Generator Reactances

[mbk2k3]

I often run into power system studies where I have the following setup:

generator that is rated at a certain kVA (lets say 1375kVA) some refer to it as "type load".
genset output, usually a lower kVA obviously (lets say 846kVA).

the generator manufacturers provide reactances in the format of:
- unsaturated reactances based on genset output (includes Xd,Xq,Xd',Xq',Xd",Xq",X2,X0,Xlu,Xrlu)
- saturated reactances based on genset output (includes Xd,Xq,Xd',Xq',Xd",Xq",X2,X0,Xls)
- smaller set of unsaturated reactances at a "type load" (Xd only)
- smaller set of saturated reactances at a "type load" (Xd,Xd',Xd",X2,X0 only)

So my questions:
1. I've read that I'm supposed to use Saturated reactances in ETAP/Short Circuit Modeling. Why is this the case?
2. In my mind, I should be using the reactances based on genset output (since my prime mover limits the kVA output to a lower value). Is this correct? Or should I be using the reactances based on "type load" which I think refers to the full kVA rating of the generator?

I wouldn't mind some good IEEE (or other) references to read up on this.

 

[davidbeach]

A loaded machine will be in the saturated region prior to the fault, so that's why they're typically used. If you're interested in faults on a unloaded machine then use the unsaturated values.

Other than saturated/unsaturated, the kW loading has little impact on the fault values. Reactive output will influence fault currents, but by changing the internal voltage rather than changing the impedances.

Mostly just use the saturated values, hopefully you have an R value to go with all those Xs, and don't worry too much about the rest. Fault clearing time will determine whether the impedance of importance is synchronous, sub-transient, or transient.

 

[RRaghunath]

With regard to your 2nd question, the reactances given in generator data sheets also indicate the reference kVA which is the basis for these % or pu reactance values.

Rompicherla Raghunath