The Reserch Of Control Strategy Based On Three Phase There-Level PWM Rectifier

In recent years, with the vigorous development of solar energy and wind power and other green energy technologies, power converters based on modern power electronics technology has become a research focus of many scholars. PWM rectifier has been widely used because of the ability to decrease or eliminate the impacts of reactive power and harmonics on grid voltage, the ability to effectively adjust dc bus voltage and other advantages.Since the limitation of rated voltage and rated volume of switching devices, the two-level converter cannot be directly applied to the high-voltage and high-capacity applications. The three-level converter can be used in high-voltage applications to control the conversion of high level voltage through switching devices with low-voltage switching devices because conduction voltage drop of switching devices are halved. In this thesis, three-level PWM is studied. The main works of this thesis are shown as follows:This thesis briefly introduces the basic switching states and working principle of three-phase three-level PWM rectifier and develops mathematical model of three-phase voltage-source PWM rectifier. With double closed-loop dq decoupling control algorithm, the PWM Rectifier control is realized. And the inner current loop controller and outer voltage loop controller of the control structure are analyzed, the parameters are also designed.As to the disadvantage of traditional SVPWM, such as complicated arithmetic and high resources occupancy, a rotating coordinate system transformation based SVPWM modulation strategy applied to three-phase three-level converter is studied. By matrix transformation, a new coordinate system is obtained by compress vertical axis and rotating clockwise the coordinate system utilized in traditional SVPWM modulation strategy by 45°. In the above coordinate system, the basic vector is located in the integer point. Then, the new SVPWM modulation strategy is realized in the obtained coordinate system, the operational complexity is decreased greatly. Besides, the unbalance of the capacitor in the DC link has a great impact on the convert. On the basis of opposite effects that every couple of positive and negative small vectors have on charging and discharging of the neutral point voltage, the voltages of the dc bus capacitors are kept balanced by adjusting the action time of positive and negative small vectors. Its implement method is simple and it can effectively balance the two voltages of the capacitors or keep them balanced.Because of the unsatisfactory performance of the DC link voltage under traditional PI controller, fuzzy PI controller is obtained by combining the outer voltage loop PI controller and fuzzy control. The ranges of baseline values and corrective values of proportion parameter and integral parameter in fuzzy PI controller are determined by simulation comparison, which could avoid blindness in designing parameters of fuzzy PI controller. And the superior performance in static and dynamic state of the fuzzy PI controller over traditional PI controller is verified.A FPGA experimental platform of three-phase three-level converter is implemented. Using Verilog HDL language, three-phase PLL modules, abc-dq conversion modules, the three-phase three-level SVPWM modulation modules and fuzzy PI control modules in new coordinate system and other modules are programmed. On the experimental platform, the experiments of and SVPWM, voltage balancing control and the three-phase three-level rectifier with fuzzy PI in new coordinate system is implemented, thus the correctness of the relative theory is verified.