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Dry type transformers

Dry type transformers

Dry type transformers

(OP)
Has anybody experienced failure of dry type distribution transformers. If yes any particular reason for failure.

Thanks

RE: Dry type transformers

One 750kVA 12.47kV:480V. The cause was not clearly determined.  There was apparently a flash-over between primary and secondary.

RE: Dry type transformers

A number of 13.2kV - 4160V transformers, 2000kVA....lamination failure.


Mike

RE: Dry type transformers

Yes. Several 11 kV dry resin (red) transformers in the 1200 - 1800 kVA range. Looks loke ozone build-up causing flashing in primary winding. Resin more or less exploded so winding exposed. We found heavy 8 kHz ringing on the 11 kV. Geafoil transformers used now. They do not explode.

Gunnar Englund
www.gke.org

RE: Dry type transformers

The main problem with conventional dry-type transformers is dirt build-up because the windings have to be ventilated.  If you've ever opened up your PC and seen all those dust bunnies inside, you'll understand how this can be problem when you have 12,000V instead of 12V, and 150 C temp rise.  

Most failures I have encountered were due to dirt build-up causing overheating or arcing and moisture.

Cast coil primary windings are supposed to cure most of these ills, but apparently it didn't work out well for skogsgurra.

RE: Dry type transformers

dpc,

(big grin). It wasn't me that made them transformers. An Italian company (now defunct) did. We found out that there were voids between resin and primary windings. That is where ozone built up.

The process was accelerated by the heavy 8000 Hz ringing because the local field strength in the sub-millimetre voids was well over corona inception levels and the 8000 Hz forced a much greater current than the normal 50 Hz would have done.

There was no dirt on these transformer windings (not enough lifetime to collect dirt).

Gunnar Englund
www.gke.org

RE: Dry type transformers

The possibility that there are voids is the reason why rigourous partial discharge testing is usually done on cast resin transformers. If skogsgurra's transformers had been tested that way he would have had less problems.

If you enquire for cast resin transformers there is a wide range of prices you will get. but, nothing is fro free. If you choose a cheap one you may end up wizh problems like described here.

RE: Dry type transformers

True, Uwe.

The paper mill now uses Geafoil (expensive, but proven) dry transformers in that application.

I am not sure that PD testing would have revealed this particular fault. There are identical transformers in another part of the paper mill (without the 8000 Hz ringing) and they have survived. We had a specialist company doing ultrasound PD testing on them - they found nothing.

Gunnar Englund
www.gke.org

RE: Dry type transformers

I've had three under 5kW dry type buck/boost transformers fail because the leads were miss labeled by the factory.

RE: Dry type transformers

Is the transformer primary winding switched by a vacuum type contactor or breaker.

RE: Dry type transformers

Dear mpparent,

Could you give us more detail about the lamination failures? Is it possible to identify it via winding visual inspection?

Thanks in advance !

fvincent

RE: Dry type transformers

Dear skogsgurra,

How did you find the voids beetween resin and primary windings  (inspection, testing)? Were them caused by problems in the manufacturer's vacuum process ?

Thanks in advance !

fvincent

RE: Dry type transformers

fvincent,

As far as I know, usually, there is no indication that a lamination failure could be visually verified (someone please chime in if you think this is incorrect).  From the folks I've talked to, it can only be determined via power factor testing, or some other type of insulation testing (not Megger though!).

Mike

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