Modular Multi-level Converter for Medium Voltage Applications with Mixed Sub-module Voltages within Each Arm

Modular Multi-level Converters (MMC) for medium voltage applications need to be compact and low-loss but also must have good waveform quality. This makes the choice of Sub-Module (SM) voltage rating and quantity a challenging compromise. An extra degree of design freedom is possible if modules in each arm are a mixture of nominal voltage ratings. A design is investigated here with several high voltage SMs for bulk power processing and two low voltage SMs for waveform following control. Power loss, efficiency and total harmonic distortion are compared against two designs of conventional MMC of equivalent power rating and waveform distortion level, but different SM voltage choices. A new low-level controller for SM rotation is devised which deploys different SM types for the different purposes, but also ensures charge balancing. Simulation results show reduced power loss for an MMC with mixed voltage-rated SMs within the same arm, when compared against the conventional MMCs.