Offset PWM in modular multilevel converters for stored arm energy reduction

Abstract

Modulation techniques for the modular multilevel converter consider that the total number of sub-modules in the phase-leg is, either instantaneously or on average, equal to N balancing the capacitor voltages to a constant reference. This paper proposes an offset PWM technique that utilises the total number of the SMs in the arms of the MMC, particularly under low modulation indices. The result is a reduction in the average voltage of the capacitor and consequently, the total energy stored in the converter. The proposed technique is investigated under open and closed-loop circulating current control and simulation and experimental results are provided to illustrate the operation of the converter.