A PWM rectifier control technique for three-phase double conversion UPS under unbalanced load

PWM rectifiers are widely used in three-phase ac-dc-ac systems due to its capability in dc voltage boost and regulation, input power factor correction, and input current harmonic control. However, with the conventional rectifier control technique, the input current tends to be unbalanced under unbalanced inverter load, which contaminates input power source and is therefore undesirable. In this paper, the cause of the unbalancing is disclosed by evaluating the spectra of the switching functions of the full bridge three-phase inverter analytically using Bessel function under standard space vector PWM switching scheme, which relates the dc link current and voltage ripples to the inverter load balancing. The analysis shows that the dc link voltage contains significant second order harmonic component which affects the voltage loop of the rectifier controller, especially when the control gain is high. A notch filter based voltage control loop is proposed to eliminate the second harmonic component in the dc-link voltage feedback signal and achieve balanced three-phase input currents. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed control technique in decoupling the rectifier and the inverter under unbalanced load.