Performance analysis of flexible multi-level converter for high voltage photovoltaic grid-connected system

Multi-level converters are widely employed in medium voltage grid-connected PV systems, since they feature better power quality and higher conversion efficiency compared with two-level converters. The conventional two-stage PV converter configuration with additional high step-up elements can satisfy the wide input voltage requirements, but results in reduced conversion efficiency and increased cost. In this paper, a flexible multilevel converter is proposed and its topology derivation process is introduced. The proposed flexible multilevel converter can adaptively work as a single-stage three-level NPC converter under high PV voltage and as a two-stage five-level converter under low PV voltage. Therefore, it realizes high conversion efficiency during a wide PV voltage variation. Furthermore, the modulation scheme of the presented converter is analyzed in detail. Moreover, the power losses with MATLAB/PLECS simulation and fault-tolerant performance of this converter is explored. Finally, a test bench is built to validate the theoretical analysis of the proposed flexible multilevel converter.