Power-cell Switching-Cycle Capacitor Voltage Control for the Modular Multilevel Converters

This paper presents a power-cell Switching-Cycle Capacitor Voltage Control (SCCVC) approach to control the capacitor voltage ripple for the Modular Multilevel Converters (MMC). The capacitor voltage will be following the reference in one switching cycle by taking advantage of the degree of freedom in the control, the circulating current. The capacitor voltages resonate with the inductors in the converter circuit when the capacitors are inserted into the converter arm, and can reach the reference voltage at the end of the insertion time interval by proper control of the initial arm current. The SCCVC decouples the capacitor voltage ripple magnitude apart from the line frequency to certain extent, which not only provides a significant reduction of the capacitance value in designing the MMC parameters, but also enable the MMC to deliver very low frequency or even DC line currents. The control principle of the SCCVC and the resonance behavior between the capacitors and inductors are shown in this paper and finally the simulation results validate the control feasibility.