Abstract: During the operation of a 600 MW power plant, when the water supply pump is started, the failure of the excitation system triggers the unit jump, resulting in the unsafe equipment of the unit and the economic loss of a large amount of load. During the period, the reasons for switching the dual power supply in the excitation system and the mechanism of switching the dual power supply switch are in the middle position. After determining the reasons, the measures such as switching time of the dual power supply switching device of the excitation system and checking the switching value of the main road loss voltage, adjusting the position of the low-voltage Transformer voltage regulating and separating joint, and improving the remote transmission function of the fan power switching signal of the excitation system are formulated. The problem of cooling fan failure in excitation system is solved, and the stable operation of excitation system is guaranteed, which lays a solid foundation for the stable and continuous operation of the unit.