Multilevel Inverter with Three Stage Hybrid Cascading using PWM Control Method

Electrical and Electronics Engineering

The inverter power circuit is made up of 18 level hybrid cascaded multilevel scheme. The three hybrid inverter stages are the high, medium, and low voltage stages. The high-voltage stage is made of a threephase conventional inverter to reduce dc source cost and losses. The medium- and low voltage stages are made of three-level inverters constructed using cascaded Hbridge units. The aim of this control is to avoid undesirable high switching frequency for high and medium-voltage stages despite the fact that the inverter’s dc sources are selected to maximize the inverter levels by eliminating redundant voltage states. Switching algorithms of the high and medium-voltage stages have been developed to assure fundamental switching frequency operation of the high-voltage stages and not more than few times this frequency for the medium-voltage stage. The low-voltage stage is controlled using PWM control to set the order of dominant harmonics as desired. The main and H-bridge cells are fed by isolated dc sources of 9V s, 3V s, and V s. This voltage ratio provides 18-level inverter. In this design, the high voltage stage has only one dc source that operates with reduced current ripple compared to the three dc sources of CHB design. The inverter has been designed with one main dc source to reduce the dc supply cost, and the supply voltage ratio has been selected to maximize the number of symmetrical levels.

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