Large-Signal Stability Analysis for VSC-HVDC Systems Based on Mixed Potential Theory

Stability studies of voltage-source converter HVDC (VSC-HVDC) systems are mainly based on small-signal linearization techniques. However, when VSC-HVDC systems become unstable due to large-signal disturbances, the small-signal analysis method would be no longer available. In this paper, the large signal stability of VSC-HVDC system is studied based on mixed potential theory. A simplified state-space averaging model of VSC-HVDC system is established under the synchronous rotating reference frame. Based on this model, the stability prediction criteria for VSC-HVDC system under large-signal disturbances is obtained through Brayton-Moser's nonlinear potential method. On this foundation, the influence of system parameters on the large-signal stability is studied and the boundaries of stability are located as well. Time-domain simulations conducted in Matlab/Simulink are used to test the stability of the working points next to the stable boundary, which aim to prove the correctness of the obtained stability criterion. Aiming at the unstable situation, the time-domain simulation results prove the feasibility of the method of adjusting the system parameters according to the stability criterion to make the system stable, which further verifies the correctness of the stability criterion.