Topology and Modulation Strategy of Asymmetric Three-unit Thirteen-level Inverter

Compared with symmetrical cascade H-bridge multilevel inverters, hybrid cascade H-bridge multilevel inverters can use fewer H-bridge units to obtain higher output voltage. However, the traditional hybrid cascade H-bridge multilevel inverters have the phenomenon of current inversion during the unit time. In order to solve the above problems, this paper improves the type Ⅲ inverters with 1:3 voltage ratio, and puts forward a kind of asymmetric three-unit 13-level inverters with 3:2:1 voltage ratio. At the same time, a hybrid modulation strategy is proposed for the inverter, which combines the advantages of ladder wave and PWM wave. The partial pulses obtained under this modulation strategy are logically calculated and distributed, which solves the problem of current inversion in traditional type Ⅲ inverters and improves the power quality of output voltage. With this hybrid modulation strategy, the high-voltage unit works at a lower frequency and the low-voltage unit works at a higher frequency to ensure the output voltage quality of the inverter. Finally, the validity and feasibility of the proposed inverter topology and the hybrid modulation strategy are verified by simulation and experiment.