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Large oscillations of the rotor speed are often observed in floating offshore wind turbines (FOWTs) operating in above-rated wind speed region. This phenomenon drastically increases the mechanical fatigue loads and even causes damage to the generator. To address this problem, this paper first establishes a Reduced-Order Speed-Control-Oriented Model (ROSCOM), which demonstrates the nonlinear coupling of the platform pitch motion and the turbine rotor rotation. The oscillation mechanism is then revealed that the platform pitch motion acts as the zero dynamics of ROSCOM, which can produce limit cycles and result in large oscillations of rotor speed. Based on the Bendixson criterion and Hopf bifurcation theorem, limit cycles and Hopf bifurcation in the platform pitch motion are identified and proved. Additionally, a nonlinear generator torque compensation strategy, named Non-Minimum Phase Platform Pitch Compensation (NMP 3 C), is designed to eliminate the limit cycles in the FOWT system. The rotor speed control performance is thus greatly improved. Electromagnetic power variation, platform pitch variation and tower base fore-aft fatigue loads are also mitigated. Digital simulations based on OpenFAST
Li et al. (Tue,) studied this question.