An improved CPS-SPWM and unified modulation strategy for multilevel converter

Due to the modularity, scalability and low loss compared with the traditional VSC converter, MMC based converter has raised a research hotspot in the field of direct-current transmission and direct-current distribution. Different from the direct-current transmission, the voltage level for direct-current distribution is relative low, resulting in a relative low level of the output waveforms. So the stair-case modulation, which may introduce numerous harmonic in low level, is not applicable to the direct-current distribution. To resolve this problem, we propose an improved carrier phase-shifted SPWM (ICPS-SPWM) in this paper, which is easier for digital implementation and has a better harmonic performance compared with traditional one. Besides, based on the improved carrier phase-shifted SPWM (ICPS-SPWM), a unified modulation is proposed, which can switch between ICPS-SPWM and space-vector PWM modulation (SVPWM) for multilevel easily and flexibly. In the end, simulation based on Matlab is presented to validate the effectiveness of the proposed modulation strategy.