Second Harmonic Suppression for DC Output Voltage of Three-Phase Four-Leg PWM Rectifier Under Unbalanced Grid Voltage Conditions

This article proposes a control strategy for three-phase four-leg PWM rectifier under unbalanced grid voltage conditions, which aims at achieving not only no second harmonic in dc output voltage, but also unity power factor operation for each phase. First, the amplitude-phase-locked loop is designed based on three-dimensional unbalanced coordinate transformation to detect the phases of three-phase unbalanced grid voltages, which can be selected as the reference phases of three-phase currents to achieve unity power factor operation for each phase. The second harmonic in dc output voltage can be detected based on Fourier transform principle, and the linear control laws of current amplitude ratios can be designed to suppress second-harmonic ripple completely. The above control objectives for four-leg PWM rectifier can both be achieved with a small dc-link capacitor and without the introduction of an active power compensator, which helps simplify circuit structure and improve power density. The simulation and experimental results are provided in the end to verify the feasibility and effectiveness of the proposed control strategy.