Abstract: The backup grounding impedance protection of the 1# main transformer in a certain 110 kV station tripped the 110 kV buscouple breaker with the first time limit action. At the same time, the zero sequence overcurrent section Ⅱ and distance section Ⅱ protection of a 110 kV line were activated. After successful reclosing, the distance section Ⅱ tripped the line breaker again, causing a voltage loss in the entire station and reducing the power supply reliability of the power grid. This article provides a detailed analysis of the reasons for the protective action of the 110 kV main transformer caused by developmental faults on the 110 kV line and proposes rectification suggestions.