“Hole one and the oil leaks out, then the transformer explodes”
My guess is that the transformers will shut down if they overheat.
The oil is for insulating as well as cooling. Once the oil drops below the windings, there will likely be an internal fault between the winding and case, or between the primary and secondary winding, resulting in a tripping of the transformer. In such a case, internal damage is a major concern and will require much testing (after the holes are plugged and oil is either replaced, filtered, or added to the proper level).
There will be insulation testing, ratio testing, combustible gas and oil sample testing - a few other things, but they won’t just top them off and energize them because if there is internal damage, it will only be worse.
Incidentally, the steel walls on these large transformers are very thick 1/2-1 inch thick. Ordinary rifle rounds won’t penetrate, and I don’t know that ordinary AP rounds will penetrate either. The steel on the radiators is thinner, however, and could be penetrated.
Whoever did this had a working knowledge of the power system in that area, most definitely.