Abstract: This paper introduces an accident of the first protection differential protection action and the second protection starting unaction of a main transformer caused by an external fault of a 110 kv substation, the cause of the first set of protection differential protection action of the main transformer is the three-phase short-circuit grounding fault of the 10 kv line on the low-voltage side of the substation, at the same time, the main transformer's first set of protection for the 10 kv side B phase differential current circuit is damaged at the secondary terminal connection of the current transformer, the current sampling of the first differential current circuit of the main transformer in 10 kv line fault is incorrect, which is not consistent with the current increase of 110 kv side and 35 kv side, cause the first set of protection ratio differential protection b-phase action jump off the main transformer three-side switch. According to the actual situation of the site and the analysis of the cause of action, the relevant solutions are put forward, which provide a reference for the prevention and solution of similar accidents.