Direct Pulsewidth Modulation Technique for Modular Multilevel Converters Based on Full-Bridge Submodules

Modular multilevel converters (MMCs) are garnering increasing attention in both academia and industry. Concerning submodule (SM) topologies, the half-bridge (HB) and fullbridge (FB) SMs stand out as the most widely adopted options. In comparison with the MMC based on HB-SMs (HB-MMC), the MMC based on FB-SMs (FB-MMC) exhibits the capability to ride through dc faults, thereby offering advantages in terms of safe operation and fault tolerance. For a medium-voltage MMC, the direct pulsewidth modulation (DiPWM), in which only one SM per arm operates in PWM mode while the remaining SMs are completely inserted or bypassed throughout the entire carrier period, emerges as a suitable modulation method due to its commendable harmonic performance and straightforward implementation. While the DiPWM has been extensively studied for HB-MMCs, its proper implementation for a FB-MMC remains unclear. This article studies the application of the DiPWM to the FB-MMC. The investigation reveals that, through proper carrier configuration and duty-ratio determination, two operating modes can be derived to optimize harmonic characteristics in either the phase voltage or the circulating current, similar to HB-MMCs. Additionally, the dc-link voltage can be well selected to further enhance the harmonic performance in overmodulation scenarios. The corresponding implementation is described in detail, and the effectiveness of the proposal is validated through both simulations and experiments.