Abstract: A single-phase grounding occurred in the area of the bus bridge on the 10 kV low-voltage side of the 110 kV main transformer. Only the grounding transformer protection action tripped the low-voltage switch and locked the standby automatic switch, resulting in load loss. The bus bridge grounding fault continued until the operator manually cut off the circuit breaker at the high-voltage side of the main transformer, which had the potential of expansion. This paper analyzes the differential protection of main transformer, high and low backup protection of main transformer, grounding transformer and automatic switching of backup during fault, and confirms the correctness of relevant protection actions. In order to avoid the load loss caused by the fault in the main transformer area, the criterion optimization scheme of changing the low self-produced zero sequence current reverse blocking is proposed for the standby automatic switching. In order to cut off the fault in time and avoid the expansion of the fault, the action optimization of the standby automatic switching trip transformer from low to high is proposed, which has a positive reference significance for improving the reliability of power supply and equipment safety.