Dynamic interaction analysis among different control loops and stability enhancement strategy for MMC-HVDC systems

This paper presents the dynamic interaction analysis method among different control loops of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems, and proposes an improved control strat-egy with feedback regulation loop and low-pass filter loop for stability enhancement. The small-signal cascade model of MMC with dc voltage control loop, ac current control loop and differential-mode ripple capacitor voltages loop is firstly established, which clearly shows the analytical interactions of inner capacitor voltage response and system input power's dynamics with simplified closed/open-loop transfer functions. Then the small-signal dynamic analysis of MMC is conducted based on the derived closed/open-loop transfer functions, which reveals the interactions between the system stability and key control loops, as well as the main parameters of the MMC-HVDC system by both the Nyquist criteria and the pole-zero theory. Next, the feedback regulation loop and low-pass filter loop are proposed and introduced in the MMC control system to maximize stability enhancement performance during disturbances, and system dynamic response under the proposed control with different parameters are also explored through theoretical analysis. Finally, several cases on a benchmark two -terminal MMC-HVDC simulation model are carried out in PSCAD/EMTDC to verify the proposed control strategy.