This paper analyzes three power grid fault range enlargement accidents in the Northwest Power Grid caused by the failure of protection to operate on command. It is pointed out that one of the reasons for this is the insufficient sensitivity of line protection setting because of the existing backup protection coordination method which is based on extreme grid operation modes. A method for coordinating the settings of distance protection for extra-high-voltage lines based on non-extreme grid operation modes is proposed. In this method, zone II of the distance protection is coordinated based on modes close to actual maintenance situations, while zone III is coordinated using the mode where the opposite-side fault is successfully removed. This method can clarify the coordination relationship of adjacent relays and thus optimize a relay's backup protection function for the line where it is installed and its downstream equipment. The effectiveness of the proposed method is verified through experiments. This work is supported by National Natural Science Foundation of China (No. 51777081).