Research On Unbalance Control Strategy Of Wide Frequency Convertion Three-phase Buck Rectifier

In recent years,the rapid development of aircraft AC power system has made PWM rectifier with power factor correction function widely used.Three-phase Buck rectifiers have features that three-phase Boost rectifiers do not have,such as wider output voltage adjustment range,fast dynamic response,controllable DC current,reliable short-circuit current limiting capability,and easy parallel connection to increase capacity.In the actual power supply system,various factors will inevitably cause unbalanced three-phase input voltage.If the control method under balanced conditions is used,the input current on the AC side of the rectifier will have high harmonic content and serious distortion,and the output voltage on the DC side will contain a large number of even-order non-characteristic voltage components.In order to ensure the stable and reliable operation of the three-phase Buck rectifier when the input is unbalanced,and to ensure the good input and output performance of the rectifier,it is of great theoretical and practical significance to carry out research on the three-phase Buck rectifier when the input is unbalanced.This paper takes the three-phase Buck rectifier as the research object,and analyzes its topological structure and working principle in detail.On this basis,different mathematical models are established in the three-phase a-b-c static coordinate system,and the cycleaveraged model suitable for the dynamic and static characteristics analysis and controller design of the three-phase Buck rectifier control system is obtained.A 12-sector symmetrical five-segment SVPWM modulation of three-phase Buck rectifier is studied,and its working principle and implementation are analyzed in detail.Aiming at the over-modulation phenomenon of SVPWM technology,its influence on the three-phase Buck rectifier is analyzed in detail.On this basis,a simple overmodulation suppression algorithm is proposed,which can ensure that the three-phase Buck rectifier works in the linear modulation region under all conditions from both steady-state and dynamic aspects.The unbalance control strategy of three-phase Buck rectifier under wide frequency conversion conditions is mainly studied.Using the switching function matrix and the analysis method of positive and negative sequence decomposition,the mechanism of even-order harmonic generation in the DC output voltage when the three-phase input voltage is unbalanced is determined.On this basis,in order to eliminate the harmonics in the DC output and obtain high-quality input current,a switching function calculation method with unbalance suppression effect is proposed.The influence of the control strategy is suitable for wide frequency conversion conditions.An implementation method of the control strategy in digital system is introduced from the aspects of SVPWM technology and software algorithm design,and the influence of unbalance degree on the maximum DC output voltage of the rectifier is briefly described.Finally,the power device stress of the three-phase Buck rectifier and parameters such as the LC filter on the AC side,the inductance on the DC side,and the output capacitor are calculated,the software and hardware design of the three-phase Buck rectifier is completed,and the principle of a 1500 W three-phase Buck rectifier is built.Prototype,the validity and feasibility of the overmodulation suppression algorithm and the unbalance control strategy are verified by experiments.