Reduced switching-frequency modulation and circulating current suppression for modular multilevel converters

Qingrui Tu, Zheng Xu, Lie Xu

Research output: Contribution to journalArticlepeer-review

1087 Citations (Scopus)


This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.
Original languageEnglish
Pages (from-to)2009-2017
Number of pages9
JournalIEEE Transactions on Power Delivery
Issue number3
Publication statusPublished - Jul 2011


  • switching-frequency modulation
  • circulating current suppression
  • modular multilevel converters
  • switching frequency
  • voltage control
  • capacitors
  • converters
  • pulse width modulation
  • switches

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