Generator steady state fault current

[KJvR]

I would appreciate some help on determining the steady state fault current for Diesel Generators. The Diesel Generators (415V) is feeding through step-up transformers (Dyn - 11/0.4kV) onto an high impedance earthed 11 kV network (isolated from utility supply). The manufacturer provides the Direct Axis Synch reactance (Xd) for the generator but also the "Three phase Decrement Curves". These curves indicate the AVR to control L-L fault current on 4.5 x nominal current. How would I calculate the steady state single phase to ground fault current for faults on the 11 kV network? Do I need the negative sequence dierect axis sync reactance or is it the same as Xd? What role does the AVR plays or will the Generator saturate for high impedance faults?

KJvR

 

[motorspert]

The AVR has a big effect on the current for a sustained short circuit. If the generator is at no load at the time of the short cct and excitation is kept constant then the sustaine current at the end of the short cct is 1/Xd - this is the definition for Xd.

For L-N I think the current is 1/(Xd+Xo) but I'll need to find a text book to check.
RBE

 

[KJvR]

Thanks for your reply. Will the generator be able to supply the fault current even with an high impedance fault such as an earth fault on the high impedance earthed 11 kV network or should I use 1/(Xd+Xo+XN1+XN2+XNo) where XN is the network impedance between the generator and the fault? Will Xo play any role in the fault current if you are using a Dy transformer. The L-G fault is on the HV side of the transformer and no zero sequence current passes through from the HV to the LV network.

KJvR

 

[mpparent]

Someone please correct me if I'm wrong, but the info. I have says:

I(L-N) = 3*Irated/(2*xq+xo){This is the maximum value}

Mike