Adaptive robust observer-based control for structural load mitigation of wind turbines

Abstract. With growth in the physical size of wind turbines, an increased structural loading of wind turbine components affecting operational reliability is expected. To mitigate structural loading in wind turbines, a novel strategy for structural load mitigation and rotor speed regulation of utility-scale wind turbines in above-rated wind speed region is proposed in this contribution. Spatial and temporal variation of wind speed is responsible for fatigue loading during power production. Previous attempts have proposed advanced control schemes incorporating disturbance models for cancelling the effects of wind disturbances. These controllers are usually designed based on reduced order models of nonlinear wind turbines, hence do not account for modeling errors and nonlinearities. Although robust controllers have been proposed to handle nonlinearities during wind turbine operation, these controllers are designed about specific operating points, hence suffer performance deterioration in changing operating conditions. In this contribution, a robust disturbance accommodating controller (RDAC), which is robust against modeling errors and nonlinearities, is combined with an adaptive independent pitch controller (aIPC), designed to be adaptive to changing operating points due to wind speed variability, to mitigate structural loads in rotor blades and tower and to regulate rotor speed. The proposed control scheme is tested on a 1.5 MW National Renewable Energy Laboratory (NREL) reference wind turbine (RWT). Simulation results show that the proposed method successfully mitigates structural loading in rotor blades and tower without sacrificing rotor speed and power regulation performance in the presence of model uncertainties and changing operating conditions.