Transformer-less single-phase unified power quality conditioner of no circulating current

This study presents a unified current-source power quality conditioner of inherent no circulating current. It is formed of a three-leg current-source ac/ac converter and a specially designed small size dc-link. Owing to the transformers-less feature and small size dc-link, the proposed topology has the benefit of high power density. In addition, without the need to address the circulating current issue, it is easier to design the controllers and filters for the circulating current compared to other existing transformer-less unified power quality conditioners. To handle the harmonic currents compensation, voltage sags/swells elimination, power factor correction simultaneously and achieve excellent dynamic and steady-state performance, a direct control method based on an extended repetitive disturbance observer is proposed. Detailed analysis of topology and operation principles is given, followed by the modulation scheme and the developed control method. Simulations and experimental results are presented to verify the effectiveness of the conditioner.