Abstract: Transformer differential current protection is a type of relay protection that operates based on the current difference on both sides of the transformer. When a transformer experiences a phase to phase short circuit fault or a single-phase turn to turn short circuit fault, the differential current is greater than the set current of the differential protection. The protection trips the circuit breakers on each side of the transformer and removes the faulty equipment from the power system. This article takes the differential current exceeding limit accident of a 35 kV substation main transformer as an example to deeply analyze the fault causes and improvement measures.