Negative Sequence Impedance of an Induction Generator.

[Distribution73]

Hi,
I am trying to study the short circuit contribution of an Induction Generator. Even if the S.C. current will be sustained during a brief period of time, I would like to assess the amount of current it will be able to deliver.
I have the generator hort circuit reactance (Xd'') which I suppose could use to calculate the contribution during 3Ph faults. However for Ph-Ph faults I am unsure of what value to use as the generator negative sequence impedance:
I have read in different sources that during short circuit conditions an induction generator can be treated as an induction motor. In the book "Symmentrical Components for Power Systems Engineering" I have also found a model for the negative sequence impedance of a motor. It appears that, when stalled, Z2 is the same as the positive sequence and similar to Xd''.
This leads me to think that for short circuit calcs I could use Xd'' as negative sequence impedance when using simmetrical components to do an analysis.
Any feedback is much welcome.
Thank you

 

[davidbeach]

X2=Xd’’ is a workable approximation if nothing better is available.

 

[mc5w]

Short circuit current will not exceed the following:

1. The amount of current that it takes to saturate the iron structure of a generator. Source: IEEE Orange Book which effectively states that full coordination is impossible when running on a standby or emergency generator. This is because the output voltage of the generator will drop to zero when the iron is saturated.

2. Short circuit contribution of an induction motor is usually calculated to be 100% of locked rotor current. However, inrush current for an induction motor will be the greater of 200% of locked rotor or 1,400% of full load current. If the induction generator full load current is less than or equal to 1% of the interrupting rating of the least short circuit rated circuit breaker then US National Electrical Code deems motor contribution to negligible.

 

[dpc]

What David Beach said.

 

[Distribution73]

Thank you very much for the feedback. It is really helpful.
Thank you