Obtaining Low Distortion Load Voltages from Y-Connected Staircase Modulated Multilevel Inverters for AC Microgrids

Dc sources such as photovoltaic arrays and fuel cell stacks are of relatively low voltage. So using multilevel inverters to interface these clean sources to an ac microgrid is an approach that has several advantages. While much research has been performed on multilevel inverters during the past twenty years or so, a study of the total harmonic distortion attainable by the line-load neutral voltages produced by staircase modulated three-phase inverters with more than five levels has not yet been performed. But such knowledge is important in light of the IEEE 519-2014 standard, which prescribes distortion limits. This paper presents results on finding the minimum total harmonic distortion (and the necessary waveform parameters) achievable by such voltages produced by Y-connected staircase (block) modulated multilevel inverters (up to nine levels per phase), firstly for equal dc sources, secondly for unequal dc sources, and finally for the case of partially unequal dc sources, which yields an inverter with increased modularity. The results show that 7- and 9-levels are the minimum necessary to satisfy IEEE 519-2014 for low- and medium-voltage ac microgrids, respectively. In addition, it is shown that the total harmonic distortions of the line-load neutral voltages and of the corresponding line-line voltages are equal. Simulation results are presented to support the analysis, along with experimental results that corroborate the simulations.