Effect of redundant sub-module utilization on modular multilevel converters

Abstract

This paper discusses the operation of the modular multilevel converter (MMC) when redundant switching sub-modules are utilized within the arms of the converter. The redundant switching sub-modules in the arm are not used in order to achieve a higher number of levels at the output of the converter, but to improve the reliability of the topology and reduce the switching frequency of the individual components. Moreover, the use of the redundant switching sub-modules leads to lower second order harmonics in the leg circulating currents and reduced switching frequency in the sub-modules while avoiding consecutive switchings of the same sub-module. Simulation results of a 21-level MMC using configurations with 20, 22 and 24 sub-modules per arm are presented.