In late years, the renewable energy use is promoted, and, additionally, large-scale windfarms are constructed on a global scale by development of large and high efficiency wind turbine generator system (WTGS). As a WTGS grows larger, safety of WTGS tower is becoming more important. Wind loads acting on a WTGS consist of extreme wind condition and normal operating condition. About an extreme wind condition, there are already some studies, and evaluation way is suggested, but very few studies related to a wind load at normal operating conditions. In this study, evaluation of the equation for WTGS tower base moment suggested by Ishihara et al. and comparison with actual survey value in JSW WTGS J70 at normal operating condition are carried out.
Site-specific fatigue evaluation of wind turbine support structures has been required by Japanese domestic rule since the revision of the Building Code in 2007. Fatigue loads are strongly affected by wind turbine controllers, as well as by the characteristics of wind and the wind turbines themselves. It is difficult for anyone but expert wind turbine controller engineers to design wind turbine controllers. For this project, blade pitch and generator torque controllers for a typical land-based variable speed- and pitch-controlled wind turbine were formulated using a Loop-Shaping Method, considering wind turbine characteristics such as rotor aerodynamics, tower vibration, and control systems. Controller gains were calculated by defining the gain cross frequency and phase margin for pitch control as well as the peak gain and damping ratio for generator torque control, without introducing control design tools such as the Bode diagram or the Nyquist diagram. Controllers that are compatible with the fatigue loads of towers and blades as well as pitch rate were investigated through case studies of three kinds of towers. Fatigue loads of the support structures were shown to depend not on gain cross frequencies themselves but on those frequencies first normalized by tower mode-bending frequencies. The normalized gain cross frequencies of pitch controllers were recommended to be about 20%. Furthermore, gain scheduling, which reduces pitch commands to filtered pitch angles, was shown to be effective in reducing fatigue loads with improved characteristics.
As there are frequent needs to install a micro wind turbine generator in a residential area, we have developed such a downwind-type turbine rotor having the following specifications: The rotor has the tip diameter of 1.5 m and three two-dimensional NACA0018 blades of 0.15 m chord whose material is light, soft and pliable foam plastic for perfect safety. From the wind tunnel test under the wind speed less than 14 m/s, it is clarified that the downwind turbine generator having soft blades has the low noise and the self-output-control characteristics, which are regarded as favorable for its actual operation. The latter feature is primarily attributed to bending and torsional elastic-deformation of the soft blade. The remarkable aerodynamic performances are summarized in this paper.
A prototype 3 kW wind turbine generator of 4 m in diameter has been designed and examined under real wind conditions. Two control methods have been applied: a variable blade pitch angle, and regulation of the generator field current. To improve the control program over a higher wind speed range, a LQG (Linear Quadratic Gaussian) control method with a wind model of low-pass filters was adopted for the pitch angle controller. This paper describes the LQG controller and shows numerical simulation results. The control program was able to vary the pitch angle and maintain an optimum rotational speed to prevent over rotation past 300 rpm.
The aerodynamic performance of a wind turbine depends on operating conditions such as tip speed ratio and pitch settings. Generally, the wind turbine blade shapes are designed based on two-dimensional airfoil characteristics which are measured under static conditions in terms of angle of attack. However, wind turbines operated under complex condition in field environment, frequently experiencing suddenly fluctuation of wind direction and velocity. It causes fluctuations of angle of attack for the blade element. Therefore, the clarification of airfoil characteristics under dynamic conditions is important for accurate estimation of the power output and aerodynamic loads on wind turbine. In this study, wind tunnel experiments under dynamic and static conditions for different turbulence intensities are conducted for a wind turbine airfoil. As the result, the lift coefficient for dynamic condition shows the hysteresis loop when the range of angle of attack during the pitching motion includes the stall angle. The flow turbulence changes the stall characteristics of the airfoil. The size of hysteresis loop depends on the stall characteristics and the range of pitching motion.
To obtain basic understanding for the development of atmospheric turbulence assessment technique for wind-energy application, we examine the performance of turbulence simulation using the large-eddy simulation technique, focusing on grid dependency of predicted turbulence statistics. We test two different types of model codes, one derived from a numerical weather prediction (NWP) model and the other from a computational fluid dynamics (CFD) model. Both model types have advantages and disadvantages while applied to the atmospheric boundary layer with complex terrain, and it is the purpose of this study to examine their capability for use in simulating wind-farm turbulence. The first simulation uses an NWP model for an ideal atmospheric flow and the other uses a CFD model for flows over complex surface. The horizontal grid spacing ranges from 50 m to 300 m. The results show that a horizontal grid spacing of 50 m for both model types can reasonably capture the energy containing eddies and represent coherence structures and turbulence statistics, such as intensity, anisotropy and spectra of wind fluctuations. This study provides a guideline for using numerical simulations for turbulence assessment at wind turbine locations. It also suggests that the combination of NWP and CFD models may provide a better approach to assess atmospheric turbulence, for example by using an NWP model with fine grids to provide turbulent inflow boundary conditions for a CFD model.
Testing of a low Reynolds number airfoil designed for low speed horizontal axis wind turbine (HAWT) blades was performed to study its aerodynamic characteristics. Experiments were conducted at Reynolds numbers (Re) of 38,000 to 200,000 at angles of attack from -2° to 20°. The airfoil geometry was chosen after testing a number of profiles with XFOIL software. The pressure distribution, lift and drag coefficients and the flow characteristics were also studied with ANSYS-CFX software. The freestream turbulence level was increased from 1% to 5% and 10% which shifted the transition point on the upper surface upstream, resulting in increased skin friction drag for lower angles of attack due to a larger turbulent boundary layer region. The slope of the lift curve did not change much at higher turbulence levels; however, for higher angles of attack, the separation from the upper surface was delayed resulting in an increase in lift and a reduction in drag. The lift to drag ratio increased by 8% to 15% as a result of increasing the turbulence level in the angles of attack-range of interest.
As a floating foundation for wind turbine, dynamic response of offshore structure to external forces should be as small as possible because the motion of the structure imposes inertial force to a wind turbine system. Generally, a TLP (Tension Leg Platform) has favorable characteristics that its dynamic response in waves is negligibly small compared to other type of floating foundation. Our conceptual design of TLP for wind turbine was carried out in consideration of mooring force and easy maintenance. Numerical analyses were performed on dynamic response and tension deviation of leg in waves, natural frequency of vibration, and dynamic response to seismic load. As a result, it is found that natural periods of heave and pitch are rather short and amplitudes of motions are very small. It is also found that it keeps sufficient safety factor to extreme environmental conditions including earthquake, and is free from resonance with a wind turbine system.
We demonstrate a procedure for determining the configuration of a tension leg platform (TLP) on which a 2.4 MW wind turbine can be installed. Various sizes of the structural elements of the TLP are examined to investigate their effects on tendon tension and response acceleration at the top of the tower. It is shown that although increasing the volume of the peripheral column reduces the acceleration response, it amplifies the tendon tension, while increasing the span length between the center and peripheral columns reduces both of them. Furthermore, the relevant determination of the initial tension is investigated.
Dynamic response due to breaking wave impact force on an offshore wind turbine support structure is investigated, especially the effects of higher modes. From comparison between two representative breaking wave impact force models, i.e. the Goda model and the Wienke model, it is clarified that their impulse response functions in the low-frequency region and high-frequency region reverse their magnitudes. For convenience in the primitive design stage, a procedure for prediction of the dynamic response by a breaking wave impact force acting on a support structure is proposed using the impulse response function. By taking only a few vibration modes, the maximum values of the section forces on the support structure can be estimated without any cumbersome time dependent analyses. Furthermore this paper proposes an approximation of the curling factor in terms of the surf similarity parameter.
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