Abstract: In the process of power grid operation, the large-scale power outage accidents caused by the protection device tripping have seriously affected the safety and economic operation of the power grid. These accidents have resulted in substantial economic losses to society and negatively affected the corporate image and the power business environment. In this paper, the causes of the accident are carefully analyzed and studied. The analysis covers the setting calculations of protection values on the high, medium, and low voltage sides of 110 kV substations, verification tests of protection plugins, transformer excitation inrush currents, main transformer current matching, and the operational status during daily inspections. Various causes of over-tripping accidents are identified. At the same time, higher requirements are put forward for the daily operation and inspection work of 110 kV substations.