(1. Shool of Electrical Engineering, Wuhan University, Wuhan 430072, China; 2. Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, China)

The three-phase voltage source converter(VSC)is a strongly nonlinear system for a large number of power electronic switching devices and corresponding control links contained. Disturbed by the grid side fault or DC side load change in the AC/DC hybrid system, the large-capacity converter will generate complex response, which brings about unpredictable influences on the power grid. This paper studies the transient response of a three-phase VSC under typical grid fault conditions. Firstly, based on the average mode, the nonlinear characteristic of the inner current loop in the three-phase VSC is analyzed. And the Lyapunov stability criterion is adopted to analyze the stability of VSC after a grid fault. Then the fault depth and duration are taken as indices to evaluate the fault tolerance ability of VSC. Finally, the effectiveness of theoretical analysis and calculation is verified by cycle-by-cycle simulations of the grid-connected system. This work is supported by National Natural Science Foundation of China(No. 51507118), Hubei Provincial Science and Technology Support Program(No. 2015BAA109), and Fundamental Research Funds for the Central Universities(No. 410500078).