In consideration of the frequent mal-operation of transformer differential protection based on the principle of waveform symmetry

this paper analyzes the internal relationship of the waveform symmetry principle and the secondary harmonic restraint principle based on the generating mechanism of the magnetic inrush current of the three-phase transformer. The root cause of the mal-operation with the conventionally adopted differential protection is pointed out by considering the two phase compensating methods in current use. For the 220 kV and above transformer with current transformer equipped on the delta side

this paper provides a new approach based on the phase current differential relay. This approach has made it possible to overcome all the disadvantages of the conventional scheme in theory

and is capable of implementing genuine phase constraint. The drawback of the proposed method is that the protective zone is reduced and needs other relays for coordination.