1000KVA Transformer: Unusual sounds

Cracked HV insulator perhaps. Does it appear one or more of the HV insulators is under mechanical strain?

What's the oil level like now its operating, you say its leaking but is it getting low enough to expose internal connections that could be resulting in arcing and subsequent tripping? I believe a dissolved gas analysis of the oil now it has been operating and tripping in this way should pick up if it is tripping because of internal arcing.

With regard to the oil leakage, perhaps the old transformer just has some perished HV bushing seals and the real problem of the reason for tripping is elsewhere in the transformer.

HV insulators are perfectly intact and under no mechanical strain.

Oil level has dropped but not enough to allegedly expose internal connections.

Oil sample for DGA has been sent to the laboratory.

Bushing seals might be a problem, I'll get them replaced after inspection.

Thanks for the insight.

Yes. Replace it.

Transformers don't malfunction, they fail, "...it tripped the HT supply...", on what, overcurrent, overload, differential, etc.? As previously stated, do a DGA (Dissolved Gas Analysis) on it for a clue on what's happening inside. Unusual buzzing could be as simple as a loose mounting bolt, or as complex as intermittent arcing; you should also see if the something has changed in the load such as unusual current spikes, increasing THC/D (Total Harmonic Content/Distortion), etc.

Surprisingly, it malfunctioned. It still runs fine under load. I put it on a test run for 30 minutes, attached to a different load.

Yes, I have sent the oil sample for DGA, I just wanted to have a look at the possible causes of the failure/malfunction till I get back the results.

Nothing changed in load i.e., Electrolyzer. But electrolyzer is turned on and off on a regular basis. Maybe the startup causes some unusual current spikes, but I don't think I have any possible way to look at it, now that the transformer's been malfunctioning.

Thanks for your insight.

RAM has asked now I'll ask "what did the 11kV trip on?"
What protection is fitted?
"Oil level has dropped but not enough to allegedly expose internal connections" is there no sight glass?
Have you replenished the oil? I think it's safe to assume no buchholtz unit is fitted.
Are the 11kV terminals in open air or in a compound filled box? If in compound oil leaking in to it will cause problems.

Try and answer the above questions and we may be able to help.

Better do the oil test...maybe the insulation in oil very low...