Worst thing that can happen to a transformer?

[Skogsgurra]

Inspired by thread238-378765 and a real case in a 10 kV/400 V station, I ask what would the worst possible failure be? A case that makes primary current increase above any expected short-circuit current in the primary. So high, in effect, that the protection relay caught fire.

I know one case. And I have recordings of what happened on the LV side.

What are your worst cases? And what caused it?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

The off-circuit tap selector of a generator's unit auxiliary transformer was operated under load, with the moving pole of the selector causing a dead short between adjacent taps. The transformer was connected across the main terminals of a 365MVA generator with additional infeed to the fault from the transmission system via the GSU transformer. The enormous fault current at the OCTS destroyed the switch, evolving a bubble of ionised gas which caused a flashover and a massive secondary explosion. The transformer tank ruptured ejecting tonnes of burning oil and gas, killing three and seriously injuring a fourth.

 

[Skogsgurra]

That is truly a much worse accident than the one I saw. "My" transformer survived. But the mechanism that caused the unexpectedly high primary current was interesting. I wait for more input on this.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

Scotty UK,
When the OCTC was moved to the next position what happened was not shorting of taps-if so it would have been a minor failure,most probably, the relays would have cleared it.In the case mentioned it was an opening of the circuit,just as in a circuit breaker, causing a massive arc and consequent failure mode.

 

[ScottyUK]

Hi prc,

The relays didn't see the fault because of increase in line current was small due to the auto-transformer effect - this fault was akin to an inter-turn fault which are notoriously difficult to see using CTs at the terminals.

Interesting thoughts on source of the arcing. The conclusion at the time was that the switch broke up because of the forces due to fault current and that initiated the arc. There was heavy damage to the adjacent poles where the short occured, but your hypothesis is certainly plausible as a source of the ionised gas. Thank you for the idea - I don't know if that was considered at the time because I was on the team trying to get the station operational, while others were involved with the investigation.

Will see if I can find a photo or two.

 

[LSpark]

Was that a well known one in the UK Scotty? Remember that one well, we had to change all of our documentation to make sure that the difference between off load and off circuit was clear.

The worst one I heard of was a transformer that had an internal fault during energisation that caused the tank to split (not UK). The ensuing fire was like a flame thrower and killed a couple of guys leaving those next to them virtually uninjured. That's why I hide round the corner if I'm nearby when we energise a transformer.

 

[ScottyUK]

Hi LSpark,

Well-known in the industry certainly.

One of the less emotion-laden sources on the internet is here:

http://www.corporateaccountability.org.uk/Deaths/teeside.html#HigginsSurteesSherwood

and there are links to the popular press if you want them.