In order to increase the transmission capacity of the new energy grid-connected transformer,a two-voltage three-winding transformer with star-angle-angle connection form is adopted. The sensitivity of the differential protection is insufficient when an inter-turn fault occurs on the low-voltage side of the transformer. At the same time,the transformer inrush current blocking criterion blocks the differential protection and extends the fault removal time when the transformer is closed at no-load during the inter-turn fault. Based on the zero-sequence equivalent loop of the new energy grid-connected transformer,the inter-turn fault identification criterion is proposed by using the characteristics of the out-of-area fault and the inter-turn fault. The proposed criterion which is not affected by the magnetizing inrush current has higher reliability and quicker action than the principle of differential protection does. The electromagnetic transient real time simulation system (RTDS) is used to perform fault simulation,which verifies the validity and reliability of the new criterion. Therefore,the new criterion can be used to identify transformer inter-turn faults and make up for the deficiency of differential protection.