Optimisation studies of a wind power generation system

Abstract

Studies on wind power generation systems are becoming a topical research area because of the increasingly scarcity of hydrocarbon fuels. The rise in petroleum price is also creating heavy demands on renewable energy sources such as wind, hydro, solar, bio-mass and so on. Hitherto, wind energy is most promising in terms of cost effectiveness. Among existing wind generation systems, variable speed wind turbines with doubly fed induction generators (DFIGs) are most common because of their high energy yields and ease of implementation that allows the users to control the reactive power of these wind generators with relatively simple and low power rating control gears. Only simple algorithms are needed for effective and robust control of wind power generation using DFIG. In this research, direct control algorithms for normal operations of DFIGs, including synchronization and power generation, are developed. Mathematical models, computer simulation, and experimental results are included for the validation of various schemes being studied. The algorithms developed in the research are model-based designs with direct feedback of the control variables to minimize the number of parameters and to simplify numerical operations, with no compromise in performance. A direct voltage control scheme for the synchronization of DFIGs to grids is therefore presented. The beauty of the proposed methodology is that the scheme is a single loop design with no current control loops, and only those parameters which are required for tuning the voltage controller are needed. In comparison with conventional control schemes for DFIGs, the proposed controller requires no mathematical coordinate transformation of currents and hence is simpler and faster. Direct torque control of DFIGs with constant switching frequency is also developed in the research. The control scheme utilizes direct feedback of torque and reactive power to avoid the need to build current control loops. Hence, the scheme does not require alignment of currents to the synchronous frames, as torque and reactive power can be evaluated in any coordinate frames. The proposed scheme inherits the simplicity of classical direct torque control scheme, but not the disadvantages of classical schemes such as variable switching frequencies and relatively poor steady state accuracy. Overall, the power quality of power generation is improved in the proposed scheme. Direct torque control of DFIGs in grids with large source impedances is proposed. The control method automatically adjusts the controller parameters in accordance to changes in grid voltage, in that the control performance and dynamics of the power generation systems are decoupled from fluctuations in the grid voltage, which is a common constraint in wind farms having weak connection to grids due to the presence of long transmission lines. A control scheme for DFIGs to operate in grids with voltage unbalance is also developed. Variables of the DFIGs are separated into sequence components and are independently controlled. As the degree of freedom is increased by the separation of sequence components, coordination of the components can be performed in the scheme to achieve different objectives, to suit the needs of grids with different operating conditions.