Transformer Explodes!

I agree with dzenizo, as we don't have enough information to accurately explain why the transformer exploded under those specific conditions.

Yet, I wouldn't conclude that the equipment was indeed a dry-type transformer, only on the assumption that there was no Buchholz relay. I personally believe that the equipment could have been an oil cooled transformer, and it could have exploded even if it had a Buchholz relay installed. We have to remember that pressure gradients developed during low impedance faults are too fast for mechanical devices, therefore the importance of Pressure Relief Valves.

There was probably an arc across the coils, originated by the high transient current associated to the load connection to the generator. As BruceFlorida stated, perhaps the generator was out of sync (in terms of frequency and/or voltage magnitude). Evidently, the stress associated to the transient current combined with an insulation degradation due to the extended transformer lifetime, was a key condition for the arc to appear, and ultimately leading to a non prevention of a short circuit between the coils. Under these conditions, what I explained above could be a valid trigger for the explosion to happen.

Of course, we also have to consider the following question: Why didn't the main circuit breaker trip during the event? Two conditions may be considered: 1) The MCB was oversized for the short-circuit current of the transformer. 2) Event happened too quickly for the MCB to react. It has been proven that the best breaker technology trips in 50 ms, way too late to prevent an explosion. TP tests have also demonstrated that the first MJ in a low impedance failure produces 80ft3 of gas during the 1st ms.

Recommendations:

- Always use a coordination of protections using a Buchholz Relay and Pressure Relief Valves.

- Perform short-circuit studies and correctly calculate and coordinate all the protections associated to the system.

- Constantly check the transformer's insulation levels. The best recommendation for oil cooled transformers, is to extract oil samples and have them tested. This should be done at least once a year.

Best regards to you all.