A state-of-charge equalisation technique of super-capacitor energy storage systems using sub-module dc-dc converter control within modular multilevel converter (MMC) for high speed traction drive applications

This paper proposes a new technique to balance the state-of-charge of supercapacitor cells within a modular multilevel converter (MMC) for high speed traction drive applications. The proposed configuration uses an H-bridge sub-module with an integrated DC-DC converter and it controls the voltage of the sub-module capacitor according the state of charge of the storage cells. Each phase leg of the MMC is also controlled to achieve the balancing of supercapacitor cells. This paper presents in details the advantages of this voltage based control method and corresponding key challenges. At last, the validity of the proposed control technique is validated by numerical simulations using a full closed-loop model of the MMC.