This paper studies the remote backup protection performance of zoneⅢ of differential current protection. It discusses in details the performance of remote backup protection of the transformer when short-circuit occurs on another side of the transformer. The analysis indicates that the sensitivity of zoneⅢ of differential current protection is lower than that of phase current protection when two-phase and three-phase faults occur separately on the other side of transformer. If the subordinate neighboring equipment of the line also is the transmission line， when two-phase and three-phase faults occur separately on the line， the sensitivity of zoneⅢ of differential current protection is higher than that of phase current protection. This paper proposes， if the subordinate neighboring equipments of the line are the transformer with Y，d11 type and the transmission line， zoneⅢ of over-current protection may select two protection sets， which export with‘or' gate. One set is phase current protection adopting the scheme of two current transformers and three relays， that can enhances the sensitivity of remote backup protection of transformer. Another set is differential current protection， that can enhances the sensitivity of remote backup protection of neighboring transmission line.