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