Dynamical measurement system for wind turbine fatigue load

A measurement system specially used in wind turbine fatigue load assessment is developed based on Labview platform and Control Area Network (CAN). By applying CAN bus communication technology, the system can perform data automatic acquisition, data stable transmission and data real-time monitoring. By adopting the technology of virtual instrument modular design, the system is designed to analyze the wind turbine mechanical load levels against wind and power, equivalent loads and lifetime fatigue loads, etc. Considering the effects of small load strengthening, low amplitude load damaging and multilevel load interaction, a novel fatigue lifetime prediction model is proposed to obtain more accurate and reliable prediction of blade fatigue life. With the developed measurement system, the in-field load measurements are performed and the results showed the system has satisfactory accuracy and good adaption, convenient operation, high integration, low cost and great practicality to load measurement of large wind turbine. And based on the proposed model the fatigue life of WT blade can be estimated more trustworthily and reliably.     

Advanced control algorithms for reduction of wind turbine structural loads

To enable further growth of wind turbine dimensions and rated power, it is essential to decrease structural loads that wind turbines experience. Therefore a great portion of research is focused on control algorithms for reduction of wind turbine structural loads, but typically wind turbine rotor is considered to be perfectly symmetrical, and therefore such control algorithms cannot reduce structural loads caused by rotor asymmetries. Furthermore, typical approach in the literature is to use blade load measurements, especially when higher harmonics of structural loads are being reduced. In this paper, improvements to standard approach for reduction of structural loads are proposed. First, control algorithm capable of reducing structural loads caused by rotor asymmetries is developed, and then appropriate load transformations are introduced that enable presented control algorithms to use load measurements from various wind turbine components. Simulation results show that proposed control algorithm is capable of reducing structural loads caused by rotor asymmetries.     

Wind turbine aerodynamics and loads control in wind shear flow

Wind turbine is subjected to some asymmetrical effects like wind shear, which will lead to unsteady blade airloads and performance. Fatigue loads can lead to damage of turbine components and eventually to failures. It is evident that the variation of the velocity over the rotor disc has an influence on the blade and introduces both flap-wise and edge-wise fatigue damage on the blade as a result of moment fluctuations in the two directions. The flap-wise moments on the blade are the origin of the rotor yaw and tilt moments which transmit to the turbine structure through the drive train to the yaw system and the tower. A lifting surface method with time marching free wake model is used to investigate the periodic unsteady nature in the wind shear. Individual pitch control (IPC) that is applied nowadays is the most advanced active control to reduce the fatigue. The blade airloads and performance of the turbine are also predicted under IPC control. It is found that IPC of the fluctuating blade root flap-wise moment can reduce the flap-wise fatigue damage remarkably while the blade root edge-wise moments are less sensitive to the varying blade pitch than the blade root flap-wise moments.     

Uncertainty analysis of wind energy potential assessment

This study presents a framework to assess the wind resource of a wind turbine using uncertainty analysis. Firstly, probability models are proposed for the natural variability of wind resources that include air density, mean wind velocity and associated Weibull parameters, surface roughness exponent, and error for prediction of long-term wind velocity based on the Measure–Correlate–Predict method. An empirical probability model for a power performance curve is also demonstrated. Secondly, a Monte-Carlo based numerical simulation procedure which utilizes the probability models is presented. From the numerical simulation, it is found that the present method can effectively evaluate the expected annual energy production for different averaging periods and confidence intervals. The uncertainty, which is 11% corresponding to the normalized average energy production in the present example, can be calculated by specifically considering the characteristics of the individual sources in terms of probability parameters.     

Effective turbulence and its implications in wind turbine fatigue assessment

The effective turbulence approximation is widely used in the wind energy industry for site‐specific fatigue assessment of wind turbines with reference to loads. It significantly reduces the amount of aero‐elastic simulations required to document structural integrity by integrating out the directional variation of turbulence. Deriving the effective turbulence involves assumptions related to load effect histories, structural dynamics, and material fatigue strength. These assumptions may lead to low accuracy of fatigue load assessments by the effective turbulence compared with full directional simulations. This paper quantifies the implications of the effective turbulence for a multimegawatt wind turbine during normal operation. Analyses based on wind measurements from almost one hundred international sites document that the effective turbulence provides accurate results compared with full sector‐wise simulations, but only when linear SN ‐curves are assumed. For a more advanced steel tower design approach using a bilinear SN ‐curve, a reduction of the cross‐sectional design parameters by almost 10% is achieved. Additional 10% reduction can be obtained if fatigue damage is estimated utilizing the wind direction information. By applying a probabilistic approach, it is shown that this reduction in the design parameter of the steel tower does not compromise the structural integrity when the current IEC 61400‐1 standard is followed. The results presented may improve decision making in site‐specific fatigue assessments of wind turbines and prevent overconservative design, which results from the use of the effective turbulence, and thereby reduce the cost of wind energy.     

A Monte Carlo simulation study of wind turbine loads in thunderstorm downbursts

The simulation of thunderstorm downbursts and associated loads on a utility‐scale wind turbine is the focus of this study. Using a deterministic–stochastic hybrid model, downburst‐related wind fields are generated separately from non‐turbulent and turbulent parts. The non‐turbulent part builds on available analytical models developed from field data that include recorded downburst events; the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions adjusted by information on parameters such as the downburst's translation velocity. Key thunderstorm downburst‐related parameters include the maximum radial velocity, the height and radial distance to the maximum radial velocity, the downburst intensity, the downburst translation velocity and the downburst translation direction. In addition, the streamwise ambient (environmental) velocity and the downburst touchdown location relative to the wind turbine are also important in turbine load computation. A utility‐scale 5‐MW wind turbine model is selected, and loads are generated using stochastic simulation of the aeroelastic response. Information available in the literature on recorded downbursts is used to define the cases studied. A single downburst simulation and associated turbine response simulation is first discussed to illustrate loads computation and highlight downburst‐related parameters of interest. Next, a Monte Carlo simulation study is performed to investigate the influence of touchdown locations and translation direction on turbine extreme loads. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of fatigue life prediction method and effect of 10-minute mean wind speed distribution on fatigue life of small wind turbine composite blade

This study aims to develop a fatigue life prediction method and to identify the effect that a 10-minute mean wind speed distribution has on the fatigue life of a small-scale wind turbine composite blade. First, combining the von Karman isotropic turbulence model and the Weibull distribution for a 10-minute mean wind speed provided us with the 1-Hz full wind history for a specific time period. Accordingly, the fatigue stress spectra at the blade's fatigue-critical locations (FCLs) were created by applying a stress tensor, in which the interaction between flapwise and edgewise bending moments was taken into consideration. The fatigue life of a composite blade can be predicted with a reliability R = 95% by applying the P–S–N curve obtained from the constant amplitude fatigue tests and rainflow cycle counting, and cumulative damage rule to the fatigue stress spectra. To acquire the second-order regression equation, nonlinear regression analysis was performed on the fatigue lives, which were simulated by using the proposed fatigue life prediction method. In this equation, the variables were the shape parameter, K, and the scale parameter, C, of the Weibull distribution for a 10-minute mean wind speed. The effects of the Weibull parameters on fatigue life were evaluated through the sensitivity analysis of the equations.     

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.     

On wind turbine loads during the evening transition period

The late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This “evening transition” (ET) period is often when energy demand peaks. This period also corresponds to the time of day that is a precursor to late‐afternoon downbursts, a subject of separate interest. To capture physical characteristics of wind fields in the atmospheric boundary layer (ABL) during this ET period, particularly the interplay of shear and turbulence, stochastic simulation approaches, although more tractable, are not suitable. Large‐eddy simulation (LES), on the other hand, may be used to generate high‐resolution ABL turbulent flow fields. We present a suite of idealized LES four‐dimensional flow fields that define a database representing different combinations of large‐scale atmospheric conditions (characterized by associated geostrophic winds) and surface boundary conditions (characterized by surface heat fluxes). Our objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine‐scale wind field variables. We then employ the database of these LES‐based inflow wind fields in aeroelastic simulations of a 5‐MW wind turbine. We discuss how turbine loads change as the ET period evolves. We also discuss maximum and fatigue loads on the rotor and tower resulting from different ABL conditions. Results of this study suggest that, during the ET period, the prevailing geostrophic wind speed affects the mean and variance of longitudinal winds greatly and thus has significant influence on all loads except the yaw moment which is less sensitive to uniform and symmetric incoming flow. On the other hand, surface heat flux levels affect vertical turbulence and wind shear more and, as a result, only affect maximum blade flapwise bending and tower fore‐aft bending loads.     

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.     

LCA sensitivity analysis of a multi-megawatt wind turbine

During recent years renewables have been acquiring gradually a significant importance in the world market (especially in the Spanish energetic market) and in society; this fact makes clear the need to increase and improve knowledge of these power sources. Starting from the results of a Life Cycle Assessment (LCA) of a multi-megawatt wind turbine, this work is aimed to assess the relevance of different choices that have been made during its development. Looking always to cover the largest possible spectrum of options, four scenarios have been analysed, focused on four main phases of lifecycle: maintenance, manufacturing, dismantling, and recycling. These scenarios facilitate to assess the degree of uncertainty of the developed LCA due to choices made, excluding from the assessment the uncertainty due to the inaccuracy and the simplification of the environmental models used or spatial and temporal variability in different parameters. The work has been developed at all times using the of Eco-indicator99 LCA method.     

Comparative assessment of performance of foreign and local wind turbine manufacturers in China

Compared to the conventional fossil fuel energy, wind power provides clean energy which can mitigate the impacts of greenhouse gas emission and optimize the electric power source structure. During the last decade, the strong support of the Chinese government has contributed to the rapid development of the Chinese wind power sector which has in turn resulted in a significant growth of the Chinese wind turbine manufacturing industry. This growth went through several phases including the initial approval of several leading global Wind Turbine Manufacturers (WTMs) to enter the Chinese market through various methods which included the establishment of wholly foreign-owned enterprises from 2005. Similarly, several government policies have contributed to the significant expansion in terms of both productivity and quantities by local Chinese WTMs. The entrance of foreign WTMs into the Chinese market coupled with the rapid growth of local WTMs has contributed to intense competition in China’s wind turbine manufacture market. This paper analyzes the characteristics of the wind turbine manufacturing industry in China and establishes a hierarchical structure of the WTMs’ competitive priority system. This system consists of 5 indicators and 10 sub-indicators. By comparing the different performances of each indicator, the competitive advantages and disadvantages of the foreign and local WTMs in the Chinese market are identified. The findings provide a valuable reference for the WTMs to improve their competitive priorities and to formulate their competitive strategy in the Chinese wind turbine market. This paper provides inputs for the sustainable development of wind power industry in other countries.     

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.     

Calculation of hoisting forces of the wind turbine rotor based on wind conditions

The assembly and hoisting process of the wind turbine rotor in an open wind environment are regarded to improve the hoisting safety, efficiency and quality. The wind turbine rotor model of a 1.5 MW wind turbine are given, and the hoisting forces of the wind turbine rotor in different poses with various azimuth angles, yaw angles and pitch angles in 3D coordinate system are calculated based on the defined wind conditions model. The maximum and minimum hoisting forces of the wind turbine rotor are acquired and the corresponding azimuth angle, yaw angle and pitch angle of the wind turbine rotor are obtained with respect to the wind conditions in the hoisting process. For four specific poses with particular azimuth angles, yaw angles and pitch angles of the wind turbine rotor, the hoisting forces of the wind turbine rotor are calculated along its hoisting height increment. The change processes of the hoisting forces of the wind turbine rotor in the hoisting process are analyzed and the conclusions are drawn.     

Design and kinetic analysis of wind turbine blade-hub-tower coupled system

A typical 1.5 MW wind turbine suitable for Xuzhou City is designed and simulated in this paper. The wind turbine blade-hub-tower coupling system and most of the parameters are designed and calculated in the design process. In the kinetic analysis process, the force analysis under 4 different situations are taken to verify the structure design, which are under quiescent condition, under random angle and random wind turbine, under maximal wind speed and over maximal wind speed. At last, the modal analysis selected the turbine hub and tower to solve the inherent frequencies and vibration modes. The first 5 order inherent frequencies and vibration modes of the hub and tower are solved to verify the design rationality.     

S-Transform and its contribution to wind turbine condition monitoring

Condition monitoring (CM) has long been recognised as one of the best methods of reducing the operation and maintenance (O&M) costs of wind turbines (WTs). However, its potential in the wind industry has not been fully exploited. One of the major reasons is due to the lack of an efficient tool to properly process the WT CM signals, which are usually non-stationary in both time and frequency domains owing to the constantly varying operational and loading conditions experienced by WTs. For this reason, S-transform and its potential contribution to WT CM are researched in this paper. Following the discussion of the superiorities of S-transform to the Short-Time Fourier Transform (STFT) and Wavelet Transform, two S-transform based CM techniques are developed, dedicated for use on WTs. One is for tracking the energy variations of those fault-related characteristic frequencies under varying operational conditions (the energy rise of these frequencies usually indicates the presence of a fault); another is for assessing the health condition of WT gears and bearings, which have shown significant reliability issues in both onshore and offshore wind projects. In the paper, both proposed techniques have been verified experimentally, showing that they are valid for detecting both the mechanical and electrical faults occurring in the WT despite its varying operational and loading conditions.     

Wind turbine fatigue loads as a function of atmospheric conditions offshore

In recent years, there has been a growing interest by the wind energy community to assess the impact of atmospheric stability on wind turbine performance; however, up to now, typically, stability is considered in several distinct arbitrary stability classes. As a consequence, each stability class considered still covers a wide range of conditions. In this paper, wind turbine fatigue loads are studied as a function of atmospheric stability without a classification system, and instead, atmospheric conditions are described by a continuous joint probability distribution of wind speed and stability. Simulated fatigue loads based upon this joint probability distribution have been compared with two distinct different cases, one in which seven stability classes are adopted and one neglecting atmospheric stability by following International Electrotechnical Commission (IEC) standards. It is found that for the offshore site considered in this study, fatigue loads of the blade root, rotor and tower loads significantly increase if one follows the IEC standards (by up to 28% for the tower loads) and decrease if one considers several stability classes (by up to 13% for the tower loads). The substantial decrease found for the specific stability classes can be limited by considering one stability class that coincides with the mean stability of a given hub height wind speed. The difference in simulated fatigue loads by adopting distinct stability classes is primarily caused by neglecting strong unstable conditions for which relatively high fatigue loads occur. Combined, it is found that one has to carefully consider all stability conditions in wind turbine fatigue load simulations. Copyright © 2016 John Wiley & Sons, Ltd.     

Study of damage and repair of blades of a 300 kW wind turbine

The inspection of damages detected in some blades of 300 kW wind turbines revealed that the nature of these damages was probably due to a fatigue mechanism. The causes that had originated the failure (superficial cracks, geometric concentrator, abrupt change of thickness) have been studied, verifying, by means of the simplified evaluation procedure of fatigue life of the “Germanischer Lloyd” (GL) standard, that these causes can explain the failure detected in the period of time in which it happened.     

Outlier robustness for wind turbine extrapolated extreme loads

Methods for extrapolating extreme loads to a 50 year probability of exceedance, which display robustness to the presence of outliers in simulated loads data set, are described. Case studies of isolated high extreme out‐of‐plane loads are discussed to emphasize their underlying physical reasons. Stochastic identification of numerical artifacts in simulated loads is demonstrated using the method of principal component analysis. The extrapolation methodology is made robust to outliers through a weighted loads approach, whereby the eigenvalues of the correlation matrix obtained using the loads with its dependencies is utilized to estimate a probability for the largest extreme load to occur at a specific mean wind speed. This inherently weights extreme loads that occur frequently within mean wind speed bins higher than isolated occurrences of extreme loads. Primarily, the results for the blade root out‐of‐plane loads are presented here as those extrapolated loads have shown wide variability in literature, but the method can be generalized to any other component load. The convergence of the 1 year extrapolated extreme blade root out‐of‐plane load with the number of turbulent wind samples used in the loads simulation is demonstrated and compared with published results. Further effects of varying wind inflow angles and shear exponent is brought out. Parametric fitting techniques that consider all extreme loads including ‘outliers’ are proposed, and the physical reasons that result in isolated high extreme loads are highlighted, including the effect of the wind turbine controls system. Copyright © 2011 John Wiley & Sons, Ltd.     

Prediction of wind‐turbine fatigue loads in forest regions based on turbulent LES inflow fields

Large‐eddy simulations (LES) were used to predict the neutral atmospheric boundary layer over a sparse and a dense forest, as well as over grass‐covered flat terrain. The forest is explicitly represented in the simulations through momentum sink terms. Turbulence data extracted from the LES served then as inflow turbulence for the simulation of the dynamic structural response of a generic wind turbine. In this way, the impact of forest density, wind speed and wind‐turbine hub height on the wind‐turbine fatigue loads was studied. Results show for example significantly increased equivalent fatigue loads above the two forests. Moreover, a comparison between LES turbulence and synthetically generated turbulence in terms of load predictions was made and revealed that synthetic turbulence was able to excite the same spectral peaks as LES turbulence but lead to consistently lower equivalent fatigue loads. Copyright © 2016 John Wiley & Sons, Ltd.