Loud Bang When Air-Break Switch Was Opened

You don't give much information about those two transformers. Are they identical or one installed later than the other one? One possible explanation is circulation current between these two transformers due to the different short circuit impedance, bus impedance and the difference between transformer secondary voltages... These currents exists even when transformer operate parallel in no-load conditions.

Dear Sir,

The phenomenon of hearing " A loud bang with a thick ball of flame ", this only happens when the electrical load is not isolated 100%.

Especially when we operate any breaker, we should also check varify whether all the 3 phases are physically isolated otherwise there may take place inevitable mishap.

In elctrical circuit we should make 100% sure that the equipment is 100% isolated from both the sides of the electrical load just to avoid possibility of any electrical hazard to the existing power system. Also both sides of the circuit breaker under maintenance should be properly earthed .

Regards,

Ulhas Pradhan

Blowed up Sir.

Yes its all about circulating currents because an air break is to interupt the magnetising currents just as the isolator dose inturupt the line charging currents.

upon closer inspection it was reavealed that the two transformers had been set on widely different taps.These transformers had 7 percent impedance. Because the turns ratios were unequal a circulating current was set up even if the secondary loads were removed.The current through the air break switch supplies the reactive losses of both transformers and therefore lags the primary voltage by 90 degrees.The resulting current can exceed the interupting rating of the switch, causing it to fail.