[Willie Green]

Thus we now have transformers that use mineral oil which is very flammable.

Actually, mineral oil is considered only slightly combustible.
It will burn, but it does not readily ignite.

[Jotmo]

But it is flammable. And the larger problem is not the oil itself but what is gassed off if there are any faults (arcing) inside the transformer. Acetylene is one. When the concentration gets high enough, you have a big bomb waiting to go off.