Modulation, Control and Performance Analysis of Asymmetrical Modular Multilevel Converters (A-MMCs)

Abstract

Modular multilevel converters (MMCs) are preferred converters for implementing high-power multilevel systems. The penalty they impose is the high number of devices needed to build them. To offset this challenge, this thesis introduces Asymmetrical Modular Multilevel Converters (A-MMCs). Unlike MMCs, each A-MMC module comprises of two half-bridge submodules, rated at asymmetric voltages. This system offers benefits like: (a) generation of four distinct voltage levels using one module; (b) 33% lesser semiconductor and gate drive requirement; (c) higher system efficiency; (d) reduction in overall cost and size. Hybrid Pulse-Width Modulation (Hybrid PWM) has been deployed to generate the drive pulses. To maintain asymmetric voltages, a novel voltage balancing algorithm has been proposed. Circulating current controller has been designed as well. The operation and performance validation was done using MATLAB Simulink and PLECS Blockset. A comparison, vis-`a-vis the MMC, was also performed, based on thermal performance and essential circuit voltages and currents.