Abstract
The operational environment of offshore wind turbines is much more complex than that of onshore wind turbines. Facing the persistent wind and wave forces, offshore wind turbines are prone to vibration problems, which are not conducive to their long-term operation. Under this background, first, how the wave affects the vibration characteristics of offshore wind turbines is analyzed. Based on the existing wave and wave load models, we analytically show that there exist fluctuating components related to the hydrodynamic frequency in the aerodynamic load and aerodynamic torque of offshore wind turbines. Simulation results based on a GH Bladed platform further validates the analysis. Second, in order to reduce the joint impacts of the wave, wind shear and tower shadow on the wind turbine, a variable pitch control method is proposed. The integrated tower top vibration acceleration signal is superimposed on the collective pitch reference signal, then the triple frequency (3P) fluctuating component of the wind turbine output power and the azimuth angle of each blade are converted into the pitch angle adjustment signal of each blade, which is superimposed on the collective pitch signal for individual pitch control. The simulation results show that the proposed pitch control strategy can effectively smooth the fluctuation of blade root flap-wise load caused by wind and wave, and significantly reduce the fluctuation of aerodynamic torque and output power of offshore wind turbines.
Highlights
Offshore wind power is inevitably becoming an important clean energy source in coastal areas of China due to its advantages of high wind speed, low turbulence intensity and no occupation of land area [1]
The research on the influence of wind on the vibration characteristics of wind turbines shows that with the continuous expansion of wind turbine blades, wind shear and tower shadow cause a great imbalance of the wind speed distribution in the rotating plane of wind turbines, which causes the aerodynamic load of wind turbines to pulsate mainly at the 1P frequency [2], and the aerodynamic torque to pulsate mainly at the 3P frequency [3]
Simulation results demonstrate the effectiveness of the proposed individual pitch control strategy (IIPC) in the smoothing fluctuation of the wind turbine blade root flap-wise load and aerodynamic torque caused by wave impact, wind shear and tower shadow, which is accomplished by aggravating the fatigue damage of the pitch mechanism via frequent pitch angle adjustment
Summary
Offshore wind power is inevitably becoming an important clean energy source in coastal areas of China due to its advantages of high wind speed, low turbulence intensity and no occupation of land area [1]. Tian et al [18] proposed a new individual pitch control strategy for three-bladed wind turbines to reduce the 1P fluctuation component in the aerodynamic loads and the multiples of the 3P fluctuation component in the aerodynamic torque caused by wind shear and tower shadow. Simulation results show that the aerodynamic loads can be decreased with this control strategy, which in turn reduces vibration in tower fore-aft and platform pitching motion of the wind turbine. In these studies, the mechanism of how waves affect the load of wind turbines and aerodynamic torque is not systematically analyzed.
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