A Modular Multilevel Converter With Active Power Filter for Submodule Capacitor Voltage Ripples and Power Losses Reduction

Abstract

Modular multilevel converter (MMC) topology has been widely used in the medium-/high-voltage applications. However, large capacitance is needed to suppress submodule (SM) capacitor voltage ripples. This article presents a modified MMC with the active power filter (APF-MMC) paralleled between the N th SM of the upper arm and the first SM of the lower arm in each phase without changing the original topology. It is intended to decrease the capacitor voltage fluctuations and power losses of SMs by diverting the phase low-frequency ripple powers into the storage capacitor of the additional active power filter circuit. Both the detailed SM capacitor voltage fluctuations, which can be reduced up to 50% of the original value in the traditional MMC topology, and the SM average currents, which are proportional to SM switching losses and can be reduced up to 87.83% of the original value, are analyzed. The arm rms currents are also derived, which are related to SM conduction losses. Moreover, the operating principle, modulation method, and control strategy are presented in detail. Finally, the APF-MMC and the traditional MMC topology simulation and experimental platforms were built. The simulation and experimental results demonstrate the validity and performance of the APF-MMC topology and control strategy by comparing with the traditional MMC topology.