Research On Control Strategy Of Three-phase Four-wire High Power Density VIENNA Rectifier

With the increasing consumers for electricity and requirement to the electric energy quality, the issues of harmonic currents, EMI and the decrease of power factor become more and more serious due to the wide use of power electronics. Rectifiers play a vital role in harmonic pollution treatment of electric power systems as an important interface to grid. The three-phase four-wire VIENNA topology has been widely used in medium-power applications like the communication field, aircraft equipment and UPS systems for its unique structure and advantages over unity power factor, high efficiency and power density.In order to reduce the volume of the VIENNA rectifier and increase the power density, the LCL filter is used here. The inductance of the input filter is also reduced to increase the power density, which makes the converter operating in mixed conduction mode(MCM). Based on the decoupling characteristics of three-phase four-wire VIENNA rectifier, the equivalent single-phase three-level VIENNA rectifier can be obtained. Then the mathematical model is derived. Since transfer function of the current loop jumping between CCM to DCM mode, the rectifier cannot get good performance by the traditional control strategies in MCM mode.Therefore, an improved control strategy with feed-forward control in CCM mode is proposed to eliminate the influence of input voltage and output voltage on the inductor current. Comparing the difference between small-signal models in CCM and DCM mode, a correction factor is added to the feed-forward control strategy in CCM mode as a new control strategy. This control strategy can switch smoothly between different operation modes without the need of mode detection circuit or complex extraction operation, which is the burden of digital controller. This correction factor is from the correction of inductor's average current sampling. After making these corrections, the new control strategy improves the stability and performance of the VIENNA rectifier working in MCM. What's more, the range of the load is also broadened by the use of improved control strategy.This paper presents a mathematical model of VIENNA rectifier in CCM and DCM mode. Improved feed-forward control scheme and LCL filter design method are given out in detail. Finally, the Simulink simulation results further illustrate and verify that the new control loop can ensure a higher power factor and lower input current harmonics when rectifier operates in mixed-conduction mode.