Low switching frequency active harmonic elimination in multilevel converters with unequal DC voltages

This paper presents an active harmonic elimination modulation control method for the cascaded H-bridges multilevel converter when supplied by unequal DC sources. First, the multilevel converter is decoupled into individual unipolar converters, and the low order harmonics (such as the 5th, 7th, 11th, and 13th) are eliminated by using resultant theory while at the same time the minimum specified harmonic distortion (HD) of a combination of unipolar converters for multilevel converter control is found. Next, the switching angle sets corresponding to the minimum harmonic distortion are used as initial guesses with the Newton climbing method to eliminate the specified higher order harmonics. If the solutions are not available for some modulation indices, the unipolar switching scheme is used to eliminate high order harmonics and the active harmonic elimination method is used to eliminate low order harmonics. This method has lower switching frequency than that of the previously proposed active harmonic elimination method.