Isolated single-phase single-stage DC-AC cascaded transformer-based multilevel inverter for stand-alone and grid-tied applications

Abstract

This paper proposes a new single-phase DC-AC Cascaded Transformer-based Modular Multilevel Inverter (CTMLI) with its PWM switching technique considering minimum components count per output voltage level. The proposed CTMLI converts the constant input DC voltage to 19-level AC voltage at its output terminals in a single-stage using only three full-bridge (FB) circuits in addition to three single-phase cascaded transformers. The three FB circuits are connected in parallel to the same DC supply, whereas the output terminals of the cascaded transformers are connected in series. In order to realize the 19-level output voltage, each FB circuit is controlled by different switching frequency and each single-phase transformer has different turn’s ratio. Detailed mathematical model of the proposed CTMLI is presented considering the optimal transformer turn’s ratio in addition to the power distribution among them. Also, cost estimation model for the MLI is presented in details in order to investigate the cost reduction in the proposed CTMLI. In addition, a detailed comparison with the recently published MLI is presented to confirm the merits of the proposed inverter. The validity of the proposed CTMLI has been investigated experimentally using laboratory prototype (220 V, 1.2 kW) controlled with dspace DS-1103 digital controller in stand-alone applications considering closed-loop voltage control and in grid-tied applications considering closed-loop current control.