The function of the circuit breaker module provided in the Simulink simulation platform is simple and cannot meet the needs of building distribution simulation. The main functional units of low-voltage AC circuit breakers for line protection are short-delay release and long-delay release. By analyzing the functions of the two trip units, two types of release models were built in Simulink and were verified in the AC distribution system, The simulation results show that the two new releases meet the requirements of protection functions, and are suitable for building distribution simulation.