Three-Phase Four-Wire Transformerless Unified Power Quality Conditioner Based on AC-DC-AC Nine-Leg Converter and Shunt Converter

In this paper, a three-phase transformerless unified power quality conditioner (UPQC) based on a nine-leg converter and a four-wire shunt converter is proposed and investigated. The system ensures grid power factor compensation with low harmonic content and feeds the load with a sinusoidal waveform. The shunt converter operates with a low switching frequency and provides balanced currents, which allows the nine-leg converter to operate with a wide range of unbalanced loads, while compensating for the harmonic content. In addition, the shared-leg currents are minimized, reducing the power losses. Compared with the conventional transformerless UPQCs, the proposed converter achieved lower overall semiconductor losses under a severely unbalanced load. Furthermore, it has also been able to reduce the size of DC-link capacitors when compared with conventional converters. The system model, pulse-width modulation (PWM) techniques, and control strategy are discussed in this work. Simulations and experimental results are presented to confirm the feasibility of the proposed structure and the correctness of the design methodology.