AC-DC-AC multilevel converters based on three-leg converters

This paper presents ac-dc-ac multilevel converters for single-phase and three-phase applications obtained from the cascade of three-leg converters. Differently from the more usual series connection based configurations, like dc-ac modules in H-bridge multilevel configurations, the series connection arrangement treated in this work means in series connections of the ac-dc-ac modules. The converters are suitable to be used as either uninterrupted power supply (UPS) or unified power quality conditioner (UPQC) without isolation transformer. PWM and control strategies suitable to provide the balanced control of the dc-link capacitors and the optimization of the harmonic distortion and converter losses are proposed. Simulated and experimental results are presented to validate the theoretical considerations.