Oral presentation at IEEE Applied Power Electronics Conference (APEC’05) Austin, Texas, USA, March 6-10, 2005 A PWM Rectifier Control Technique for Three-Phase Double Conversion UPS under Unbalanced Load Min Dai, Mohammad N. Marwali, Jin-Woo Jung, and Ali Keyhani The Ohio State University 205 Dreese Laboratory, 2015 Neil Avenue Columbus, OH 43210, USA Keyhani.1@osu.edu Abstract— PWM rectifiers are widely used in three-phase ac- dc-ac double conversion UPS systems due to its capability in dc voltage boost and regulation, input power factor correction, and input current harmonic control. However, with the conven- tional 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. I. I NTRODUCTION A three-phase ac-dc-ac voltage stiff double conversion UPS system consists of a front-end rectifier, a dc link with a ca- pacitor, and an inverter. A PWM controlled front-end rectifier is desirable due to its capability of dc voltage boost and regulation, input power factor correction (PFC), and input current harmonic control. Conventional PFC boost rectifier controllers usually have two feedback loops - an outer voltage loop and an inner current loop, where the voltage regulator generates current command for d-axis current while the q- axis current command is zero for unity power factor control as shown in Fig. 1. Under normal operating conditions, steady state dc bus voltage is a constant and the voltage regulator output, i.e., the d-axis current command, is also a constant, which yields a constant power drawn from the input ac stage and balanced three-phase input currents. However, once the inverter load is not balanced, the output power is no longer a This research is supported by National Science Foundation under the grant NSF ECS0105320. constant, which leads to fluctuation of the dc link voltage. On the rectifier side, the ripple corrupted dc link voltage is fed back to the voltage regulator which generates a fluctuating d- axis current command under a constant dc voltage reference. If the current regulator of d-axis has high bandwidth, it yields fast current tracking and consequently a fluctuating rectifier output current which causes unbalanced front-end input current and high total harmonic distortion (THD) in the input current. This is considered contamination of power system if the front-end is fed by utility and therefore undesirable. Although this particular problem has not been addressed in literature, related researches have been conducted on rectifier control under unbalanced input voltage conditions [1]–[4]. Kamran et al. [5] have mentioned dc voltage ripple problem caused by either unbalanced inverter load current or unbal- anced input voltage supply. However, their control goal was to minimize the dc link voltage ripple instead of improving the input power quality. Some other researches focused on improving instantaneous power balance between the input and output of a rectifier-inverter system and minimizing the dc coupling capacitance to reduce the cost [6]–[10]. The less the dc coupling capacitance is, the better instantaneous power balance the system could yield. However, this is only desirable under balanced load. Once the inverter load is unbalanced, it is apparent that the steady state inverter output power is no longer a constant, and neither is the inverter input dc power. Due to the existence of the dc link capacitor, the rectifier side steady state power can be decoupled from the inverter side and controlled to be a constant without being affected by the power fluctuation on the inverter side, which will lead to balanced front-end three-phase input currents. To achieve this goal, a new rectifier controller has to be designed to enforce constant input power and not to respond to the dc voltage ripple. In this paper, a switching function concept will be used under standard space vector PWM to quantify the harmonic components in the dc link. According to the analysis result, a notch filter will be designed and applied to eliminate the