Research On Control Strategy Of Three-phase PWM Rectifier Under Unbalanced Grid Voltage

Three-phase voltage-type PWM Rectifier(VSR) control strategies are designed to be balanced by the state power grid design as constraints, However, grid voltage imbalance often happens, if the grid voltage unbalance, the traditional control strategy of three-phase VSR, lead to negative sequence current of three-phase VSR AC, result in unbalanced power grid is more serious, will appear in the DC side voltage of the second harmonic, causing severe distortion, DC voltage by pulse width modulation control, can produce three times in a large number of harmonics on the AC side, seriously affect the control effect.In this paper, the mathematical model of three-phase VSR in unbalanced grid is established,and on the basis of expand the calculation method of positive and negative sequence current component and the control strategy of the research, in order to improve the control performance.Firstly, the first problem of the unbalanced power grid control strategy is separation of positive and negative sequence component calculation. The analysis is based on the traditional trap method and T/3, and in view of the defects of the two methods, puts forward the T/4 delay method,which is based on two phase static(alpha, beta) coordinates, the synthesis of the grid voltage vector delay T/4 fundamental wave cycle to achieve separation of positive and negative sequence components in power grid voltage, thus simplifies the algorithm and to a great extent improve the response speed of the system, and the feasibility and the effectiveness of the proposed method is validated by computer simulation;Secondly, in this paper, the existing grid unbalanced three-phase VSR control strategy are studied, aiming at the existing problem of double current independent control strategy of the inner ring, such as predictive current control strategy was proposed to solve the disadvantage of relatively complex control strategy structure and algorithm and slow dynamic response, the method constitute unbalanced control system in the(alpha, beta) static coordinate system, and we does not need to separate detection of positive and negative sequence current component in the current tracking control, so as to simplify the control algorithm, the strategy greatly eliminate the rectifier dc voltage ripple component, implements the positive and negative sequence current astatic tracking, and improve the system stability, further improve the performance of the system, and the feasibility of this method is verified by simulation.Finally, according to the system requirements, design of three-phase VSR system, control software and hardware design, using real-time RT-LAB-the-loop simulator as a controller test system, to further verify the feasibility of the proposed control method.