A Nearest Level PWM Method for the MMC in DC Distribution Grids

For modular multilevel converters (MMCs) applied to medium-voltage dc distribution grids, using the traditional nearest level modulation (NLM) as in HVdc systems can lead to severe current distortion due to significantly reduced module numbers. This paper proposes a hybrid modulation method combining NLM and pulse width modulation (PWM) where only one module per arm operates under PWM mode. The proposed nearest level PWM (NL-PWM) method not only significantly reduces the current distortion, but also avoids the complicated voltage balancing control in each module. The harmonic characteristics of NL-PWM are derived using double Fourier transform, which provides a theoretical basis for selecting the module number and switching frequency for medium-voltage applications in accordance with grid harmonic requirements. Finally, the harmonic characteristics and feasibility of the proposed modulation method are validated by simulation and experimental studies on an MMC with six modules per arm. The simulated and experimental results reveal that NL-PWM has better voltage and current harmonic characteristics over NLM and CPS-PWM, thereby suiting the application of MMC with few models.