Power fluctuation and power loss of wind turbines due to wind shear and tower shadow

The magnitude and stability of power output are two key indices of wind turbines. This study investigates the effects of wind shear and tower shadow on power output in terms of power fluctuation and power loss to estimate the capacity and quality of the power generated by a wind turbine. First, wind speed models, particularly the wind shear model and the tower shadow model, are described in detail. The widely accepted tower shadow model is modified in view of the cone-shaped towers of modem large-scale wind turbines. Power fluctuation and power loss due to wind shear and tower shadow are analyzed by performing theoretical calculations and case analysis within the framework of a modified version of blade element momentum theory. Results indicate that power fluctuation is mainly caused by tower shadow, whereas power loss is primarily induced by wind shear. Under steady wind conditions, power loss can be divided into wind farm loss and rotor loss. Wind farm loss is constant at 3a（3a- 1）R^2/（8H^2）. By contrast, rotor loss is strongly influenced by the wind turbine control strategies and wind speed. That is, when the wind speed is measured in a region where a variable-speed controller works, the rotor loss stabilizes around zero, but when the wind speed is measured in a region where the blade pitch controller works, the rotor loss increases as the wind speed intensifies. The results of this study can serve as a reference for accurate power estimation and strategy development to mitigate the fluctuations in aerodynamic loads and power output due to wind shear and tower shadow.

基于等效风速的风电机组参数对输出功率波动和功率损失特性的影响

以大型三叶风电机组为例,研究发电机组参数对输出功率波动及功率损失的影响。首先建立包括风剪切和塔影效应的等效风速模型,推导输出功率和功率系数的数学模型,研究叶轮半径、塔筒高度、塔筒直径和叶片到塔筒中线的悬垂距离等相关参数对功率波动的频率、幅值、波长、波动区域等特性影响规律。然后建立功率损失的数学模型,研究风电机组参数通过风剪切和塔影效应对功率损失造成的影响规律。结果表明:不同参数通过风剪切和塔影效应对输出功率周期性波动及功率损失的影响规律不同,功率周期性波动的幅值及波长受塔筒直径和悬垂距离2个参数影响较大,而功率损失受叶轮半径和塔筒高度2个参数影响较大,但其作用效果相反:功率损失程度随叶轮半径的增大而增大,但随塔筒高度的增大而减小。

基于等效风速的风机功率波动特性分析

自然界的风随机性强,风速和风向变化频繁。风机在实际运行中,经常因风的变化和控制策略问题引起风机频繁偏航、振动超限、故障停机等一系列工程问题。通过建立包含风剪切、塔影效应及偏航误差的等效风速模型,采用Matlab进行仿真分析,与风场实际运行数据进行对比,研究风机在等效风速下功率波动特性。结果表明,等效风速各分量对机组功率特性影响差异明显。可据此进行相关控制策略的优化设计以提高风机运行稳定性和风能利用率。

中小型风机功率波动的影响因素及量化指标

风电系统输出功率波动导致电能品质下降,并影响系统设备的使用寿命。风电输出功率的波动主要来源于非恒定转矩的变化,为研究中小型风机转矩波动的影响因素,文章从风速特性和机组自身结构特性两方面入手,在时域范围内,研究风速的变化对功率波动幅值的影响,根据实测数据计算得出湍流强度与功率幅值波动随时间尺度的增大而增大;在频域范围内,利用Matlab仿真,得出风剪切和塔影对风速变化均有周期性影响,引起的波动频率为风轮转速频率的3倍。

风切变和塔影效应对风力机输出功率的影响

以NREL 5 MW风机为例,基于叶素动量理论(Blade Element Momentum,BEM)研究风切变和塔影效应对风力机输出功率的影响。用三阶Taylor展开描述指数型风切变模型;针对现代大型风力机的锥状塔架结构,对塔影效应模型进行适当修正。计算不同来流风速下的输出功率,并从功率波动和周期内平均输出功率两方面研究风切变和塔影效应的影响。结果显示,风切变和塔应效应都是周期性功率波动和周期内平均功率下降(功率损失)的来源。其中塔影效应是功率波动的主要原因,而风切变是功率损失的主要原因。功率损失由风场损失和风轮损失构成,其中风场损失是一个与风轮结构参数及风速轮廓系数相关的常数,而风轮损失与风力机控制策略密切相关,在变桨距控制阶段,风轮损失随风速的增加而增加。