Why does an electric transformer sound like a bomb going off when the electricity is suddenly cut off?

I've never heard one when that happens but I could tell you what's probably happening. Transformers are basically just big electromagnets that induce current from one coil of wire to the other. AC voltage switches from 120V to -120V to 120V again 60 times per second so when the power is cut off, the voltage across the transformer is probably not zero. Same with the current. There is current still going through the windings and the magnetic field is still there. That field has to break down somehow, typically with a spark, and with such big transformers, that spark is probably big.

So that's it. Big magnetic field breaks down and causes a spark. At least that's what 2 years of college has lead me to guess.

I have never heard the sound you describe and I have de energised thousands of transformers up to 400MVA. When the power is cut, they just go silent.

Of course, when you energise a 400MVA TX, they make a hell of a racket....

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