Sync Equivalent of an Induction machine

[GiSH]

Hey
Does anyone know any good references for the method of producing a synchronous equivalent of an induction machine?

 

[electricpete]

Can you describe a little further what you are doing?

Modeling something? If so what type of study?

Looking for a motor for an application? What application?


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[ccamts]

Hey,
I need this too, I am modelling two induction generators in paralle and I have to calculate their three phase fault current and single phase fault current.

 

[rcw retired EE]

Initial three phase fault current = locked rotor current. Model the induction generator as an induction motor.

Usually the induction generator/motor neutral is not connected to ground so it does not contribute to a single phase to ground fault.

 

[GiSH]

Ok, I'm looking at a generation project and (although its not ideal) i've been requested to give the Synchronous equivalent of the induction generator parameters.
It is for the purposes of modelling and so that the the model won't have to include a model of the Doubly Fed Induction Generator power electronic converter.

Any thoughts?

 

[electricpete]

It's been awhile since I messed around with fault currents but I believe the ungrounded generator can still contribute to SLG fault current if there are other ground-connected loads or transformers in the circuit.

One way I think about it is if the generator were feeding fault through a transformer with grounded wye on secondary. The fault tries to bring the faulted phase voltage to neutral/ground. But that would create unbalanced phase voltages and the generator is going to try to feed current to keep the phase voltages balanced.

Also I'm remembering the SLG symmetric components analysis would depend on three sequence networks in series. The Zero sequence network has in parallel all paths to ground. Other loads connected to ground in parallel can cause a non-infinite zero sequenc eimpedance. The ungrounded generator doesn't contribute anything toward decreasing this zero sequence impedance but it does provide the driving voltage source in the positive sequence network (connected in series with the zero sequence network) which drives the fault current.

I'm reaching way back and could be off base but that's how I'm remembering it.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Bung]

The power electronic converter means that an equivalent synch machine model is not possible. There will be little or no fault contribution as the electronics will back right off and limit the output.

Bung
Life is non-linear...

 

[GiSH]

Hey,
The DFIG only has power electronics on the Rotor Circuit so it is not totally isolated from the grid. But (and I assume this is why i'm being asked for this) the 'worst case' seems to be to consider a normal induction machine (I know, a very worst case indeed). So, if we ignore the fact its a DFIG how would you produce an equivalent synchrnous machine to a normal induction machine?

 

[Bung]

The only contribution an induction machine can make to a fault is the "dumping" of the magnetic field energy into the fault. This will depend on the machine time constants and its size, but in any case is only a few cycles. And I still think the electronics will have an impact - unless the electronics short out completely, the machine will never appear as squirrel cage (or short-circuited wound rotor) induction machine. So trying to find a synchronous equivalent of squirrel cage machine as a proxy for calculating fault contributions of a wound rotor machine with a simultaneous control system fault seems doubly flawed.

Perhaps something like IEC 909 or similar standards on fault level calcs could give some guidance?

Bung
Life is non-linear...

 

[Mstrvb19]

As I understand it, the DFIG is a kind of "partial syncronous" machine, in that the rotor circuit is boosted by the electronics but not to full synchronouse speed. I would think that if you pretend it is truly synchronous, this would be conservative, as a true synchronous machine would contribute to the fault longer. Perhaps if you use the locked rotor current to establish an equivalent impedance, you could enter this as the subtransient reactance and then enter high values for the transient and synchronous reactances during the fault simulation....