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Phase-shifted pulsewidth modulation (PS-PWM) has long been adopted as a salient modulation strategy for modular multilevel converters (MMCs). Regarding the design of PS-PWM for MMCs, various degrees of freedom including the phase-shift angles between carriers of different submodules and converter arms have been extensively studied. However, very little attention has been paid to the phase displacement between the carriers and the fundamental reference waveform. This article investigates the effect of the carrier-to-fundamental phase displacement (CtF-PD) on the performance of PS-PWM applied to an MMC. Through theoretical double Fourier analysis and simplified numerical emulation tests considering a wide range of scenarios, it is revealed that the CtF-PD can have a significant impact on the harmonic performance of the MMC under certain conditions. Utilizing this effect, the conventional PS-PWM for MMCs can be better designed with greatly reduced switching frequency/losses, while maintaining or enhancing the harmonic performance thus also reducing the ac filtering requirement. Simulation and experimental results are provided for validation. Design procedures and implementation issues of the proposed enhanced PS-PWM are also described and discussed.

For a modular multilevel converter (MMC) operating in the medium-voltage range, carrier-based pulsewidth modulation (PWM) methods are good options for modulating the converter, as harmonic contents of the phase voltage/current can be centered around the carrier frequency and/or its multiples. However, in the relevant literature, arm voltage references are mostly normalized by the submodule (SM) capacitor voltage reference, leading to the so-called direct modulation, while the indirect modulation considering the capacitor voltage ripples receives much less attention. This article investigates the indirect modulation of MMCs, focusing on a specific PWM method where only two SMs per phase operate in PWM mode in each control period. It is found that the direct PWM (DiPWM) generates phase voltages/currents that contain high-amplitude low-order harmonics, while the indirect PWM (IndiPWM) presents well-suppressed low-order harmonics but significant distortions around the carrier frequency. The cause of the above problem of the IndiPWM is revealed theoretically through Fourier analysis, and a solution with simple implementation is developed by rearranging the switching pulses of the PWM-mode SMs. Both simulations and experiments are carried out. The results show that the proposed method combines the advantages of both DiPWM and IndiPWM, achieving low harmonic distortions simultaneously in the low frequency and around the carrier frequency.