Asymmetrical multilevel inverter using ratio-three based sources

Abstract

This paper proposes a base-three ratio for the selection of asymmetrical multilevel inverter voltage source. The advantage is the optimum output voltage quality for a particular power circuit configuration. The series H-bridges topology with four-cell per arm inverter has been investigated. This configuration conventionally forms a 9-level inverter when used in symmetrical multilevel inverter. However with the proposed voltage selection in asymmetrical multilevel inverter, the number of levels increases to 81. A lookup table-based controller has been designed and shown to provide high quality output voltage for a wide range of modulation index. Simulation has been conducted to show that the optimum output voltage quality has been produced. Examination of the switching signals reveals that the higher voltage cells operate at lower frequency. Investigation on the input currents of the inverter shows that the higher power H-bridge cells require unidirectional current if the load power factor is high enough.