A wind turbine generally comprises a generator housed in a nacelle, mounted on top of a tower so that the nacelle is situated relatively high above ground or sea level. A high tower is preferred, since wind speed increases with increasing altitude. Wind turbine towers can exceed 70 m in height. A tower is generally a closed structure to provide structural stability and to afford protection for components arranged in the tower itself, such as electrical systems, cooling arrangements, control equipment, etc. One type of widely used tower construction comprises a number of steel sections connected together and mounted to a foundation. Another type of tower structure is made of concrete, for example reinforced concrete sections stacked one on top of the other, or cast in situ. The nacelle is generally mounted to the top of the tower by means of a yaw ring, so that the nacelle can be moved in order to have the rotor face into the wind. Here, the term “rotor” is to be understood as an arrangement of rotor blades connected to a hub, which in turn is mounted to a rotatable component of the generator such as a shaft or field, depending on the type of generator that is used.
The combined weight of the nacelle, generator and rotor rests on the top of the tower, which is usually simply regarded as a weight-bearing structure. However, regardless of the manner in which the tower was constructed, the tower is not entirely rigid, and can oscillate in any direction of a horizontal plane, for example a horizontal plane through the top of the tower, and the tower can oscillate sideways as a result of the forces acting on it during operation of the wind turbine or as a result of high winds. The vibrations or oscillations can ultimately compromise the structural stability of the wind turbine, since repeated vibration can result in fatigue. Furthermore, vibrations of the nacelle can also manifest as unacceptably loud levels of acoustic noise. The nature and extent of the oscillations are difficult to predict before commencing the actual construction of a wind turbine. Usually, complex simulation programs are used to model various aspects of wind turbine construction, and the results of a software simulation are used to refine various design aspects. However, such software simulations are only of limited use, since the results depend entirely on the accuracy of the modelling input information, and it is simply not possible to exactly model every aspect of a tower, nacelle, generator and rotor under every conceivable operating condition. As a result, when the wind turbine has been constructed and put into operation, it may be observed to perform poorly under certain conditions, with detrimental side effects that cannot be corrected. Particularly in the case of a wind park comprising many wind turbines, for example an offshore wind park, a favourable performance of the wind turbines is of great importance—in all weather conditions and for all levels of power output.