Abstract: In recent years, with the increasing of long distance transmission lines and transformers in substation, line backup protection action range is usually extended to the other side of the transformer bus when multiple transformers operate in parallel, which potentially leads to serious accident that an operation-failure of line protection in one voltage level grid brings about protection trip in another grid. To solve this problem, the protection of each side of the transformer should play a role similar to the firewall and limit the influence of faults in the local grid. To realize this, impedance protection for each side of the transformer should have adequate sensitivity to the fault at the remote end line and is in coordination with the distance protection of the local side line. In addition, the distance protection of the line should coordinate with the impedance protection at the other side of the transformer. This method improves the selectivity, sensitivity and speed of the related protection greatly. Finally the setting calculation method for relevant protection at power grid interface is presented and problems for attention in engineering application are discussed as well.