Generator Positive Sequence Impedance

[tiewrc]

I got my generator data as below but not sure which value to use as Gen. Positive Sequence Reactance.
1.Xd (unsaturated)=2.46pu
2.X'd (unsaturated)=0.247pu
3.X''d (unsaturated)=0.188pu
4.X'dv (saturated)=0.225pu
5.X''dv (saturated)=0.137pu

what is the difference between saturated direct axis transient reactance and unsaturated direct axis transient reactance?

Can i assume my Gen Positive Sequence Reactance as Negative Sequence Reactance?


Please advice

 

[dpc]

If you are wanting to calculate maximum 1/2 cycle short circuit current, use the saturated X" dv. (Subtransient)

The X2 is not generally exactly equal to the X1 positive sequence, but the assumption is commonly made if no additional information is available.

 

[davidbeach]

To expand on dpc's comment - you can use X''dv(saturated) for X2 of the values given; it will be slightly less. You really need the q axis values also if you wish to fully quantify the generator performance. X0 will be even less than X2, can be considerably less.

 

[JJHorak]

Which you use depends on the time frame you are looking at. For a few cycles of fault current, use item 5. For time overcurrent response beyond maybe 10 cycles, item 4 gives a closer representation.
Generally I think the saturated is used. I think the difference is how the mfr tests the impedance. I think they get the unsaturated value by seeing how much excitation it takes to create rated current on a machine with shorted phase conductors, and hence it is at low excitation, and not too representative of normal excitation during a fault.