Abstract: The abnormal phenomenon of total shutdown of cooler fan of main transformer in a substation is studied. Combined with the drawing of PLC secondary circuit, the fault points are analyzed and the causes of abnormal phenomena are found out. Since the power supply of the manual/automatic start control loop of the cooler is from loop 301 output by the power transmitter, when the power transmitter fails, loop 301 loses power. K6 and K7 coils lost power, causing the multi-function protection switch to be turned off. Therefore, the multifunctional protection switch bq1 of the first group of fans is in the stop position. The multifunctional protection switch BQ2 of the second group of fans may be damaged due to excessive impulse current at the moment of shutdown or aging of its own components. This paper puts forward some measures to improve the lifting, and suggests that designers should use two transmitters for power supply and other treatment methods. It is hoped that designers can pay enough attention to avoid similar problems in the design of PLC secondary circuit.