Three-phase Voltage-type Pwm Rectifier Control Strategy Study

The traditional rectifiers are the main source of harmonic pollution in power system. Three-Phase PWM voltage rectifier can produce constant output voltage to achieve unity power factor operation, and it also has the capability to feedback the line power. This paper introduces three-phase PWM voltage rectifier, including the three-phase PWM voltage rectifier principle, the topology, the mathematical model and the control strategies. The mathematical model of PWM rectifier is an effective mean to investigate the rectifier. The mathematical model is established under three-phase stationary coordinate system, two-phase stationary coordinate system and the two-phase rotating coordinate system respectively. This paper studies voltage vector oriented control strategy and the direct power control strategy, the control strategy of three-phase PWM rectifier under unbalanced grid voltage is also discussed.Methods of voltage vector oriented control strategy including indirect current control and direct current control. This paper introduces their principles, and establishes the simulation model of control system.The advantages of direct power control are simple structure, fast dynamic response, and the ability to resist disturbance. This paper introduces the concept of instantaneous power and virtual flux, the direct power control principle, the inconstant switching frequency control and the constant switching frequency control. The switching table in conventional direct power control will lead ineffectiveness of reactive power control and ac current distortion. We improved the switching table and the simulation results show the effectiveness of the improved method. This paper proposes a simple method for calculating the switching time of SVPWM algorithm; it works well in direct power control strategy.When the grid voltage is unbalanced, it will seriously affect the operation of three-phase PWM rectifier, and even the system will collapse. This paper introduces the control strategy of PWM rectifier under unbalanced grid voltage and studies methods to suppress the dc output voltage harmonic. We also establish a simulation model to testify the strategy.The research results are separately published in Journal of Qingdao University (engineering & technology edition) and The First International Symposium on Power Electronics for Distributed Generation Systems (Hefei, China Aug.7-9,2007).The main contents of this thesis are as follows:Chapter 1 reviews the development of three-phase PWM rectifiers. Chapter 2 describes the principle of three-phase PWM voltage principle; we use Matlab/Simulink simulation software to establish the system model, we also introduces the voltage vector oriented control strategy, discusses the indirect current control and direct current control separately. Chapter 3 introduces direct power control strategy, including the inconstant switching frequency control and the constant switching frequency control .We improved the switching table of inconstant switching frequency control and the SVPWM algorithm of constant switching frequency control. The simulation system was established to verify the strategy. Chapter 4 introduces the control strategy of PWM rectifier under unbalanced grid voltage. Chapter 5 summarizes the whole thesis, and prospects some further research issues.