Power loss minimization in cascaded multi-level converters for distribution networks

This paper compares two structures of cascaded multilevel converter for 11kV and 33kV distribution networks for the two main competing IGBT technologies (Non Punch Through and Punch Through) with the intention of minimizing power loss. The traditional cascaded converter, which has equal-sized cells in its chain and the recently proposed chain with a ternary relationship between its dc-link voltages are both investigated. Models with 81 and 27 levels are developed for both kinds of converter after selection of suitable IGBT devices for comparative analysis. The total power losses in the IGBTs and diodes of each cell in the chain are estimated by simulation (160 separate cases) and it has been concluded that the conduction losses are dominant in both types of converters in NPT and PT IGBTs for 11kV and 33kV systems. The results have shown that the equal-sized converter is only likely to be useful in one case (27-levels in the 33kV system) whereas the ternary-sequence converter produces lower losses in all the other cases. Further, it was found that the PT IGBT is the better option for ternary sequence whereas the NPT IGBT is better for the equal-sized converter.