A nearest level PWM method for the MMC in DC distribution grids

Yi Wang, Can Hu, Ruoyu Ding, Lie Xu, Chao Fu, Erlin Yang

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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 number. 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 theoretical basis for selecting module number and switching frequency for medium-voltage application 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 a MMC with 6 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.
Original languageEnglish
Number of pages17
JournalIEEE Transactions on Power Electronics
Early online date11 Jan 2018
Publication statusE-pub ahead of print - 11 Jan 2018


  • harmonic analysis
  • pulse width modulation
  • voltage control
  • power system harmonics


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