Quasi two-level operation of modular multilevel converter for use in a high-power DC transformer with DC fault isolation capability

I. A. Gowaid, G. P. Adam, Ahmed M. Massoud, Shehab Ahmed, Derrick Holliday, B. W. Williams

Research output: Contribution to journalArticlepeer-review

331 Citations (Scopus)


DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.
Original languageEnglish
Pages (from-to)108-123
Number of pages16
JournalIEEE Transactions on Power Electronics
Issue number1
Early online date14 Feb 2014
Publication statusPublished - 31 Jan 2015


  • DC-DC power convertors
  • power transformers
  • power transmission control
  • power transmission faults
  • voltage control
  • DC fault isolation capability
  • DC fault protection
  • DC voltage regulation
  • DC-DC transformer
  • cell capacitance requirements
  • common-mode dc component
  • galvanic isolation
  • high-power DC transformer
  • low-switching frequency
  • modular multilevel converter
  • multiterminal DC grids
  • protection zone
  • quasi two-level operation
  • transient intermediate voltage levels
  • voltage matching
  • Capacitance
  • Capacitors
  • Circuit breakers
  • Harmonic analysis
  • Power transformer insulation
  • Voltage control
  • Dual active bridge (DAB)
  • dc fault
  • dc transformer
  • dc/dc power conversion
  • modular multilevel converter (MMC)


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