Oil leak from transformer secondary bushing

[stzatzop73]

On site we have a 2 MVA oil filled transformer 6.3kV/0.42kV. There is a small leak form the bushing base connection to the transformer tank. We want to replace the gasket of the bushing. Is there any procedure for this kind of activity? I guess that the bushing needs to be lifted somehow, take out the old gasket and replace it with the new one. But also the tank oil will be exposed to atmosphere also we might need oil recirculation and o2 measurement and of course during the work execution to make a small kiosk around the area with the use of dehumidifier. Any proposal will be appreciated.

 

[electricpete]

How to do it: very carefully.

I don't think bushing diassebmly/maintenance is a typical field job. If you're doing it, you certainly should have on hand someone that has already done it. One paramount concern is that when you're done there is no moisture remaining in the bushing paper, oil or gas space.

There are other standard procedures for opening transformers, considering moisture exclusion, personnel safety etc. This also is something not to be done without someone onhand that knows how to do it.

Sorry if my response wasn't productive. In general I'm not a fan of motherhood responses and maybe I'm not quite understanding your question/situation. Also maybe I am erring on the conservative side as far as bushing maintenance, I know we considered it once and elected to swap it and have maintenance done by OEM instead. I have worked with big oil-filled transformers, not small ones.

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(2B)+(2B)' ?

 

[gattie]

What are the climatic conditions likely to be when you intend undertaking the repair?

 

[stzatzop73]

gattie, it would be 30 oC , without rain, 50-70 relative humidity....

 

[prc]

Let me suggest simple methods.

There are many good quality epoxy sealing compounds available in market.You can apply and seal the leakage.Drain oil from tank till just below the bushing to stop the oil leak before applying sealing glue.

Most probably the type of bushing will be such a type that you can change the gasket from outside.Please see the instruction book for bushing details. or post a photo of the leaking bushing so that I can suggest.

After changing gasket or after curing of seal, top up the drained oil back in to transformer. Leave it for atleast two days before energising.

 

[electricpete]

There is a small leak form the bushing base connection to the transformer tank. We want to replace the gasket of the bushing.

First time I read that, I was thinking it is (A) - a leak from the bushing (connection area at bottom of bottom-connected bushing) to the tank. i.e. oil leaking between bushing and tank in sealed bottom connected bushing. Then you would have to be replacing a gasket integral to the bushing. That is the one I had reservations about.

Now I see it can also more logically be interpretted as (B) a leak from between bushing and tank. No need to tamper with a gasket integral to the bushing. May also be a draw-lead bushing.

If it is in fact B, I withdraw my comments.

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(2B)+(2B)' ?

 

[prc]

Pete, these may be plain porcelain bushings ( not high voltgae sealed type that you have in mind )that can be fitted from outside,usually as per DIN design in Europe and India,may be cemented type in certain areas eg US

 

[Jay27]

Being that it is 2MVA at the given voltages, I assume this is large 3ph distribution unit. We routinely do this kind of work in our shop and we do not take 'special' precautions to limit moisture, etc... I'm not saying we take 'no precautions', we just don't do anything special like run dehumidifiers, etc. Plan to replace the whole bushing, not just the gasket. The parts are cheap enough and the labor is intensive enough that it will just tick you off if replacing the gasket doesn't fix the problem. Drain/pump enough oil out to get below the bushing. As I said, we do this kind of work in our shop instead of in the field... too many 'environmental contamination' issues should you spill some oil... then again, we'd do it in the field if we had to, just make sure to have oil spill control products on hand. Changing a bushing is not a very difficult task. The manufacturer should be able to furnish instruction manuals.

you can't fix stupid

 

[DTR2011]

if doing the work in the field, consider pumping dry nitrogen in the tank during the repair time. At 6 kV I wouldnt be too worried using the N2 method. I'd also be prepared with extra gasket material and gasket cutting tools. Many times (after unit is taken down and drained) you find out you have the wrong gasket ordered / shipped.

 

[ejrampe]

We regularly replace bushings and bushing gaskets onsite at our power station, and I would agree with the points listed above by prc and Jay27.
Typically once the transformer is proven de-energised, the biggest risk we need to manage is of oil spills and subsequent environmental cleanup issues.

The point I wanted to raise was with Smallgreek's suggestion of using dry nitrogen during the repair - be very, very careful of using nitrogen in this manner (ie with work crew in vicinity of open transformer flange) as the nitrogen will alter the breathable atmosphere surrounding the bushing flange and can lead to nitrogen asphyxiation. There are numerous documented cases of fatalities when using Nitrogen as a purge gas:

http://www.csb.gov/newsroom/detail.aspx?nid=261

http://www.csb.gov/assets/document/SB-Nitrogen-6-11-03.pdf

If our risk assessment identifies the need for controlling the ingress of moisture to the transformer, we would use dehumidified air.

 

[defreitas]

As previously mentioned, make sure you have someone that has done this type of work before.

Drop oil level to below bushing level (have spill kits ready, just in case).

Make sure it's a "dry day" (I would not be doing it in 60% humidity).

Account for the loss of a bushing (break during handling).

Undo lid, replace gasket, making sure you use you use rubberised cork gasket (TG70 / 70% rubber)...

Clean your surfaces with a solvent like ISOWash or something similar...

Put everything back together... Maybe take the change to get an oil sample for DGA and DAM analysis before and after, if you can...

I hope that helps...

Dan de Freitas