35kV Metal Clad Switchgear Partial Discharge 1

[AfricanAmber9]

thread238-439155
Our company has had its share of partial discharge issues within its 15kv and 35kv meatal clad switchgear. We've had a internal bus failure with 2 weeks of cleanup and re-covering the flat conductor with Raychem tubing. Other subs have had issues with the 'Stab' connection with the breaker into the Bus connection, the metal ring, Corona Ring, which is intended to mitigate partial discharge but actually influences or rather produces corona. I maybe co mingling my terms, but you get it. Right now on a 35kv that I am currently investigating, we have partial discharge located in back of our compartments. There is a barrier board and where the outgoing circuit and bus penetrate to the circuit breaker, we have a difference of material and this where we have the surface partial discharge. It created a smoky white substance on the barrier board, material breakdown I imagine. We have cleaned it, scrubbed it off with scotchbrite and put it back in service, where it subdued it for a while but slowly came back. I am trying to minimize clearances and looking for a long term solution for our problem for the next time we get in there. I am entertaining the thought of wrapping the bus even moreso, taking the insulating boot off and installing another Bus wrap directly over the bus. Suggestions are welcome if you have them, thank you.

 

[wcaseyharman]

We’ve had similar issues with 4000A 13.8kV non-segregated Busswork. In our case it was due to moisture intrusion, which led to a major bus explosion. Since then, better weatherproofing and ensuring the heaters are in service has seemed to prevent further issues.

 

[RRaghunath]

I too have found keeping heaters on and controlling humidity thus, in the switchgear room helps. Our company changed cable routing in to the switchgear from top, eliminating the cable trenches under the switchgear thus.
The observation there was water in the cable trenches, in spite of arrangements being there to pump out water that finds itself in to the trenches during monsoons.
Applying silicone coating over the insulators is also helpful.

R Raghunath

 

[davidbeach]

I would think that water in the cable trenches would be a good indication that the cables should enter from the bottom so that any water coming in along the cables doesn't have a path into the electrical equipment. Had a project once where I did exactly that and there was always a steady seep of water into the trenches from the cable entrances, wouldn't have wanted that into the top of the gear.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[AfricanAmber9]

wcaseyharman - That sounds so familiar. We had the same exact problem on our same size Bus at another sub. Heaters were actually turned off in the summer. And it was a common practice every summer, every sub. This I believe is a contributing factor and one I am hoping to minimize. Thanks.
RRaghunath - Looking into a silicon product from 3M and hoping this just might be the application for this. Thanks.

I appreciate the responses, when I am able to go forward with our repairs I will let you know how it turned out. Thanks again.

 

[Gr8blu]

Partial Discharge (PD) and Corona are two distinctly different entities. PD occurs INSIDE the insulation, wherever a void is present. Corona occurs OUTSIDE the insulation and is essentially a tracking phenomenon.

There are several things which influence the speed and amount of corona observed. One, as mentioned in previous posts, is the presence of moisture (which may be ANY liquid). Liquids may accumulate on the exterior of the insulation (leading to increased tracking, and thus corona) or on the interior (i.e. moisture wicking along between insulating layers and/or the conductor itself). A second is the routing of conductors - insufficient space between phases or between phases and ground plane will increase corona. A third is the elevation of the installation: as the air density gets lower, the voltage at which corona appears is also reduced (at typical sea level installations, corona starts around 7500 V. At 1500 m, it drops to 4000 V. At 10700 m - or a typical commercial aircraft flight path - it starts somewhere between 24 and 30 V.) And finally, a fourth influencer is the presence of conductive contaminants (other than the corona itself) - usually external to the insulation.

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