The slip power of the chopper cascade speed system is very small at high duty ratio. The DC current waveform of the inverter presents sparse pulse shape when inverter fault occurs, which results in that the current inverter protection can not detect the fault signal and perform the protection action, and it is easily lead to the inverter devices damage. Aiming at the existing problems, by analyzing the steady-state current characteristics and the transient process of inverting fault at high duty ratio, the theoretical basis of inverter protection failure is obtained. The protection optimization is carried out, and the optimal scheme of voltage detection and under-voltage criterion with DC circuit is put forward. The simulation and experiment results show that the theoretical analysis is correct and the optimized protection strategy can effectively protect the inverter side and can improve the system reliability.