Modular multilevel converter designs for medium-voltage machine drives

  • Mohamed Said Ibrahim Mahmoud Diab

Student thesis: Doctoral Thesis


Various industry sectors are now adopting high-power applications that involve power electronics on a large scale. This increases the power level requirement of the adjustable-speed drives utilized in such applications to scores of megawatts, triggering the need for high-power driving at the medium-voltage (MV) level. Thus, this thesis addresses the modular multilevel converter (MMC) as a promising topology for MV machine drives.;A special focus is given to the variable-speed operation of MMC-fed three-phase machines, where the MMC experiences a serious challenge when operating at low frequencies. The MMC's unique operation principles imply its floating submodule (SM) capacitors experience voltage fluctuation at the fundamental operating frequency. At lower operating frequencies, MMC capacitor voltage fluctuations can reach extreme levels, inversely proportional to the operating frequency, where increasing the SM capacitance for voltage ripple compensation is not viable.;In this context, this thesis proposes an approach for SM capacitor voltage ripple compensation, which employs magnetic chain links involving dual half-bridges (DHBs) to interface the SMs, for ripple power decoupling. The approach is able to transfer the ripple power of one SM to another in a bidirectional fashion, countering the energy fluctuation. This allows for an even capacitive energy distribution across MMC SMs, independent of the operating frequency, resulting in a significant reduction in SM capacitance. A class of different DHB modules allocations relevant to the MMC SMs is investigated.;Also, this thesis addresses MMC application for multiphase machine drives, where reduced topological designs derived from the standard MMC are presented for six-phase machine drives. The topologies are enhanced to realize a more-economic six-phase machine drive without any performance compromise.;Analytical investigations are derived for the different approaches presented in this thesis, while necessary control algorithms are analyzed. The viability of proposed concepts and approaches are substantiated through both simulation and experimentation.
Date of Award29 Apr 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorDerrick Holliday (Supervisor) & Barry Williams (Supervisor)

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