Research On PWM Rectifier And Its Nonlinear Control Strategies

Traditional un-controlled diode rectifier and phase controlled rectifier consisted of half-controlled thyristor, have some drawbacks:low ac-side power factor, big current ripple and un-revisable power flow. PWM rectifier self-turn-off-device can make the grid current approach to sine wave,grid power factor come to 1,four quadrant circulate,actively eliminate ac-side harmonic pollution of inverter and economize energy resources. It has many advantages such as rapid motion response,DC voltage controllable and load disturb resistible.In this paper, the author lays a strong emphasis on study of three-phase voltage source PWM rectifier and its control methods. The four-quadrant working principle of PWM rectifier has been explained here, and the control goals of PWM rectifier will be achieved as long as the ac-side current has been controlled. The paper deduces the math model based on switch function under reference frame (a, b, c), then deduces the two-phase quiescent reference frame and two-phase rotary reference frame math model after using the reference frame transform. It can be found that three-phase voltage source PWM rectifier is a time-varying,strong-coupling,complex coupled nonlinear system. Traditional control scheme usually designs controller based on the small-signal linearized model near the steady-state work point. But it needs a larger DC-side capacitor, and the global stability of the system can not be proved theoretically.This paper applies nonlinear control theories to design the controller for the converter system consisted of PWM rectifier. The implementation of specific is: According to the systems control goal, the double-closed-loop control structure consisted of inner current loop and outer voltage loop has been designed, and then using voltage oriented control, the inner current loop controller and outer voltage loop controller of three-phase voltage source PWM rectifier can be designed by input-output feedback linearization and the sliding mode control method.For three-phase asynchronous motor AC speed-adjustment system, this paper lays a strong emphasis on study of asynchronous motor AC speed-adjustment system consisted of dual-PWM converters. The design and control strategies of the whole speed-adjustment system has been given here after analyzing the the mathematical model and the principle of vector control of three-phase asynchronous motor. The author applies cascade structure of nonlinear control scheme based on input-output feedback linearization to design controller for ac-side rectifier and applies rotor-flux-oriented Vector Control to design controller for motor-side inverter.Many simulation experiments have been finished by the MATLAB simulation toolbox for verifying the validity of above schemes. The simulation results show that: three-phase voltage source PWM rectifier dc-side voltage is steady,controllable and load disturb resistible, ac-side power factor is controllable (such as unit power factor) and power can flow bidirectionally; Besides, for three-phase asynchronous motor AC speed-adjustment system, the control goals of three-phase asynchronous motor's speed can be achieved and the robustness of the system is strong for load fluctuation.