Small-signal stability analysis and optimal control parameters design of MMC-based MTDC transmission systems

The stability of the modular multilevel converter-based multi-terminal direct current (MMC-based MTDC) transmission system is strongly affected by the coupling among multiple elements and the interaction between the control systems. Improper designed control parameters may deteriorate the dynamic response or even reduce the stability margin of the MMC-based MTDC. This study proposes an optimal control parameters design approach to enhance the stability of MMC-based MTDC. The dynamic model of an MMC-based MTDC consisting of main circuit, control system, AC system and DC system is established, and the small-signal stability model is derived. The impact of control systems on the stability of MMC-based MTDC is studied through participation factor analysis. An optimal control parameters design problem with an eigenvalue-based objective function is formulated and a sensitivity analysis-based optimisation algorithm is proposed to optimise the control parameters jointly. The backtracking line search is applied to determine the feasible step size in each iteration. Stability assessment and time-domain simulations are performed to verify the effectiveness of the proposed method.