Partial Discharges in a generator

[Yuma]

We have open a hydrogen-cooled 350 MW generator for inspection. To gain access to the basket winding, we have to remove an inner shield which is made of glass fiber and whose purpose is to conduct the flow of hydrogen. This shield is rather close to the basket winding, only a few centimetres of separation. Now,once the shield is removed, we found some stains and deterioration on one small area of the winding, and the same kind of stains in the part of the shield which is right in front of that area of the winding.

I am told that this damage has been caused by partial discharges / corona effect. It would make sense to me if the shield was made of iron/steel, but it sounds strange to me that you can have partial discharges from a coil to a glass fiber shield. Do you think this is possible?

On the other hand, I can't think of any other way that damage could be produced - rubbing between the coil and the shield could be another possibility, but both are static parts, and the damage doesn't really look like rubbing anyway...

Any ideas will be welcome.
Thanks

 

[edison123]

PD is known to occur even in the end winding areas where there is nothing but insulation. It may be due to contamination or insufficient airgap between the coils. In your case, it is porbably due to insufficient gap between the shield and the nose of the basket winding.

Have you talked to the OEM ? Is the stator voltage around 15 KV ?

Muthu

http://www.edison.co.in

 

[Yuma]

Thanks Muthu,

The voltage is 17 kV and, yes, the OEM thinks like you that the gap should be bigger in order to avoid PD. It's just that I wanted to have some more opinions about it. If you say that it is possible to have PD towards a non-magnetic material, then it's fine. That was the point I wasn't very sure about.

 

[edison123]

The OEM thinks only now that the gap should be bigger ?

What is the gap anyway ?

PD is more about small packets of air getting ionized and breaking down under excessive dielectric stress than about insulation per se.

Muthu

http://www.edison.co.in

 

[electricpete]

From my understanding pd is less common in hydrogen cooled machine than aircooled machine.

In 13.2kv air-cooled motors, I have seen the fiberglass air baffle literally touching the end turns. There was no pd in that case.

Also (again in air cooled machines), there is non-conducting material laid between phases in the endwindings (surge ring, blocks and ties). No pd occurs there in general.

I have a hard time understanding why you would see pd. The one possibility I can think of is there is some conductive contamination on the surface that facilitates some kind of tracking between phases.

To confirm I would try to identify the voltage source and see if it follows a pattern between phases (I assume there is no ground plane in sight). Also if it is pd it should be most evident at locations between line end coils, less evident toward the neutral. If you can't see any of these patterns I personally would be skeptical that it is pd. DISCLAIMER: I don't inspect hydrogen cooled machines... not claiming to be an expert.

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[ScottyUK]

I agree with Pete - pd activity in a hydrogen machine is really unusual. I won't say 'impossible', but very unusual. With a gap of a few centimetres in hydrogen I would say that you are seeing something other than PD activity. From the rating of the set I guess this is not a water-cooled stator - correct?

Got any photographs?


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