Abstract: The transformer plays an indispensable role in the transmission and distribution of power system. Once the transformer is out of order, it not only affects the normal operation of this circuit, but also may spread to affect other users even leading to the power grid collapsing. Therefore, the protection of transformer is very important. Based on the 220 kV substation grid simulation system to simulate the relay protection of the fault acting at the transformer in electric power system, and make predictions and observations of the protection operation situation, and study all kinds of transformer protection movement situation in fault, and to further analyze the interaction between the various protection and transformer longitudinal differential protection works.