With the maturity of the electronic transformer and advance of the intelligent substation, electronic transformers have been widely used in digital substation for current and voltage signal measurement. Based on the features of inrush current, the traditional transformer differential protection applies its secondary harmonics to distinguish inrush current from fault current to restrain the differential protection. However, if some of transformer faults occur, such as low percentage interturn faults, when a transformer is switched on or the power supply is restored after a fault, the differential protection may delay action even refuse to move. It is because the inrush current of non-fault phase restrained the differential protection. Regarding to the defect in principle, a new unblock criterion of inrush current secondary harmonic is proposed. The results show that the improved scheme can significantly improve the transformer differential protection operation in internal fault.