Six-Leg AC-DC-AC Single-Phase Multilevel Converter for Grid Overvoltage Mitigation

This paper proposes a six-leg single-phase ac-dc-ac multilevel converter that is capable to mitigate fundamental overvoltage and voltage harmonics in the grid. This configuration is composed of two three-leg converters series-connected throughout one transformer at the grid side. From this topology, a generalized configuration is also analyzed. The converters ensure regulated load voltage (with fixed amplitude and frequency) and grid current with high power factor and low harmonic content. Thus, the converters are suitable to operate as a unified power quality compensator (UPQC) or uninterrupted power supply (UPS). System equations, two pulse-width modulation techniques and a control system for regulating the dc-link voltages and to decrease the harmonic distortion are developed. The proposed topologies are compared with the conventional in terms of harmonic distortions and semiconductor losses. Simulation and experimental results are provided to validate the feasibility of the system under grid fundamental overvoltage.