A Level-Increased Nearest Level Modulation Method for Modular Multilevel Converters

Abstract

The modular multilevel converter (MMC) is emerging to be a promising approach for medium- and high-voltage applications. This paper proposes a modified nearest level modulation (NLM) method for the MMC, with which the level number of ac output voltage increases to 2N + 1 (where N is the submodule numbers per arm) as great as that of the carrier-phase-shifted PWM and the improved submodule unified PWM methods. The level number is almost doubled and the height of the step in the step wave is halved. Therefore, the ac output voltage waveform quality using the proposed method is better than that using the conventional NLM method. Besides, the proposed method keeps the conventional NLM method's characteristics such as low switching frequency and simple implementation. The performance of the proposed method is verified by simulations and experiments using a laboratory-scale prototype.