Single-Phase AC-DC-AC Multilevel Converter Based on Three-leg Modules Series-Connected to H-bridges through Transformers

This paper investigates two single-phase ac-dc-ac multilevel converter topologies composed of a three-leg module series-connected to two H-bridges through transformers, sharing the same dc-link. The proposed converters can be used to mitigate grid voltage disturbances such as sags/swells and voltage harmonics because the transformer at the grid side increases the available voltage in the input of the converter. Besides, the turn ratios of transformers can be chosen in order to increase the number of levels of voltages synthesized by the converter and to reduce the total harmonic distortions. They are appropriated for applications such as uninterrupted power supply and unified power quality conditioner with equal or different voltage levels at the input and output. Circuit model equations, converter ratings and a space vector PWM strategy are discussed. The proposed system control ensures fixed load voltage with constant amplitude and frequency, maximizes the grid power factor and regulates the dc-link voltage. Comparison in terms of harmonic distortions, semiconductor power losses and switching frequency are carried out. Simulation results validate the theoretical studies.