On the integration of lightning protection with stealth coated wind turbine blades

In this paper, we consider the problem of reducing the radar cross section of a wind turbine blade through the application of radar absorbing material (RAM). One problem encountered by these techniques is the integration of the RAM solution with the existing lightning protection system, which is mandatory requirement to protect the blade when in operation. A common form of lightning protection is the use of conducting lightning receptors on the surface of the blade. To ensure the protection system is effective, a clearance area around the receptor may be required before any RAM treatment is applied. The size of the clearance area and the number of lightning receptors therefore potentially reduce the effectiveness of the RAM treatment. Design guidelines are given in this paper for a generic 40 m blade geometry. Some modelling results of the radar cross section and Doppler signature from a RAM treated blade are presented, and a comment is also made on the importance the blade edges have in reducing radar effects. ©2013 The Authors. Wind Energy published by John Wiley & Sons, Ltd.     

Analysis and suppression measures of lightning transient overvoltage in the signal cable of wind turbines

Lightning strikes are a major threat to the secure operation of wind turbines. When lightning strikes a wind turbine, the lightning current flows through the blade and the tower and then the induced overvoltage will damage sensors and signal cables. In this study, a comprehensive transient surge impedance model of a wind turbine was built to analyze the causes of the overvoltage in the signal cable. The model that studies the overvoltage caused by both capacitive coupling and electromagnetic induction included the blade, nacelle, tower, signal cable, power cable, and grounding system using π networks. The influences of the cable shielding layer, soil resistivity, and lightning current waveform on the overvoltage were also analyzed. Then, 2 overvoltage suppression measures, ie, grounding at 2 ends of the outer shielding layer and installation of a surge protective device, were tested. Results show that a signal cable with double shielding layers reduced the overvoltage in the signal cable, and higher soil resistivity resulted in increased voltage on the tower base. In addition, the peak and the front time of the lightning current significantly influenced the overvoltage on the tower and the cable. The effectiveness of the 2 suppression measures was also verified. The calculation results will provide guidance for a reasonable lightning protection design.     

风电场风电机组的接地设计

较系统地介绍了风电场风电机组对接地电阻的要求、接地设计思路及方法,并提供实际工程中接地网布置图实例作为参考。     

Lightning protection for wind turbine blades and bearings

The protection of wind turbines from lightning damage is increasingly important as they increase in size and are placed in locations where access to carry out repairs may be difficult. As blades are the most common attachment point of lightning, they must be adequately protected. In addition, the passage of lightning current through wind turbine bearings introduces a risk of lightning damage to these vital components. Investigations relating to the improvement of blade lightning protection systems have been carried out, including experiments designed to address the difficult problems involved in the protection of hydraulic cylinders used for tip brake control. Work has also focused on the ability of lightning current to cause damage to wind turbine bearings. The work has been a mixture of computer simulations and experimental testing using high‐voltage and high‐current facilities. Copyright © 2001 John Wiley & Sons, Ltd.     

A new method for spatial allocation of turbines in a wind farm based on lightning protection efficiency

Existing studies of the spatial allocation of wind farms are typically based on turbine power generation efficiency and rarely consider the damage caused by lightning strikes. However, lightning damage seriously affects the economic performance of wind farms because of the high cost of repairing or replacing damaged blades. This paper proposes a method for the spatial optimization of multiple turbines based on lightning protection dependability. Firstly, the lightning protection efficiency of turbine blade protection systems is analyzed by combining the physical mechanisms of lightning leader progression with a conventional electro‐geometric model to develop an electro‐geometric model of turbine blades (EGMTB). Then, the optimized spatial allocation of multiple turbines in a wind farm is investigated using the EGMTB. The results are illustrated from an example wind farm with 1.5 MW turbines, which shows that the optimal spacing between two turbines perpendicular to the prevailing wind direction L_⊥ is 4R‐6R, where R is the length of a turbine blade. This spacing is shown to effectively shield turbine blades from lightning damage over a wide range of lightning currents (>26‐60 kA). Note that, the suggested L_⊥ will be smaller considering the influence of lightning polarity as it takes more difficulty developing upward leader (UL) in the condition of positive lightning striking. Experiments verify the effectiveness and correctness of this method.     

Calculation of electromagnetic induction inside a wind turbine tower struck by lightning

An efficient algorithm is described in this paper for calculating the magnetic transient effects in a wind turbine tower struck by lightning. An electrical network model of the tower is established to compute the transient responses on its surface, and the electrical parameters of the network are determined considering the effect of ground. Based on the transient current responses along the tower conductors, the evaluation of magnetic field distribution is presented directly in time domain, and the transient overvoltage induced in a loop is also calculated according to the law of electromagnetic induction. The results are helpful in determining possible hazards and designing lightning protection systems. © 2009 John Wiley & Sons, Ltd.     

海上风力发电机组基础的选择

介绍了海上风力发电的发展现状,结合海上采油平台形式,对海上风电机组采用的基础定义、基础类型及其选择进行了介绍。     

Estimation of lightning current amplitudes incident to wind turbines in exposed locations

This paper proposes a novel and comprehensive methodology for estimation of lightning current amplitudes, which are incident to wind turbines (WT) at lightning‐exposed locations, i.e., hilltops. The proposed methodology takes into the account the following aspects related to WT lightning incidence: (i) site topology and keraunic level, (ii)statistical nature of lightning currents, (iii) WT effective height, (iv) dependence of lightning current amplitudes on the WT effective height, (v) influence of an upward‐initiated connecting streamers interaction with a downward‐propagating step leader on the WT lightning attractiveness, and (vi) both downward and upward lightning strikes and their relative contribution in relation to the WT effective height. This methodology could be perceived as beneficial in providing relevant lightning‐current amplitudes—particularly at lightning‐endangered locations and wind farm sites characterized by high soil resistivity—for wind turbine and wind farm overvoltage protection and backsurge studies, as well as the WT grounding systems transient analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Survey of modelling methods for wind turbine wakes and wind farms

This article provides an overview and analysis of different wake‐modelling methods which may be used as prediction and design tools for both wind turbines and wind farms. We also survey the available data concerning the measurement of wind magnitudes in both single wakes and wind farms, and of loading effects on wind turbines under single‐ and multiple‐wake conditions. The relative merits of existing wake and wind farm models and their ability to reproduce experimental results are discussed. Conclusions are provided concerning the usefulness of the different modelling approaches examined, and difficult issues which have not yet been satisfactorily treated and which require further research are discussed. Copyright © 1999 John Wiley & Sons, Ltd.     

Adjustment of wind farm power output through flexible turbine operation using wind farm control

When the installed capacity of wind power becomes high, the power generated by wind farms can no longer simply be that dictated by the wind speed. With sufficiently high penetration, it will be necessary for wind farms to provide assistance with supply‐demand matching. The work presented here introduces a wind farm controller that regulates the power generated by the wind farm to match the grid requirements by causing the power generated by each turbine to be adjusted. Further, benefits include fast response to reach the wind farm power demanded, flexibility, little fluctuation in the wind farm power output and provision of synthetic inertia. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessment on energization sequence for reducing sympathetic interaction between wind turbine transformers

Increasing installations of large offshore wind farms are being designed and commissioned. The collection grids of these wind farms usually accommodate hundreds of wind turbine transformers. Energization of these transformers is likely to cause sympathetic inrush due to the close electrical distances between them. The wind turbine transformers are consequently subjected to long duration mechanical and thermal stresses under various energization conditions. In this paper, a network model of wind farm collection grid, developed and validated against field measurement results, is utilized to assess various energization sequences for reducing the sympathetic inrush. Deterministic assessment under a critical energization condition and stochastic assessment under stochastic energization condition were conducted. Both indicate that the optimum sequence is to energize wind turbine transformer from the one closest to the offshore platform to the one farthest from the platform. Copyright © 2014 John Wiley & Sons, Ltd.     

Assessment of wind characteristics and wind turbine characteristics in Taiwan

Wind characteristics and wind turbine characteristics in Taiwan have been thoughtfully analyzed based on a long-term measured data source (1961–1999) of hourly mean wind speed at 25 meteorological stations across Taiwan. A two-stage procedure for estimating wind resource is proposed. The yearly wind speed distribution and wind power density for the entire Taiwan is firstly evaluated to provide annually spatial mean information of wind energy potential. A mathematical formulation using a two-parameter Weibull wind speed distribution is further established to estimate the wind energy generated by an ideal turbine and the monthly actual wind energy generated by a wind turbine operated at cubic relation of power between cut-in and rated wind speed and constant power between rated and cut-out wind speed. Three types of wind turbine characteristics (the availability factor, the capacity factor and the wind turbine efficiency) are emphasized. The monthly wind characteristics and monthly wind turbine characteristics for four meteorological stations with high winds are investigated and compared with each other as well. The results show the general availability of wind energy potential across Taiwan.     

Electrical and thermal performance of different core materials applied in wind turbine blades under lightning strikes

Wind turbine blades are subject to lightning strikes, which may result in severe damage of the blade materials. The withstanding performance of different blade materials needs to be classified in order to maximize their operational safety. Lightning impulse voltages were applied to model blades with different core materials (polyvinyl chloride [PVC], polyethylene terephthalate [PET], and balsa wood) to characterize the breakdown points (electrically vulnerable areas) on the blades. It has been found that the areas subjected to puncturing are located in the back part (toward the trailing edge) of the sandwich structure, especially at locations close to the main beam. Model blades made of balsa wood are more susceptible to puncturing breakdown than PVC and PET. High impulse currents were imposed at the most probably stricken spots in the impulse voltage tests to compare the severity of damage for PVC, PET, and balsa wood in an attempt to understand the thermal effect of lightning discharge following the final jump. Results show that balsa wood is most resistive to while PVC suffers most damage due to the thermal impact of lightning. Molecular simulation of the chemical degradation process and thermal gas production dynamics was performed at atomic level to explain the damage mechanisms of the three core materials. The performance data of the blade core materials against lightning strike obtained in the present work provide strong guidance on the optimal design of wind turbine blade structure and selection of blade core material.     

风电场风机初步选型方法研究

市场上可供选择的风机种类众多,各类风机在性能、价格、维修及制造商服务等多方面都存在明显差异,因此在选择风机时有必要综合考虑多方面因素。鉴于工程上对风机作初步选型时采用的方法大多具有局限性,文章在确定影响风机选型的主要因素后提出一套风机初步选型方法,该方法根据拟建风电场的情况和设计人员的要求,综合考虑各因素后选出符合条件的风机。文章基于该选型方法将Visual Basic语言与Microsoft Access数据库相结合,开发出相应的选型系统,使整个选型过程更为方便、快捷。     

Blade sections for wind turbine and tidal current turbine applications–current status and future challenges

The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design of blade sections appropriate for specific applications. In modern large wind turbines, the blade tip is designed using a thin airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support. A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefficient directly contributes to a higher aerodynamic efficiency, an increase in the lift coefficient does not have a significant contribution to the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with LS and SERI airfoils, which are followed by many series of airfoils, including the new CAS airfoils. After nearly two decades of extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the freestream turbulence level affects the performance of the blade. A number of active and passive flow control devices have been proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to lead to thicker sections, particularly near the root, which will cause a higher drag coefficient. A bigger challenge in the design of blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift coefficient. Another challenge for the designers is to design blades that give consistent output at varying flow conditions with a simple control system. The performance of a rotating blade may be significantly different from a non‐rotating blade, which requires that the design process should continue till the blade is tested under different operating conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal turbine spacing in fully developed wind farm boundary layers

As wind farms become larger, the asymptotic limit of the ‘fully developed’, or ‘infinite’, wind farm has been receiving an increased interest. This limit is relevant for wind farms on flat terrain whose length exceeds the height of the atmospheric boundary layer by over an order of magnitude. Recent computational studies based on large eddy simulation have identified various mean velocity equilibrium layers and have led to parameterizations of the effective roughness height that allow the prediction of the wind velocity at hub height as a function of parameters such as wind turbine spacing and loading factors. In the current paper, we employ this as a tool in making predictions of optimal wind turbine spacing as a function of these parameters, as well as in terms of the ratio of turbine costs to land surface costs. For realistic cost ratios, we find that the optimal average turbine spacing may be considerably higher than that conventionally used in current wind farm implementations. Copyright © 2011 John Wiley & Sons, Ltd.     

Nonstationarity in correlation matrices for wind turbine SCADA-data

Modern utility-scale wind turbines are equipped with a Supervisory Control And Data Acquisition (SCADA) system gathering vast amounts of operational data that can be used for analysis to improve operation and maintenance of turbines. We analyze high-frequency SCADA-data from the Thanet offshore wind farm in the United Kindom and evaluate Pearson correlation matrices for a variety of observables with a moving time window. This renders possible a quantitative assessment of nonstationarity in mutual dependencies of different types of data. We show that a clustering algorithm applied to the correlation matrices reveals distinct correlation structures for different states. Looking first at only one and then at multiple turbines, the main dependence of these states is shown to be on wind speed. This is in accordance with known turbine control systems, which change the behavior of the turbine depending on the available wind speed. We model the boundary wind speeds separating the states based on the clustering solution. Our analysis shows that for high-frequency data, the control mechanisms of a turbine lead to detectable nonstationarity in the correlation matrix. The presented methodology allows accounting for this with an automated preprocessing by sorting new data based on wind speed and comparing it to the respective operational state, thereby taking the nonstationarity into account for an analysis.     

Resonant overvoltage assessment in offshore wind farms via a parametric black‐box wind turbine transformer model

The protection of offshore wind farms (OWFs) against overvoltages, especially resonant overvoltage, is of paramount importance because of poor accessibility and high repair costs. In this paper, we study how switching overvoltages at the wind turbine transformer (WTT) medium voltage (MV) side can lead to high overvoltages on the low voltage (LV) side. The effect of overvoltage protective devices is analyzed. A detailed model of an OWF row is developed in electromagnetic transients program–alternative transients program (EMTP‐ATP), including interconnecting cables, WTT, surge arresters and resistive–capacitive filters. A parameterized black‐box WTT model is obtained from measurements and is used for investigating the transfer of resonant overvoltages from the MV to the LV side. The model is capable of shifting systematically the frequencies and adjusting the transformer input impedance. Simulation results show that wind turbine energization in an OWF can lead to overvoltages on the LV terminals. The rate of rise of overvoltages (du/dt) is in the range of 300–500 pu/µs. It is found that resistive–capacitive filters should be installed on both MV and LV terminals of WTTs to decrease both resonant overvoltages and du/dt, which is unachievable by surge arrester alone. Copyright © 2014 John Wiley & Sons, Ltd.     

大连近海风电场风机机组的选型与布置初探

风机的选型与布置是风电场建设可行性研究的重要内容,对风电场的建设造价和投产后的发电效益有重要的影响。文章在大连近海风资源评估的基础上,综合考虑国内外风力发电机组的制造水平、技术成熟程度,选择4种机型,布置在两个参考场址,预测其理论发电量,通过技术经济比较,选出最佳机型。一号场址的水文地质条件比较利于风机布置,可安装34台单机容量为3 MW的风力发电机组,布置方式为2排17列,年上网发电量约为26 262万kWh。     

风电机组防雷保护和接地设计

通过防雷保护设计的理论分析,根据风电机组不同的防雷保护区进行的专项设计,结合工程实际,系统地阐述了风电系统中雷电保护、防雷接地设计方案及其实现方法,可为风电机组和风电场防雷措施和接地设计提供参考。