Six-Leg Single-Phase AC–DC–AC Multilevel Converter With Transformers for UPS and UPQC Applications

This article investigates a single-phase ac–dc–ac multilevel converter composed of two three-leg converters series-connected throughout two transformers (one on the grid side and one on the load side), sharing the same dc-link. The proposed converter allows to feed the load with sinusoidal voltages (keeping constant amplitude and frequency) under grid voltage disturbances, as sags, swells, and harmonics. Besides, the converter can operate with a sinusoidal grid current with a high power factor and can be applied to systems with either equal or different voltage levels at the input and output. Circuit model equations, pulse width modulation (PWM) technique based on space vector approaches, and a control system are presented. The proposed topology is compared with the conventional one in terms of harmonic distortion, semiconductor losses, transformers losses, and total power losses. Simulation and experimental results are presented under the same conditions to validate PWM and control strategies.