Controller Design to Reduce Mechanical Fatigue in Offshore Wind Turbines Affected by Ice and Tide

This paper presents a novel control structure to mitigate the mechanical fatigue in towers of onshore and offshore wind turbines. A general wind turbine dynamic model for both, (1) onshore and (2) offshore systems with the effects of ice and tide is included. These weather conditions further contribute to the uncertainties in the model, most importantly, in the values of tower equivalent mass, stiffness, and damping and increase the amplitude of the velocity of tip-tower vibrations at some particular frequencies, which creates greater mechanical fatigue. A novel control technique to attenuate such a mechanical fatigue is presented in the paper. It is based on the variation of the generator torque in the above rated wind speed region. The control strategy, designed by using Quantitative Feedback Theory (QFT), decreases the velocity of the nacelle movement due to the wind turbulences, thus reducing the associated mechanical fatigue. The new strategy is validated with a realistic nonlinear simulator under a set of different input scenarios and a Monte Carlo method for the uncertainty selection.