Proposal for a lower limit control of a generator’s torque based on the nacelle wind speed and demonstration results using a full-scale spar-type floating offshore wind turbine

Among the issues of floating offshore wind turbines are the platform-pitching vibrations generated by blade pitch angle motions of the variable speed control. Control of blade pitch angle based on the platform-pitching motion (floating platform vibration control) can suppress these vibrations. This study investigates the impact of floating platform vibration control on variable speed control and the generator torque control, which control interferences cause the fluctuation of generator speed and platform-pitching angle at the transition region between below and above rated operating conditions. This study also proposes a new control to reduce the impact; in this method, the lower limit of generator torque is adjusted linearly depending on the nacelle wind speed, and the conventional feedback control for the generator torque mainly adjusts the generator torque. Simulation and demonstration results using a full-scale spar-type FOWT showed that the proposed method can stabalize suppress the fluctuations of generator speed, platform-pitching angle, and power at the transition region.