Dynamic Performance Control of Modular Multilevel Converters in HVDC Transmission Systems

This paper focuses on dynamic performance control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. To achieve this objective, a new mathematical model including six state variables of ac-currents and dc-link voltage of MMC, and circulating currents of converter arms are proposed for MMC in d-q reference frame. In addition, a robust control technique with three sub-control loops is designed to provide the stable operation of MMC. In the overall structure of the proposed controller, three outer, central and inner loops have the duties of 1) making the state variables error zero with changeable convergence rate, 2) adding robustness characteristic to the proposed controller, and 3) generating the appropriate reference values for MMC's currents, respectively. The effectiveness of the proposed control algorithm is investigated via MATLAB simulation. The simulation results highlight the capability of the proposed control algorithm in offering an accurate active and reactive power tracking through the control method of MMC, a stabilized dc-link voltage, capacitor voltage balancing of sub-modules, and minimization of circulating currents of converter arms during dynamic transitions and steady state operation.