A Comprehensive Study on the Modulation Ratio for Modular Multilevel Converters

Modulation ratio is an important index for power converters, which is related to the circuit parameter selection, converter transformer taps changing, control scheme design, etc. Nevertheless, the modulation ratio in modular multilevel converters (MMC) has not been well studied yet, and in most cases, it is used by referring to the modulation ratio of two-level converters. However, considering the particular features of MMCs, whether the modulation ratio can likewise be applied for MMCs is required to be studied. This article found that there are three problems lying in the modulation ratio of MMCs; as a result, the modulation ratio cannot always accurately reflect the modulation characteristics of MMCs. The study concluded that, in evaluating the overmodulation risk of MMCs, the modulation ratio is only accurate in the situation where the circulating current suppression controller is adopted in the control scheme. Using modulation ratio as the substitution of the amplitude of fundamental modulation signal can introduce large errors (from -15.63% to 12.74% in case studies), except when the MMC mainly outputs/consumes the active power. Finally, the analyses and conclusions are verified by the Zhangbei HVDC project based on the RT-Lab platform and by the 5 kVA/400 V downscaled MMC prototype.