Journal of Power Electronics

The Korean Institute of Power Electronics (전력전자학회)
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Modulation signal analysis and parameter selection for modular multi-level converters with harmonic injection method

  • Zeng, Qifan (National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering) ;
  • Xiao, Fei (National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering) ;
  • Ren, Qiang (National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering) ;
  • Ai, Sheng (National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering)
  • Received : 2019.09.16
  • Accepted : 2019.11.07
  • Published : 2020.03.20
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Abstract

The harmonic injection method is widely used in modular multilevel converters (MMCs) to suppress the significant amplitude of the submodule (SM) capacitor voltage fluctuation under low-frequency operation. A method is required to inject high-frequency common-mode voltages and circulating currents, which both occupy modulation space and are prone to over-modulation. This paper focuses on the modulation signal required for circulating current injection, which has been neglected in previous studies. In addition, this paper redefines the amplitude references of the injected harmonics to reserve modulation space for the injected circulating current. By analyzing the characteristics of an MMC system with the harmonic injection method, the relationship between the modulation signal for the circulating current and other variables (such as the output modulation index, output frequency and injection frequency) is obtained. Using the obtained relationship, a strategy for selecting the amplitude and frequency of the injected harmonics is presented to avoid over-modulation under variable conditions while suppress the SM capacitor voltage fluctuation. The theoretical analysis, the proposed injection method and its parameter selection strategy are verified by experimental results.

Keywords

Acknowledgement

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant 51907200 and the Science Foundation for Distinguished Young Scholars of China (No. 2018-JCJQ-ZQ-002).

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