Abstract: Two 10 kV lines in a 110 kV substation have successively ground short circuit fault. Due to the fault of zero-sequence current transformer circuit, the line protection refused to operate, resulting in over-grade tripping, resulting in the loss of voltage of 10 kV bus. Under the condition that the fault is not removed, the dispatcher tried to power on several times, resulting in the system continuously suffering from fault current and voltage impact, resulting in the expansion of the accident. Through supplementary inspection and analysis of protection action information and oscillograph files, the specific cause of the accident was found, and some rectification suggestions were put forward to avoid similar accidents.