Transformer

[anandmoh]

dear fellow members,
what could be the reasons for arcing across the transformer top cover flange whenever the transformer is back charged from the 220kV side.This is a Generator step up unit connected to a Gas turbine Generator thro a GCB.During start ups the auxiliary board is energised thro the220kV breaker which also acts as a Station auxiliary transformer.The transformer tank flanges are a bolted design with equalising copper flexibles shorting links provided across the top and bottom flangeat a number of locations.

 

[ScottyUK]

Is this transformer equipped with oil-air bushings or does it have a CGIT (SF₆ bus duct) connected to it? Any chance of a photograph or two?


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Sometimes I only open my mouth to swap feet...

 

[anandmoh]

DEAR SCOTTYUK,
Iam sorry i donot have a photograph readily for sending it to you.This transformer is fitted with a typical oil impregnated paper bushing(on the 220kV side).The busduct is a pressurised air isolated phase busduct on the lv side while the 220kV side is connected to the 220kV yard thro an overhead moose ACSR conductor.The flash like arc jumping across the top cover flange(bolted construction) is visible only for a second at the instant of 220kV breaker closing for back charging the transformer.Then everything is normal.We have even tried putting more copper flexibles across the flange so as to equalise the potential, but still this phenomenon is prevelant.The manufacturer ABB is perplexed.It appears that the top tank cover acquires a potential due to some leakage flux during the unbalance condition arising out of inrush.But I can not back it up with any sound reasoning. Have you ever encountered such a happening.
regards,
anandmoh

 

[ScottyUK]

No, haven't seen anything quite like this. Sometimes bus duct image currents can find strange paths if the bonding isn't done properly, but this sounds like a different mechanism at work.

Is the arcing between the top plate and the tank body, or between the bushing flange and the top plate?


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Sometimes I only open my mouth to swap feet...

 

[waross]

Is the metal support flange on the bushing grounded or are the ground straps only from the tank top to the tank. It sounds like it may be a bad ground on the bushing support flange.
respectfully

 

[anandmoh]

Dear scottyUk
Yes I,ll have a look at the busduct image current path.But when the transformer is back energised there is no current in the busduct connected to the generator and also because the Generator circuit breaker between the transformer lv and the generator is OPEN.The arcing is between the top plate and tank and not between the bushing plate and tank.Yes i need to check the grounding between the bushing bottom flange and the tank.Thanks waross for your point.
with regards,
anandmoh

 

[ScottyUK]

Thinking aloud, the top plate will have a gasket, probably impregnated cork, between the flange on the tank walls and the top plate. It will be a good insulator. Any induced current which tries to circulate in the steelwork will be forced to flow through the bolting and the fortuitous connections to the top plate and flange.

How heavy are the bonds you have installed, and how are they arranged? I have seen bolting burned away on generator inner covers where circulating currents induced by leakage flux are huge. I wonder if this is a similar phenomena at work? The bonding we installed to reduce the current in the bolting is heavy and distributed along the joints. A single bond or couple of bonds are ineffective in the case of the generator. One question if this is happening to the transformer would be "why is the leakage flux so high?". Do you know the vector group and core design (3 limb / 5-limb) of the transformer?

Which part of ABB are you dealing with? ABB's operations out of Sweden and Finland were the European transformer centres of excellence. It might be worth getting the opinion of their engineers if you are only dealing with the local guys at present.


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Sometimes I only open my mouth to swap feet...

 

[prc]

Your query requires some more details.MVA rating,voltage ratio,any surge absober/capacitor at LV terminals at the time of back charging.Flash over at all times,duration of flash over etc.

When a large fault current passes through the transformer heavy leakge flux will overflow from the windings and impinge the tank.This is the case when a dead line fault occcurs near transformer terminal or when the transformer is under short circuit test at a test lab.If impedance of transformer is 10 %,the fault current will be 10 times normal and leakage fllux will be 10 times more than the value at normal full load working condition. The leakage flux shields provided inside the transformer will not be able to contain this flux and the overflowing flux will create a high voltage difference across the curb and cause a flash over.To avoid /reduce this ,when transformers are Sc tested additional connections will be provided across the curbs.Normally this not harmfull.

The mentioned case is a different story.Inrush current will not create the leakage flux.But back charging of GT with out surge absorbers /arresters on Lv side can sometimes cause high overvoltages due to transferred transient surges on LV or on Hv winding itself.This can charge the ground capacitances and cause a momentary flash over across the curb.It can also be due to inadeqate earthing from paints at curb etc.