Wind turbine load reduction by rejecting the periodic load disturbances

For the cost per kilowatt hour to be decreased, the trend in offshore wind turbines is to increase the rotor diameter as much as possible. The increasing dimensions have led to a relative increase of the loads on the wind turbine structure; thus, it is necessary to react to disturbances in a more detailed way, e.g. each blade separately. The disturbances acting on an individual wind turbine blade are to a large extent deterministic; for instance, tower shadow, wind shear, yawed error and gravity are depending on the rotational speed and azimuth angle and will change slowly over time. This paper aims to contribute to the development of individually pitch-controlled blades by proposing a lifted repetitive controller that can reject these periodic load disturbances for modern fixed-speed wind turbines and modern variable-speed wind turbines operating above-rated. The performance of the repetitive control method is evaluated on the UPWIND 5MW wind turbine model and compared with typical individual pitch control. Simulation results indicate that for relatively slow changing periodic wind disturbances, this lifted repetitive control method can significantly reduce the vibrations in the wind turbine structure with considerably less high-frequent control action. Copyright (c) 2012 John Wiley & Sons, Ltd.