Active Power Decoupling for Full-Bridge Submodules of a Modular Multilevel Converter

The modular multilevel converter (MMC) topology has been widely used in medium/high voltage high-power transmission and distribution and motor drive fields. The full-bridge submodule (FB-SM) MMC topology with fault-handling capability is currently receiving increasing attention and application. However, in order to suppress the SM capacitor voltage ripple, the usage of SM capacitors with larger capacitance significantly increases the hardware cost and volume of the system. In this article, a modified FB-SM with an active power decoupling circuit (APD-SM) is introduced, in which the APD circuit is integrated by sharing two power-switching devices with traditional FB-SM. By switching the power device switching mode when the FB-SM outputs zero voltage, the APD circuit operates in Buck or Boost mode to achieve the transfer of ripple powers from the FB-SM capacitor to the APD circuit, thereby achieving significant suppression of FB-SM capacitor voltage ripple or reducing the FB-SM capacitance. Its topology, operating modes, and voltage ripple are introduced in detail. In addition, the parameter design and control method of the APD circuit are presented. Finally, the simulation and experimental platform of MMC with APD-SM and FB-SM (abbreviated as APD-MMC and FB-MMC, respectively) are built. Simulations and experimental results verify the validity of the APD-MMC topology and control strategy.