×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Partial Discharges in a generator

Partial Discharges in a generator

Partial Discharges in a generator

(OP)
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


 

RE: Partial Discharges in a generator

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
www.edison.co.in

RE: Partial Discharges in a generator

(OP)
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.



 

RE: Partial Discharges in a generator

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
www.edison.co.in

RE: Partial Discharges in a generator

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.

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

RE: Partial Discharges in a generator

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?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources