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

• Published in: IEEE transactions on power electronics Vol. 33; no. 11; pp. 9209 - 9218
• Main Authors: Wang, Yi, Hu, Can, Ding, Ruoyu, Xu, Lie, Fu, Chao, Yang, Erlin
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-11-2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Computer simulation; Converters; DC grid; Distortion; Electric potential; Electronics; Fourier transforms; Harmonic analysis; Metal matrix composites; modular multilevel converter (MMC); nearest level modulation (NLM); Phase modulation; Power system harmonics; Pulse duration; Pulse duration modulation; Pulse width modulation; pulse width modulation (PWM); Voltage control
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2018.2792148

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.