I should then use the genset kVA. But you're right, since the genset kVA cancels out the conversion isn't even necessary.

 

[xptpcrewx]

I followed the Cummins white paper PT-6008 which says to convert the p.u. reactance from the alternator base to the genset base. One it's in the genset base I should then use the genset kVA. But you're right, since the genset kVA cancels out the conversion isn't even necessary.

Just looked it up. Doesn’t make a whole lot of sense to me unless the alternator base given is different than the actual alternator kVA rating. I can’t imagine a different prime mover making a difference.

Anyway, since you already seem to have failing equipment, it would be a good idea to ensure you are actually maximizing fault generation conditions to expose any other problem equipment.

You can temperature correct cable resistance (for cold resistance values), adjust the driving point voltage to 1.05 pu, model all motor contributions and consider the worst case switching scenario. You’ll find the momentary duty will be higher than 17.2-kA.

What you don’t want to do is produce convenient results to avoid having to replace equipment. Let the pieces fall where they may. Establishing an upper limit with maximum fault generation conditions provides room for future expansion and other contingencies. Establishing this base line and uncovering issues is the whole point of a short circuit study.

 

[EC Dan]

I want nothing more than to get the most accurate values for maximum and minimum fault current.

I have one more question regarding motor contribution fault current on which I can't get a good answer. If the fault is anywhere on the primary side of a transformer, do the motor loads on the secondary side of the transformer add to the fault current? I would think so, but for example the Busssmann calculator doesn't seem to propagate that current back upstream across the transformer to the upstream fault locations.

 

[xptpcrewx]

I want nothing more than to get the most accurate values for maximum and minimum fault current.

I have one more question regarding motor contribution fault current on which I can't get a good answer. If the fault is anywhere on the primary side of a transformer, do the motor loads on the secondary side of the transformer add to the fault current? I would think so, but for example the Busssmann calculator doesn't seem to propagate that current back upstream across the transformer to the upstream fault locations.

Absolutely, motor contributions pass through transformers (be it upstream or downstream).

Another one of my pet peeves is when someone does a “partial” study only for new equipment and completely ignores the motor contributions from the rest of the system.