Power Quality-Oriented Multiloop Design of Floating Wind Turbines Pitch Control

Floating offshore wind turbines (FOWTs) face inherent control limitations in above-rated operating conditions due to right-half-plane zeros in the blade-pitch-to-rotor-speed dynamics, which restrict achievable control bandwidth and may lead to closed-loop instability. Conventional mitigation strategies based on controller detuning preserve stability at the expense of performance, while auxiliary feedback loops improve regulation but introduce additional dynamic coupling that complicates tuning and gain scheduling. This work proposes a novel gain-scheduling methodology based on an output-feedback linear quadratic regulator (LQR) formulation for pitch control of FOWT. The approach leverages a reduced-order linear model capturing the dominant coupled dynamics of the drivetrain and platform pitch motion and synthesizes optimal gains scheduled as functions of operating conditions. The resulting gains are directly mapped onto the standard Reference Open-Source Controller (ROSCO) pitch control parameters, without modifying the controller structure or introducing additional feedback loops. Nonlinear aero-servo-hydro-elastic simulations performed with OpenFAST on the IEA 15-MW reference turbine demonstrate that the proposed approach significantly improves rotor speed regulation and power quality while enhancing closed-loop stability compared with conventional detuning strategies. These improvements are accompanied by reduced fatigue loads on the tower and blades across a wide range of wind, turbulence, and sea-state conditions, highlighting the effectiveness and practical applicability of the proposed methodology.