An Improved PSC-PWM Method for Modular Multilevel Converters with Reduced Harmonic Content in Common-Mode Voltage

Phase-shifted carrier pulse-width modulation (PSC-PWM) method is a salient option for the modulation of modular multilevel converters (MMCs). Very good harmonic characteristics can be obtained in terms of the phase voltage or the circulating current. However, in the conventional PSC-PWM alternatives, very little attention has been paid to the harmonic feature of common-mode voltage (CMV) of the ac-terminals of the MMC. This paper proposes a new PSC-PWM method for the MMC, with the harmonic content of the CMV of the MMC also taken into account, which is optimized through phase shifting the carriers among different phases. Such a modulation technique is quite suitable for medium-voltage dc distribution systems. The effectiveness is verified through simulation results.