Control-oriented modeling and design of tower velocity feedback for floating wind turbines

The control design of floating wind turbines is challenging due to the strong coupling between floater dynamics and rotor speed regulation. A common approach to mitigating negative damping is detuning the onshore blade pitch controller, which lowers bandwidth but increases rotor speed overshoot and reduces power tracking performance. To address this, some studies introduce tower velocity feedback loops with a torque compensator or pitch damper. However, many control designs rely on models with implicit parameters, where the floater's mass, damping, and stiffness matrices are extracted from aeroelastic codes but are not always explicitly defined or easily interpretable. This paper develops a first-principles-based, control-oriented model of a floating wind turbine that explicitly incorporates mooring and hydrodynamic forces. In addition, the paper also proposes a novel model-based tuning method for the tower velocity feedback loop. Unlike conventional approaches, this method ensures specific pole placements and meets defined performance requirements for rotor speed regulation. A three-degree-of-freedom linear model is formulated and validated through open- and closed-loop simulations on a 15-MW turbine with a spar buoy. The controller tuning for both the torque compensator and the pitch damper shows that the pitch damper fails to ensure theoretical stability, whereas the torque compensator significantly enhances platform pitch stability and rotor speed regulation under varying wind and wave conditions. Besides the spar buoy, the simulations are also conducted on a semi-submersible platform to demonstrate applicability.