Control design for floating wind turbines: a novel feedback control structure

The generator speed feedback control of onshore wind turbines, via the pitch controller to feather the blades, is well established, but employing the same controller gains with floating offshore wind turbines causes the turbines to become unstable. Such instability is attributed to the coupling between the nacelle fore-aft motion and the wind turbine controller, which makes the wind turbine negatively damped. The non-minimum phase zeros existing in the transfer function from the blade pitch to the generator speed impose a fundamental limitation on the closed-loop bandwidth, posing a challenge to the operation of the floating turbines. This paper gives an overview of the control strategies and their tuning techniques employed for floating wind turbines in the presence of the negative damping instability. It discusses the different available strategies. Moreover, we propose a new controller that can alleviate the adverse effects of the negative damping while preserving the standard proportional-integral control structure. Contrary to the multi-input, multi-output controllers that have been proposed, the proposed controller is more robust as it does not require additional signals of the floating platform, which often makes controllers sensitive to unmodelled dynamics. The controller is compared against the previously proposed controllers using the non-linear simulation tool OpenFAST. The proposed controller excels in regulating generator speed, surpassing other controllers in performance. Additionally, it effectively mitigates the platform pitch in addition to the tower and blade loads. However, achieving a balance between power quality, actuator usage, and structural loading presents inherent trade-offs that need to be carefully addressed.