Simulation Study On Stability Of Grid-Connected Large-Scale Doubly Fed Induction Generator

As the most potential for development of renewable energy, wind power is developing rapidly all over the world driving by the increasingly exhausted fossil energy and deteriorating global environment. But Statistics show that most of established wind farms in our country can’t achieve grid connection. The "Difficult grid-connection" problem has become a bottleneck in the development of wind power. More in-depth study of the impact on grid stability by both of the wind farm and grid becomes quite significant at this time.In this thesis, the doubly-fed wind turbine is taken as the research object, which is most widely used at present. The variable speed constant frequency principle and the mathematical models of modules which include wind turbine, shaft drive mechanism, doubly-fed generator and converter are analyzed in detail. The control technology of pitch, the maximum power point tracking and vector orientation are studied in-depth.In view of the modeling and simulation of wind power generation, several commonly used power system simulation tools are compared. And the simulation model of the power system with doubly-fed wind farm is established in Matlab/Simulink and DIgSILENT respectively. A relatively simple power grid is adopted in Matlab/Simulink. The model is mainly used for research on the operating characteristics of the wind turbine itself. The maximum power point tracking operating characteristic of the doubly-fed wind turbine and combination wind speed model are simulated and analyzed. The IEEE9node power system is adopted in DIgSILENT. In the model, the use of a long transmission line between wind farm and power grid, and wind power capacity accounts for up to13.6percent of all power system capacity, which can well simulate the large scale and long transmission distance characteristics of wind power generation in our country. The impact of wind speed disturbance on the operation of power system with doubly-fed wind farm is simulated and analyzed.The low voltage ride through capability of wind farm is the core content in grid connection requirements. In the case of power grid under large disturbances, the low voltage ride through capability of wind farm is mainly achieved by increasing the crowbar protection circuit at present. And the switching time of the crowbar protection circuit has important effect on the low voltage ride through capability of wind farm. However there is not yet a good quitting control strategy of crowbar circuit. In this thesis, the dynamic response characteristics of voltage and current, active and reactive power, speed, and other important parameters of doubly-fed wind generator under various short circuit fault conditions are simulated systematically. The characteristic of generator reactive power is analyzed deeply. And a crowbar protection circuit quitting control method based on the reactive power judgment is proposed, which can realize crowbar protection circuit quit immediately after fault clearance, so the low voltage ride through capability of doubly-fed wind turbine is improved.The recovery of electric network voltage after fault clearance is improved through increasing the dynamic reactive compensation device and making the grid side converter to operate in STATCOM mode during power grid fault by improving its control strategy. The over-voltage of converter DC bus during power grid fault is reduced effectively and quickly by increasing the DC-link chopper. Through improving the control strategy of rotor side converter by adding a speed stabilizer can get the speed quickly back to the steady-state value after the fault is cleared.The influence of different crowbar resistance on the operating characteristic of doubly-fed wind turbine is simulated and analyzed. It is pointed out that the conventional method for setting the value range of energy dissipation resistance of crowbar circuit does not take into account the interaction between energy dissipation resistance and the generator speed. And the crowbar resistance setting method is improved.In this thesis, the low voltage ride through capability of doubly-fed wind turbine is improved from four aspects:crowbar circuit quitting control strategy improvement, generator terminal voltage stability base upon dynamic reactive power compensation, DC bus fault over-voltage suppression and generator speed stability.