When the power transformer high voltage side outgoing line occurs single-phase or two-phase ground fault, in the process awaiting reclosing after the fault phase is cut off or the main transformer high voltage side outgoing line occurs one or two phase broken, generator transformer group is in the open phase running state. This paper analyzes the characteristics of voltage and current of generator using sequence component, verifies the analytical results according to the recorded data, and proposes the effect on generator transformer relay protection. According to the results of the analysis, it puts forward some attention and prevention measures on open phase operation, which is conducive to the safe and reliable operation of power system.