Induction Generators Unbalanced Current 1

[jplinn]

A 3-phase induction generator has balanced terminal voltages but an unbalance in the currents. One phase current is about 25% lower than the other two. Voltages are balanced to the low-side of the step-up transformer.

What can cause this and what tests (simple) can be performed to find out what the problem(s) could be?

 

[buzzp]

Uneven loads could cause this. Have you checked into this?
Does history tell you that what your seeing is 'abnormal'?

 

[jplinn]

The induction generator is connected to a infinite bus. The bus voltages are balanced.

The current unbalanced condition is new.

 

[QUAVIET]

It has been my experience with three phase motors that when you have two phases pulling close to the same current and one pulling 25% lower then I look for a poor connection either at the peckerhead or the parts feeding the motor. A poor motor starter contact or bucket contact could cause this. Also I would meg the motor taps to ground and if there are more than three taps on the motor I would break all the taps and meg between windings. I assume that you have already ohmed the winding resistance looking for equal resistances from the motor starter/VFD.

 

[buzzp]

Outside of loose connections, or other obvious reasons there is not much else it can be except the loading. Are you able to confirm the currents on the high-side of the transformer with the transformer ratio? There may be other locations to confirm currents as well to perhaps indicate where there may be a problem.
We see unequal loading like this all the time on our generators and is a result of unequal single phase loads somewhere on the transmission/distribution system. It is not easily fixed but since you have not seen this in the past it could be indicated a potential problem. Sorry I can not help pinpoint potential problem areas. But check all connections with a resistance test and it may not hurt to check your transformer as well (doble or other).

 

[aolalde]

Identify the generator and load terminals. Record the currents.
Rotate the generator lines connected to the load.
If the unbalanced currents follow the generator terminals you have some impedance problem in the generator. If the unbalanced current stays in the load terminals, your “infinite bus” is unbalanced and the correction is out of your control.

 

[baltic]

Nobody mentioned so far disbalance that may come from excitation capacitors

 

[brmamor2]

I have some experience with this kind of problem, what kind of winding connection in the transformer you have? i beleive that a WYE-WYE connection will transmit the unbalance from medieum volatge to the generator bars and it should be better to have a WYE-Delta connection

Also, i beleive tha MG standard allow derating generator starting 1% of voltage unbalnace, it seems for starting as very restrictive limit but this will be uppon the strenght of the grid and i beleive for 5% voltage unbalance you have to derate your generator of about 25%