Low Speed Shaft Torque Research Articles

The performance of two control strategies for floating wind turbines: lidar-assisted feedforward and multi-variable feedback

In this paper, we analyze the performances of two control strategies and their combination on a floating turbine through OpenFAST simulations. The floating wind turbine is modeled based on the demonstrative FLOATGEN, which consists of a 2MW wind turbine mounted on the Damping Pool platform designed by BW Ideol. The lidar-assisted control utilizes lidar wind preview to achieve blade pitch feedforward control. The multi-variable feedback additionally uses the platform pitch rate to determine blade pitch. By simulations with the above-rated wind and irregular wave conditions, both control strategies and their combination has the potential to reduce turbine vibrations. Especially, combining lidar-assisted control and multi-variable feedback control brings the most significant reduction in the standard deviations of rotor speed (>36%), low-speed shaft torque (>30%), and blade root moment (>15%).

Enhanced energy extraction in small-scale wind turbines through slot-based passive blowing

The increasing demand for sustainable energy has spurred research and development of wind turbines. In this study, a passive flow-control technique using a non-uniform curved slot is proposed. The slot is engraved on the mid-chord region of the National Renewable Energy Laboratory (NREL) Phase VI wind turbine blade, between the quarter-span and tip radial stations. The impact of slot-induced flow control is investigated using three-dimensional transient computational fluid dynamics (CFD) modeling across a range of wind speeds, including pre-stall and deep-stall conditions. Aerodynamic analyses are performed to ascertain the influence of slot-flow on the complex three-dimensional flow dynamics around the blade. The slotted blade demonstrates significant improvements in low-speed shaft torque and power compared to the baseline blade. Additionally, the slotted blade experiences increment in the inherent thrust and flapwise bending moments. Detailed wake flow field and surface flow visualization indicate that the slot promotes a more streamlined attached flow across the blade span, delaying flow transition and suppressing turbulence on the blade surface and downstream wake. The slot-flow also reduces radial flow and tip-vorticity anomalies along the blade. Overall, flow control enhancement achieved by the slotted blade significantly boosts the annual energy production of the wind turbine.

Performance enhancement of small-scale wind turbine featuring morphing blades

The demand for renewable energy is driven by the depletion and adverse environmental impacts of fossil fuels. There is a growing global consensus for research and development of renewable energy, including wind. In the current study, National Renewable Energy Laboratory (NREL) Phase VI wind turbine blade is integrated with morphing trailing-edge, installed on the aft-30% blade chord, across outboard 75% blade span. The morphing trailing-edge generates unique topology for each wind speed such that the glide ratio is maximized along the blade span. Three-dimensional transient computational fluid dynamics (CFD) analyses are conducted over low to medium wind speeds to investigate the blade aerodynamics. The analyses exhibit significant increments in the low-speed shaft torque and power of the morphed blades compared to the baseline. The integration of morphing trailing-edge high-lift flow control mechanism on the NREL Phase VI blade enhanced energy harvesting and reduced the wind turbine cut-in wind speed. Comparative investigations are also conducted to assess the improvements in thrust, bending moment, and aerodynamic load distribution, as well as alterations in the pressure, flow field, turbulence, surface flow, and wake. The aeroacoustics directivity of the wind turbines exhibits marginal far-field noise increment in case of morphing trailing-edge integrated blades.

Impact mechanism of frequency response on wind turbine fatigue load

Wind turbines' participation in frequency response is known to improve the frequency stability of power systems, but it can also have a negative impact on the fatigue load of wind turbines. The objective of this paper is to investigate the effect of frequency response on the fatigue loads experienced by various components of a wind turbine, including the low-speed shaft, tower, and blade. To achieve this goal, the authors develop a model of a variable speed horizontal-axis wind turbine based on a doubly fed induction generator. They derive explicit analytical equations of low-speed shaft torque, tower bending moment, and blade bending moment to describe the fluctuations of torque and moment related to the operating states of wind turbines, such as generator torque, rotor speed, and pitch angle, under frequency response. These equations allow for the evaluation of the impact of frequency response on torque and moment changes and fatigue load. Spectral density analysis and modal analysis are used to further analyze the analytical equations, examining the influence of frequency response on different operating conditions of wind turbines and determining the mechanism by which frequency response affects fatigue load qualitatively and quantitatively. The authors use the FAST V8 Code based on the NREL offshore 5-MW baseline wind turbine to demonstrate the effectiveness of the proposed analysis method in evaluating fatigue loads affected by frequency response. The results show that the fatigue load on the low-speed shaft and the lateral side of the tower will significantly increase due to wind turbine participation in frequency response.

Influence of drivetrain efficiency determination on the torque control of wind turbines

Decreasing the levelized cost of energy is a major design objective for wind turbines. Accordingly, the control is generally optimized to achieve a high energy production and a high-power coefficient. In partial load range, speed and torque are controlled via the generator torque but the rotor torque determines the power coefficient of the turbine. High uncertainties for the uncalibrated low-speed shaft torque measurement and varying drivetrain efficiencies which depend on the speed, load and temperature lead to a torque control error that reduces the power coefficient of the wind turbine. In this paper the rotor torque control error and the impact on the power coefficient of wind turbines is quantified. For this purpose, the variation of drivetrain efficiency is analyzed. An efficiency model for the wind turbine drivetrain is build and validated on the test bench. Then, the influence of the drivetrain speed, torque loads, non-torque loads, and temperature on the efficiency is quantified. Finally, the influence of the rotor torque control error on the power coefficient was simulated with an aerodynamic model. The results show that of all examined influences only torque and temperature significantly impacting the efficiency leading to rotor torque control errors that reduce the power coefficient and consequently increase the levelized cost of energy. Improved efficiency measurement on WT test benches or drivetrain efficiency modelling can reduce the rotor torque control error and therefore decrease the LCOE.

Aerodynamic performance of horizontal axis wind turbine with application of dolphin head-shape and lever movement of skeleton bionic airfoils

In order to continuously develop the special airfoil for wind turbine and further improve the energy efficiency of wind turbine, based on the bionic airfoil Dol-Rot 24° that was proposed by the streamline contour of the Phocoenoides Dalli, another novel bionic airfoil is constructed combined with the lever movement shape of the dolphin skeleton, and the airfoil is named Dol-Rot 24°-2. For the cross section of NREL phase VI horizontal axis wind turbine blade (S809 airfoil), Dol-Rot 24°, and Dol-Rot 24°-2 are adopted and numerically simulated to quantitatively analyze from both aspects of aerodynamic performance and noise characteristics. Using the above two novel airfoils profiles, two 3D bionic blades named Dol-Blade-1 and Dol-Blade-2 suitable for NREL phase VI HAWT are established. The results show that: compared with S809 airfoil, Dol-Rot 24°-2 airfoil can greatly improve the lift coefficient, while Dol-Rot 24° airfoil can more effectively inhibit the flow separation on the suction surface. In terms of noise characteristics, compared with S809 airfoil, the noise of the two types of dolphin airfoils does not increase significantly. In addition, Dol-Blade-1 can comprehensively improve the low-speed shaft torque of the blade, while the load of the wind turbine hardly changes. The low-speed shaft torque of Dol-Blade-2 is even better than that of Dol-Blade-1. However, the blade root bending moment of Dol-Blade-2 increases obviously, which inevitably increases the load of the wind turbine. In this paper, the new bionic method is proposed to improve the wind energy utilization of wind turbine on the premise of strictly controlling the aerodynamic noise and load level of blade. The research results of this paper can provide a reference for the optimization of energy efficiency and noise research of NREL phase VI HAWT.

Estimation of Damage Equivalent Loads of Drivetrain of Wind Turbines using Machine Learning

In the recent years machine learning techniques have attracted the attention of wind energy community to make use of the large amount of available data produced from the running wind turbines. These modern wind turbines are typically equipped with measurement systems and sensors that can provide a wealth of information about the operating conditions of the machine. Nevertheless, not all the acquired raw data can be used effectively to enhance the operation of a turbine. This work addresses the question of estimating the damage equivalent loads (DEL) of different components of a drivetrain. The estimation is based on low frequency sampled typically available SCADA measurements. Typical SCADA measurements that are used as input for the estimation model are generator rotational speed, low speed shaft torque and generator torque as well as, wind speed and direction. Several machine learning methods as random forests (RF), support vector machines (SVM), linear regression (LR), decision trees and neural networks (NN) were developed, exhibiting different behavior for each approach. The qualitative and quantitative performance of each algorithm are evaluated and compared against each other. Furthermore, analysis of importance of the input features is presented.

Dynamic load frequency control for high-penetration wind power considering wind turbine fatigue load

The frequency stability of power systems can be improved by the participation of high-penetration wind power in grid frequency regulation. However, the wind turbine (WT) fatigue load will be increased after the wind power participates in the load frequency control (LFC). This paper proposes a control method that takes into account the WT fatigue load and the frequency response of the power system. First, wind turbines are added to the two-areas LFC model including thermal power and hydropower models to participate in frequency regulation. And considering the fatigue load experienced by the tower bending moment and the low-speed shaft torque of the WT, a WT model capable of reflecting the main dynamics is established. Then, the model identification adaptive Proportional-Integral-Differential (PID) controller is designed to be applied to LFC of wind power participating in the grid considering both frequency response and fatigue load. In order to achieve better control effects, the differential evolution (DE) algorithm is used to optimize the PID parameters based on multi-cost functions. The controller parameters are optimized by considering both the fatigue load and frequency deviation. To verify the effectiveness of the proposed method, the controller is applied to the established model and the simulation results are compared.

A Simplified Free Vortex Wake Model of Wind Turbines for Axial Steady Conditions

A simplified free vortex wake (FVW) model called the vortex sheet and ring wake (VSRW) model was developed to rapidly calculate the aerodynamic performance of wind turbines under axial steady conditions. The wake in the simplified FVW model is comprised of the vortex sheets in the near wake and the vortex rings, which are used to replace the helical tip vortex filament in the far wake. The position of the vortex ring is obtained by the motion equation of its control point. Analytical formulas of the velocity induced by the vortex ring were introduced to reduce the computational time of the induced velocity calculation. In order to take into account both accuracy and calculation time of the VSRW model, the length of the near wake was cut off at a 120° wake age angle. The simplified FVW model was used to calculate the aerodynamic load of the blade and the wake flow characteristic. The results were compared with measurement results and the results from the full vortex sheet wake model and the tip vortex wake model. The computational speed of the simplified FVW model is at least an order of magnitude faster than other two conventional models. The error of the low-speed shaft torque obtained from the simplified FVW model is no more than 10% relative to the experiment at most of wind speeds. The normal and tangential force coefficients obtained from the three models agree well with each other and with the measurement results at the low wind speed. The comparison indicates that the simplified FVW model predicts the aerodynamic load accurately and greatly reduces the computational time. The axial induction factor field in the near wake agrees well with the other two FVW models and the radial expansion deformation of the wake can be captured.

Application of a turbulent vortex core model in the free vortex wake scheme to predict wind turbine aerodynamics

Developing suitably generalized models of rotor blade vortices that accurately predict their evolution continues to be a challenge for wind turbine analysis. The choice of vortex core model in a free vortex wake (FVW) method is essential to accurately predict rotor aerodynamic performance. In this work, a turbulent vortex core model (β-Vatistas model) was considered and improved. A formula was developed that approximates the key parameter β, which represents the degree of turbulence in the β-Vatistas model. The improved β-Vatistas model was used in the FVW model, and the β value was calculated based on the tip vortex circulation in each iteration. The wake flow characteristic and low speed shaft torque were calculated using the FVW model coupled with the turbulent vortex core model or two laminar vortex core models. The use of the improved β-Vatistas model provided smaller maximum vorticities at the rolled-up angle and a slower dissipation speed of the tip vortex than those calculated by two laminar vortex core models. The FVW model with the improved β-Vatistas model can accurately capture the tip vortex positions, either in the near wake or in the far wake. The results indicate that the application of the β-Vatistas turbulent vortex core model in the FVW model is feasible and can better capture the wake structure than the laminar vortex core models.

Investigation on aerodynamic performance of horizontal axis wind turbine by setting micro-cylinder in front of the blade leading edge

For NREL Phase VI horizontal axis wind turbine, a flow control method to suppress the flow separation by setting micro-cylinder in front of the blade leading edge is proposed, and the corresponding numerical simulation analysis for the aerodynamic performance of wind turbine is conducted. Firstly, the results predicted by simulation are confirmed experimentally. Under the same operating condition, the simulation and experimental results of low-speed shaft torque are compared, along with results from other studies. It can be found that the simulation results can accurately reflect the basic physical characteristics of flow field for NREL Phase VI wind turbine. Secondly, the influence of different diameters and positions of micro-cylinders on aerodynamic performance of wind turbine is discussed. Numerical results suggest that under different stall conditions, setting appropriate micro-cylinders in front of the blade leading edge can effectively suppress flow separation on wind turbine blades without increasing the load of wind turbine. Moreover, under different wind speeds, micro-cylinders with different diameters and positions have various impacts on aerodynamic performance of wind turbine. Through numerical calculation, the blade torque can maximally have an increase of 27.3% by setting a micro-cylinder with proper diameter and position in front of the blade leading edge.

Aerodynamic performance enhancement analysis of horizontal axis wind turbines using a passive flow control method via split blade

Horizontal axis wind turbines (HAWTs) are being widely used for green energy generation. Hence, aerodynamic performance enhancement of these machines is a key point for their overall performances. This paper numerically investigates effects of a passive flow control method on aerodynamic performance of a HAWT by splitting its blades along the span. First, 2-D simulations are conducted on S809 airfoil in order to study effects of split width and Reynolds. Later, original blade (without split) and two split blades are simulated, and effects of split location on power coefficient, low speed shaft torque and flow patterns are investigated at different tip speed ratios. The results reveal that for an attached flow, torque value are sensitively dependent upon split location and injected flow angle. This is because injected flow triggers an early separation and reduces aerodynamic performance. For a partially separated case, results show that split location may impose either positive or negative effects on torque generation. Finally, for highly separated/stall situations, blade aerodynamic performance becomes less sensitive to the location of split. Also, torque per unit span analysis shows that as tip speed ratio decreases the split part of the blade generates a higher share of the total power.

Wind Climate Parameters for Wind Turbine Fatigue Load Assessment

Site-specific assessment of wind turbine design requires verification that the individual wind turbine components can survive the site-specific wind climate. The wind turbine design standard, IEC 61400-1 (third edition), describes how this should be done using a simplified, equivalent wind climate established from the on-site distribution functions of the horizontal mean wind speeds, the 90% quantile of turbulence along with average values of vertical wind shear and air density and the maximum flow inclination. This paper investigates the accuracy of fatigue loads estimated using this equivalent wind climate required by the current design standard by comparing damage equivalent fatigue loads estimated based on wind climate parameters for each 10 min time-series with fatigue loads estimated based on the equivalent wind climate parameters. Wind measurements from Boulder, CO, in the United States and Høvsøre in Denmark have been used to estimate the natural variation in the wind conditions between 10 min time periods. The structural wind turbine loads have been simulated using the aero-elastic model FAST. The results show that using a 90% quantile for the turbulence leads to an accurate assessment of the blade root flapwise bending moment and a conservative assessment of the tower bottom for-aft bending moment and low speed shaft torque. Currently, IEC 61400-1 (third edition) neglects the variation in wind shear by using the average value. This may lead to a nonconservative assessment of blade root flapwise fatigue loads, which are sensitive to wind shear. The results in this paper indicate that using a 75% quantile for the wind shear at each wind speed bin leads to an appropriate, but conservative, assessment of the fatigue loads. However, care should be taken when using this approach for components where low or negative wind shears can lead to large fatigue loads. This is the case for some drivetrain components where a lower quantile may be required.

Load Mitigation and Optimal Power Capture for Variable Speed Wind Turbine in Region 2

This paper proposes the two nonlinear controllers for variable speed wind turbine (VSWT) operating at below rated wind speed. The objective of the controller is to maximize the energy capture from the wind with reduced oscillation on the drive train. The conventional controllers such as aerodynamic torque feedforward (ATF) and indirect speed control (ISC) are adapted initially, which introduce more power loss, and the dynamic aspects of WT are not considered. In order to overcome the above drawbacks, modified nonlinear static state with feedback estimator (MNSSFE) and terminal sliding mode controller (TSMC) based on Modified Newton Raphson (MNR) wind speed estimator are proposed. The proposed controllers are simulated with nonlinear FAST (fatigue, aerodynamics, structures, and turbulence) WT dynamic simulation for different mean wind speeds at below rated wind speed. The frequency analysis of the drive train torque is done by taking the power spectral density (PSD) of low speed shaft torque. From the result, it is found that a trade-off is to be maintained between the transient load on the drive train and maximum power capture.

Development and application of a dynamic stall model for rotating wind turbine blades

In unsteady conditions of wind turbines, both the dynamic stall phenomenon and the three-dimensional (3D) rotational effect affect the rotor aerodynamics. The dynamic stall mechanism for rotating wind turbine blades is first investigated. Through the comparison of the aerodynamic data between the rotating blade and the two-dimensional (2D) airfoil, the normal force slope in the attached flow and the separation point expression in the separated flow are modified in the Beddoes-Leishman (B-L) dynamic stall model for rotating NREL wind turbine blades. The modified model is validated by the comparison between the calculation results and the experimental results of the lift and drag coefficients at different radial positions. Both the hysteresis loop shapes and the calculation values are closer to the experiment than the 2D dynamic stall model. The present dynamic stall model is then coupled to a free vortex wake model. The coupled model is used to calculate the unsteady blade aerodynamic loads and the low speed shaft torque of the NREL wind turbine in a yawed condition. The accuracy is greatly improved by the corrections presented in the paper.

Sliding Mode Dynamic Torsional Load Control of Double-Fed Wind Turbine Drive-Train

Wind energy is one of the most competitive renewable energies, load has become the problem which has to be taken into consideration with larger capacity and flexibility. Due to drive-train damping was small and torsional load was large by traditional constant torque control (CTC) at above rated wind speed, a dynamic torsional load control (DTLC) was presented based on state feedback and sliding mode control. The method reduced the fluctuation of low speed shaft torque by decreasing equivalent torsional displacement difference. The model and controller were simulated in MATLAB/Simulink, the results demonstrate its effectiveness.

Comparison of Strategies for Enhancing Energy Capture and Reducing Loads Using LIDAR and Feedforward Control

In this paper, we investigate strategies to enhance turbine energy capture and mitigate fatigue loads using pulsed light detection and ranging (LIDAR) system-enabled torque control strategies. To enhance energy capture when a turbine is operating below rated wind speed, three advanced LIDAR-enabled torque controllers are proposed: the disturbance tracking control (DTC) augmented with LIDAR, the optimally tracking rotor (OTR) control augmented with LIDAR, and LIDAR-based preview control. The DTC with LIDAR and LIDAR-based preview control is combined with a linear quadratic regulator in the feedback path, while OTR is a strategy adapted from a quadratic <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> Ω <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> torque feedback control. These control strategies are simulated in turbulent wind files and their performance is compared against the baseline <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> Ω <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> control scheme. We also consider the effects of different LIDAR update rates and range gates. It is shown that LIDAR-enabled controllers have only a small effect on energy capture at the cost of increased control action and low-speed shaft torque load. However, when considering a combination of fatigue load mitigation, power capture enhancement, and control authority requirements, the LIDAR-enabled preview controller outperforms the baseline <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> Ω <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> controller.

1.5MW Wind Turbine Structural Dynamic Analysis

In recent decades, the rapidly development of wind energy in China and the increasing of size and complexity of wind turbine have requested the improvement in wind turbine systematic design technology. A reasonable systematic dynamic model is an important part for systematic design of MW-class wind turbine. In structural dynamic model, the flexibility of blade and tower is represented by presumed mode shapes. In this paper, based on presumed mode shape method, the structural dynamic equations of wind turbine were constructed. Along with the wind field model, the wind turbine aeroelastic systematic dynamic model was constructed. Using the model, the deflections and load of blade, low speed shaft torque of a 1.5MW wind turbine have been calculated. Therefore, the construction of wind turbines systematic dynamic model has an important significance for the development of wind systematic design and manufacturing capacity.

A numerical study of the effects of atmospheric and wake turbulence on wind turbine dynamics

Although the atmospheric sciences community has been studying the effects of atmospheric stability and surface roughness on the planetary boundary layer for some time, their effects on wind turbine dynamics have not been well studied. In this study, we performed numerical experiments to explore some of the effects of atmospheric stability and surface roughness on wind turbine dynamics. We used large-eddy simulation to create atmospheric winds and compute the wind turbine flows, and we modeled the wind turbines as revolving and flexible actuator lines coupled to a wind turbine structural and system dynamic model. We examined the structural moments about the wind turbine blade, low-speed shaft, and nacelle; power production; and wake evolution when large 5-MW turbines are subjected to winds generated from low- and high-surface roughness levels representative of offshore and onshore conditions, respectively, and also neutral and unstable atmospheric conditions. In addition, we placed a second turbine 7 rotor diameters downwind of the first one so that we could explore wake effects under these different conditions. The results show that the turbulent structures generated within the atmospheric boundary layer wind simulations cause isolated loading events at least as significant as when a turbine is waked by an upwind turbine. The root-mean-square (RMS) turbine loads are consistently larger when the surface roughness is higher. The RMS blade-root out-of-plane bending moment and low-speed shaft torque are higher when the atmospheric boundary layer is unstable as compared with when it is neutral. However, the RMS yaw moments are either equal or reduced in the unstable case as compared with the neutral case. For a given surface roughness, the ratio of power produced by the downwind turbine relative to that of the upwind turbine is 15–20% higher when the conditions are unstable as compared with neutral. For a given atmospheric stability, this power ratio is 10% higher with the onshore roughness value versus the offshore one. The main conclusion is that various coherent turbulent structures that form under different levels of atmospheric stability and surface roughness have important effects on wind turbine structural response, power production, and wake evolution.

Research on Non-contact Torque Sensor with Half-Ring Base on Fe-base Amorphous Alloy

New non-contact measuring dynamic torque sensor based on magneto-elastic effect of Fe-base amorphous alloy is discussed.The principle and structure of sensor are presented,and its output characteristic equation is derived.By making an dynamic torque measuring experiment,effect of the gap between sensor probe and shaft surface,and rotational speed on measurement accuracy and sensitivity is analyzed,and U 0-δ curve which is the change of sensor output voltage with gap,the best air gap rang and dynamic output curve in three kinds of rotation speed of the shaft are got.The experimental result shows when gap is 0.5 mm,maximal repeatability error is 1.71%,maximal nonlinearity error is 2.48% and sensitiveness is 1.385 2 mV/(N?m).Sensor designed is feasible,especially for the needs of fixed non-contact monitoring of some heavy-duty low-speed shaft torque.