A Single-Phase ac-dc-ac Unidirectional Three-Leg Converter

This article presents a single-phase ac-dc-ac unidirectional three-level three-leg converter. It is composed by a unidirectional T-type leg at the input side, a bidirectional T-type leg at the load side and a bidirectional T-type leg that is shared between the input and load. The proposed converter allows feeding the load voltage with a sinusoidal voltage that has both constant amplitude and frequency. It also keeps the well-known merits of a T-type leg, such as lower conduction losses and better power quality, associated with the reduced component count provided by the shared-leg. Therefore, the proposed converter is a good candidate for ac-dc-ac low-medium-voltage applications, in which efficiency and power quality are major selection criteria. A detailed analysis of the converter is provided, including the converter model, a complete control strategy and a space-vector pulsewidth modulation (PWM) technique. The space-vector PWM technique was developed aiming to reduce switching losses, while ensuring five-level multilevel voltages at both the grid and load sides, and equalizing the unbalance caused by the mid-point dc-link capacitor's connection. In order to validate the theoretical analysis, simulated and experimental results are presented and discussed.