Abstract: In view of the abnormal closing time of phase C circuit breaker of the AC filter group in the converter station, the circuit breaker body test and secondary control circuit inspection were carried out respectively.Through analyzing the mechanical characteristics, gas composition, transmission test results and closing circuit voltage monitoring results of the circuit breaker, the causes were concluded that the long closing circuit of the circuit breaker led to the voltage reduction at both ends of the closing coil.Long time low-voltage operation reduced the cooperation performance between the closing coil and the armature of the phase C mechanism of the circuit breaker, resulting in 100 ms action delay.According to the analysis results, the control circuit of the circuit breaker is optimized, and the improved circuit is continuously monitored.The three-phase closing action of the circuit breaker was reliable without delay, and the voltage at the end of the closing coil restored to the normal voltage.