Switched Capacitor MMC Submodule Voltage Balancing with Reduced Number of Voltage Sensors

Abstract

This paper presents a submodule (SM) capacitor voltage balancing scheme for the Switched Capacitor Modular Multilevel Converter (SC-MMC), which employs a middle submodule connecting the upper and lower arms and does not require large physical arm inductors. Only small stray/parasitic inductance is enough to limit inrush current during SM’s switching instants. This topology is directly derived from the generalized multilevel converter. The SC-MMC has the benefit of providing the same number of output voltage levels than the conventional MMC using one less submodule per phase leg. A Y-matrix modulation (YMM) that guarantees automatic SM capacitor balance without any SM feedback voltage has been proposed for an MMC topology with extremely low arm inductance. It is shown here, that the direct application of YMM cannot achieve automatic capacitor balancing for SC-MMC with middle SM. Thus, a strategy based on a modified Y-matrix modulation that requires only sensing and feedback of the middle SM voltage and output current is proposed to ensure balancing of all SM capacitor voltages. Simulation of a 6-level SC-MMC is provided to verify the voltage balancing capability with proposed control in steady state and transient conditions.