Abstract: This paper discusses in detail a protection abnormal tripping fault caused by the operation of 220kV circuit breaker under the condition of closed loop operation, and analyzes the accident case of protection action tripping of another power supply line caused by the opening of circuit breaker on the user side under the condition of closed loop operation, resulting in voltage loss of the whole station. Based on the analysis of operation, fault description, protection action report, wave recording analysis and logical reasoning, combined with on-site patrol inspection, this paper finds the main causes of abnormal tripping of line protection during the operation of 220kV circuit breaker on the user side, and puts forward specific preventive technology and management measures based on the patrol status of operators on the user side, so as to avoid the recurrence of similar accidents.