Research On The Influences Of Integration Of Wind Farm On Regional Power Grid Stability Containing Front-end Speed Controlled Wind Turbines

At present, the regular wind generators are doubly-fed induction wind generator(DFIG) and permanent magnetic synchronous wind generator(PMSG) in China, with the development of wind power technology, new generators have emerged, for example, the front-end speed controlled wind turbine(FSCWT) has been applied in Hexi area of Gansu Province because of its power grid friendly characteristics. As the overall scheme of FSCWT different from the backend constant frequency scheme of DFIG or PMSG, FSCWT uses a hydraulic torque converter to adjust the speed of the unit in the front, and electric brushless excitation synchronous generator linked with power grid directly. The unit has the powerful reactive power output ability, high low voltage ride through capacity, good electric energy quality and high reliability, ect. So that the influence of the wind farm contain FSCWT connected to the system is very different from that of the traditional unit. So, the research on the influence of the wind farm contain FSCWT integration on grid voltage level and the stability of the power grid has important theoretical research value and guidance significance.The mathematical model of wind turbine is established on the basis of the structure and operation principle of FSCWT and the regulating principle and characteristic of hydraulic torque converter are described. The dissertation focused on the achievement of variable speed constant frequency of hydraulic torque converter and the mathematical model of guide vane is derived. According to the actual situation and operation characteristics of wind farm contain FSCWT, the geographic location of wind farm is introduced and the output characteristic of wind farm is analyzed. Then, the thesis summarizes the operation requirements of wind farm and power grid. On the basis, the scheme of wind farm connected to regional power grid is introduced and the equivalent model of wind farm is introduced.Secondly, Combined with a typical example of wind farm contain FSCWT integration Minqin regional power grid, the impact of wind farm integration on grid voltage level is studied. The wind farm model and regional grid model are built in DIgSILENT/PowerFactory. To research the problem of reactive power and voltage of wind farm, the reactive power allocation principle is discussed, and the reactive power compensation capacity of wind farm is calculated. Load flow analysis and short circuit calculation are carried out, and the network structure is checked by load flow calculation, and the reactive voltage regulator, reactive power compensation equipment and so on are selected. The influence on the short circuit current of wind farm is analyzed by the short circuit calculation. On the basis, the impact of wind farm integration on grid voltage level is studied and an analysis of the cause of most node voltage rise in power grid. By analyzing the stability mechanism of wind farm contain FSCWT, this dissertation also analyses the influence of wind farm consisting of FSCWT and DFIG on power system LVRT and transient stability. Then, the transient stability of different fault points is studied. Research shows that the established model of wind farm is accurate and effective.It also show that power system stability is improved because of the integration of wind farm contain FSCWT and the wind farm integration based on FSCWT into power system is better than that of DFIG as far as transient stability and LVRT is concerned.Moreover, the dissertation analyses the voltage stability of wind farm contain FSCWT by bifurcation theory. Combined with a typical example of wind farm contain FSCWT integration three node system, the basic knowledge of bifurcation theory and the relationship between the bifurcation theory and the stability of power system are discussed. Then, the differential algebraic equations(DAE) describe the dynamic characters of wind generator is established. On the basis, the bifurcation analysis method combined time domain simulation method is used to research the one-parameter and two-parameter bifurcation in MATLAB/MatCont simulation environment, the process of voltage instability and collapse of the system are analyzed. The nonlinear dynamic nature of voltage stability and the solution structure with the parameter variation is revealed and the bifurcation point and the critical point of voltage collapse under different control parameters are determined, based on which, wind speed fluctuation, the point of time of wind disturbance and the excitation voltage amplitude on the voltage stability are analyzed. Then the variation of node voltage along with the wind speed under different excitation is studied. Research shows that the number of equilibrium points and the node voltage will change in pace with the wind speed variation and excitation voltage regulation. When the wind speed fluctuates significantly, the voltage stability margin of the system and its ability of carrying reactive power can be improved by increasing the excitation voltage, which ensures the stable operation of the FSCWT at high wind speed, and thus provides a theoretical basis of the stable operation control of FSCWT.