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A selective harmonic elimination (SHE) control strategy on a three-phase four-leg inverter is reported in this paper. The control signals of the four legs are calculated as follows: 1) control signals of three legs are mathematically expressed using Fourier-based equations on line-to-line basis as conventional SHE technology, which eliminates lower order nontriplen harmonics, and 2) the fourth leg produces the harmonics that are equal to the lower triplen harmonics produced by the other legs. The inverter has almost symmetric three-phase output voltage with unbalanced load because the fourth leg provides a low-impedance path for the triplen harmonics. The switching-to-fundamental frequency ratio is only 13 and 27 (per unit) p.u. for the three legs and the fourth leg in the proposed prototype, which is with the total harmonic distortion of output voltage lower than 4%. Selected solutions for the switching angles are presented and verified by simulation and experimentation in order to confirm the effectiveness of the proposed scheme.
Date of Publication: July 2009