Feasibility of capacitor voltage regulation and output voltage harmonic minimization in cascaded H-bridge converters

Multilevel converters have found popularity in electric-drive vehicles. In cascaded H-bridge multilevel converters, it is advantageous to use only one DC voltage source for the main H-bridge cell and supply the rest of the cells with capacitors. However, regulating the capacitors' voltage level becomes a challenging issue. This paper studies the existing constraints on regulating the capacitor voltage levels especially with respect to the elimination of the fifth and seventh harmonics. Detailed as well as simplified analyses are presented and verified. In addition, the constrained nonlinear optimization method is used to find the switching angles that regulate the capacitor voltage, eliminate the fifth harmonic, and minimize the combination of seventh and eleventh harmonics.