Abstract: When UPS (Uninterruptible Power Supply) system is put into operation in a 660 MW unit, switching of thermal power supply from UPS power supply circuit to security section power supply circuit, tripping of power switch controlled by spoon tube of three feed pump hydraulic couplers, low-low protection of feedwater flow action, makes the unit trip. Through the inspection and analysis of UPS, thermal control power supply, spoon control circuit of feed water pump, etc., the reason is that the long switching time of UPS power supply bypass operation, power loss of UPS control power supply in thermal control main circuit, the phase difference between thermal UPS power supply and security section power supply is large, over-impulse current generated in switching process. Some measures are put forward and perfected, such as improving the switching mode of UPS operation and maintenance, changing the phase of thermal control power supply, air switch of feed pump, increasing power supply damping, etc. The impulse current event caused by power switching has been resolved.