A novel PWM rectifier control technique for on-line UPS under unbalanced load based on DSP

Abstract

Three-phase ac-dc-ac voltage stiff systems consisting of a front-end rectifier, a dc link with a capacitor, and an inverter are widely used in on-line UPS, motor drive, and distributed generation systems. A PWM controlled front-end rectifier is desirable in three-phase ac-dc-ac systems. With the conventional PWM 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, a novel PWM rectifier control technique for on-line UPS under unbalanced load is presented, 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. First of all, three-phase on-line UPS structure based on DSP is given. secondly, the principles of this system is analyzed, which 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. Finally, the effectiveness of the proposed control technique in decoupling the rectifier and the inverter under unbalanced load are presented to demonstrate through both simulation and experimental text results.