Transformer backup protection action because:
(1) transformer high side short circuit.
(2) transformer short circuit low voltage bus.
(3) due to a failure within the differential scope of protection, differential protection malfunction.
(4) backup protection malfunction.
(5) Low line fault, outgoing protection tripping, causing a transformer overload trip.

Principles are:

(1) If the overcurrent protection operation, it found that the voltage drop, shock, arc, sound and so on, to deal with external transformer to be checked; if timely troubleshooting, you can try to send again, or should be ready to take security measures to repair; if not found Problems can also be sent once again; no differential protection for transformers, in addition to the external checks should be carried out to check the insulation measurement.
(2) If the low-pressure outlet malfunction, line protection tripping, you can manually switch knock down the fault line, and then send the power transformer.
(3) If a fault occurs in the differential scope of protection, differential protection fails, the differential protection should be treated.
(4) If the secondary circuit failure, the case of malfunction or misuse of touch, the duty officer can immediately test transmission.