Modern Multi-megawatt Wind Turbines Research Articles

Numerical study of the unsteady blade root aerodynamics of a 2 MW wind turbine equipped with vortex generators

Abstract. In order to design vortex generators for modern multi-megawatt wind turbines accurately, the 3D behaviour of the boundary layer has to be considered. Due to the rotation of the blade, the lift-enhancing rotational augmentation has a considerable impact, especially in the inner blade sections. To investigate the interaction of vortex generators and rotational augmentation, high-fidelity computational flow simulations by means of unsteady Reynolds-averaged Navier–Stokes methods are presented for a rotating blade of a generic 2 MW horizontal-axis wind turbine. The inner blade section is analysed with and without vortex generators for two different pitch settings, including one causing largely separated flow. Two ways of placing the vortex generators on the blade with different radial starting positions are investigated in order to find out if the coexistence of the two lift-enhancement methods (i.e. rotational augmentation and vortex generators) is beneficial. All simulations are performed with the flow solver FLOWer, and the vortex generators are modelled by the introduction of source terms into the computational domain through a so-called BAY (Bender–Anderson–Yagle)-type model. For the case without vortex generators, it is found that the strength of rotational augmentation largely depends on the effective angles of attack (i.e. the pitch setting). For the case with lower effective angles of attack, rotational augmentation is a cyclic phenomenon, whereas for the case with higher effective angles of attack, it generates large loads in the inner root section due to a constant centrifugal pumping mechanism in time. The results from the cases with vortex generators display a rather destructive interaction of vortex generators and rotational augmentation on the torque. For low effective angles of attack and thus attached flow conditions, vortex generators exhibit slight losses compared to the case without vortex generators, as they inhibit spanwise flow through rotational augmentation. For high effective angles of attack, the vortex generators placed over 30 % of the blade produce an increase of 3.28 % in torque compared to the case without vortex generators and high centrifugal pumping.

On the extension of streamwise turbulence intensity profile beyond the atmospheric surface layer under neutral to unstable stratifications

Turbulence intensity (TI) is a key factor that affects both the energy production and the performance of individual wind turbines and wind farms. In particular, accurate energy production estimation from wind sources requires a reliable TI model at turbine hub height. Modern multi-megawatt wind turbines operate at heights well above the atmospheric surface layer (ASL). However, most existing analytical TI models are merely valid in the ASL or under neutral atmospheric conditions. Therefore, this paper aims to derive an analytical expression for streamwise TI under neutral to unstable stratifications, which can reasonably model TI at heights beyond the ASL. To this end, the Monin-Obukhov similarity theory, Townsend's attached eddy hypothesis, and a modification proposed in the literature to the eddy population density of Townsend for unstable conditions are utilized. The proposed models are validated against datasets from four tall meteorological towers in Denmark, Sweden, and China. The sites constitute a considerable difference in Coriolis parameter and surface roughness. Results show that the proposed models reasonably follow the measured velocity variance and TI data under neutral to unstable conditions in the ASL and beyond.

Multi-objective optimization of composite airfoil fibre orientation under bending–torsion coupling for improved aerodynamic efficiency of horizontal axis wind turbine blade

Modern multi-megawatt wind turbines have long, slender and flexible blades, which experience significant vibration due to aerodynamic loads. These blades are generally made of composite layups with cambered airfoil sections, which induce elastic and dynamic coupling between bending and torsional modes. In addition to these phenomena, the aerodynamic pitching moment also arises due to the offset of the blade pitch axis from the aerodynamic centre. With this in mind, the present work aims to develop a computationally efficient complete multi-body dynamic aero-servo-elastic model of an onshore horizontal axis wind turbine, including bending–torsion coupling and aerodynamic pitching moment. 3D wind fields are generated using TurbSim, and the responses of the coupled system are solved under different operational scenarios, which signify the impact of the aforementioned coupling. It is found to be particularly detrimental at higher wind speeds than the rated value. But, proper design and implementation of structural layup can enhance the aerodynamic performance of the blade. Finally, the optimal design of composite fibre orientation of a benchmark turbine blade is investigated to show its beneficial effects in load/response mitigation.

Pitch Control of Three Bladed Large Wind Energy Converters—A Review

Modern multi-megawatt wind turbines are currently designed as pitch-regulated machines, i.e., machines that use the rotation of the blades (pitching) in order to adjust the aerodynamic torque, such that the power is maintained constantly throughout a wide range of wind speeds when they exceed the design value (rated wind speed). Thus, pitch control is essential for optimal performance. However, the pitching activity is not for free. It introduces vibrations to the tower and blades and generates fatigue loads. Hence, pitch control requires a compromise between wind turbine performance and safety. In the past two decades, many approaches have been proposed to achieve different objectives and to overcome the problems of a wind energy converter using pitch control. The present work summarizes control strategies for problem of wind turbines, which are solved by using different approaches of pitch control. The emphasis is placed on the bibliographic information, but the merits and demerits of the approaches are also included in the presentation of the topics. Finally, very large wind turbines have to simultaneously satisfy several control objectives. Thus, approaches like collective and individual pitch control, tower and blade damping control, and pitch actuator control must coexist in an integrated control system.

Weight-scheduling for linear time-variant model predictive wind turbine control toward field testing

Modern multi-megawatt wind turbines require powerful control algorithms which consider several control objectives at the same time and respect process constraints. Model predictive control (MPC) is a promising control method and has been a research topic for years. So far, very few studies evaluated MPC algorithms in field tests. This work aims to prepare a real-time MPC system for a full-scale field test in a 3 MW wind turbine. To this end, we introduce a weight-scheduling scheme for a linear time-variant MPC in order to ensure control operation over the entire operating range from the partial to the full load range. We use a rapid control prototyping process, in particular with comprehensive software-in-the-loop (SiL) tests, in order to design and validate the MPC system for the field test.In this contribution, we present the implementation of the linear time-variant MPC with weight-scheduling to be tested in the field test. With the weight-scheduling for the optimization problem inside the MPC, we achieved good performance over the entire operating range of the wind turbine. In the SiL tests, the proposed MPC algorithm achieved loads, comparable to the baseline controller of the wind turbine and improved the reference tracking of the power output and the rotational speed. The proposed linear time-variant MPC with weight-scheduling is fully validated in the presented software-in-the-loop tests and is ready for full-scale field test in the 3 MW wind turbine. We present the experimental field test results of the introduced MPC system in a separated contribution.

Vibration control of horizontal axis offshore wind turbine blade using SMA stiffener

Modern multi-megawatt wind turbines have large flexible blades that often exhibit significant vibration when exposed to turbulent wind. The present study proposes longitudinal stiffening of blades to address this issue using tendon made of shape memory alloy. A reduced order model of the combined system is developed that incorporates blade stiffening. The nonlinear material behavior of the stiffener is modeled by combining the principle of thermodynamics with the constitutive model as proposed in the literature. Thus, super-elastic effects are utilized in active mode (i.e. using Jule heating with the help of current flow) to apply actuation force that opposes blade deformation. Three-dimensional wind field passing through the rotor plane is simulated using TurbSim package freely available from National Renewable Energy Laboratory, USA. Aerodynamic loads are computed using modified Blade Element Momentum theory and wave loads are simulated using Morison’s equation where wave time histories are simulated from the JONSWAP spectrum. Using these loads as input, the response of a benchmark wind turbine is simulated to show the performance of the proposed control strategy. Numerical results presented in this paper clearly demonstrate the efficiency and advantage of the SMA based stiffener to improve the vibration characteristics of the large turbine blades.

Improved reliability of wind turbine towers with active tuned mass dampers (ATMDs)

Modern multi-megawatt wind turbines are composed of slender, flexible, and lightly damped blades and towers. These components exhibit high susceptibility to wind-induced vibrations. As the size, flexibility and cost of the towers have increased in recent years, the need to protect these structures against damage induced by turbulent aerodynamic loading has become apparent. This paper combines structural dynamic models and probabilistic assessment tools to demonstrate improvements in structural reliability when modern wind turbine towers are equipped with active tuned mass dampers (ATMDs). This study proposes a multi-modal wind turbine model for wind turbine control design and analysis. This study incorporates an ATMD into the tower of this model. The model is subjected to stochastically generated wind loads of varying speeds to develop wind-induced probabilistic demand models for towers of modern multi-megawatt wind turbines under structural uncertainty. Numerical simulations have been carried out to ascertain the effectiveness of the active control system to improve the structural performance of the wind turbine and its reliability. The study constructs fragility curves, which illustrate reductions in the vulnerability of towers to wind loading owing to the inclusion of the damper. Results show that the active controller is successful in increasing the reliability of the tower responses. According to the analysis carried out in this paper, a strong reduction of the probability of exceeding a given displacement at the rated wind speed has been observed.

Wind Turbine Blade Fault Detection Using Wavelet Power Spectrum and Experimental Modal Analysis

The dynamic behavior of modern multi-Megawatt wind turbines has become an important design consideration. The power generated by wind turbine depends mainly on the interaction between the blades and the flowing wind. Therefore, the major aspects related to the operational reliability of a wind turbine are the aerodynamics characteristics and integrity of the blades. This paper demonstrates the results of the application of wavelet transform combined with experimental modal analysis as a tool for wind turbine blade condition monitoring. An experimental model analysis with different vibrational excitations performed for healthy and cracked cantilever stepped beam, which approximate the wind turbine blade when attached to the wind turbine hub. The beam response signals acquired for three different locations of vibration sensor. The cantilever beam with breathing crack response exhibits a non-stationary vibrational signal because of crack open-close cycles during the oscillation of the beam. The wavelet transform as a time-frequency signal processing technique is an effective tool to extract the characteristics of the non-stationary signals; therefore, used in this paper to analyze the cracked beam vibrational signals. The wavelet power spectrum (WPS) calculated for different beam health conditions and for different locations of the vibration sensors. The results reveal the effectiveness of the wavelet power spectrum over the traditional FFT spectrum to identify the existence of the crack in the wind turbine blade. Moreover, a mathematical model, which correlates the variation of the blade natural frequencies with the crack severity and location, is presented and validated with an experimental modal analysis data.

Technical and Economic Feasibility study on Repowering of Wind Farms

Objectives: The main objective of the paper is to perform a repowering assessment of an old wind farm consisting of 2 bladed turbines located at Gudimangalam near Coimbatore in Tamil Nadu which is a good wind potential site. Methods/Statistical Analysis: Repowering refers to the replacement of first generation small capacity wind turbines with modern multi megawatt wind turbines that can produce more energy than before from the same site. Technical analysis of the site is done using WAsP software with the help of wind data collected from a met mast at the site and includes the calculation of Net Annual Energy Production and Wake losses. Net Annual Energy Production and Wake losses are the primary indicators for selecting the best options for repowering. The option selected is further supported by calculating the economic performance indices. The total repowering has been accomplished through partial repowering executed in three stages. Partial repowering refers to the replacement of only few of the turbines from the existing wind farm rather than replacing the whole farm. The economic feasibility of every repowering option has been studied based on the calculation of the economic performance indices like ALCoG, Payback Period and IRR. Findings: The study mainly results in framing a procedure for performing total repowering for any site through the optimization of best possible partial repowering options. The existing site comprises 18 turbines of 450 kW rating and is having a net AEP of 14.5 GWh with a wake loss of 3.82%. The Plant Load Factor (PLF) is obtained to be 20.44%. The best possible total repowering option is found to be producing a net AEP of 65.287 GWh with a wake loss of 2.36% and included 9 Suzlon S95 2.1 MW turbines which are four times the AEP of the existing farm. The three stages of partial repowering also include replacement of 6 old turbines by 3 new turbines in each stage. Finally the total repowering is reached through the execution of partial repowering options in three stages and it results in close proximity to the best possible total repowering option in both technical and economic aspects. The final repowering eventually attained results in a net AEP of 65.28 GWh with a wake loss of 2.36% which is the exact match of the best possible total repowering option found in the beginning. The total repowering also leads to an improvement in PLF i.e. almost doubles the PLF to 39.42%. Application/Improvements: This procedure can be very useful where the initial investment is an important factor. The study infers that performing total repowering through optimization process is more beneficial than performing partial repowering randomly.

Cable connected active tuned mass dampers for control of in-plane vibrations of wind turbine blades

In-plane vibrations of wind turbine blades are of concern in modern multi-megawatt wind turbines. Today׳s turbines with capacities of up to 7.5MW have very large, flexible blades. As blades have grown longer the increasing flexibility has led to vibration problems. Vibration of blades can reduce the power produced by the turbine and decrease the fatigue life of the turbine. In this paper a new active control strategy is designed and implemented to control the in-plane vibration of large wind turbine blades which in general is not aerodynamically damped. A cable connected active tuned mass damper (CCATMD) system is proposed for the mitigation of in-plane blade vibration. An Euler–Lagrangian wind turbine model based on energy formulation has been developed for this purpose which considers the structural dynamics of the system and the interaction between in-plane and out-of-plane vibrations and also the interaction between the blades and the tower including the CCATMDs. The CCATMDs are located inside the blades and are controlled by an LQR controller. The turbine is subject to turbulent aerodynamic loading simulated using a modification to the classic Blade Element Momentum (BEM) theory with turbulence generated from rotationally sampled spectra. The turbine is also subject to gravity loading. The effect of centrifugal stiffening of the rotating blades has also been considered. Results show that the use of the proposed new active control scheme significantly reduces the in-plane vibration of large, flexible wind turbine blades.

Repowering of Wind Farms — A Case Study

The main objective of the paper is to devise a method for assessing the repowering potential and to improve the energy output from the wind farms and also to understand the impact on the power quality due to repowering. With repowering, the first-generation wind turbines can be replaced with modern multi-megawatt wind turbines. To carry-out the study an old wind farm located at Kayathar, Tamilnadu is selected. The wind farm was commissioned in 1990's with a capacity of 7.35 MW, which consists of 36 Wind Turbines each with the capacity of 200 kW and 225 kW. The present annual energy generation of the wind farm is 7350 MWhr with the plant load factor of 11.41%. The intent of this study is to predict the annual energy output of the wind farm after the repowering using WAsP (Wind Atlas Analysis Application Program). Further this study analyses the power quality issues of the various Wind Turbines. In addition, the main feeder, in which the wind farm which is taken for the study also modeled and the impact on power quality due to repowering also studied. Simulations were carried out using MATLAB. The results are analyzed to understand the significance of repowering to overcome the energy crisis of the nation since the best locations for wind in India are occupied by old wind turbines. The following are the observations and conclusions from the above study. Plant load factor (PLF) increased to 24%, Energy yield increased to more than 4 times and the capacity of the wind farm became double. And in the view of power quality, comparing to the existing Feeder, Repowered Feeder having less reactive power consumption, and voltage variations except the harmonic distortion.

Actuator control of edgewise vibrations in wind turbine blades

Edgewise vibrations with low aerodynamic damping are of particular concern in modern multi-megawatt wind turbines, as large amplitude cyclic oscillations may significantly shorten the life-time of wind turbine components, and even lead to structural damages or failures. In this paper, a new blade design with active controllers is proposed for controlling edgewise vibrations. The control is based on a pair of actuators/active tendons mounted inside each blade, allowing a variable control force to be applied in the edgewise direction. The control forces are appropriately manipulated according to a prescribed control law. A mathematical model of the wind turbine equipped with active controllers has been formulated using an Euler–Lagrangian approach. The model describes the dynamics of edgewise vibrations considering the aerodynamic properties of the blade, variable mass and stiffness per unit length and taking into account the effect of centrifugal stiffening, gravity and the interaction between the blades and the tower. Aerodynamic loads corresponding to a combination of steady wind including the wind shear and the effect of turbulence are computed by applying the modified Blade Element Momentum (BEM) theory. Multi-Blade Coordinate (MBC) transformation is applied to an edgewise reduced order model, leading to a linear time-invariant (LTI) representation of the dynamic model. The LTI description obtained is used for the design of the active control algorithm. Linear Quadratic (LQ) regulator designed for the MBC transformed system is compared with the control synthesis performed directly on an assumed nominal representation of the time-varying system. The LQ regulator is also compared against vibration control performance using Direct Velocity Feedback (DVF). Numerical simulations have been carried out using data from a 5-MW three-bladed Horizontal-Axis Wind Turbine (HAWT) model in order to study the effectiveness of the proposed active controlled blade design in reducing edgewise vibrations. Results show that the use of the proposed control scheme significantly improves the response of the blade and promising performances can be achieved. Furthermore, under the conditions considered in this study quantitative comparisons of the LQ-based control strategies reveal that there is a marginal improvement in the performances obtained by applying the MBC transformation on the time-varying edgewise vibration model of the wind turbine.

Minimum DC link voltages for the generator bridge converter of a SCIG based variable speed wind turbine with fully rated converters

Squirrel Cage Induction Generator (SCIG) based variable speed wind turbine with Fully Rated Converters (FRC) is a popular choice in the industry for the modern multi mega-watt wind turbines. Typical FRC system uses a fixed DC link voltage that allows operation in all steady state and dynamic operating conditions while allowing the modulation index of the PWM scheme to vary. However, the analysis made in this paper shows that at steady state, in the maximum power point tracking region where the turbine is operated at variable speeds with generator controlled using Rotor Flux Oriented Control (R-FOC), it is possible to operate the Generator Bridge (GB) converter with significantly lower DC link voltages than the fixed value used, by maintaining maximum modulation index in the PWM scheme. This paper presents a methodology of determining the minimum DC link voltages for such a system supported by simulation results showing the successful operation of a GB converter with minimum DC link voltages in the maximum power point tracking region.

Control of flapwise vibrations in wind turbine blades using semi-active tuned mass dampers

The increased size and flexibility of modern multi-Megawatt wind turbines has resulted in the dynamic behaviour of these structures becoming an important design consideration. The aim of this paper is to study the variation in natural frequency of wind turbine blades due to centrifugal stiffening and the potential use of semi-active tuned mass dampers (STMDs) in reducing vibrations in the flapwise direction with changing parameters in the turbine. The parameters considered were the rotational speed of the blades and the stiffness of the blades and nacelle. Two techniques have been employed to determine the natural frequency of a rotating blade. The first employs the Frobenius method to a rotating Bernoulli-Euler beam. These results are compared with the natural frequencies determined from an eigenvalue analysis of the dynamic model of the turbine including nacelle motion, which is developed in this paper. The model derived considers the structural dynamics of the turbine and includes the dynamic coupling between the blades and tower. The semi-active control system developed employs a frequency-tracking algorithm based on the short-time Fourier transform technique. This is used to continually tune the dampers to the dominant frequencies of the system. Numerical simulations have been carried out to study the effectiveness of the STMDs in reducing flapwise vibrations in the system when variations occur in certain parameters of the turbine. Steady and turbulent wind loading has been considered. Copyright © 2010 John Wiley & Sons, Ltd.