不同强度湍流风对风力机气动载荷的影响

湍流强度是影响风力机载荷的重要因素之一。基于OpenFAST软件对NREL-5 MW风力机进行了不同强度湍流风为入流条件下的数值计算,利用Turbsim软件生成的5%、10%、15%和20%这4种湍流强度的湍流风作为入流条件,探究了不同湍流强度对风力机叶根处的剪切力和弯矩的影响,并分析了不同湍流强度下风力机叶片整体的载荷分布情况。结果表明：随着湍流强度的增加,风力机的气动载荷和气动功率波动幅值相应出现规律性的增加,但大体变化趋势不变。10%、15%和20%湍流强度下的气动功率标准差比5%湍流强度下的分别高出87%、163%和243%,摆振弯矩标准差比5%湍流强度下的分别高出30%、64%和95%;叶片上的弯矩载荷从叶根向叶尖方向逐渐减小,且随着湍流强度的增加,叶片上的弯矩标准差值也随之增加,在叶根处差别最大,从叶根向叶尖方向逐渐减小,到叶尖处完全重合。且10%、15%和20%湍流强度下的叶根处第一个节点位置的挥舞弯矩比5%湍流强度下的摆振弯矩分别高出44.94%、93.1%和137%。湍流强度的增加对叶片的摆振弯矩影响最大。     

风力机塔架在偏心载荷作用下的屈曲分析

以底部开门洞的大型风力机塔架为研究对象,研究了塔架在风轮、机舱荷载和塔架自身重力作用下引起的塔架屈曲问题的工程算法和有限元数值分析方法。对一个1.5MW风力机塔架在这类偏心荷载作用下的塔架屈曲值和屈曲模态进行了计算。计算结果表明:门洞的存在对薄壁筒体塔架的屈曲临界载荷有较大影响;相关标准提出的工程算法存在一定的局限性,需要结合有限元数值分析方法进行分析。在进行风力机塔架的设计时,该文计算结果具有一定的参考作用。     

变速变桨风力机叶片载荷控制策略

针对变速变桨风力机叶片受力特点,提出叶片载荷控制策略。将叶片载荷分为静态和动态两部分。静态部分采取单神经元PID控制器对桨距角和电机转矩进行控制;动态部分采取多叶片坐标转换理论将变量从旋转坐标系转换到固定坐标系,再将力矩转换为桨距角,然后通过多叶片坐标逆变换将控制得到的参数转换为旋转坐标系中的参数。最后将动态部分的参数值和静态部分的单神经元PID控制器的输出桨距角相加,一起构成风力机变桨所需的控制值。采用风力机专业软件Bladed开发外部控制器,并对控制策略进行仿真研究。将外部控制器和内置控制器的控制性能进行了仿真对比,仿真结果表明:外部控制器能够有效降低叶片根部的倾斜力矩和叶片拍打力矩,外部控制器的控制性能更优,验证了所采用的叶片载荷控制策略的正确性。     

某1 MW水平轴风力机叶片气动设计及载荷计算

基于片条理论,考虑了叶尖损失、叶根损失、叶柵影响和重载荷下对片条理论参数修正的情况下,完成了某1 MW水平轴风力机叶片的气动设计,并对其气动性能进行了评估;最后根据IEC规范对叶片在不同风况状态下进行载荷计算,所得结果可为同类风力机气动设计和结构设计提供参考。     

风力机变桨故障叶片载荷特征及仿生分形与原始结构对比研究

为探究切出风速下风力机变桨故障叶片的载荷及动力学特征,利用FAST软件进行数值计算,发现载荷及叶尖位移分别为变桨成功叶片的13.6和14.2倍,可知变桨故障叶片极可能发生结构失效断裂事故。为改善结构力学性能,提出结合仿生方法及分形理论的新构型叶片,并通过有限元分析与原始叶片进行对比,其中载荷边界条件来自以数据库方式存储FAST计算结果的叶片17段载荷数据。结果表明：仿生分形叶片重量及叶尖位移分别比原始叶片减少23.4%和19.7%;同时,结构屈曲分析显示,仿生分形叶片一阶屈曲因子为1.858,比原始叶片提高33.6%,极大增加了叶片结构安全性。     

极端台风下停机姿态对海上风力机叶片气动载荷影响

基于计算流体力学(CFD)方法,选取美国可再生能源实验室(NREL)5 MW风力机叶片主要设计参数,重点研究台风风速大于切出风速(风力机停机)时,不同台风入射风向及不同叶片停机桨矩角对风力机叶片气动载荷的影响特征,揭示最不利叶片停机姿态及对应风载系数;进一步对比分析可能出现的最不利叶片变桨故障停机工况与正常顺桨矩停机工况下的风力机气动载荷的特征,提出2种停机姿态下海上风力机最大安全设计风速的对应建议值。     

局部瞬态风况下风力机叶片载荷计算

对叶素动量理论(BEM)进行改进,动量理论的计算仍在风轮坐标系下进行,而叶素理论的计算在叶片的局部坐标系下进行。在局部坐标系下能更准确地利用二维翼型的特性来计算气动载荷,同时也能更好地调用Beddoes-Leishman(B-L)模型,从而全面考虑到风轮锥角、局部风况甚至叶片弯曲变形的影响。再将动量理论和叶素理论统一在风轮坐标系下完成迭代。采用B-L模型预测二维翼型的动态气动性能时应用切向力分离点计算分离流切向力,从而提高模型对动态气动阻力的预测精度。结合改进的BEM理论和动态效应模型对Tjaereborg 2 MW风力机进行仿真,所得叶根挥舞弯矩相比经典BEM更接近测试值。     

1.5MW风力机叶片变形的分析

影响风力机叶片变形的载荷源主要是空气动力载荷、离心力载荷以及重力载荷。讨论了一种风力机叶片变形的计算方法,首先将叶片简化为悬臂梁进行初步分析并得出剪力和弯矩,然后进一步将悬臂梁离散化,通过对离散化节点的分析得出叶片各截面的变形。静载实验针对某1.5MW叶片的6个截面展开,测试了某叶片模型在其设计风速时各剖面的变形量。通过实验验证计算方法的可靠性,并对实验和计算结果进行对比分析。     

1.5MW风力机叶片变形的分析

影响风力机叶片变形的载荷源主要是空气动力载荷、离心力载荷以及重力载荷.讨论了一种风力机叶片变形的计算方法,首先将叶片简化为悬臂梁进行初步分析并得出剪力和弯矩,然后进一步将悬臂梁离散化,通过对离散化节点的分析得出叶片各截面的变形.静载实验针对某1.5MW叶片的六个截面展开,测试了某叶片模型在其设计风速时各剖面的变形量.通过实验验证了文中计算方法的可靠性,并对实验和计算结果进行对比分析.     

风力机等效载荷的评估

通过神经网络建立数学模型,利用Multibrid M5000风力机实测的运行信号对叶片拍打弯矩、叶片弦向弯矩、塔筒弯矩和塔筒转矩的等效载荷进行了评估.同时分析了不同运行参数对评估效果的影响程度.通过与实测结果比较表明,该方法可以在避免测试风力机载荷的情况下获得载荷的评估结果,具有较高的精度,可以用来预测风力机的寿命.     

风力机叶片在不同载荷作用下的应力特性分析

针对额定功率为100 W的水平轴风力机叶片为研究对象构建实体模型,利用ANSYS有限元方法,模拟分析叶片在气动力、离心力及重力作用下的应力特征,最后由实验数据和模拟计算做对比分析。额定工况单独载荷作用研究,得出叶片应力主要受气动力与离心力的作用,重力影响不大;施加复合载荷,同一截面相对位置,叶片迎风面应力值大于背风面应力值,且随风速增加应力值逐渐增大,最大应力值处始终保持最大。     

Atmospheric turbulence and its influence on the alternating loads on wind turbines

Analysis of measurements on atmospheric turbulence with respect to the statistics of velocity increments reveals that the statistics are not Gaussian but highly intermittent. Here, we demonstrate that the higher quantity of extreme events in atmospheric wind fields transfers to alternating loads on the airfoil and on the main shaft in the form of torque fluctuations. For this purpose, alternating loads are discussed with respect to their increment statistics. Our conjecture is that the anomalous wind statistics are responsible for load changes, which may potentially contribute to additional loads and may cause additional fatigue. Our analysis is performed on three different wind field data sets: measured fields, data generated by a standard wind field model and data generated by an alternative model based on continuous time random walks, which grasps the intermittent structure of atmospheric turbulence in a better way. Our findings suggest that fluctuations in the loads might not be reflected properly by the standard wind field models. Copyright © 2010 John Wiley & Sons, Ltd.     

Model of wind shear conditional on turbulence and its impact on wind turbine loads

We analyse high‐frequency wind velocity measurements from two test stations over a period of several years and at heights ranging from 60 to 200 m, with the objective to validate wind shear predictions as used in load simulations for wind turbine design. A validated wind shear model is thereby proposed for flat terrain and that can significantly decrease the uncertainty associated with fatigue load predictions for wind turbines with large rotors. An essential contribution is the conditioning of wind shear on the 90% quantile of wind turbulence, such that the appropriate magnitude of the design fatigue load is achieved. The proposed wind shear model based on the wind measurements is thereby probabilistic in definition, with shear jointly distributed with wind turbulence. A simplified model for the wind shear exponent is further derived from the full stochastic model. The fatigue loads over different turbine components are evaluated under the full wind measurements, using the developed wind shear model and with standard wind conditions prescribed in the IEC 61400‐1 ed. 3. The results display the effect of the Wöhler exponent and reveal that under moderate turbulence, the effect of wind shear is most pronounced on the blade flap loads. It is further shown that under moderate wind turbulence, the wind shear exponents may be over‐specified in the design standards, and a reduction of wind shear exponent based on the present measurements can contribute to reduced fatigue damage equivalent loads on turbine blades. Although the influence of wind shear on extreme loads was found to be negligible, the IEC 61400‐1 wind shear definition was found to result in non‐conservative estimates of the 50 year extreme blade deflection toward the tower, especially under extreme turbulence conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Aerodynamic loads on wind turbine nacelles under different inflow turbulence conditions

The aerodynamic loads on wind turbine nacelles for range of inflow turbulence conditions are investigated. To this end, a series of wind tunnel experiments are conducted to investigate pressure field distributions over the surface of scaled models of rectangular and ellipsoidal nacelles. It is found that the mean pressure fields on the roof of the rectangular nacelle are similar for all the inflow turbulence cases for respective yaw angles. However, when yaw angle is around 90°, the mean pressure coefficient becomes more negative close to upstream edge with increasing inflow turbulence. For the ellipsoidal nacelle, the magnitude of the mean pressure coefficient increases with decreasing inflow turbulence in the adverse pressure gradient region, although the minimum peak pressure coefficient remains unaffected by inflow turbulence. The overall effect of wind‐induced load on the nacelle surfaces is evaluated by computing force coefficients from the pressure data. It is observed that the peak force coefficients for both rectangular and ellipsoidal nacelles increase with increasing inflow turbulence. The models for the estimation of peak aerodynamic loads on the nacelle surfaces are proposed as functions of inflow turbulence and mean force coefficients and show satisfactory agreement with measurements. Finally, the loads calculated by the GL guideline are compared against the measurements. It is found that the guideline estimation is conservative for design load case (DLC) 6.1, but it underestimates the load by about 35% for DLC 6.2 when the inflow turbulence intensity is 13.2%.     

Estimation of wind misalignment and vertical shear from blade loads

This paper describes the formulation and verification of a novel observer of wind parameters. The general idea behind the proposed approach is to consider the wind turbine rotor as an anemometer. In fact, the rotor responds to varying wind conditions; by properly interpreting this response, one can indirectly measure some desired wind characteristics, as for example yaw and shear, as described here.Measurements of wind conditions obtained this way are not affected by the usual disturbances of existing sensors, for example when installed in the nacelle or in the rotor wake. Furthermore, the approach provides rotor-equivalent quantities, and not the typical point information provided by wind vanes, anemometers or other similar sensors, whose information might be too local for large rotors.The proposed method is here formulated for the observation of wind direction and vertical shear. The new observer is demonstrated first in a comprehensive simulation study using high-fidelity aeroservoelastic models, and then experimentally using an aeroelastically-scaled wind tunnel model.     

风力机桨叶的三维建模与动力学特性有限元计算

以结构动力学基本方程为理论基础,将桨叶看成是根部完全固定的悬臂结构,建立了桨叶运动的物理模型和数学模型。以15 kW风力机桨叶为例,在AutoCAD软件中建立风力机桨叶实体模型,并应用国际通用的大型有限元分析软件ALGOR对桨叶在零转速下的振动模态和不同转速下的振动模态进行了计算与分析,分别算出其前5阶的固有频率和振型。对风力机桨叶的频率谱响应进行了分析,得到了激振力作用下叶片的振动位移与应力分布。     

Assessment of the strain gauge technique for measurement of wind turbine blade loads

Blade load measurement errors are assessed by numerical simulation and full‐scale laboratory tests. A theoretical justification of standard experimental practices through strain measurements is presented and applied as a design tool for detailed laboratory tests. The error sources affecting measurements on the composite blade material are cross‐talk effects and the influence of temperature deviations on interpretation of strain measurements. Calibration practices and measurement configurations are considered. The analysis indicates that axial and shear forces may be neglected as sources of cross‐talk in measuring the blade root bending moments. The cross‐talks of the flap bending moment on the edge signal and vice versa should always be quantified in calibration practices. The temperature effect is the most significant source of error and appears to be influenced by the load and thermal condition of the blade and the timescale of temperature variations. The temperature compensation methods are discussed and recommendations are provided to assist in the improvement of the blade load measurement quality. Copyright © 2000 John Wiley & Sons, Ltd.     

Gaussian vs non‐Gaussian turbulence: impact on wind turbine loads

From large‐eddy simulations of atmospheric turbulence, a representation of Gaussian turbulence is constructed by randomizing the phases of the individual modes of variability. Time series of Gaussian turbulence are constructed and compared with its non‐Gaussian counterpart. Time series from the two types of turbulence are then used as input to wind turbine load simulations under normal operations with the HAWC2 software package. A slight increase in the extreme loads of the tower base fore‐aft moment is observed for high wind speeds when using non‐Gaussian turbulence but is insignificant when taking into account the safety factor for extreme moments. Other extreme load moments as well as the fatigue loads are not affected because of the use of non‐Gaussian turbulent inflow. It is suggested that the turbine thus acts like a low‐pass filter that averages out the non‐Gaussian behaviour, which is mainly associated with the fastest and smallest scales. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of load reduction mechanisms on loads and blade bearing movements of wind turbines

The power control of wind turbines is usually realized via a change in the pitch angle of the rotor blades. Pitching facilitates the exact control of the turbines and the reliable deceleration of the rotor when required. Pitch movements can moreover be used for load control. One of these methods is called individual pitch control (IPC). IPC controls the blades individually and brings about a significant reduction in the fatigue loads and extreme loads placed on the structural components, while at the same time reducing the yield of the turbine only slightly. The lower loads reduce material costs, and thus, the cost of energy (CoE) is reduced, despite the slight reduction in yield. The method is nevertheless not used everywhere since the additional movement cycles put the rotor blade bearings in particular under stress. Special attention must be paid to small amplitude oscillating movements, which carry a high risk of inducing surface damage in the rolling contacts of the blade bearings. This paper uses a cycle analysis of the IWT7.5‐164 reference turbine to illustrate the differences in the movement patterns of wind turbine blade bearings with and without IPC. Moreover, model calculations with single contacts are used to show which of the movement patterns carries a risk of inducing surface damage. The use of IPC leads to the expected load reduction at the blade root. In current literature, IPC is usually assumed to have a negative influence on the life expectancy of blade bearings, but the findings of this study contradict this. The summed blade bearing movement is increased, although the number of very small pitch angles occurring is reduced. This reduction reduces the risk of wear in the blade bearings.     

风电机组低电压穿越能力影响因素分析与实例

我国风电大规模汇集地区多处于电网末端,电压波动性强,随着对风电并网安全的关注和风电机组并网性能要求的提高,低电压穿越能力已成为衡量风电机组并网性能的重要指标。文章调研了同一区域内多个风电场实际运行中的低电压穿越故障情况,分析了整机制造厂家、高校及相关研究机构对风电机组低电压穿越技术的研究现状。通过分类统计测试过程中遇到的风电机组低电压脱网故障和总结56台风电机组的低电压穿越测试结果,分析了造成风电机组低电压穿越能力不足的原因,并对引起风电机组低电压穿越能力不足的影响因素进行了阐述。目前,软件版本控制、保护定值的设置与管理、硬件的维护水平已经成为并网风电机组低电压穿越能力的主要影响因素。