Dual-Phase-Shifted Pulsewidth Modulation of Three-Phase Modular Multilevel Converters

The modular multilevel converter (MMC) has been a crucial component for medium/high-power energy conversion systems due to its modular structure. In the literature, the harmonic analysis of MMC with different modulation methods, including the conventional carrier phase-shifted pulsewidth modulation (CPS-PWM) method, have been extensively investigated. However, the influence of the carrier displacement angles between three-phase legs on the output performance of an MMC is normally ignored. This article proposes a dual-phase-shifted PWM (DPS-PWM) for a three-phase MMC, which further enhances the performance of the conventional CPS-PWM scheme through the proper selection of carrier displacement angles between three phases. First, the operation principle and implementation of the proposed DPS-PWM method are described. Then a two-dimensional Fourier model is derived to analyze how the carrier displacement angles between stacks and legs affect the harmonics of output voltage and circulating current. It is found that DPS-PWM can suppress specific harmonics in the line-to-line voltage by coordinating the carriers' phase shifting between the three-phase legs without affecting phase voltage and circulating current. Finally, the effectiveness of the DPS-PWM method is verified by simulation and experimental results.