Grid-voltage-sensorless Control Strategy Of Three-Phase PWM Rectifier

Due to the higher harmonic standard of power appliances and the development of the techniques in power electronics and microcontrollers, PWM rectifiers gradually show much more advantages than the traditional diode or thyristor rectifiers, for they have the advantages of lower harmonics and higher power factor. However, the cost of PWM rectifiers is the limit on its popularization and application. Thus the key issues to promote PWM rectifiers focus on reducing the cost and improving the reliability. Based on this, this thesis mainly studied the grid voltage sensorless control strategies under balance and unbalance grid conditions, and suppression of the harmonic current caused by the PWM dead-time.The mathematical models of the PWM rectifier in different coordinate systems are analyzed based on its main circuit. Through analyzing relations of the power, voltage and current between DC and AC side, the operating principle is clarified, and the design of current controller and DC voltage controller is introduced. Before the controlled mode, the connecting phasic sequence and grid imformation are detected by phase locking of ac current. In the balanced condition, grid voltage estimation methods are analyzed in both stationary coordinates and synchronous rotating coordinates, and a virtual flux estimation method with a modified integrator is proposed. Based on the estimation, a grid voltage sensorless unity power factor control strategy is proposed. In the unbalanced condition, a method to estimate and separate the positive sequence and negative sequence grid voltage is introduced. As the unbalanced grid voltage has adverse influence on the voltage, current and power in both AC and DC side, methods of suppressing negative sequence current and eliminating DC voltage component are proposed respectively. Besides, the influence of the PWM dead-time on AC side voltage and current is analyzed, and the PI+PR regulator is used in the current loop to suppress the harmonic interference caused by the PWM switching dead-time.The PWM rectifier simulation model is established in Matlab/Simulink, and the physical experiment platform is constructed with TMS320F2808. The detail results of experiments and simulations prove the effectiveness of these proposed methods.