partial discharge erosion of grading/semicon interface on hv motor 1

[electricpete]

We have a 13.2kv motor in the shop. Visual inspection shows apparent partial discharge degradation at the overlap/boundary between the semi-con tape and the grading paint about 1" outside the slot. It appears this created a band which electrically isolates the grading from the semi-con.

Questions:
#1 - Is there easy repair available for this condition.
#2 - Is this condition highly destructive? From my perspective it defeats the purpose of the grading paint (to eliminate stress concentration at the end of the semi-con). But I have heard/read that others consider it to be a relatively insignificant problem.

 

[DougMSOE]

If you can physically see this on the outside of the insulation, it is not partial discharge, it is corona discharge. Partial discharge is internal to the insulation and requires instrumentation to "see".
Easy repair, well...
Clean up the area and re-paint with grading paint
or
Clean up the area, retape with semi-con tape and re paint with grading paint.
Corona discharge can be distructive but in most cases it is not. The idea with any repair of this nature is to relieve the electrical stress in that area.

 

[motorman]

There could be partial discharge and corona. The repair I have seen done is to seal the effective areas with silicone.
An interesting website:

http://www.infraspection.com/thermography/corona.html

 

[electricpete]

Thx for the comments motorman. From my perspective the concern is that electrical contact will be lost between the semi-con and the grading coating. That means the grading coating can no longer perform it's function. I believe the function of the grading paint is to provide a gradual termination of the grounded portion of the surface of the coil in order to prevent stress concentration which would arise if the ground plane is abruptly terminated. (similar to the practice of terminating a medium voltage cable in a stress cone).

So I would be inclined to think that silicon would not help, but reapplying grading and/or semi-con as suggested by Doug would help. Unfortunately there is a thick build of resin over the top of these stress control coatings (burnt away by pd in places), so it will be difficult to ensure good contact of the repair coating with the original coatings.