Research On Control Strategy Of Three-phase Voltage Source PWM Rectifier

With the development of science and technology, power electronic equipment are widely used in national life, but the increasing reactive power and high-order harmonics which generated by these equipments have resulted severe grid pollution, which affect the power quality of grid. PWM rectifier using full controlled devices has the characteristics of unit power factor operation, electric energy bidirectional circulation and low current THD%, which realize the "green" electric energy conversion. PWM rectifier is widely used in UPS, flexible AC transmission system(AFG, SVG, UPFC), HVDC, new energy power generation, superconducting magnetic energy storage, electrical drive and so on. Control algorithm is the core part of PWM rectifier, which determines the performance of the rectifier, this paper mainly studies the improvement of control algorithm, and the specific content is as follows:Firstly, based on the two-phase rotating coordinate system, the VSR dq mathematical model of three-phase voltage source PWM rectifier is established. Based on VSR dq mathematical model, PI vector control with current closed-loop and the direct power control are analyzed emphatically, including the design of PI control principle and its parameter setting, the calculation of instantaneous power of DPC and grid connected control strategy. Then, the parameters of the DC side capacitor and filter inductor of VSR are analyzed.Secondly, when dealing with the load power fluctuation and high THD% of the grid, the traditional PI vector control is difficult to meet the requirements of the rectifier. By using the load current feed-forward control which can timely reflect the change of the load power, the VSR can adjust the power in time to reduce the fluctuation of DC voltage. But feed-forward control requires inner current loop which must have fast response speed, thus sliding mode variable structure control based on PID reaching law is used in current inner loop. The scheme not only has fast response speed and the bility to suppress harmonic disturbance, but also can sufficiently reduce chattering problem of SMC and enhance the stability of system. MATLAB simulation results prove that the optimization scheme proposed in this paper that can effectively solve the voltage fluctuations and high current THD%, which is caused by the load power of non-periodic dramatic changes and harmonic interference of power grid, the VSR has achieved good performance.Thirdly, the principle of model predictive direct power control(MPDPC) and voltage vector selection of switching table are introduced. MPDPC which is based on the combination of model predictive control(MPC) and direct power control(DPC), can effectively solve uncertainty and nonlinear problems in control process. It is easy to handle controlled variables and operating variables for any constraint conditions and solve the problem of DPC steady state ripple. However, when the variation of the load power changes violently, the fluctuation of the DC voltage and the AC input power is large, and the adjustment time is too long. The reason is that the setting of objective function is not reasonable. When load power change, the control variables produce mutual interference, which affects the performance of the rectifier. In this paper, we put forward the strategy of adding weighing factor in the objective function, which can effectively solve the problem of the interference between variables. MATLAB simulation results show that the voltage fluctuation and the power fluctuation of MPDPC with weighting factor are small, and the expected results are obtained.Finally, a multi-function experimental platform is established, and then the software algorithm is designed. The experimental results of two schemes show that both schemes have achieved the desired experimental results and verified correctness of the theory analysis and simulation.