Controller Design for the Cancellation of the Tower Fore-aft Mode in a Wind Turbine

With the increase in size of wind turbines, there is increasing interest in exploiting the pitch control capability of variable speed turbines to alleviate tower fatigue loads. The most direct method is to modify the blade pitch angle in response to a measurement of tower acceleration. It is shown that the flap mode has a central role in determining whether this approach is effective since there is a strong interaction between the blade flap-wise mode and the tower fore-aft mode. Several different approaches to the design of the controller for the tower speed feedback loop are investigated.