IEC short circuit Calculation

[HamidEle]

In IEC60909, 4.6.4, it says that in all cases for steady-state unbalanced short circuits, the flux decay in the generator is not taken into account and the folowing equations should be used.

Ik2=Ik2"
Ik2E=Ik2E"
IkE2E=IkE2E"
Ik1=Ik1"

It is totally different from three phase steady state short circuit, which decay after 30cycles.

In this case, steady case phase-ground fault is higher than three phase steady case fault. This means, we should be use steady case phase-ground fault, which is higher value to rate the SWGR based on short time circuit withstand. But some engineers insist that three phase steady case value is good enough. Do you agree with me?

Thank you.

 

[Modula2]

It appears that IEC does not have a method to decay the unbalanced fault currents, consistent with the way it is done for the three phase fault. Is that correct?

 

[HamidEle]

No. That could be true. Because the flux decay in the generator is not taken into account for L-G,L-L,L-L-G faults, which is indicated in IEC 60909.