SINGLE-PHASE TRANSFORMERLESS UNIFIED POWER QUALITY CONDITIONER BASED ON A FIVE-SWITCH TWO-LEG CONVERTER (FSTL-UPQC).
dc-ac converters, inverters, power conditioning, power quality, UPQC.
The SSTL-UPQC and TL-UPQC, with control strategies based on the traditional space vector modulation technique for three-phase VSI (Voltage Source Inverter) AC-AC converters, are two topologies applicable to single-phase transformerless UPQC. Proposed by Genu (2020) and Lu (2016), respectively, they are cost-effective alternatives compared to the traditional FB[1]UPQC, due to the reduced number of semiconductor switches — six instead of eight — and the elimination of the transformer. However, the SSTL-UPQC is a four-wire topology, which introduces a circulating current through the converter, adding another variable to be controlled and minimized for loss reduction and increased converter efficiency. In this context, there is a need to explore an alternative converter topology for UPQC, also focused on reducing the number of semiconductor devices but, as a means of simplifying implementation, without the circulating current issue observed in the SSTL-UPQC and with a control strategy based on that proposed for TL-UPQC by Lu (2016). This work proposes a new converter topology called FSTL-UPQC, applicable to single-phase transformerless UPQC. The topology has five semiconductor switches and connects to the grid through an LC passive filter and a series capacitor. Two pulse width modulation (PWM) techniques are presented: space vector modulation and scalar modulation, both generalized, i.e., capable of reproducing any PWM technique. In its control strategy, the series and shunt units of the FSTL-UPQC are naturally decoupled, allowing existing control techniques for active filters and DVR to be directly applied. The theoretical details of the proposed topology, along with the presented simulation and experimental results, confirm the functionality of the FSTL-UPQC as a cost-effective, efficient, and simpler implementation alternative when compared to the SSTL-UPQC, TL-UPQC, and FB-UPQC topologies.