Three-Phase Unified Power Quality Conditioner Based on High-Frequency Link

In this article, a unified power quality conditioner (UPQC) that does not employ line-frequency transformers (LFTs) is proposed. In the proposed UPQC, the dc links of the series H-bridge cells are fed from the dc link of the three-phase shunt cell with a unidirectional dc–dc converter isolated with a multiwinding high-frequency transformer (HFT). The three-phase shunt cell is able to compensate reactive power, current harmonics, and unbalances from the load, while the H-bridge series cells compensate voltage disturbances such as sags and swells. A control strategy of the dc-link voltages of the series cells is proposed to allow the compensation of unbalanced disturbances, and strategies to extend the operation of the converter to also compensate voltage swells are discussed. Compared with the UPQC topologies available in the literature, the proposed one does not require LFTs and does not present the operational problems of the transformerless solutions such as limitations of series compensation and balancing issues. Also, it requires fewer controlled switches than the HFT-based solutions in the literature and does not require control in the dc–dc converter. Converter model is presented as well as pulsewidth modulation, control strategy, and experimental results for validation.