Single-Phase Transformerless Unified Power Quality Conditioner Based on Three-Leg and Standby Converters

In this paper, a transformerless single-phase-to-single-phase unified power quality conditioner (UPQC) based on three-leg and standby converters is proposed. The system consists of a two-leg module series connected to a three-leg module. The proposed control system provides shunt and series compensation and can be designed to operate with a high modulation index under rated conditions and voltage sag events. In addition, a low standby power consumption and a new decoupling method to compensate voltage swells using the two-leg module are proposed. Compared to conventional solution, the proposed converter exhibits lower semiconductor losses and reduced waveform harmonic distortion, while maintaining equal or superior operational features, including voltage swell compensation capability and the elimination of circulating currents and line-frequency transformers. The system model, the scalar PWM technique, the applied standby concept, and the overall control system strategy are discussed. Simulations and experimental results are presented to evaluate the feasibility of the proposed system and the correctness of the design methodology.