SF6-oil bushing failure

[ScottyUK]

I have an unusual problem which I wonder if anyone has ever seen before. I would appreciate some advice.

We have a 275/11kV station auxiliary transformer which has developed a fault on one of the HV bushings. These bushings are SF6-oil, Micafil type RTKG 275-1050/1250 rated for 1420A.

The failure mode is that SF6 is present in the transformer tank and collects in the Buchholz relay where it may be released and tested at the sample point. The gas is definitely SF6, and this is corroborated by the loss of SF6 pressure on the associated busbar gas zone. The conclusion at present is that the bushing has suffered mechanical failure of a seal and this is causing the gas leakage.

Questions:

I have never considered repairing a bushing seal before, so this is new territory to me. We have been informed that the bushing is a write-off and a replacement is required, although this would be to the advantage of the source of that information. As the bushing appears to be electrically sound, I'm hoping for some independent opinion. I am not remotely interested in a Heath-Robinson repair on the bushing - it must be done properly, or not at all.

The oil within the transformer (presumably) has a quantity of SF6 dissolved in it. I say presumably because I don't know the solubility of SF6 in mineral oil. Our oil lab's response was "you've got what in it?", but they are pursuing this information for us. Is dissolved SF6 likely to be removed using standard vacuum processing plant (Ilovac or similar)? I am reluctant to replace 33 tonnes of oil if it can be salvaged, but I'm equally reluctant to re-use oil of unknown characteristics in an important transformer.

We have a possible spare bushing which we can 'borrow' from another transformer, but I think the bushing may not have been stored in especially good conditions and may have been exposed to some degree of humidity. We intend to carry out a tan delta test on the bushing: on the assumption that it is degraded, it has been suggested that the bushing may be dried under vacuum in a similar manner to a motor winding prior to impregnation. I'm sceptical, but only from gut feeling. Any comments are appreciated.





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

 

[RalphChristie]

Scotty

Sorry, have never worked on SF6-oil bushings. However, I've seen on the ABB-site similar stuff, maybe you'll find it helpful?

http://www.abb.com/global/abbzh/abb...e=us&m=9F2&c=E6EDE587894A218485256DC200453A9B

and especially:

http://search.abb.com/library/ABBLi...LanguageCode=en&DocumentPartId=&Action=Launch

I've noticed also contact-information on some of the pdf-files. Maybe you can contact them for aditional info.

Regards
Ralph

 

[busbar]

 
You may want to contact Freddi Jakob in the western US. I believe he may be interested in resolution of an SF6-in-oil situation, or at very least address the oil-solubility characteristic.
fjakob@weidmann-acti.com

http://www.actiinc.com/leadership.html

Weidmann-ACTI Inc. Sacramento, CA USA 95826 +1(916)455-2284 Fax +1(916)455-0191 [US-Pacific time zone]

 

[ScottyUK]

Ralph,

Thanks for the links. I'm already working with ABB Power T&D at Dundee and Stone, and they are internally in touch with ABB HOSAG in Switzerland who originally built the substation. Their package of engineering drawings is pretty comprehensive, so we have most of what we need, but at this stage I can't get a good answer on the questions above. ABB, as you would expect, have some first-class engineers looking at our problem, but at this stage it seems to be a new problem to them.

Obviously ABB would like to replace the bushing and oil with new as this is easier from their warranty point of view, plus it is more lucrative. I'd do the same in their position.

Busbar,

Thanks for the link - it is an interesting-looking website. I'll drop him an e-mail if we are struggling to make progress, or if we turn up any information on how to process the oil to remove the SF6.



----------------------------------

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

 

[prc]

Why dont you contact Micafil directly?They will give the correct solution.Most probably these will be RIP bushings with double o-rings to prevent gas leak in to trf.I had used these bushings earlier but never came across such a problem.RIP bushings are not prone to moisture absorbtion .Clean thoroughly the outer surfaces ,do C and tan delta measurements at 10 KV and if values are comparable with test report values you can go ahead for replacement .
Trf oil has to be degassed by sending the oil through vacuum-filter machine for at least 48 hours.Substantial quantity of SF-6 may be in oil as gas in GIS may be at 4-6 bar.
Micafil will be able to salvage the bushing by putting new sealant.
If the plant is under guarantee go for the advice of the supplier.