Generator Fault Current Contribution

[mbk2k3]

Say you are doing a hand-calc for the fault contribution of a generator.

If an alternator is rated for 2000kVA, but the genset max output kW is 1500kW at a worst case PF of 0.8:

Which number do you use for short circuit fault contributions? The 2000kVA or the 1875kVA (1500/0.8)?

I would suspect that the fault contribution by the alternator is determined by the construction and winding impedances rather than the genset driving power.

I've asked two alternator manufacturers, and got conflicting answers: one said you can't contribute more than the genset driving power, and the other said genset driving power doesn't matter, its the inherent characteristics (impedances/reactaces/construction) of the alternator that determines the fault contribution.

So which one is it?

Thanks.

 

[RRaghunath]

2000kVA.
The generator parameters like Xd" are all referenced to kVA of the generator.
Hence, it is 2000kVA or whichever kVA to which the generator reactances are referenced to will be considered in SC calculations.

 

[waross]

The initial fault current will be driven by inertia of the set.
The fault current is highly reactive and the undersized prime mover will generally be able to support the fault current.
2000 KVA at 5% or so Power factor is not much KW.
But compare the actual voltage with the voltage at which the 2000KVA is rated.
I had to deal with undersized sets a couple of times when sales men supplied sets at the required KVA but at the wrong voltage.
eg: 50 KVA at 240 Volts is only 43.3 KVA at 208 Volts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mbk2k3]

thanks guys for the input!

I was confused because the impedance the genset provided gave was the adjusted impedance based on genset output.