A Novel Modulation Method for the MMC applied to Medium Voltage Distribution Grids

When the Modular Multilevel Converter (MMC) adopts the Nearest Level Modulation (NLM) method in the medium voltage (MV) DC distribution grids, the current distortion is serious due to the smaller number of modules. This paper proposed a Nearest Half-Level Modulation (NHLM) method for the MMC. This method can double the equivalent levels by introducing the square-wave modulation, thereby reducing the current distortion. Contrarily, it slightly increases switching losses and the switching frequency. However, the NHLM method introduces square-wave modulation in partial intervals, so its increase in switching loss and switching frequency is much smaller than the Carrier Phase Shift PWM (CPS-PWM) which can also reduce current distortion. This paper proposes a voltage balance control strategy suitable for the NHLM method. Finally, the simulation of the MMC using Matlab/Simulink is implemented to verify harmonic characteristics of the NHLM.