Disturbance Interaction Analysis and Suppression Strategy of MMC-HVDC Systems Considering Sub-Module Capacitor Voltage Ripples

This paper presents the interaction analysis of external disturbances and internal dynamics of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems considering sub-module (SM) capacitor voltage ripples, and proposes an enhanced control strategy for disturbance suppression. The disturbance equation indicating the interaction of the external MMC output currents and internal SM capacitor voltage ripples are firstly established. Then based on the disturbance equation, the dynamic analytical expressions of the differential-mode capacitor ripple voltages and the converter currents are derived in the d-q reference frame, which explains that the resultant current term i(0) can be regarded as an equivalent disturbance that will lead to transients in the capacitor voltages during external perturbations. Next, the disturbance observer is introduced to track the resultant disturbance current term and an enhanced control for the MMC based on disturbance suppression strategy is proposed to improve the system dynamic response during external transients. Furthermore, system stability characteristics under the enhanced control with different parameters are explored through theoretical analysis. Finally, a benchmark single-terminal MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the proposed control.