reactive current inrush on induction generators 1

[Karim02]

Hi,
Could anyone tell me why does induction generators, like those installed on wind turbines,draw large reactive currents following a fault at its terminal ?
Thanks in advance.

 

[dpc]

"Following" a fault, or during a fault?

Power factor of faults is usually very low meaning there will be a lot of reactive power required. This is nearly always true and is not unique to induction generators.

Induction generators in normal operation require a lot of reactive power for excitation.

 

[Karim02]

Thanks dpc for your help.
My question was of course large reactive currents of induction generators during faults. I just read in a paper that in some cases, caps are unable to support reactive power requirement of induction generators during faults. So dpc, you said that power factor is very low during a fault, isn't there a physical explanation to that ?

 

[dpc]

Sure - during a fault, the only impedance seen by the source is the system impedance. This impedance is highly inductive (transmission lines, power cables, etc. So the fault current is highly lagging which is the same thing as saying it requires a lot of reactive power.

Actually, the load impedance is still there, but it is being shunted by the fault.

I think the paper may have been referring to external faults, not faults in the generator.

Induction generators will never be able to provide reactive power - they consume reactive power.

 

[jghrist]

Induction generators will never be able to provide reactive power - they consume reactive power.

While this is true in the steady state, induction machines (motors and generators) will contribute current to a power system fault and this current is inductive. Induction generators can be treated the same as motors in fault calculations.

 

[dpc]

Yes, I agree, for a few cycles at best.

 

[Karim02]

Yes, that's true dpc, the fault was an external one , somewhere in the neighbourhood of the point of common coupling. So following your explanation, not only induction generators draw large reactive currents during faults in the network, but Synchrounous Generators as well, only, SG, do have a reactive power source which make the situattion less problematic !

 

[dpc]

Right. It is the fault that is actually drawing the reactive current - the generators are supplying the current. With a synchronous generator, it can provide the reactive current and its voltage regulator can increase the excitation to sustain this contribution for an extended period of time.

If the synchronous generator gets its excitation power from the ac mains (such as with a static exciter), it can also experience voltage collapse for very close faults.

 

[Karim02]

Yes, that's what I was thinking about, it's actually the fault that is taking large reactive current due the inductive nature of AC lines. That's really great,dpc, thanks a lot, you really helped me clarify things in my mind.