Elimination of Subtractive and Quasi-Redundant Levels in Multi-Cell Converters

The multi-cell converter, which is becoming increasingly popular in the medium voltage drives market, possesses a number of undesirable switching states which diminish efficiency of the system. These conditions lead to circulating power among the cells, DC-link instability and superfluous switching between states. In this paper the undesirable states are identified and the concept of subtractive levels is introduced. Calculations show the impact of eliminating these states on the number of available output voltage levels for three-and five-cell converters. Through simulations, optimized DC-link configurations are identified and investigated in a power converter model. DC-link stability can be significantly improved by the exclusion of subtractive levels, while THD is not affected much. Converters which use four-quadrant supplies for the cells can achieve efficiency improvements by avoiding power conversion loops between the cells and simpler converters using single quadrant supplies can minimize DC-link capacitive energy storage.