Active Power Decoupling for a Modified Modular Multilevel Converter to Decrease Submodule Capacitor Voltage Ripples and Power Losses

The usage of too many sizeable capacitors significantly increases the volume and cost of the modular multilevel converter (MMC), and limits its spread in medium- and high-voltage applications. By adding a buck-type active power filter (APF) circuit into a particular MMC topology [of which each phase multiplexes one submodule (SM)], a modified MMC (APF-M3C) topology, which inherently has the ability to suppress capacitor voltage fluctuations and decrease the power losses compared with conventional MMC topology, is proposed in this article. Both the SM capacitor voltage ripples, which can be reduced up to 50% of original value and are related to the SM capacitance, and the arm currents, which are related to SM power losses, are calculated in detail. Its topology, and operating principle is presented in detail, and a novel modulation method for the APF-M3C topology are proposed. In addition, an improved control method for the voltage balancing is also recommended to further reduce system cost and power losses. Finally, the APF-M3C topology and traditional MMC simulation and experimental platforms were built. Simulations and experimental results verify the feasibility and effectiveness of the APF-M3C topology and control strategy by comparing with the conventional MMC.