Comparison of phase-shifted and level-shifted PWM in the modular multilevel converter

This paper reports a comparison study of different carrier-based PWM techniques applied to the modular multilevel converter. Phase-disposition PWM (PD-PWM) and phase-shifted pulse-width modulation (PS-PWM) with non-interleaving and interleaving are considered in this study. In PS-PWM, two cases are evaluated. In the first case, the particular SMs that have to be activated/deactivated are defined by a voltage balancing algorithm, which is the same one implemented in PD-PWM. In addition, an algorithm to restrict the number of switching SMs is also implemented to reduce the switching frequency of the power devices. In the other case of PS-PWM, each sub-module (SM) has a carrier signal associated to it and capacitor voltage balance is achieved by individual control of its capacitor voltage. The circulating current is controlled to be a dc component in all the cases. Simulation and experimental results are presented to evaluate the quality of the line-to-line output voltages and SM capacitor voltage ripples for the different cases under study.