Capacitor Voltage Balancing Control Strategies for Hybrid Cascaded Multilevel Converters

The hybrid cascaded multilevel converter (HCMC) high voltage direct current (HVDC) transmission system offers smaller foot-print and DC-side pole-to-pole fault blocking capability compared to the commonly used modular multilevel converter (MMC) based HVDC systems. One of the technical challenges associated with the control system of the HCMC is to control the submodule capacitor voltage ripples without significantly increasing the submodule switching frequency. Various efforts have been dedicated to developing the capacitor voltages sorting methods for the conventional MMC. However, the use of full-bridge submodule in the HCMC requires advanced voltage balancing control (VBC) schemes for proper operations during steady-state and dynamic transients. This paper proposes a new scheme to control submodule capacitor voltage ripples and enable low submodule switching frequency of the HCMC. The performance of the proposed VBC strategy is validated using dynamic simulation of 1GW HCMC-HVDC system. The simulation results demonstrate the improvement of the new approach over the conventional VBC approaches.