Transformer faults can be divided into internal and external faults.
Internal faults of transformers refer to various faults that occur inside the transformer oil tank. Faults inside the oil tank include phase-to-phase short circuits between phase windings, turn-to-turn short circuits between some turns of single-phase windings, single-phase grounding faults of single-phase windings or lead wires through the outer shell, and core burnout. These faults are very dangerous for transformers. The arc generated by the fault inside the oil tank will cause violent gasification of the insulating material, which may cause an explosion. These faults should be removed immediately.
External faults of transformers refer to faults outside the oil tank, mainly phase-to-phase short circuits and grounding short circuits on bushings and lead wires. For internal faults, the transformer is mainly removed by gas and differential protection actions; for external faults, the transformer is generally removed by differential protection actions. The fast-acting protection (gas and differential) cuts off the faulty transformer without delay. When the busbars on each side of the transformer and the lead-out equipment in the connected intervals fail, if the faulty equipment is not equipped with protection or the protection refuses to operate, the transformer backup protection will delay the opening of the corresponding switch to disconnect the transformer from the fault.