THD-optimal line-load neutral voltages from three-phase multilevel inverters

For relatively low voltage dc sources such as photovoltaic arrays and fuel cell stacks to supply power for servicing typical AC loads, using multilevel inverters is an approach with several advantages. Under staircase/block modulation of these inverters (to reduce switching losses), it is not yet known what´s the minimal total harmonic distortion achievable by the line-load neutral voltages obtained from such inverters. This paper describes the analysis of the distortion of line-load neutral voltages obtained from three-phase staircase modulated multilevel inverters, first for equal dc source ratios, then for unequal source ratios, and finally for a partially unequal source ratio case, which is motivated by the advantages obtained by increased modularity. Simulation results are presented to support the analysis.