Research On Grid Connection Technology For VSC-HVDC Based Wind Farm

Nowadays, people are taking on many challenges that menace the sustainable development of the world, such as pollution, global climate change and security of energy supply. These challenges, on the other hand, are also the main drive forces behind the development and exploitation of renewable energy sources among which more and more importance has been attached to wind energy that is growing much faster than any other energy sources. The total installed wind power capacity reached 74.22GW worldwide till the end of 2006, with an increase of 25.6% over 2005. World Wind Energy Association (WWEA) predicts that the cumulative capacity will reach 160GW by the end of 2010. China is making great efforts to develop its wind power industry. By the end of 2006, the total installations of mainland had reached 2599MW, with an annual increase of 105.3%. The Chinese government will have increased its total installation capacity to 30GW by 2020. In these circumstances, the dissertation addresses several crucial problems in the field of wind power technology, e.g. modeling and control of wind turbine generator, wind farm grid interconnection via voltage source converter based HVDC (VSC-HVDC), and so on. The main achievements of the research include:(1) A continuous-time state-space model of VSC-HVDC and the linear decoupling control schemes for supplying both active and passive networks are investigated in the dissertation. Decoupling control of DC voltage and reactive power, active and reactive power is realized through feed-forward compensation technique for electrifying active systems. For feeding passive systems, the AC voltage controller is designed based on the concept of space vector.(2) Based on the principle of multi-input multi-output feedback linearization, a nonlinear decoupling control scheme for VSC-HVDC is proposed. The concepts of decoupling matrix and relative order are introduced. The application of the nonlinear scheme to VSC-HVDC is analyzed, from which the nonlinear controller is derived. Simulation results indicate the nonlinear scheme has better performance than its linear counterpart.(3) Modeling of wind turbine with doubly fed induction generator (DFIG) and directly driven wind turbine with permanent magnet synchronous generator (PMSG) is completed with MATLAB/Simulink. The models of DFIG with different orders are compared. Control strategies for the rotational speed of DFIG and PMSG are developed. (4) A new type of wind farm grid connection method based on multi-terminal VSC-HVDC is initiated. This method of grid connection features economy, environmental friendliness, voltage support of host grid and modularity of converter stations.(5) The concepts of full output feedback linearization (FOFL) and simplified input and output linearization (SIOL) are developed for connecting multi-terminal VSC-HVDC based wind farm with grids. FOFL has the best performance due to its thorough consideration of the interaction of current, grid voltage and DC voltage. SIOL features its succinctness in both application and principle without sacrificing much of control properties. The two schemes can enhance the fault ride-through capability of wind farms.