Wind turbines dynamics loads alleviation: Overview of the active controls and the corresponding strategies

Structural loads vibration is a critical problem in WTs (wind turbines) due to many reasons such as the complicated dynamics, structural coupling and diversity of the driving sources such as gravitational, inertial, aerodynamic and operational loads. These loads cause pressures forces along the span of the blade and alternate bending moments of the chord-wise and flap-wise axes per revolution, thus leading to fatigue loads in WT components. The accumulation of these loads over time cause degradation and lifespan reduction of the overall system. Monitoring, analysis of the dynamics loads and mitigation of their impact are a straightforwardly challenge in modern WTs. In this regard, this article explores a deep research on dynamics loads mitigation for WTs applications. In the first time, baseline control of WTs, based on electromagnetic torque and collective pitch angle control (CPC), is presented. Thereafter, the paper presents the vibration control methods including passive, active and semi-active controls. The control based on the active method is economic, reliable and the generated forces are based on the control algorithms. This control includes drive-train damper, tower damper and indi-vidual pitch angle control (IPC). Subsequently, the various techniques and control strategies to implement these vibration control methods are given out and discussed in detail under various operating conditions such as wind speed fluctuation, high non-linearity, un-modeled dynamics, perturbations, disturbances ... etc. These methods cover Disturbance accommodation control (DAC), Model Predictive Control (MPC), combined feed-forward and feedback control, robust and multi-variable control, and adaptive control. It can be stated that DAC allows to improve loads rejection under spatial varying wind, while that MPC allows to optimize a cost function containing a set of WTs objectives. Combined feed-forward and feedback control allows to actuate the WT in anticipation of the incoming disturbances. Multivariable and robust controls allow to accomplish WTs objectives under dis-turbances and uncertainties. Adaptive control allows to adjust its parameters to the WTs operating conditions under un-modeled dynamics and uncertainties. The paper finishes by conclusions and perspectives.