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Windmills have been used for at least 3000 years, mainly for grinding grain or pumping water, while in sailing ships the wind has been an essential source of power for even longer. From as early as the thirteenth century, horizontal-axis windmills were an integral part of the rural economy and only fell into disuse with the advent of cheap fossil-fuelled engines and then the spread of rural electrification. To use wind energy efficiently and to concentrate the visual impact of modem wind turbines, the regions with a good wind climate, a tendency to group turbines in wind farms can be observed. These wind farms are connected to high voltage transmission grids and thus directly influence the dynamic behavior of the electrical power system [1].
Modern electricity-generating wind turbines now use three-bladed upwind rotors although two-bladed, and even one-bladed, rotors were used in earlier commercial turbines. Reducing the number of blades means that the rotor has to operate at a higher rotational speed in order to extract the wind energy from wind turbine. Initially these turbines were small, sometimes rated at only 30 kW, but were developed over the next 15 years to around 40-m diameter, 800-1,000 kW. However, by the mid-1990s it was becoming clear that for larger wind turbines it would be necessary to move away from this simple architecture and to use a number of the advanced concepts (e.g. variable-speed operation, pitch regulation, advanced materials) that had been investigated in the earlier research work. Large wind turbines are up to 100 m in diameter, rated at 3 to 4 MW are used. Most of the largest wind turbines now being installed operate at variable-speed, as the power electronic converters also allow much greater control of the output power and it is easier to comply with the requirements of the power system network operator.
