The Research On Control Strategy Of Variable-Speed Constant-Frequency Wind Power Conversion Using AC-excited Generator

With the lack of general energy resource, electric power and the worsening of power supply circumstance problems increasingly, it is provided with very important meanings to partially solve energy lack by using renewable energy resource such as wind energy as the one of the most important clear substituted energy resource in the future. This dissertation chose the most attractive and booming approach—Doubly-fed induction generator (DFIG) as reseach object, the approach of this system reduced the capacity of inverter, the generator has a very wide speed range which can be adjusted from sub synchronous speed to super synchronous speed. The stator voltage and its frequency keep constant, so the active power and reactive power can be controlled freely. Besides, the DFIG can be equipment for reactive power compensation of the Power Network, then improve its stability.Firstly, this dissertation analyses DFIG operation theory, including mathematic model of DFIG, coordinate transformation, DFIG operation characteristics, DFIG power relationship, which establishes the theoretical foundation for succeeding study. The dissertation also deduced the calculation method of reference power, applied the stator flux-oriented vector control method to the operating control of DFIG, then formed a active power and reactive power decoupled strategy using stator flux-oriented vector control method.PWM converter which is the key equipment to realize the control of DFIG has a decisive influence to the control characteristics and the power quality. By using the operating characteristics of AC-excited VSCF wind power generator, this dissertation designed a dual PWM converter which can let the power flow to the two direction, and also discussed the function and control difference of the two converter. The grid-side PWM converter can realize unity power factor and keep dc-bus voltage constant by using grid voltage vector orientation control method and load current feedforward control method based on power balance between the two PWM converters. In order to realize decoupled control of DFIG active & reactive power and to capture the maxim wind power, field oriented vector control technique was applied to DFIG control. A general control strategy based on stator flux oriented vector control was proposed as well. According to the different operation conditions below and above the synchronous speed, the two converter can shift automatically at rectifier/ inverter (or inverter/ rectifier) state, and then bi-directional power flow is realized.Finally, simulating system of variable speed constant frequency wind generating is designed based on DSP and simulating. It is composed of a 7.5kW DFIG and grid-side and rotor-side converter which controlled by dual DSP controllers. The design of signal processing circuit, communications interface circuit, quadrature encoder pulse circuit and protection circuit has been detailed.