Generator Rotor inter turn short test.

[smnellore]

Dear all,

Recently we did pole comparision test to find out the inter turn short of the rotor winding. It is 210MVA Synchronous Generator. Basically in this test we inject 100V ac in to the rotor winding (with exitation brushes off and rotor stand still)and measure the voltage drop between the positive slip ring and pole cross over of the rotor winding and similarly between -ve slip ring and pole cross over. If there are no interturn short the readings will be same. If you find 5% difference between the readings it shows the inter turn short of the winding.
My question is let us suppose we have one turn short in the winding . If we inject 100V AC and consider rotor winding as a transformer then the shorted turn will act as secondary winding and a huge current will flow thru that turn and may damage the winding more. Can some body give their comments on my thoughts. Please correct me if my understanding is wrong.

 

[TurbineGen]

I could be wrong, but your theory seems correct. Will it damage the winding further on your test? Maybe. It depends on the system's impedance and the time duration of the test. If you do find a short, are you prepared to repair it? If not, it might be a better idea not to conduct the test.

We always used an impulse test. Each coil was isolated and shorted out to a charged capacitor and the waveform was recorded. The capacitor/pole winding becomes an LC oscillator. You will see a very different waveform if there is a short as the period will be shorter and the waveform will be noticably distorted. This test is non-destructive, but takes more set up time.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[edison123]

The rotor winding does not act as secondary winding since you are supplying it with AC. Since the rotor winding highly inductive, you won't get much of an overall current at 100 V though you may have high local circulating current at the turn shorts. Since the test duration is short (typically lees than a minute), there is no danger to the rotor winding per se.

Of course, if you maintain the voltage long enough, you can actually "smoke out" the faulty zone.

 

[davidbeach]

edison123, I think he is talking about the transformer effect within the rotor as the shorted turn functions as a single turn winding being excited by the remainder of the rotor. Similar to what happens with a shorted turn in a transformer.

 

[smnellore]

Yes David you are right. thanks for understanding my question.

 

[smnellore]

No answer from forum experts???

 

[pwrengrds]

From my experience with smaller generators (typically <50MVA) I wouldn't expect more than 10 amps AC, I would not expect 10 amps to hurt a rotor rated for 500 amps DC. I have run this test quite a few times looking for shorted turns, 5 amps was typical at 120vac.