An Improved NLM Modulation Scheme for Modular Multilevel Converters

This paper addresses the issues of reduced output voltage levels and imbalanced fluctuation of submodule capacitor voltages in modular multilevel converters (MMC) used in power transmission applications when employing the Nearest Level Approximation (NLM) modulation technique. To tackle these problems, an improved NLM modulation scheme is proposed. Firstly, the scheme modifies the rounding threshold of NLM to achieve jump synchronous modulation, effectively doubling the number of output voltage levels. Secondly, a bubble sorting method is introduced to perform charging and discharging sorting of submodule capacitors. Simulation experiments demonstrate that this scheme effectively suppresses the fluctuation of submodule capacitor voltages in modular multilevel converters.