Research On Dynamic Model Of Variable Speed Wind Turbine With Doubly-Fed Generator And Control Strategy Connected With Grid

In recent years, with the great development in manufacture of power electronic converter and its application technology, the variable speed wind turbine(VSWT) with large capacity has become one primary technology directions of wind turbines. The VSWT with doubly-fed induction generator(DFIG) which can flexibly join wind turbine to power system has turned into mainstream machine in the field of exploring wind energy in the world now. It is therefore important to study the technology, operation characteristics, safety and stability in an electric power system with this type wind turbines. In order to explore the new technology of wind power in our country, a project named "The VSWT with large capacity of MW level" has been supported by National High Technology Research and Development Plan (863 plan). In this dissertation, the dynamic modeling, operation characteristics of VWST with doubly-fed generator and voltage-reactive power control strategy of a wind farm in an electric power system are studied based on the sub-project that is entitled as research on operation technology of VSWT with large capacity in an electrical power system and the full analysis on the references from domestic and aboard. The primary contents and original contributions of this dissertation are as follows:Firstly, a simply 3rd order dynamic model for DFIG is developed. The proposed model is adequate to describe dynamic behaviors and is easy to simulate by computer. Since control variables have taken place step changes and gradual changes, a detailed comparison of dynamic behaviors is made between the proposed model and exact model in the dissertation. The results are shown that the proposed model has a favorable definition and can substitute the exact model in research on DFIG with high power factor and large range of rotor speed. Therefore, the 3rd model could be applied in study on VSWT with DFIGSecondly, based on 3rd order dynamic model of DFIG, a linear model in state space formulation and transfer functions fitting for analysis on small disturbance stability of DFIG are presented. Moreover using open transfer function and eigenvalue locus for the first time, the relationship between original operation point and dynamic characteristics are discussed in the dissertationThirdly, a model of frequency converter that is expressed by four control parameters, that is, modulation ratio and phase difference of converter and inverter is originally presented in this dissertation Meanwhile, a aggregated dynamic model of VSWT is established based on the physical characteristics of composing componentsThen a simulation tool of VSWT is originally developed by using MATLAB.Fourthly, the operation characteristics of VSWT are discussed in this dissertation. At the aspects of steady characteristics, the active power, reactive power and P-Q capacity limit are analyzed. At the aspects of dynamic characteristics, the dynamic behaviors are simulated contrastively between VWST and constant speed wind turbine (CSWT). The results show that VSWT has more superior behaviors. Furthermore, taking the example of VSWT with DFIG connected straight with large grid, the operation characteristics are simulated under the different types of wind. It is shown that the proposed model in this dissertation is correct, the design of control objects is rational and it is also indicated that VSWT has a potential to regulate reactive power and voltage.Fifthly, the operation areas of the VSWT with DFIG are analyzed and the general control objects in the ideal situation are made certain in this dissertation. Moreover, a power-voltage control strategy of frequency converter at the coordinate system of rotating stator magnetic field is originally developed and the general controller is designed in the dissertation.Sixthly, a real optimized operation control strategy of VSWT operation with grid is presented originally in this dissertation. The control strategy is divided into two subsections according to wind speed, and two layers according the amending control references and tracing control references. Its aims are to ensure safe operation. Simultaneous, it can improve operation efficiency of VSWT, take part in the reactive power optimization of the grid and enhance the voltage stability of power systems. Then taking the example of a wind park lied in north of china, the operation characteristics of single VSWT and voltage characteristics of grid are simulated and analyzed when some disturbances about wind speed emerge and some faults in the systems take place under the control strategy. The results show that the proposed control strategy not only can carry out optimal operation of VSWT but also adequately exert the regulation ability of reactive power of DFIG and so disturbances are balanced and the voltage stability of grid is availably improved, optimal operation of the electric power system is reached.In summary, based on the research status of DFIG at home and aboard, the dynamic modeling and operation characteristics of VSWT are fully analyzed in this dissertation. Then the control strategy of VSWTs in the power systems is accomplished and a simulation tool is developed by using MATLAB correspondingly. Thereby, the work makes up of a integrated research at aspect of control strategy about "Operation technology of VSWT with large capacity in the grid".