B1: Wind-tunnel modeling of the tip speed ratio influence on the near wake evolution
Victor Stein | Lehrstuhl für Aerodynamik und Strömungsmechanik | Germany
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Authors:
Victor Stein | Lehrstuhl für Aerodynamik und Strömungsmechanik | Germany
Hans-Jakob Kaltenbach | Germany
Wind-tunnel measurements on the near-wake evolution of a three bladed horizontal axis wind turbine model in the scale of 1:350 operating in uniform flow conditions and within a turbulent boundary layer at different tip speed ratios are presented. Operational conditions are chosen to exclude Reynolds number effects regarding the turbulent boundary layer as well as the rotor performance. Triple-wire anemometry is used to measure all three velocity components, covering the range from the near- to the far-wake region. It is confirmed that realistic modelling of the wake evolution is not possible in a low-turbulence uniform approach flow. For nearly constant thrust coefficients differences in the evolution of the near-wake can be identified for tip speed ratios in the range from 6.5 to 10.5. It is shown that with increasing downstream distances mean velocity profiles become indistinguishable whereas for turbulence statistics a subtle dependency on the tip speed ratio is still noticeable.
B2: Large Eddy Simulation for Atmospheric Boundary Layer Flows over Flat and Complex Terrains
Prof. Jonathan Naughton | University of Wyoming | United States
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Authors:
Prof. Jonathan Naughton | University of Wyoming | United States
Yi Han | United States
Michael Stoellinger | United States
In this work, we present Large Eddy Simulation (LES) results of atmospheric boundary layer (ABL) flow over complex terrain with neutral stratification using the OpenFOAM-based simulator for on/offshore wind farm applications (SOWFA). The complete work flow to investigate the LES for the ABL over real complex terrain is described including meteorological-tower data analysis, mesh generation and case set-up. New boundary conditions for the lateral and top boundaries are developed and validated to allow inflow and outflow as required in complex terrain simulations. The turbulent inflow data for the terrain simulation is generated using a precursor simulation of a flat and neutral ABL. Conditionally averaged met-tower data is used to specify the conditions for the flat precursor simulation and is also used for comparison with the simulation results of the terrain LES. A qualitative analysis of the simulation results reveals boundary layer separation and recirculation downstream of a prominent ridge that runs across the simulation domain. Comparisons of mean wind speed, standard deviation and direction between the computed results and the conditionally averaged tower data show a reasonable agreement.
B3: Analysis of Turbulent Coherent Structures in a Flow Over an Escarpment using Proper Orthogonal Decomposition
Horia Hangan | Canada
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Authors:
Ryan Kilpatrick | Western University | Canada
Kamran Siddiqui | Canada
Horia Hangan | Canada
An analysis of turbulent coherent structures was conducted on the flow over a 1:25 scale model of the Bolund Hill escarpment by means of Proper Orthogonal Decomposition (POD) using Particle Image Velocimetry (PIV) data. Mapping of flow energy at various modes in the vicinity of the escarpment, provided an insight into the influence of inflow conditions on the underlying behaviour of turbulent coherent structures.
B4: Investigating Coherent Structures in the Standard Turbulence Models using Proper Orthogonal Decomposition
Dr. Lene Eliassen | NTNU | Norway
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Authors:
Dr. Lene Eliassen | NTNU | Norway
Søren Andersen | Norway
Proper Orthogonal Decomposition (POD) is used to characterize the large coherent structures of the stochastically generated inflow turbulence with the two recommended turbulence models in the wind turbine design standards. The turbulence models are used to estimate design loads, and the coherent turbulent structures in the generated turbulence play a significant role for the load calculations.
B5: CFD modelling approaches against single wind turbine wake measurements using RANS
Nikolaos Stergiannis | Vrije Universiteit Brussel | Belgium
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Authors:
Nikolaos Stergiannis | Vrije Universiteit Brussel | Belgium
Chris Lacor | Belgium
Jeroen van Beeck | Belgium
Rory Donnelly | Belgium
Numerical simulations of two wind turbines including the exact geometry of their blades and hubs are compared against a simplified actuator disk model (ADM). The wake expansion of the upstream rotor is investigated and compared with measurements. CFD simulations have been performed using the open-source platform OpenFOAM. The MRF approach was used to model the inner rotating reference frames in a stationary computational mesh and outer reference frame for the full wind turbine rotor simulations. The standard k-ε and k-ω turbulence closure schemes have been used to solve the steady state, three dimensional RANS equations. Results of near and far wake regions are compared with wind tunnel measurements along three horizontal lines downstream. The ADM under-predicted the velocity deficit downstream with both turbulence models. Full wind turbine rotor simulations showed good agreement against the experimental data at the near wake, amplifying the differences between the simplified models.
B6: Turbulence Impact on Wind Turbines: Experimental investigations on a wind turbine model
Youjin Kim | SGB | Germany
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Authors:
Youjin Kim | SGB | Germany
Ali Al-Abadi | Germany
Özgür Ertunç | Germany
Antonio Delgado | Germany
Experimental investigations have been conducted by exposing an efficient wind turbine model to different turbulence levels in a wind tunnel. The developments of turbulence scales in the flow direction at various Reynolds numbers and the grid mesh size are measured. Performance measurements are conducted with an experimental setup that allow measuring of torque, rotational speed from the electrical parameters. The study shows the higher the turbulence level, the higher the power coefficient. This is due to the interaction of turbulence scales with the blade surface boundary layer, which in turn delay the stall. Thus, suppressing the boundary layer and preventing it from separation, and hence, enhancing the aerodynamics characteristics of the blade. In addition, higher turbulence helps in damping the tip vortices. Therefore, reduces the tip losses. Adding winglets to the blade tip is proven to reduce the tip vortex. Further investigations of the near and far wake-surrounding intersection are performed to understand the energy exchange between the wake and the free stream.
B7: An analysis of offshore wind farm SCADA measurements to identify key parameters influencing the magnitude of wake effects
Niko Mittelmeier | Senvion | Germany
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Authors:
Niko Mittelmeier | Senvion | Germany
Tomas Blodau | Germany
Gerald Steinfeld | Germany
Andreas Rott | Germany
Martin Kühn | Germany
Atmospheric conditions have a clear influence on wake effects. Stability classification is usually based on wind speed, turbulence intensity, shear and temperature gradients measured partly at met masts, buoys or LiDARs. The objective of this paper is to find a classification for stability based on wind turbine Supervisory Control and Data Acquisition (SCADA) measurements in order to fit engineering wake models better to the current ambient conditions. Two offshore wind farms with met masts have been used to establish a correlation between met mast stability classification and new aggregated statistical signals based on multiple measurement devices. We found a good correlation between the standard deviation of active power divided by the average power of wind turbines in free flow with the ambient turbulence intensity when the wind turbines were operating in partial load.
B8: PIV measurements in a real time controlled wind turbine wake simulator
Ricardo Castillo | United States
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Authors:
Ricardo Castillo | United States
Yeqin Wang | United States
Travis Monk | United States
Stephanie Vasquez | United States
Dr. Suhas Pol | Texas Tech Univeristy | United States
Beibei Ren | United States
Andy Swift | United States
Fazle Hussain | United States
Casrten Westergaard | United States
A wind tunnel based “Hyper Accelerated Wind Farm Kinematic-Control Simulator (HAWKS) is being built at Texas Tech University (TTU) to emulate controlled wind turbine flow physics. The HAWKS model turbine has pitch, yaw and speed controls that could be operated in real time with different power coefficient (Cp) conditions. The purpose of HAWKS is to simulate control strategies at much faster turnaround times. Currently, the fundamental building blocks of the simulator are being tested. A few salient tests are presented here.
B10: Calculating the sensitivity of wind turbine loads to wind inputs using response surfaces
Dr. Jennifer Rinker | Technical University of Denmark | Denmark
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Author:
Dr. Jennifer Rinker | Technical University of Denmark | Denmark
This paper presents a methodology to calculate wind turbine load sensitivities to turbulence parameters through the use of response surfaces (high-order polynomial surrogate models). The proposed methodology is demonstrated by calculating the sensitivity of the maximum blade root bending moment of the WindPACT 5 MW model to four turbulence parameters: mean wind speed, turbulence intensity, Kaimal length scale, and a nonstationarity parameter. The fit of the calibrated response surface is evaluated in terms of error between the model and the training data and in terms of the convergence. The Sobol SIs are calculated using the response surface, and the convergence is examined. The SIs reveal that the variance caused by the Kaimal length scale and nonstationarity parameter are negligible. Thus, the findings in this paper represent the first systematic evidence that stochastic wind turbine load response statistics can be modeled purely by mean wind wind speed and turbulence intensity.
B11: Probability density function selection based on the characteristics of wind speed data
Nurseda Yildirim Yurusen | CIRCE-Universidad de Zaragoza | Spain
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Authors:
Nurseda Yildirim Yurusen | CIRCE-Universidad de Zaragoza | Spain
Julio J Melero | Spain
The probabilistic approach has an important place in the wind energy research field as it provides cheap and fast initial information for experts with the help of simulations and estimations. Wind energy experts have been using the Weibull distribution for wind speed data for many years. Nevertheless, there exist cases, where the Weibull distribution is inappropriate with data presenting bimodal or multimodal behaviour which are unfit in high, null and low winds that can cause serious energy estimation errors. This paper presents a procedure for dealing with wind speed data taking into account non-Weibull distributions or data treatment when needed. The procedure detects deviations from the unimodal (Weibull) distribution and proposes other possible distributions to be used. The deviations of the used distributions regarding real data are addressed with the Root Mean Square Error (RMSE) and the annual energy production (AEP).
B12: Wind turbine wake structure in forest and neutral plane wall boundary layer large-eddy simulations
Josef Schröttle | German Aerospace Center DLR | Germany
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Authors:
Josef Schröttle | German Aerospace Center DLR | Germany
Zbigniew Piotrowski | Germany
Antonia Englberger | Germany
Andreas Dörnbrack | Germany
Thomas Gerz | Germany
Wind turbine wake flow characteristics are studied in a strongly sheared and turbulent forest boundary layer. As a reference, results are presented from the neutral plane wall boundary layer flow. The results without wind turbines compare quantitatively to earlier large-eddy simulations by Shaw and Schumann (1992) and Porte-Agel et al. (2000). To use the fields from the turbulent boundary layers on the fly as inflow fields for the wind turbine wake large-eddy simulations a new and highly efficient methodology was developed for the multiscale geophysical flow solver EULAG. With this method, fully developed turbulent flow fields can be simulated upstream of the wind turbine. These fields are independent of the wake flow contrary to previous approaches in literature. The large-eddy simulations reproduce the measured eddy dissipation rates from the atmospheric boundary layer. Similar to these measurements, a significant increase of the dissipation rate is simulated in the wind turbine wakes. The velocity deficit is strongly asymmetric above the forest and recovers faster downstream compared to the velocity deficit in the plane wall boundary layer. This is due to the inflection point in the mean streamwise velocity profile with corresponding turbulent coherent structures of high turbulence intensity in the strong shear flow above the forest.
B13: Transient growth of perturbations in a vortex ring
Fei Chen | GoldWind | China
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Authors:
Fei Chen | GoldWind | China
Daniel Wacks | China
Daniel Cabezon | Spain
Xuerui Mao
Bofu Wang | China
Perturbations to a vortex ring with largest transient energy growth are calculated and applied to drive the vortex ring to evolve. Optimal initial perturbations appear at azimuthal wave numbers β=0 and β=9. The former was described as 'axial flow' and is not related to the breakdown of the ring, while the latter is critical to the ring breakdown process as observed in previous direct numerical simulation (DNS) with random initial noise input. In the present work, DNS is conducted to study the nonlinear development of the optimal perturbations, and good agreement with past studies is achieved.
B14: IEA Wind Task 36 Forecasting
Dr. Gregor Giebel | Technical University of Denmark | Denmark
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Authors:
Dr. Gregor Giebel | Technical University of Denmark | Denmark
Joel Cline | Denmark
Helmut Frank | Denmark
William Shaw | Denmark
Pierre Pinson | Denmark
Bri-Mathias Hodge | Denmark
George Kariniotakis | Denmark
Jens Madsen | Denmark
This paper presents the new International Energy Agency Wind Task 36 on Forecasting, and invites to collaborate within the group. Wind power forecasts have been used operatively for over 20 years. Despite this fact, there are still several possibilities to improve the forecasts, both from the weather prediction side and from the usage of the forecasts. The new International Energy Agency (IEA) Task on Forecasting for Wind Energy tries to organise international collaboration, among national meteorological centres with an interest and/or large projects on wind forecast improvements (NOAA, DWD, MetOffice, met.no, DMI, ...), operational forecaster and forecast users. As first results, a mapping of the forecasting use at end users is presented, as well as a list with future research issues stemming from a recent workshop in Barcelona.
B15: The Collection of The Main Issues for Wind Farm Optimisation in Complex Terrain
Prof. Chang Xu | Hohai University | China
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Authors:
Prof. Chang Xu | Hohai University | China
Wenzhong Shen | China
Micro-sitting is an important step in the exploitation and utilization of wind energy, especially in complex terrain, i.e. mountainous regions. Since the terrain undulates greatly, the wind energy distribution is affected by many factors, which make it very difficult to select micro-locations for wind farms in complex terrain. The tortuous path of collector circuits and the number of branch lines are influenced by the layout of turbines. Depend on the arrangement of wind turbines, the number of the main circuits determines the length of its path and the size of conductor cross-section, and influences the total investment of the project. An appropriate design of pit road is significant to the investment and operation of a wind farm. The design of pit road is straightforward for a wind farm in grassland, desert or coastal areas, but it is an important part in mountainous regions.
B16: Turbulence influence on optimum tip speed ratio for a 200 kW vertical axis wind turbine
Erik Möllerström | Halmstad University | Sweden
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Authors:
Erik Möllerström | Halmstad University | Sweden
Sandra Eriksson | Sweden
Anders Goude | Sweden
Fredric Ottermo | Sweden
Jonny Hylander | Sweden
The influence of turbulence intensity (TI) on the tip speed ratio for maximum power coefficient, here called C_P_max, is studied for a 200 kW VAWT H-rotor using logged data from a 14 month period with the H-rotor operating in wind speeds up to 9 m/s. The TI - C_P_max relation is examined by dividing 10 min mean values in different turbulence intensity ranges and producing multiple C_P curves. A clear positive relation between TI and C_P_max is shown and is further strengthened as possible secondary effects are examined and deemed non-essential. The established relation makes it possible to tune the control strategy to enhance the total efficiency of the turbine.
B17: Two-scale momentum theory for very large wind farms
Dr. Takafumi Nishino | Cranfield University | United Kingdom
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Author:
Dr. Takafumi Nishino | Cranfield University | United Kingdom
A new theoretical approach is proposed to predict a practical upper limit to the efficiency of a very large wind farm. The new theory suggests that the efficiency of ideal turbines in an ideal very large wind farm depends primarily on a non-dimensional parameter Î-Cf0, where Î is the ratio of the rotor swept area to the land area (for each turbine) and Cf0 is a natural friction coefficient observed before constructing the farm. When Î-Cf0 approaches to zero, the new theory goes back to the classical actuator disc theory, yielding the well-known Betz limit. When Î-Cf0 increases to a large value, the maximum power coefficient of each turbine reduces whilst a normalised power density of the farm increases asymptotically to an upper limit. A CFD analysis of an infinitely large wind farm with aligned and displaced array configurations is also presented to validate a key assumption used in the new theory.
B18: Effects of ambient turbulence on the near wake of a wind turbine
Dr. Yusik Kim | University of Stuttgart | Germany
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Authors:
Dr. Yusik Kim | University of Stuttgart | Germany
Eva Jost | Germany
Galih Bangga | Germany
Pascal Weihing | Germany
Thorsten Lutz | Germany
Developments of the near wake behind the Avatar research turbine (radius of 102.88 m) in ambient turbulence are investigated using high fidelity numerical simulations. A moderate level of background turbulence with a wide range of scales, which has not been considered in the previous studies is applied. With ambient turbulence, a significant impact on the near wake development is observed. The mean velocity profile becomes Gaussian after 450 m distance downstream, which is a demarcation between the near and the far wake. From the spectral analysis of the wake, clear peaks in the spectra are observed at the blade passing frequency, but the distributions of the peak extend into a wide range of frequency domain. Such aspects provide useful information in classifying periodic and stochastic fluctuations, and their contributions to the momentum mixing in the wake.
B19: Validation of buoyancy driven spectral tensor model using HATS data
Abhijit Chougule | University of Agder | Norway
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Authors:
Abhijit Chougule | University of Agder | Norway
Jakob Mann | Norway
Mark Kelly | Norway
Gunner Larsen | Norway
We present a homogeneous spectral tensor model for wind velocity and temperature fluctuations, driven by mean vertical shear and mean temperature gradient. Results from the model, including one-dimensional velocity and temperature spectra and the associated co-spectra, are shown in this paper. The model also reproduces two-point statistics, such as coherence and phases, via cross-spectra between two points separated in space. Model results are compared with observations from the Horizontal Array Turbulence Study (HATS) field program (Horst et al. 2004). The spectral velocity tensor in the model is described via five parameters: the dissipation rate (ε), length scale of energy-containing eddies (L), a turbulence anisotropy parameter (Γ), gradient Richardson number (Ri) representing the atmospheric stability and the rate of destruction of temperature variance (ηθ).
B20: Wake redirection: comparison of analytical, numerical and experimental models
Jiangang Wang | Technische Universität München | Germany
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Authors:
Jiangang Wang | Technische Universität München | Germany
Carlo Bottasso | Germany
Filippo Campagnolo | Germany
This paper focuses on wake redirection techniques for wind farm control. Two control strategies are investigated: yaw misalignment and cyclic pitch control. First, analytical formulas are derived for both techniques, with the goal of providing a simple physical interpretation of the behavior of the two methods. Next, more realistic results are obtained by numerical simulations performed with CFD and by experiments conducted with scaled wind turbine models operating in a boundary layer wind tunnel. Comparing the analytical, numerical and experimental models allows for a cross-validation of the results and a better understanding of the two wake redirection techniques. Results indicate that yaw misalignment is more effective than cyclic pitch control in displacing the wake laterally, although the latter may have positive effects on wake recovery.
B21: Wake center position tracking using downstream wind turbine hub loads
Dr. Stefano Cacciola | TUM Technische Universität München | Germany
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Authors:
Dr. Stefano Cacciola | TUM Technische Universität München | Germany
Marta Bertelè | Germany
Johannes Schreiber | Germany
Carlo Luigi Bottasso | Germany
Having an improved awareness of the flow within a wind farm is useful for power harvesting maximization, load minimization and design of wind farm layout. Local flow information at each wind turbine location can be obtained by using the response of the wind turbines, which are consequently used as distributed sensors. This paper proposes the use of hub loads to track the position of wakes within a wind farm. Specifically, the downstream wind turbine nodding and yawing moments are related to horizontal and vertical shears by an ad-hoc identified physical steady model, which works in conjunction with a standard wind speed estimator based on the torque balance equation. Finally, tracking of the wake center is performed in two steps: first shears and rotor effective wind speed are estimated, and then a minimization algorithm is used to match a Larsen wake model to the estimates. An estimate of the lateral wake position is an important information that could be exploited by algorithms for wind farm control based on wake redirection. In fact, for example, the actual wake position could be used for suggesting a suitable misalignment angle to the upstream turbine or employed as a feed-back measurement in a closed-loop wind farm power/fatigue optimization strategy. Simulation experiments conducted within a high fidelity aeroservoelastic environment demonstrate the performance of the new method.
B22: Identification and quantification of vortical structures in wind turbine wakes for operational wake modeling
Yves Marichal | WaPT - Wake Prediction Technologies | Belgium
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Authors:
Yves Marichal | WaPT - Wake Prediction Technologies | Belgium
Ivan De Visscher | Belgium
Philippe Chatelain | Belgium
Grégoire Winckelmans | Belgium
The present paper describes a method to quantify the vortical structure characteristics from simulation results of the flow past a wind turbine, with the aim to develop an accurate, physics-based operational wake model. The wake centerline is first identified. Then, the flow characteristics are extracted by fitting a vorticity-based wake skeleton onto the velocity deficit profiles defined around the centerline and measured at several downstream distances from the rotor. The simulation results were obtained using a hybrid Vortex Particle-Mesh approach combined with an immersed Lifting Line technique to account for the blades. The characterization of the identified vortex wake structure lays a basis for the development of an operational wake model based on strong physical grounds.
B23: Rotor performance and wake conditions in non-uniform flow behind an obstacle
Dr. Robert Mikkelsen | Technical University of Denmark | Denmark
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Authors:
Dr. Robert Mikkelsen | Technical University of Denmark | Denmark
Ivan Kabardin | Denmark
Valery Okulov | Denmark
Jens Sørensen | Denmark
Igor Naumov | Denmark
Obstacles like forests, ridges and hills can strongly affect the velocity profile in front of a wind turbine rotor. The present work aims at quantifying the influence of nearby located obstacles on the performance and wake characteristics of a downstream located wind turbine. Here the influence of an obstacle in the form of a cylindrical disk was investigated experimentally in a water flume. A model of a three-bladed rotor, designed using Glauert's optimum theory at a tip speed ratio λ = 5, was placed in the wake of a disk with a diameter close to the one of the rotor. The flow condition in front of the rotor was measured with high temporal accuracy using LDA and power coefficients were determine as function of tip speed ratio for different obstacle positions. Furthermore, PIV measurements were carried out to study the development of the mean velocity deficit profiles of the wake behind the wind turbine model under the influence of the wake generated by the obstacle.
B24: Multi-fidelity wake modeling based on Co-kriging method
Yimei Wang | North China Electric Power University | China
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Authors:
Yimei Wang | North China Electric Power University | China
Pierre-Elouan Réthoré | China
Paul van der Laan | China
Juan Pablo Murcia Leon | China
Yongqian Liu | China
Li Li | China
The article presents an approach to combine wake models of multiple levels of fidelity, which is capable of giving accurate predictions with only a small number of high fidelity samples. The G. C. Larsen and k-ε-fP based RANS models are adopted as ensemble members of low fidelity and high fidelity models, respectively. Both the univariate and multivariate based surrogate models are established by taking the local wind speed and wind direction as variables of the wind farm power efficiency function. Various multi-fidelity surrogate models are compared and different sampling schemes are discussed. The analysis shows that the multi-fidelity wake models could tremendously reduce the high fidelity model evaluations needed in building an accurate surrogate.
B25: Defining wake characteristics from scanning and vertically pointing full-scale lidar measurements
Prof. Rebecca J Barthelmie | Cornell University | United States
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Authors:
Prof. Rebecca J Barthelmie | Cornell University | United States
Paula Doubrawa | United States
Hui Wang | United States
SC Pryor | United States
This paper describes the use of lidar to capture full-scale wake characteristics. Measuring wake characteristics such as velocity deficit, wake width and asymmetry as well as wake meander with scanning Doppler lidar requires an efficient scan geometry in which wake volumes are comprehensively scanned while empty volumes are excluded and also requires optimization for maximum spatial and temporal coverage. Some examples are given from a field experiment in Prince Edward Island in 2015 that show wake characterization from both scanning and vertical lidar.
B26: A numerical study on the flow upstream of a wind turbine in complex terrain
Alexander Raul Meyer Forsting | DTU Wind Energy | Denmark
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Authors:
Alexander Raul Meyer Forsting | DTU Wind Energy | Denmark
Andreas Bechmann | Denmark
Niels Troldborg | Denmark
The interaction of a wind turbine with the upstream flow-field in complex and flat terrain is studied using Reynolds-averaged Navier-Stokes (RANS) simulations with a two equation turbulence closure. The complex site modelled is Perdigão (Portugal), where a turbine is located on one of two parallel running ridges. Simulating various wind directions with and without rotor, the impact of the rotor on the flow-field upstream is determined. This is compared and related to simulations with sheared and uniform inflow. The induction zones forming with sheared and uniform inflow agree to such an extent, that shear could be interpreted as linear perturbation to the uniform inflow solution. However, for complex terrain this is not the case, as it is highly dependant on flow features caused by the topography and their interaction with the rotor. Separation plays a crucial role, as it dictates the wake trajectory which in turn governs the orientation of the induction zone.
B27: Analyzing complex wake terrain interactions and its implications on wind-farm performance
Dr. Mandar Tabib | SINTEF | Norway
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Authors:
Dr. Mandar Tabib | SINTEF | Norway
Adil Rasheed | Norway
Franz Fuchs | Norway
The current study helps to understand the fundamental vortex-terrain interaction mechanism and its influence on wind-farm performance. An initial numerical set-up validation study is done to minimize numerical vortex diffusion. Subsequent Large eddy simulation (LES) of interaction of various terrain geometries with a vortex reveals a much faster decay of vortex with increasing terrain complexity. This is due to: (a) formation and interaction of secondary vortices with the main vortex and (b) due to enhanced vorticity diffusion because of increased terrain-induced turbulence. Further, a comparison of observed annual wind-farm power with LES of an industrial scale wind-farm in two different terrain layouts (the actual complex terrain and a made-up flat terrain) reveals a drop in power production for the flatter terrain wind-farm case due to terrain and wake effects. This indicates that future studies on wake-dissipation through artificial obstruction/terrain modifications can help wind-farms.
B28: Wind turbine rotor simulation using the actuator disk and actuator line methods
Dr. John Prospathopoulos | National Technical University of Athens | Greece
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Authors:
Dr. John Prospathopoulos | National Technical University of Athens | Greece
Michail Tzimas | Greece
The present paper focuses on wind turbine rotor modeling for loads and wake flow prediction. Two steady-state models based on the actuator disk approach are considered, using either a uniform thrust or a blade element momentum calculation of the wind turbine loads. A third model is based on the unsteady-state actuator line approach. Predictions are compared with measurements in wind tunnel experiments and in atmospheric environment. At low wind speeds the rotor thrust is predicted satisfactorily, whereas the predictions of the tangential force present larger deviations from the measurements. This is attributed to the fact that the estimation of the tangential force is more sensitive to the variations of the angle of attack, which is influenced from possible 3D phenomena. As the wind speed increases, such 3D effects and flow separation become more pronounced, resulting in larger deviations from the measurements, due to the fact that forces are calculated using 2D airfoil data for lift and drag coeffcients. In general, loading is better predicted using the actuator line instead of the actuator disk approach, suggesting the importance of considering the unsteady character of the flow. Regarding the predictions of the velocity deficit, a significant underestimation is observed in the near wake. This is a known weakness of the two-equation turbulence models which overestimate the normal stress and the production of turbulent kinetic energy. However, a small improvement is observed when implementing the actuator line instead of the actuator disk approach.
B29: Experimental testing of axial induction based control strategies for wake control and wind farm optimization
Jan Bartl | NTNU | Norway
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Authors:
Jan Bartl | NTNU | Norway
Lars Sætran | Norway
Wind farm control concepts aim for optimizing the overall power output of the farm taking wake interactions between the individual turbines into account. This experimental wind tunnel study examines the performance of two in-line model wind turbines when the upstream turbine's tip speed ratio or blade pitch angle is modified. The focus is directed on the wake flow behind the upstream rotor. It is shown how the radial distribution of kinetic energy in the wake area can be controlled by modifying the upstream turbine's tip speed ratio. By pitching out the upstream turbine's blades, however, the available kinetic energy in the wake is increased at an equal rate over the entire blade span. No significant increase in the array efficiency is measured as the small parts of the added kinetic energy in the wake are diffused into the freestream flow.
B30: An analytical model for a full wind turbine wake
Prof. Peter Clive | SgurrEnergy Ltd | United Kingdom
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Authors:
Prof. Peter Clive | SgurrEnergy Ltd | United Kingdom
Pablo Olmos | United Kingdom
Hannah Ferchland | United Kingdom
Aidan Keane | United Kingdom
Daniel Gallacher | United Kingdom
An analytical wind turbine wake model is proposed to predict the wind velocity distribution for all distances downwind of a wind turbine, including the near-wake. This wake model augments the Jensen model and subsequent derivations thereof, and is a direct generalization of that recently proposed by Bastankhah and Porte-Agel. The model is derived by applying conservation of mass and momentum in the context of actuator disk theory, and assuming a distribution of the double-Gaussian type for the velocity deficit in the wake. The physical solutions are obtained by appropriate mixing of the waked- and freestream velocity deficit solutions, reflecting the fact that only a portion of the fluid particles passing through the rotor disk will interact with a blade.
B31: LES of wind farm response to transient scenarios using a high fidelity actuator disk model
Maud Moens | Université catholique de Louvain | Belgium
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Authors:
Maud Moens | Université catholique de Louvain | Belgium
Matthieu Duponcheel | Belgium
Grégoire Winckelmans | Belgium
Philippe Chatelain | Belgium
Large eddy simulations coupled to Actuator Disks are used to investigate wake effects in wind farms. An effort is made on the wind turbine model: it uses the prevailing velocities at each point of the disk to estimate the aerodynamic loads and is improved using a tip-loss correction and realistic control schemes. This accurate and efficient tool is used to study the wind farm response in terms of flow and power production during an unsteady scenario: this work focuses on an emergency shutdown of one rotor inside a wind farm.
B32: Spatial correlation of atmospheric wind at scales relevant for large scale wind turbines
Lars Morten Bardal | Norwegian University of Science and Technology | Norway
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Authors:
Lars Morten Bardal | Norwegian University of Science and Technology | Norway
Lars Roar Sætran | Norway
Wind measurements a short distance upstream of a wind turbine can provide input for a feedforward wind turbine controller. Since the turbulent wind field will be different at the point/plane of measurement and the rotor plane the degree of correlation between wind speed at two points in space both in the longitudinal and lateral direction should be evaluated. This study uses a 2D array of mast mounted anemometers to evaluate cross-correlation of longitudinal wind speed. The degree of correlation is found to increase with height and decrease with atmospheric stability. The correlation is furthermore considerably larger for longitudinal separation than for lateral separation. The integral length scale of turbulence is also considered.
B33: Comparison of the far wake behind dual rotor and dual disk configurations
Prof. Jens Nørkær Sørensen | DTU Wind Energy | Denmark
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Authors:
Prof. Jens Nørkær Sørensen | DTU Wind Energy | Denmark
Robert F Mikkelsen | Denmark
Igor V Naumov | Denmark
Ivan V Litvinov | Denmark
Liza Gesheva | Denmark
Okulov Valery | Denmark
There is an increasing interest in studying the development of far wakes behind two or more interacting wind turbines in order to study the influence of wake interaction in relation to the design of wind farms. The focus of this experimental study is to investigate the resulting wake features for two rotors subjected to different operating and spatial conditions. As a part of this, comparison with the wake development behind two disks replacing the rotor models has been performed to determine the difference between the two wake systems. LDA and Stereo PIV experiments were carried out to study the development of far wakes behind configurations of dual HAWT wind turbine rotors and dual circular disks. The setups were placed in the middle of a water flume. The initial flow in the flume is subjected to a very low turbulence level, limiting the influence of all external disturbances in order to limit the study to the inherent wake instability. As a result of the investigation, we obtained decays of profiles for the velocity deficit and turbulent pulsations in the far wakes behind both dual rotor and dual disk configurations. By using regression techniques to fit the obtained velocity profiles the experimental data were approximated by identical analytical models and compared to each other. An identical rational dependence with the same powers, but with different coefficients, was found for the two configurations.
B34: Numerical Analysis of NREL 5MW Wind Turbine: A Study Towards a Better Understanding of Wake Dynamics and Torque Generation Mechanism
Dr. Muhammad Salman Siddiqui | NTNU | Norway
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Authors:
Dr. Muhammad Salman Siddiqui | NTNU | Norway
adil Rasheed | Norway
Mandar Tabib | Norway
Trond Kvamsdal | Norway
With the increased feasibility of harvesting offshore wind energy, scale of wind turbines is growing rapidly and there is a trend towards clustering together higher number of turbines in order to harvest maximum yield and to leave a smaller footprint on the environment. This causes complex flow configurations inside the farms, the study of which is essential to making offshore wind energy a success. The present study focuses on NREL 5MW wind turbine with the following objectives (a)To compare Sliding Mesh Interface and Multiple Reference Frame modeling approaches and their predictive capabilities in reproducing the characteristics of flow around the full scale wind turbine. (b)To get a better insight into wake dynamics behind the turbine in near and far wake regions operating under different tip-speed-ratio and incoming turbulence intensities.
B35: Numerical study of how stable stratification affects turbulence instabilities above a forest cover: application to wind energy
Dr. Ashvinkumar Chaudhari | Lappeenranta University of Technology | Finland
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Authors:
Dr. Ashvinkumar Chaudhari | Lappeenranta University of Technology | Finland
Boris Conan | Finland
Sandrine Aubrun | Finland
Antti Hellsten | Finland
Forest areas are of increasing interest for the wind energy industry. However, they induce complex flows with strong shear and high turbulence levels. Stably stratified atmospheric conditions, typical during nighttime and especially in winter, add to the challenge of accurately estimating wind resources. Such conditions typically imply strong wind shear and cause larger structural fatigue loads to wind turbines. In this work, large-eddy simulations are performed in neutral and stable conditions over a forest to analyze the influence of the combined effect of forest and thermal stabilities on the unsteady characteristics of the wind flow. Taking advantage of the unsteady resolution provided by the simulations, turbulent characteristics of each thermal stability including the organization of turbulent structures are presented. The resulting comparison between the two cases is put into perspective for wind energy applications.
B36: Estimation of annual energy production using dynamic wake meandering in combination with ambient CFD solutions
Dr. Seonghyeon Hahn | Vestas Wind Systems A/S | Denmark
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Authors:
Dr. Seonghyeon Hahn | Vestas Wind Systems A/S | Denmark
Ewan Machefaux | Denmark
Yavor Hristov | Denmark
Marco Albano | Denmark
Robert Threadgill | Denmark
In the present study, combination of the standalone dynamic wake meandering (DWM) model with Reynolds-averaged Navier-Stokes (RANS) CFD solutions for ambient ABL flows is introduced, and its predictive performance for annual energy production (AEP) is evaluated against Vestas' SCADA data for six operating wind farms over semi-complex terrains under neutral conditions. The performances of conventional linear and quadratic wake superposition techniques are also compared, together with the in-house implemention of successive hierarchical merging approaches. As compared to our standard procedure based on the Jensen model in WindPRO, the overall results are promising, leading to a significant improvement in AEP accuracy for four of the six sites. While the conventional linear superposition shows the best performance for the improved four sites, the hierarchical square superposition shows the least deteriorated result for the other two sites.
D1: Fatigue minimising power reference control of a de-rated wind farm
Dr. Tom Nørgaard Jensen | Aalborg University | Denmark
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Authors:
Dr. Tom Nørgaard Jensen | Aalborg University | Denmark
Torben Knudsen | Denmark
Thomas Bak | Denmark
Modern wind farms (cluster of wind turbines) can be required to control the total power output to meet a set-point, and would then profit by minimising the structural loads and thereby the cost of energy. In this paper, we propose a new control strategy for a de-rated wind farm with the objective of maintaining a desired reference power production for the wind farm, while minimising the sum of fatigues on the wind turbines in steady-state. The controller outputs a vector of power references for the individual turbines. It exploits the positive correlation between fatigue and added turbulence to minimise fatigue indirectly by minimising the added turbulence. Simulated results for a wind farm with three turbines demonstrate the efficacy of the proposed solution by assessing the damage equivalent loads.
D2: Computationally Efficient Kalman Filtering for the Medium-Fidelity Flow Model 'WindFarmSimulator'
Bart Doekemeijer | Delft University of Technology | Netherlands
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Authors:
Bart Doekemeijer | Delft University of Technology | Netherlands
Jan-Willem Van Wingerden | Netherlands
Sjoerd Boersma | Netherlands
Lucy Pao | Netherlands
Wind turbines are often grouped together for financial reasons, but due to wake development this usually results in decreased turbine lifetimes and power capture, and thereby an increased levelized cost of energy (LCOE). Wind farm control aims to minimize this cost by operating turbines at their optimal control settings. Most state-of-the-art control algorithms are open-loop and rely on a low fidelity, static flow model. Closed-loop control relying on a dynamic model and state observer has real potential to further decrease wind\'s LCOE, but is often too computationally expensive for practical use. In this work two time-efficient Kalman filter (KF) variants are outlined incorporating the medium fidelity, dynamic flow model WindFarmSimulator (WFSim). This model relies on a discretized set of Navier-Stokes equations in 2D to predict the flow in wind farms in a horizontal plane at hub height at low computational cost. The filters implemented are an Ensemble KF and an Approximate KF. Simulations in which a high fidelity simulation model represents the true wind farm show that these filters are typically several orders of magnitude faster than a regular KF with comparable or better performance, correcting for wake dynamics that are not modeled in WFSim (noticeably, wake meandering and turbine hub effects). This is a first big step towards real-time closed-loop control in wind farms.
D3: Model Predictive Wind Turbine Control for Load Alleviation and Power Leveling
Uwe Jassmann | RWTH Aachen | Germany
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Authors:
Uwe Jassmann | RWTH Aachen | Germany
Sebastian Dickler | Germany
János Zierath | Germany
Mathias Hakenberg | Germany
Dirk Abel | Germany
This contribution presents a Model Predictive Controller (MPC) with move-blocking strategy for combined power leveling and load alleviation in wind turbine operation. The wind turbine considered in this work is a commercial, state-of-the-art 3 MW wind turbine. The foremost motivation for the MPC in this work is not to utilize additional sensors or actuators, but to replace several single input single output control loops with just one controller and by this ease the controller tuning in future developments. Therefore not only the performance of the MPC regarding its control results is of importance, but also the computational time required running the MPC. In this paper a move-blocking strategy is proposed to reduce the computational burden related to the MPC by approximately 50% compared to a baseline MPC. This even allows to run the proposed MPC with move-blocking strategy on a state-of-the-art Programmable Logic Controller in real-time.
D4: Balancing rotor speed regulation and drive train loads of floating wind turbines
Dr. Boris Fischer | Fraunhofer IWES | Germany
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Authors:
Dr. Boris Fischer | Fraunhofer IWES | Germany
Peter Loepelmann | Germany
The interaction of the blade pitch controller with structural motion is particularly important for wind turbines mounted on floating platforms. A controls-based approach to overcome the related technical challenges is to feed back the nacelle's motion to the demanded generator torque. This work aims to further improve this approach by feeding back only a narrow fraction of the available frequency range. Simulations show that, in doing so, unrealistically high torque magnitudes are avoided, and better a trade-off between rotor speed regulation and drive train loads is achieved.
D5: Stability Analysis of a Single-Input/Two-Output, Variable Loop Transmission System for Wind Turbine Control
Yelena O'Brien | University of Wyoming | United States
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Authors:
Yelena O'Brien | University of Wyoming | United States
John O'Brien | United States
Large wind turbine control performance is restricted due to a host of intrinsic feedback limitations including low frequency structural modes and slow blade pitch rate. A variable loop transmission system applied to a multiple output controller smoothly transitions the limited feedback available between channels in real time to enhance performance. This work is focused on the stability analysis of a variable loop transmission system applied to a 1.5 MW wind turbine. Simulation results illustrate interesting challenges to compensator design in the form of more sophisticated loop shaping required to satisfy conditions of absolute stability.
D6: Multi-objective Extremum Seeking Control for Enhancement of Wind Power Capture with Load Reduction
Prof. Yaoyu Li | University of Texas at Dallas | United States
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Authors:
Prof. Yaoyu Li | University of Texas at Dallas | United States
Yan Xiao | United States
Mario Rotea | United States
The primary objective in below rated wind speed (Region 2) is to maximize the energy capture. Due to uncertainty, variability of turbine characteristics and lack of inexpensive but precise wind measurements, model-free control strategies those do not use wind measurements such as Extremum Seeking Control (ESC) have received significant attention. Based on a dither-demodulation scheme, ESC can maximize the wind power capture in real time despite uncertainty, variabilities and lack of accurate wind measurements. The existing work on ESC based wind turbine control focuses on power capture only. In this paper, a multi-objective extremum seeking control strategy is proposed to achieve nearly optimum wind energy capture while decreasing structural fatigue loads. The performance index of the ESC combines the rotor power and penalty terms of the standard deviations of selected fatigue load variables. Simulation studies of the proposed multi-objective ESC demonstrate that the damage-equivalent loads of tower and/or blade loads can be reduced significantly with slight compromise in energy capture.
D7: Statistical fault diagnosis of wind turbine drivetrain applied to a 5MW floating wind turbine
Mahdi Ghane | NTNU | Norway
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Authors:
Mahdi Ghane | NTNU | Norway
amir rasekhi nejad | Norway
mogens blanke | Norway
torgeir moan | Norway
Zhen Gao | Norway
Deployment of large scale wind turbine parks, in particular offshore, requires well organized operation and maintenance strategies to make it as competitive as the classical electric power stations. It is important to ensure systems are safe, profitable, and cost-effective. In this regards, the ability to detect, isolate, estimate, and prognose faults plays an important role. One of the critical wind turbine components is the gearbox. Failures in the gearbox are costly both due to the cost of the gearbox itself and also due to high repair downtime. In order to detect faults as fast as possible to prevent them to develop into failure, statistical change detection is used in this paper. The Cumulative Sum Method (CUSUM) is employed to detect possible defects in the downwind main bearing. A high fidelity gearbox model on a 5-MW spar-type wind turbine is used to generate data for fault-free and faulty conditions of the bearing at the rated wind speed and the associated wave condition. Acceleration measurements are utilized to find residuals used to indirectly detect damages in the bearing. Residuals are found to be nonGaussian, following a t-distribution with multivariable characteristic parameters. The results in this paper show how the diagnostic scheme can detect change with desired false alarm and detection probabilities.
D8: The design of nonlinear observers for wind turbine dynamic state and parameter estimation
Bastian Ritter | Technische Universität Darmstadt | Germany
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Authors:
Bastian Ritter | Technische Universität Darmstadt | Germany
Axel Schild | Germany
Matthias Feldt | Germany
Ulrich Konigorski | Germany
This contribution addresses the dynamic state and parameter estimation problem which arises with more advanced wind turbine controllers. These control devices need precise information about the system's current state to outperform the conventional industrial controllers effectively. First, the necessity of a profound scientific treatment on nonlinear observers for wind turbine application is highlighted. Secondly, the full estimation problem is introduced and the variety of nonlinear filters is discussed. Finally, a tailored observer architecture is proposed and estimation results of an illustrative application example from a complex simulation set-up are presented.
D9: Controller design for wind turbine load reduction via Multi Objective Parameter Synthesis (MOPS)
Arndt Hoffmann | German Aerospace Centre (DLR) | Germany
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Authors:
Arndt Hoffmann | German Aerospace Centre (DLR) | Germany
Felix Weiß | Germany
During the design process for a wind turbine load reduction controller many different, sometimes conflicting requirements must be fulfilled simultaneously. The requirements can be expressed as mathematical criteria and therefore as quantity, there is the possibility to solve such a design problem by a criterion-vector and a multi-objective design optimization. The software environment MOPS (Multi-Objective Parameter Synthesis) supports the engineer for such a design optimization. In this paper MOPS is applied to design a multi-objective load reduction controller for the well-known DTU 10 MW reference wind turbine. A significant reduction in the fatigue criteria especially the blade damage can be reached by the use of an additional Individual Pitch Controller (IPC) and an additional tower damper. This reduction is reached as a trade-off with an increase of actuator load.
D10: Direct Torque Control of a Small Wind Turbine with a Sliding-mode Speed Controller
Jagath Senanayaka | University of Agder | Norway
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Authors:
Jagath Senanayaka | University of Agder | Norway
Hamid Reza Karimi | Norway
Kjell Gunnar Robbersmyr | Norway
In this paper, the direct torque control method with a sliding-mode speed controller is proposed for a small wind turbine being used in water heating applications. This concept and control system can be easily extended for any grid connected or off-grid applications. Direct torque control of electrical machines has shown several advantages including very fast dynamics torque control over field-oriented control. Moreover, the torque and flux controllers in the direct torque control algorithms are based on hysteretic controllers which are nonlinear. In the presence of a sliding-mode speed control, a nonlinear control system can be made which is matched for AC/DC conversion of the converter that gives fast responses with low overshoots. The main control objectives of the proposed small wind turbine include, maximum power point tracking and soft-stall power control. This small wind turbine consists of permanent magnet synchronous generator and external wind speed and rotor speed measurements are not required for the system. But, a sensor is required to detect the rated wind speed overpass events to activate proper speed references for the wind turbine. To get the required measurement, a low cost wind speed sensor can be modified or a new low cost sensor can be designed to match with the low cost design requirement of small wind turbines. The simulation results will be provided to illustrate very good performance of the closed loop system in entire wind speed range (4-25 m/s).
D11: Design of Linear Control System for Wind Turbine Blade Fatigue Testing
Torben Knudsen | Aalborg University | Denmark
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Authors:
Torben Knudsen | Aalborg University | Denmark
Anders Toft | Denmark
Bjarke Roe-Poulsen | Denmark
Rasmus Christiansen | Denmark
This paper proposes a linear method for wind turbine blade fatigue testing at Siemens Wind Power. The setup consists of a blade, an actuator (motor and load mass) that acts on the blade with a sinusoidal moment, and a distribution of strain gauges to measure the blade flexure. Based on the frequency of the sinusoidal input, the blade will start oscillating with a given gain, hence the objective of the fatigue test is to make the blade oscillate with a controlled amplitude. The system currently in use is based on frequency control, which involves some non-linearities that make the system difficult to control. To make a linear controller, a different approach has been chosen, namely making a controller which is not regulating on the input frequency, but on the input amplitude. A non-linear mechanical model for the blade and the motor has been constructed. This model has been simplified based on the desired output, namely the amplitude of the blade. Furthermore, the model has been linearised to make it suitable for linear analysis and control design methods. The controller is designed based on a simplified and linearised model, and its gain parameter determined using pole placement. The model variants have been simulated in the MATLAB toolbox Simulink, which shows that the controller design based on the simple model performs adequately with the non-linear model. Moreover, the developed controller solves the robustness issue found in the existent solution and also reduces the needed energy for actuation as it always operates at the blade eigenfrequency.
D12: PI Controller Design of a Wind Turbine using Constrained Optimization
Dr. Mahmood Mirzaei | Technical University of Denmark | Denmark
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Authors:
Dr. Mahmood Mirzaei | Technical University of Denmark | Denmark
Carlo Tibaldi | Denmark
Morten H. Hansen | Denmark
PI/PID controllers are the dominant wind turbine controllers. Normally they are first tuned with an initial guess using methods such as pole-placement or Ziegler-Nichols and then extensive simulations are used to obtain the best tuning in terms of regulation of the outputs and reduction of the loads. In the traditional tuning approaches, the properties of different open loop and closed loop transfer functions of the system, such as robustness measures, are not normally considered. In this paper an assessment of the pole-placement tuning method is presented. Then a constrained optimization setup is suggested to automatically tune the WT controller subject to robustness constraints. The properties of the system such as the maximum sensitivity and complementary sensitivity functions (M s and M t ) along with some of the responses of the system are used to investigate the controller performance and formulate the optimization problem. The cost function is the integral absolute error (IAE) of the rotational speed from a disturbance modeled as a step in wind speed. Linearized model of the DTU 10- MW reference wind turbine is obtained using HAWCStab2. Thereafter, the model is reduced with model order reduction for the setup. The trade-off curves are given to assess the tunings of the poles-placement method and a constrained optimization problem is solved to find the best tuning.
D13: Iterative tuning of feedforward IPC for two-bladed wind turbines
Sebastiaan Mulders | TU Delft | Netherlands
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Author:
Sebastiaan Mulders | TU Delft | Netherlands
At present, the cost of offshore wind energy does not meet the level of onshore wind and fossil-based energy sources. One way to extend the turbine lifetime, is by reduction of the fatigue loads of blades and other turbine parts using Individual Pitch Control (IPC). The main goal of this article is to develop a self-optimizing feedforward IPC strategy for a two-bladed wind turbine to reduce actuator duty cycle and reduce the dependency on blade load signals. The approach uses blade load measurement data only initially for tuning of the feedforward controller, which is scheduled on the rotor azimuth angle and wind speed. The feedforward strategy will be compared to the feedback implementation. Results show that the implementation is capable of learning the optimal feedforward IPC controller in constant and turbulent wind conditions, to alleviate the pitch actuator duty cycle, and to considerably reduce harmonic fatigue loads without the need for blade load signals after tuning.
D14: Uncertainty Quantification for Robust Control of Wind Turbines using Sliding-Mode Observer
Prof. Horst Schulte | University of Applied Sciences Berlin (HTW) | Germany
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Author:
Prof. Horst Schulte | University of Applied Sciences Berlin (HTW) | Germany
A new quantification method of uncertain models for robust wind turbine control using sliding-mode techniques is presented with the objective to improve active load mitigation. This approach is based on the so-called equivalent output injection signal, which corresponds to the average behavior of the discontinuous switching term, establishing and maintaining a motion on a so-called sliding surface. The injection signal is directly evaluated to obtain estimates of the uncertainty bounds of external disturbances and parameter uncertainties. The applicability of the proposed method is illustrated by the quantification of a four degree-of-freedom model of the NREL 5MW reference turbine containing uncertainties.
D15: Tower Based Load Measurements for Individual Pitch Control and Tower Damping of Wind Turbines
Dr. Oscar Hugues-Salas | DNV GL - Energy | United Kingdom
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Authors:
Dr. Oscar Hugues-Salas | DNV GL - Energy | United Kingdom
Avishek Kumar | United Kingdom
William Keogh | United Kingdom
Barbara Savini | United Kingdom
The cost of IPC has hindered adoption outside of Europe despite significant loading advantages for large wind turbines. In this work we presented a method for applying individual pitch control (including for higher-harmonics) using tower-top strain gauge feedback instead of blade-root strain gauge feedback. Tower-top strain gauges offer hardware savings of approximately 50% in addition to the possibility of easier access for maintenance and installation and requiring a less specialised skill-set than that required for applying strain gauges to composite blade roots. A further advantage is the possibility of using the same tower-top sensor array for tower damping control. This method is made possible by including a second order IPC loop in addition to the tower damping loop to reduce the typically dominating 3P content in tower-top load measurements. High-fidelity Bladed simulations show that the resulting turbine spectral characteristics from tower-top feedback IPC and from the combination of tower-top IPC and damping loops largely match those of blade-root feedback IPC and nacelle-velocity feedback damping. Lifetime weighted fatigue analysis shows that the methods allows load reductions within 2.5% of traditional methods.
D16: Automatic weight determination in nonlinear model predictive control of wind turbines using swarm optimization technique
Elham Tofighi | University of Newcastle | Australia
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Authors:
Elham Tofighi | University of Newcastle | Australia
Amin Mahdizadeh | Australia
This paper addresses the problem of automatic tuning of weighting coefficients for the nonlinear model predictive control (NMPC) of wind turbines. The choice of weighting coefficients in NMPC is critical due to their explicit impact on efficiency of the wind turbine control. Classically, these weights are selected with respect to intuitive understanding of the system dynamics and control objectives. The empirical methods, however, may not yield optimal solutions especially when the number of parameters to be tuned and the nonlinearity of the system increase. In this paper, the problem of determining weighting coefficients for the cost function of the NMPC controller is formulated as a two-level optimization process in which the upper-level PSO-based optimization computes the weighting coefficients for the lower-level NMPC controller which generates control signals for the wind turbine. Simulations are performed comparing the performance of NMPC in controlling the NREL 5-MW wind turbine implementing empirical weights as well as weights computed by the two-level optimization method. The simulation results verify the efficiency and accuracy of the proposed method.
D17: On the functional design of the DTU10 MW wind turbine scale model within LIFES50+ project
Prof. Marco Belloli | POLITECNICO DI MILANO | Italy
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Authors:
Prof. Marco Belloli | POLITECNICO DI MILANO | Italy
Ilmas Bayati | Italy
Hermes Giberti | Italy
Luca Bernini | Italy
Enrico Fiore | Italy
Hermes Giberti | Italy
Alberto Zasso | Italy
This paper illustrates the mechatronic design of the wind tunnel scale model of the DTU 10MW reference wind turbine, for the LIFES50+ H2020 European project. This model was designed with the nal goal of controlling the angle of attack of each blade by means of miniaturized servomotors, for implementing advanced individual pitch control (IPC) control laws on a Floating O shore Wind Turbine (FOWT) 1/75 scale model. Many design constraints were to be respected: among others, the rotor-nacelle overall mass due to aero-elastic scaling, the limited space of the nacelle, where to put three miniaturized servomotors and the main shaft one, with their own inverters/controllers, the slip rings for electrical rotary contacts, the highest sti ness as possible for the nacelle support and the blades attach points, for ensuring the proper kinematic constraint, considering the rst apwise blade natural frequency, the performance of the servomotors to guarantee the wide frequency band due to frequency scale factors, etc. The design and technical solutions are herein presented and discussed, along with an overview of the building and veri cation process. Also an extensive discussion about the goals achieved and respected constraints for the rigid wind turbine scale model (Lifes50+ deliverable D.3.1) and the further possible improvements for the next IPC-aero-elastic scale model for the following project milestones.
D18: Tailoring the Employment of Offshore Wind Turbine Support Structure Load Mitigation Controllers
Binita Shrestha | University of Oldenburg | Germany
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Authors:
Binita Shrestha | University of Oldenburg | Germany
Rasoul Shirzadeh | Germany
Martin Kuehn | Germany
The currently available control concepts to mitigate aerodynamic and hydrodynamic induced support structure loads reduce either fore-aft or side-to-side damage under certain operational conditions. The load reduction is achieved together with an increased loads in other components of the turbine increasing the risk of unscheduled maintenance. The main objective of this paper is to demonstrate a methodology for support structure DEL reduction in fore-aft and side-to-side directions using already available control concepts. A multi-objective optimization problem is formulated to minimize the DELs, while limiting the collateral effects of the control algorithms. The optimization gives trigger values of sea state condition for the activation/deactivation of certain control concepts. As a result, by accepting the consumption of a small fraction of the load reserve in the design load envelope of other turbine components, a considerable reduction of the support structure loads is facilitated.
F1: Improving the Accuracy of Wind Turbine Power Curve Validation by the Rotor Equivalent Wind Speed Concept
Dr. Frank Scheurich | Siemens Wind Power | Denmark
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Authors:
Dr. Frank Scheurich | Siemens Wind Power | Denmark
Peder B Enevoldsen | Denmark
Henrik N Paulsen | Denmark
Kristoffer K Dickow | Denmark
Moritz Fiedel | Denmark
Alex Loeven | Denmark
Ioannis Antoniou | Denmark
The measurement of the wind speed at hub height is part of the current IEC standard procedure for the power curve validation of wind turbines. The inherent assumption is thereby made that this measured hub height wind speed sufficiently represents the wind speed across the entire rotor area. It is very questionable, however, whether the hub height wind speed (HHWS) method is appropriate for rotor sizes of commercial state-of-the-art wind turbines. The rotor equivalent wind speed (REWS) concept, in which the wind velocities are measured at several different heights across the rotor area, is deemed to be better suited to represent the wind speed in power curve measurements and thus results in more accurate predictions of the annual energy production (AEP) of the turbine. The present paper compares the estimated AEP, based on HHWS power curves, of two different commercial wind turbines to the AEP that is based on REWS power curves. The REWS was determined by LiDAR measurements of the wind velocities at ten different heights across the rotor area. It is shown that a REWS power curve can, depending on the wind shear profile, result in higher, equal or lower AEP estimations compared to the AEP predicted by a HHWS power curve.
F2: Design and implementation of a controllable model wind turbine for experimental studies
Jannik Schottler | ForWind, University of Oldenburg | Germany
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Authors:
Jannik Schottler | ForWind, University of Oldenburg | Germany
Agnieszka Hölling | Germany
Joachim Peinke | Germany
Michael Hölling | Germany
This technical paper describes the design and characterization of a controllable model wind turbine for wind tunnel experiments. The setup of the turbine and the implementation of the control system are described in detail and tests of the control system are shown. Finally, results of one exemplary scientific application are presented, where the model turbine was exposed to turbulent, intermittent inflow and the controller influence on torque fluctuations is investigated.
F3: Fast Ice Detection for wind turbine blades via the Langevin equation
Dr. Haijun Fang | Envision Energy | United States
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Authors:
Dr. Haijun Fang | Envision Energy | United States
Linpeng Wang | United States
In this paper, a software-based algorithm for fast detection of ice on wind turbine blades is developed. The Langevin equation is used to create an entire or partial power curve with the high frequency data of wind speed and electrical power. Such a power curve is called the Langevin Power Curve (LPC). The LPC is obtained periodically. The period can be adjusted to be from 1 minute to 1 hour. For our application, the period is set to 5 minutes to allow enough data to generate an entire or partial LPC and then ice may be detected within a short period of time. The obtained LPC is compared to a reference power curve and then an ice index is calculated given that the condition for ice accretion is met. If the ice index is much higher or lower than 1, it may be concluded that there is ice on the anemometer or the blades of a wind turbine.
F4: A new motion compensation algorithm of floating lidar system for the assessment of turbulence intensity
Dr. Atsushi Yamaguchi | The University of Tokyo | Japan
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Authors:
Dr. Atsushi Yamaguchi | The University of Tokyo | Japan
Takeshi Ishihara | Japan
In this study, a new motion compensation algorithm was proposed and verified by using numerical simulation. Compensated horizontal mean wind speed by using conventional method shows good agreement with reference wind speed regardless of the motion of the floater. However, turbulence intensity is always overestimated. The overestimation is more significant when the maximum pitch angle of the floater motion is larger. When proposed method is used, the overestimation of the turbulent intensity is improved and estimated turbulent intensity shows better agreement with reference value. There still remains underestimation of the turbulence intensity with the bias of -1.1%. This is probably caused by the low sampling frequency in LIDAR measurement and further research is needed to model the high frequency component of the wind speed for LIDAR measurement.
F5: Performance tests of a power-electronics converter for multi-megawatt wind turbines using a grid emulator
Nurhan Rizqy Averous | RWTH Aachen University | Germany
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Authors:
Nurhan Rizqy Averous | RWTH Aachen University | Germany
Anica Berthold | Germany
Alexander Schneider | Germany
Franz Schwimmbeck | Germany
Rik W. De Doncker | Germany
Antonello Monti | Germany
A vast increase of wind turbines (WT) contribution in the modern electrical grids have led to the development of grid connection requirements. In contrast to the conventional test method, testing power-electronics converters for WT using a grid emulator at Center for Wind Power Drives (CWD) RWTH Aachen University offers more flexibility for conducting test scenarios. Further analysis on the performance of the device under test (DUT) is however required when testing with grid emulator since the characteristic of the grid emulator might influence the performance of the DUT. This paper focuses on the performance analysis of the DUT when tested using grid emulator. Beside the issue regarding the current harmonics, the performance during Fault Ride-Through (FRT) is discussed in detail. A power hardware in the loop setup is an attractive solution to conduct a comprehensive study on the interaction between the power-electronics converters and the electrical grids.
F6: Spatial-temporal analysis of coherent offshore wind field structures measured by scanning Doppler-lidar
Laura Valldecabres Sanmartin | University of Oldenburg | Germany
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Authors:
Laura Valldecabres Sanmartin | University of Oldenburg | Germany
Martin Kühn | Germany
Lüder von Bremen | Germany
Wilm Friedrichs | Germany
An analysis of the spatial and temporal power fluctuations of a simplified wind farm model is conducted on four offshore wind fields data sets, 2 from lidar measurements and 2 from LES under unstable and neutral atmospheric conditions. The integral length scales of the horizontal wind speed computed in the streamwise and the cross-stream direction revealed the elongation of the structures in the direction of the mean flow. The summation of the power of two wind turbines smooths out the fluctuations of a single wind turbine at different scales under 10 minutes. This effect, which is stronger with increasing spacing between turbines, can be seen in the aggregation of the power of two wind turbines in the streamwise direction. Due to the anti-correlation of the coherent structures in the cross-stream direction, this smoothing effect is stronger when the aggregated power is computed with two wind turbines aligned orthogonally to the mean flow direction.
F7: Estimation of rotor effective wind speeds using autoregressive models with Lidar
Ashim Giyanani | TU Delft | Netherlands
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Authors:
Ashim Giyanani | TU Delft | Netherlands
Feike Savenije | Netherlands
Wim Bierbooms | Netherlands
Gerard van Bussel | Netherlands
Lidars are becoming increasingly useful for providing accurate wind speed measurements in front of the wind turbine. Wind speed measurements from distant meteorological masts change their structure and are too distorted that simply using Taylor's frozen turbulence assumption does not provide accurate representations of the rotor effective wind speed. When Lidar measurements are employed appropriately, wind turbine controllers can optimise the energy output and reduce the loads significantly, in case of extreme events. Wind speed estimations are currently derived from aerodynamic torque, pitch angle and tip speed ratio after the wind flows past the turbine and have their limitations, for e.g in predicting gusts. Nacelle-mounted Lidars measure the oncoming wind field and appropriate models could be used for deriving the rotor effective wind speed from these measurements. This article proposes an auto-regressive model combined with a blockage factor to estimate the wind speeds with low uncertainty using nacelle-mounted Lidars. The model could be extended to estimate the wind turbine fatigue loads and thus, allow the development of effective wind turbine control strategies. We used auto-regressive models to determine a transfer function, that resembles the physical evolution of wind towards the wind turbine.
F8: Fatigue life on a full scale test rig: Forged vs. cast wind turbine rotor shafts
Jenni Herrmann | Hamburg University of Applied Sciences | Germany
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Authors:
Jenni Herrmann | Hamburg University of Applied Sciences | Germany
Thes Rauert | Germany
Peter Dalhoff | Germany
Manuela Sander | Germany
To reduce uncertainties associated with the fatigue life of the highly safety relevant rotor shaft and also to review today's design practice, the fatigue behaviour will be tested on a full scale test rig. Until now tests on full scale wind turbine parts are not common. Therefore, a general lack of experience on how to perform accelerated life time tests for those components exists. To clarify how to transfer real conditions to the test environment, the arrangements and deviations for the upcoming experimental test are discussed in detail. In order to complete investigations of weight saving potentials, next to getting a better comprehension of the fatigue behaviour by executing a full scale test, a further outcome are suggestions for the usage of cast and forged materials regarding the fatigue and the remaining life of the rotor shaft. It is shown, that it is worthwhile to think about a material exchange for the forged rotor shaft.
F9: Optimal Sensor Placement for Modal Testing on Wind Turbines
Andreas Schulze | Universität Rostock | Germany
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Authors:
Andreas Schulze | Universität Rostock | Germany
János Zierath | Germany
Sven-Erik Rosenow | Germany
Reik Bockhahn | Germany
Roman Rachholz | Germany
Christoph Woernle | Germany
The mechanical design of wind turbines requires a profound understanding of the dynamic behaviour. Even though highly detailed simulation models are already in use to support wind turbine design, modal testing on a real prototype is irreplaceable to identify site-specific conditions such as the stiffness of the tower foundation. Correct identification of the mode shapes of a complex mechanical structure much depends on the placement of the sensors. For operational modal analysis of a 3 MW wind turbine with a 120 m rotor on a 100 m tower developed by W2E Wind to Energy, algorithms for optimal placement of acceleration sensors are applied. The mode shapes used for the optimisation are calculated by means of a detailed flexible multibody model of the wind turbine. Among the three algorithms in this study, the genetic algorithm with weighted off-diagonal criterion yields the sensor configuration with the highest quality.
F10: Finding the ideal strategy: Full-scale fatigue testing of wind turbine rotor shafts
Thes Rauert | Hamburg University of Applied Sciences | Germany
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Thes Rauert | Hamburg University of Applied Sciences | Germany
Jenni Herrmann | Germany
Peter Dalhoff | Germany
Manuela Sander | Germany
For the purpose of a light weight design of rotor shafts, fatigue testing is necessary. Since full-scale fatigue tests of these large components are time consuming, costly and have not been done before, much effort has to be put into the implementation of a suitable test strategy. The paper presents the boundary conditions that have to be considered to determine the finite life regime of the component S/N-curve. A statistical simulation shows how much the derived S/N-curve is influenced by the specific test procedure.
F11: SCADA alarms processing for wind turbine component failure detection
Elena Gonzalez | CIRCE - Universidad de Zaragoza | Spain
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Authors:
Elena Gonzalez | CIRCE - Universidad de Zaragoza | Spain
Maik D. Reder | Spain
Julio Javier Melero | Spain
Wind turbine failure and downtime often compromise wind farm profitability due to their high impact on the O&M costs. Early detection of failures can facilitate the changeover from corrective maintenance towards a predictive approach. This paper presents a cost-effective methodology to combine alarm analysis techniques, using data from the SCADA system, in order to detect component failures. The approach categorises the alarms according to a reviewed taxonomy, turning overwhelming data into valuable information to assess component status. Then, different alarms analysis techniques are applied for two purposes: the evaluation of the SCADA alarm system capability to detect failures, and the investigation of the relation between components faults being followed by failure occurrences in others. Various case studies are presented and discussed. The study highlights the relationship between faulty behaviour in different components and between failures and adverse environmental conditions.
F12: Wind Turbine Failures - Tackling current Problems in Failure Data Analysis
Maik D. Reder | CIRCE - Universidad de Zaragoza | Spain
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Authors:
Maik D. Reder | CIRCE - Universidad de Zaragoza | Spain
Elena Gonzalez | Spain
Julio Javier Melero | Spain
With the growing wind energy sector, there is an urgent need for cost effective operation and maintenance (O&M) strategies. Especially unplanned wind turbine (WT) downtime is one of the main cost drivers of a modern wind farm. Reliability and failure prediction models can enable operators to apply preventive O&M strategies rather than corrective ones. In order to develop these models, WT component failure rates and caused downtime have to be understood profoundly. This paper focuses on tackling three main issues related to WT failure analyses: (1) non-uniform data treatment, (2) scarcity of available failure analyses, and (3) the lack of investigation on alternative data sources. For this, a modernised form of an existing WT taxonomy is introduced. Moreover, an extensive analysis of historical failure data of over 4300 turbines is presented. Finally, the possibilities to encounter the lack of available failure data by complementing historical databases with SCADA alarms are evaluated.
F13: Experimental airfoil characterization under tailored turbulent conditions
Hendrik Heißelmann | University of Oldenburg | Germany
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Hendrik Heißelmann | University of Oldenburg | Germany
Joachim Peinke | Germany
Michael Hölling | Germany
Studies of the impact of turbulent inflow conditions on the airfoil characteristics were performed within the EU FP7 project AVATAR. The aim of this study is to provide data for the validation of simulations and the improvement of engineering tools. Chord-wise pressure distributions and highly-resolved force data of the wind turbine dedicated DU 00-W-212 profile were measured in the wind tunnel in two tailored turbulent inflow conditions generated with an active grid. A sinusoidal and an intermittent pattern with customized inflow angle fluctuations were generated providing two significantly different distributions of reduced frequencies. The obtained pressure distributions and polars from the unsteady patterns are compared to the laminar baseline case.
F14: Wireless monitoring of structural components of wind turbines including tower and foundations
Bernhard Wondra | Technische Universität München | Germany
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Authors:
Bernhard Wondra | Technische Universität München | Germany
Max Botz | Germany
Christian Grosse | Germany
Very few large wind turbines contain an extensive structural health monitoring (SHM) system. Such SHM systems could provide deeper insight into the real load history of a wind turbine along its standard lifetime of 20 years and support a justified extension of operation beyond the original intended period. This paper presents a new concept of a wireless SHM system based on acceleration measurement sensor nodes to permanently record acceleration of the tower structure at different heights. Exploitation of acceleration data and its referring position on the turbine tower enables calculation of vibration frequencies, their amplitudes and subsequently eigenmodes. Tower heights of 100 m and more are within the transmission range of wireless nodes, enabling a complete three-dimensional surveillance of the tower without the need for long cabling or electric signal amplification. Mounting of the sensor nodes is not limited to a few positions by the presence of an electric cable anymore.
F15: The effect of wakes on the fatigue damage of wind turbine components over their entire lifetime using short-term load measurements
Dr. Sarah Karlina-Barber | Fraunhofer IWES | Germany
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Authors:
Dr. Sarah Karlina-Barber | Fraunhofer IWES | Germany
Sebastian Mechler | Germany
Marco Lutz | Germany
A method is developed for quantifying the effect of neighboring wind turbines on the fatigue damage of the main components of a wind turbine over its entire operating time using short-term load measurements. This method could be used in the future for improving wind farm planning software that takes into account fatigue damage as well as energy yield or for improving lifetime extension calculations of wind turbines. The method is applied here to a measurement campaign on a Vestas V66 wind turbine located in northern Germany and the results are found to be plausible. Furthermore, the results show that the increase in total lifetime fatigue damage due to neighboring wind turbines for wind turbine separations of the order of 5D is significant and needs to be taken account of in wind farm planning software. The accuracy of the method is examined by investigating the sensitivity of the main assumptions on the results. Further investigation and standardization of the method is required.
F16: A finite difference approach to despiking in-stationary lidar velocity data
Alexander Raul Meyer Forsting | DTU Wind Energy | Denmark
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Authors:
Alexander Raul Meyer Forsting | DTU Wind Energy | Denmark
Niels Troldborg | Denmark
A novel despiking method is presented for in-stationary wind lidar velocity measurements. A finite difference approach yields the upper and lower bounds for a valid velocity reading. The sole input to the algorithm is the velocity series and optionally a far-field reference to the temporal variation in the velocity. The new algorithm is benchmarked against common despiking algorithms using a dataset acquired by three synchronised lidars in the upstream area of a full-scale wind turbine rotor and an artificially created space-time series with controlled spike contamination. By accounting for variations in space and time, this approach yields improvements in spike detection for in-stationary lidar measurements of about 25% over other more established stationary methods. Furthermore it proofs to be robust even for large numbers of spikes.
F17: Full load estimation of an offshore wind turbine based on SCADA and accelerometer data
Nymfa Noppe | Vrije Universiteit Brussel | Belgium
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Authors:
Nymfa Noppe | Vrije Universiteit Brussel | Belgium
Alexandros Iliopoulos | Belgium
Wout Weijtjens | Belgium
Christof Devriendt | Belgium
As Offshore Wind Farms grow older, the optimal use of the actual fatigue lifetime of an Offshore Wind Turbine and predominantly its foundation will get more important. In case of OWTs, both quasi-static thrust loads and dynamic loads, as induced by turbulence, waves and the turbine's dynamics, contribute to its fatigue life progression. A model is created to estimate the thrust load using SCADA data and strain measurements. From this model the thrust loads on the foundation can be estimated over the lifetime of the OWT. To estimate the contribution of the dynamic loads a modal decomposition and expansion based virtual sensing technique, using acceleration measurements recorded at accessible locations on the tower only, is applied. Superimposing both contributions leads to a so-called multi-band virtual sensing. This approach is validated using data from an operating Belgian OWF. An initial good match between measured and predicted strains for a short period of time proofs the concept.
F18: Dynamics of the interaction between the rotor and the induction zone
Dr. Mahmood Mirzaei | Technical University of Denmark
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Authors:
Dr. Mahmood Mirzaei | Technical University of Denmark
Alexander R. Meyer Forsting
Niels Troldborg
Traditionally met masts are used for power and load verification. They are normally placed 2-4 rotor diameters ahead of the turbine. However in complex terrains this can lead to complex analysis of the effect of the terrain on the flow field. A nacelle mounted lidar can provide a better tool for wind field measurements in all terrains. Provided the measurement is close enough to the rotor disc, the uncertainty in the flow field measurement can be reduced significantly and therefore complex analysis of the effect of the terrain and changes in the wind direction can be avoided. However, close distance lidar measurements are affected by the presence of the wind turbine. This is due to the induction zone of the wind turbine. In this work, the dynamic effect of changes in the operating point of the the wind turbine on the wind velocity in the induction zone is studied. Reynolds-Averaged Navier Stokes (RANS) simulations are used to investigate this phenomena. Thereafter, system identification is used to fit first order dynamic models to the simulation results and the parameters of the model are given in the induction zone upstream of the turbine. The results can be used to reduce the uncertainty in the lidar measurements because of the induction zone.
F19: The use of long term monitoring data for the extension of the service duration of existing wind turbines support structures
Christophe Loraux | MCS - EPFL | Switzerland
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Authors:
Christophe Loraux | MCS - EPFL | Switzerland
Eugen Brühwiler | Switzerland
Actual wind energy converter (WEC) are designed for a relatively short service life of 20 years and the limiting criterion is the fatigue safety. However, effective fatigue loading endured by the structural components of the wind turbines (WT) is likely to be much below design assumptions provided by current codes. This paper describes a simple but efficient long term monitoring system that allows owners to verify the fatigue safety of their existing WTs. The monitored data will also help to drastically extend the service life of existing wind turbine support structure and will thus reduce the global environmental footprint of WTs.
H1: Exploring the wakes of large offshore wind farms
Prof. Stefan Emeis | Karlsruhe Institute of Technology - KIT | Germany
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Authors:
Prof. Stefan Emeis | Karlsruhe Institute of Technology - KIT | Germany
Astrid Lampert | Germany
Jens Bange | Germany
Johannes Schulz-Stellenfleth | Germany
Thomas Neumann | Germany
Offshore meteorological characteristics set specific conditions for the operation of offshore wind farms. One specific feature is low turbulence intensity which on the one hand reduces loads on turbines but on the other hand is the reason for much longer turbine and farm wakes than over land. The German Government - in supporting the massive erection of wind farms in the North Sea - is presently funding a research project called WIPAFF (WInd PArk Far Field) which heads for the analysis of properties and impacts of offshore wind park far fields. The focus is on the analysis of the horizontal extension of wind farm wakes (which is supposed to depend considerably on the thermal stratification of the marine atmospheric boundary layer), their interaction among each other and their regional climate impact. The wake assessment in the project is planned to be done by in-situ, extensive aircraft, and satellite measurements and by operating meso-scale wind field models and an analytical wind farm model. The poster will present data from a pre-experiment, the project outline, wind statistics including thermal stratification from the North Sea, and first results from SAR satellite data analysis and numerical modelling with a coupled wind field (WRF) and ocean wave (WAM) model.
H2: Model-based fault diagnosis of blade pitch system in floating wind turbines
Seongpil Cho | NTNU | Norway
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Authors:
Seongpil Cho | NTNU | Norway
Zhen Gao | Norway
Torgeir Moan | Norway
This paper presents a model-based scheme for fault detection of a blade pitch system in floating wind turbines. A blade pitch system is one of the most critical components due to its effect on the operational safety and the dynamics of wind turbines. Faults in this system should be detected at the early stage to prevent failures. To detect faults of blade pitch actuators and sensors, an appropriate observer should be designed to estimate the states of the system. Residuals are generated by a Kalman filter and a threshold based on H-infinity optimization, and linear matrix inequality (LMI) is used for residual evaluation. The proposed method is demonstrated in a case study that bias and fixed output in pitch sensors and stuck in pitch actuators. The simulation results show that the proposed method detects different realistic fault scenarios of wind turbines under the stochastic external winds.
H3: Offshore wind turbine foundation monitoring, extrapolating fatigue measurements from fleet leaders to the entire wind farm
Dr. Wout Weijtjens | Vrije Universiteit Brussel | Belgium
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Authors:
Dr. Wout Weijtjens | Vrije Universiteit Brussel | Belgium
Nymfa Noppe | Belgium
Tim Verbelen | Belgium
Alexandros Iliopoulos | Belgium
Christof Devriendt | Belgium
The present contribution is part of the ongoing development of a fatigue assessment strategy driven purely on in-situ measurements on operational wind turbines. The primary objective is to estimate the remaining life time of existing wind farms and individual turbines by instrumenting part of the farm with a load monitoring setup. This load monitoring setup allows to measure interface loads and local stress histories. This contribution will briefly discuss how these load measurements can be translated into fatigue assessment of the instrumented turbine. However, due to different conditions at the wind farm, such as turbulence, differences in water depth and foundation design this turbine will not be fully representable for all turbines in the farm. In this paper we will use the load measurements on two offshore wind turbines in the Northwind offshore wind farm to discuss fatigue progression in an operational wind farm. By calculating the damage equivalent loads (DELs) on the two turbines the fatigue progression is quantified for every 10 minute interval and can be analyzed against turbulence and site conditions . In future work these results will be used to predict the fatigue life progression in the entire farm.
H4: Assessment of weather downtime for the construction of offshore wind farm by using wind and wave simulations
Dr. Yuka Kikuchi | The University of Tokyo | Japan
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Authors:
Dr. Yuka Kikuchi | The University of Tokyo | Japan
Takeshi Ishihara | Japan
In this study, numerical simulations for winds and waves were carried out using WRF and WW3 and the predicted wind speed, wave height and wave period were validated with measurement. Annual average values of absolute monthly error of wind speed, wave height and wave period were 4.30 %, 12.3 % and 7.8 %. The prediction accuracy were improved by bias modification in the region of low wave height and short wave period. Predicted seasonal frequency distributions showed good agreement with measurements. The criteria of experienced construction methods were investigated at Choshi and Kitakyushu wind farm and the sensitivity of environmental conditions on weather downtime were clarified. At Choshi, the weather downtime was predicted by using wind and wave simulations and showed good agreement with the actual weather downtime.
H5: Integrated Layout and Support Structure Optimization for Offshore Wind Farm Design
Prof. Turaj Ashuri | The University of Texas at Dallas | United States
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Authors:
Prof. Turaj Ashuri | The University of Texas at Dallas | United States
Chandra Varma Ponnurangam | United States
Jie Zhang | United States
Mario Rotea | United States
This paper develops a multidisciplinary design optimization framework for integrated design optimization of offshore wind farm layout and support structure. A computational model is developed to characterize the physics of the wind farm wake, aerodynamics and hydrodynamics loads, response of the support structure to these loads, soil-structure interaction, as well as different cost elements. Levelized cost of energy is introduced as the objective function. The design constraints are the farm external boundary, and support structure buckling, first modal-frequency, fatigue damage and ultimate stresses. To evaluate the effectiveness of the proposed approach, four optimization scenarios are considered: a feasible baseline design, optimization of layout only, optimization of support structure only, and integrated design of the layout and support structure. Compared to the baseline design, the optimization results show that the isolated support structure design reduces the levelized cost of energy by 0.6%, the isolated layout design reduces the levelized cost of energy by 2.0%, and the integrated layout and support structure design reduces the levelized cost of energy by 2.6%.
H6: Dynamic Effects of Anchor Positional Tolerance on Tension Moored Floating Wind Turbine
Christopher Wright | UCC | Ireland
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Authors:
Christopher Wright | UCC | Ireland
Vikram Pakrashi | Ireland
Jimmy Murphy | Ireland
For water depths greater than 60m floating wind turbines will become the most economical option for generating offshore wind energy. Tension mooring stabilised units are one type of platform being considered by the offshore wind energy industry. The complex mooring arrangement used by this type of platform means that the dynamics are greatly effected by offsets in the positioning of the anchors. This paper examines the issue of tendon anchor position tolerances. The dynamic effects of three positional tolerances are analysed in survival state using the time domain FASTLink. The severe impact of worst case anchor positional offsets on platform and turbine survivability is shown. The worst anchor misposition combinations are highlighted and should be strongly avoided. Novel methods to mitigate this issue are presented.
H7: Design and performance analysis of control algorithm for a floating wind turbine on a large semi-submersible platform
Kwansu Kim | Kangwon National University | Korea, Republic of
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Authors:
Kwansu Kim | Kangwon National University | Korea, Republic of
hyun-gyu kim | Korea, Republic of
joong-hyeok lee | Korea, Republic of
seock-hyun kim | Korea, Republic of
byeong-hee kim | Korea, Republic of
Insu paek | Korea, Republic of
A control algorithm for a floating wind turbine installed on a large semi-submersible platform is investigated in this study. The floating wind turbine is different from other typical semi-submersible floating wind turbines in that the platform is so large that the platform motion is not affected by the blade pitch control. For simulation, the hydrodynamic forces data were obtained from ANSYS/AQWA, and implemented to Bladed. For the basic pitch controller, the well-known technique to increase damping by reducing the bandwidth of the controller lower than the platform pitch mode was implemented. Also, to reduce the tower load in the pitch control region, a tower damper based on the nacelle angular acceleration signal was designed. Compared with the results obtained from an onshore wind turbine controller applied to the floating wind turbine, the floating wind turbine controller could reduce the tower moments effectively, however, the standard deviation in power increased significantly.
H8: An Experimental Study on the Effects of Base Motions on the Aeromechanic Performance of Floating Wind Turbines
Hui HU | United States
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Authors:
Morteza Khosravi | Iowa State University | United States
Hui HU | United States
An experimental study was conducted to investigate the effects of the wave-induced base motions experienced by floating wind turbines sited in offshore wind farms on their aeromechanic performance and wake characteristics, in comparison with those of a bottom-fixed wind turbine. The experimental study was performed in a large-scale atmospheric boundary layer (ABL) wind tunnel with a scaled wind turbine model placed in a turbulent boundary layer flow with similar mean and turbulence characteristics as those over a typical offshore wind farm. The wind turbine model was mounted on a translational and rotational stage, which can generate combined translation and rotation motions to simulate the dynamic wave-induced motions (i.e., surge, pitch and heave motions) experienced by floating wind turbines in offshore wind farms. In addition to measuring dynamic wind loads (both forces and moments) and the power outputs of the model turbine, a high-resolution Particle Image Velocity (PIV) system was used to conduct detailed flow field measurements to characterize the turbulent wake flows behind the turbine model with the turbine base in either surge, or pitch, or heave or combined motions. The detailed flow field measurements were correlated with the dynamic wind loads and power output data to elucidate underlying physics for higher total power yield and better durability of floating wind turbines sited in offshore wind farms.
H9: Description of an 8 MW reference wind turbine
Christopher Wright | UCC | Ireland
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Authors:
Dr. Cian Desmond | University College Cork | Ireland
Jimmy Murphy | Ireland
Lindert Blonk
Wouter Haans | Ireland
Christopher Wright | UCC | Ireland
An 8 MW wind turbine is described in terms of mass distribution, dimensions, power curve, thrust curve, maximum design load and tower configuration. This turbine has been described as part of the EU FP7 project LEANWIND in order to facilitate research into logistics and naval architecture efficiencies for future offshore wind installations. The design of this 8 MW reference wind turbine has been checked and validated by the design consultancy DNV-GL.
J1: Comparing the Brushless DFIM to other Generator Systems for Wind Turbine Drive-Trains
Dr. Henk Polinder | Delft University of Technology | Netherlands
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Authors:
Dr. Henk Polinder | Delft University of Technology | Netherlands
Udai Shipurkar | Netherlands
Henk Polinder | Netherlands
Jan Ferreira | Netherlands
In this paper, the brushless DFIM based wind turbine drive-train topology is compared to the DFIG based and PM generator based drive-train topologies, that are most commonly applied in modern wind turbines. The comparison will be based on a 3.2MW case study wind turbine. By using FE based multi-objective optimization, optimized generator designs for the different topologies are generated. Then the capital expenditures of the resulting drive-train topologies are calculated and compared. Additionally, wind turbine drive-train configurations with 1, 2 and 3 stage gearboxes as well as a direct-drive configuration are taken into account. The resulting comparison shows that the brushless DFIM based drive-train with a 2 stage gearbox configuration provides a feasible alternative in commercial wind turbine drive-train applications.
J2: Non-ideal feedforward torque control of wind turbines: Impacts on annual energy production & gross earnings
Korbinian Schechner | Technische Universität München | Germany
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Authors:
Korbinian Schechner | Technische Universität München | Germany
Christoph Hackl | Germany
We discuss non-ideal torque control in wind turbine systems. Most high-level controllers generate a reference torque which is then send to the underlying electrical drive system (generator+inverter) of the wind turbine system to steer the turbine/generator to its optimal operation point (depending on the wind speed). The energy production heavily depends on the mechanical power (i.e. the product of rotational speed and generator torque). However, since torque sensors in the MW range are not available or extremely expensive, the underlying torque control system is implemented as feedforward control and, therefore, is inherently sensitive to parameter variations/uncertainties. Based on real wind data and a wind turbine system model, we discuss causes and impacts of non-ideal feedforward torque control on the energy production and the annual gross earnings.
J3: Encoderless Model Predictive Control of Doubly-Fed Induction Generators in Variable-Speed Wind Turbine Systems
Mohamed Abdelrahem | Technische Universität München | Germany
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Authors:
Mohamed Abdelrahem | Technische Universität München | Germany
Christoph Hackl | Germany
Ralph Kennel | Germany
In this paper, an encoderless finite-control-set model predictive control (FCS-MPC) strategy for doubly-fed induction generators (DFIGs) based on variable-speed wind turbine systems (WTSs) is proposed. According to the FCS-MPC concept, the discrete states of the power converter are taken into account and the future converter performance is predicted for each sampling period. Subsequently, the voltage vector that minimizes a predefined cost function is selected to be applied in the next sampling instant. Furthermore, a model reference adaptive system (MRAS) observer is used to estimate the rotor speed and position of the DFIG. Estimation and control performance of the proposed encoderless control method are validated by simulation results for all operation conditions. Moreover, the performance of the MRAS observer is tested under variations of the DFIG parameters.
J4: Analyzing wind turbine flow interaction through vibration and SCADA data.
Prof. Francesco Castellani | University of Perugia | Italy
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Authors:
Prof. Francesco Castellani | University of Perugia | Italy
Gianluca D'Elia | Italy
Davide Astolfi | Italy
Emiliano Mucchi | Italy
Giorgio Dalpiaz | Italy
Ludovico Terzi | Italy
Wind turbines commonly undergo non-stationary flow and, not rarely, even rather extreme phenomena. In particular, rough terrains represent a challenging testing ground, because the combination of terrain-driven flow and wakes might lead to far from trivial flux acceleration and turbulence structures. It is therefore crucial to assess the impact of dynamic loads on the turbines. In this work, tower and drive-train vibrations are analyzed, from a subcluster of four turbines of a wind farm sited in a very complex terrain. The main outcome of the study is that it is possible to start from the analysis of wind conditions and interpret how wakes manifest in the vibrations of the turbines, both at structural level (tower vibrations) and at the drive-train level. This “wind to gear†approach therefore allows to build a connection between a flow phenomenon and a mechanical phenomenon and can be very precious to assess loads in different working conditions.
J5: Comparison of Life Calculations for Oscillating Bearings Considering Individual Pitch Control in Wind Turbines
Fabian Schwack | Leibniz Universität Hannover - Institute for Machine Design and Tribology | Germany
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Authors:
Fabian Schwack | Leibniz Universität Hannover - Institute for Machine Design and Tribology | Germany
Matthias Stammler | Germany
Gerhard Poll | Germany
Andreas Reuter | Germany
The fatigue life calculation of bearings under rotating conditions has been well researched and standardized. In contrast, for bearings in oscillating applications no international standards exist. As a result, pitch bearings in wind turbines are designed with different, non standardized approaches. Furthermore, the impact of individual pitch control on pitch bearings has not yet been studied. In this paper four approaches for fatigue life calculation will be applied and compared under individual pitch control conditions. For comparison, the loads and the bearing geometry of the reference turbine IWT 7.5 MW, which is individual pitch controlled, are used. This paper will show how the bearing life calculated by different approaches reacts to individual pitch control conditions. Furthermore, the factors for the modified rating life, according to the ABMA and ISO standards, calculated for the given loads and the given bearing geometry in oscillating applications.
J6: Drivetrain load effects in a 5 MW bottom-fixed wind turbine under blade-pitch fault condition and emergency shutdown
Dr. Amir R. Nejad | Norwegian University of Science and Technology | Norway
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Authors:
Dr. Amir R. Nejad | Norwegian University of Science and Technology | Norway
Zhiyu Jiang | Norway
Zhen Gao | Norway
Torgeir Moan | Norway
In this paper, the effect of the blade-pitch fault and emergency shutdown on drivetrain responses in a 5-MW bottom-fixed wind turbine are investigated. A 5-MW reference gearbox with 4-point support is employed and the decoupled analysis approach is used for the load effect analysis. The effect of this fault event is then investigated for all bearings and gears inside the gearbox as well as main bearings. The results show that the blade-pitch fault creates significant axial forces on main bearings which increases the nontorque force entering the gearbox. Due to the emergency shutdown, the rotor torque reversal occurs which causes force reversals in gears. The main bearings are more affected than gears and bearings inside the gearbox in this fault condition and emergency shutdown, but first-stage bearings may also be considerably affected. It is therefore recommended to conduct a thorough inspection of main bearings and first stage bearings in case of such blade-pitch fault condition and emergency shutdown.
J7: Statistical study of the effect of wind characteristics on wind turbine main shaft loadings
Dr. Sho Oh | ClassNK | Japan
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Author:
Dr. Sho Oh | ClassNK | Japan
The dynamic loadings of the wind turbine main shafts are important for drivetrain components as external excitation force, and the evaluation of their dependence on wind characteristics is necessary for both the understanding of the drivetrain bahavior and the extrapolation of the loadings at different sites. In this study, the load measurements of the wind turbine main shafts were performed along with the wind field measurement. Next the multivariate regression analysis was utilized to identify the influential wind parameters that affect the statistics of the dynamic loadings of the shaft. Finally, the dependence of the load statistics on the identified wind parameters was evaluated qualitatively using the observed data.
J8: Speed and Torque Control Strategies for Loss Reduction of Vertical Axis Wind Turbines
Michael Argent | University of Strathclyde | United Kingdom
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Authors:
Michael Argent | University of Strathclyde | United Kingdom
Alasdair McDonald | United Kingdom
Bill Leithead | United Kingdom
Alexander Giles | United Kingdom
This paper builds on the work into modelling the generator losses of Vertical Axis Wind Turbines that result from the intrinsic torque cycling of the VAWT rotor. In particular this paper investigates the aerodynamic inefficiencies caused by varying rotational speed arising from different torque control strategies to the cyclic torque and how this impacts on generator losses. These strategies can be fixed torque, fixed speed, or one from a range of strategies between these two extremes. The resulting effect of these strategies are evaluated by modelling the wake that builds up from the rotation of the VAWT rotor to investigate how the wake responds to a changing rotor speed and how this in turn affects the torque produced by the blades. This in turn allows for the evaluation the corresponding change in generator losses and any changes to the energy extracted by the wind turbine rotor, which can then be used in the process of optimising the generator design for the VAWT.
J9: Efficient operation of anisotropic synchronous machines for wind energy systems
Julian Kullick | Technische Universität München | Germany
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Authors:
Julian Kullick | Technische Universität München | Germany
Christoph Hackl | Germany
Hisham Eldeeb | Germany
This paper presents an analytical solution for the Maximum-Torque-per-Ampere (MTPA) operation of synchronous machines (SM) with anisotropy and magnetic cross-coupling for the application in wind turbine systems and airborne wind energy systems. For a given reference torque, the analytical MTPA solution provides the optimal stator current references which produce the desired torque while minimizing the stator copper losses. From an implementation point of view, the proposed analytical method is appealing in terms of its fast online computation (compared to classical numerical methods) and its efficiency enhancement of the electrical drive system. The efficiency of the analytical MTPA operation, with and without consideration of cross-coupling, is compared to the conventional method with zero direct current.
J10: Variation of Extreme and Fatigue Design Loads on the Main Bearing of a Front Mounted Direct Drive System
Dr. Asger Bech Abrahamsen | Technical University of Denmark | Denmark
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Authors:
Dr. Asger Bech Abrahamsen | Technical University of Denmark | Denmark
Anand Natarajan | Denmark
The drivetrain of a 10 MW wind turbine has been designed as a direct drive transmission with a superconducting generator mounted in front of the hub and connected to the main frame through a King-pin stiff assembly. The aeroelastic design loads of such an arrangement are evaluated based on the thrust and bending moments at the main bearing, both for ultimate design and in fatigue. It is found that the initial superconductor generator weight of 363 tons must be reduced to 75% in order not to result in higher extreme loads on main and yaw bearing than the 10 MW geared reference drive train. A weight reduction of 50% is needed in order to provide main bearing fatigue loads equivalent to the reference drive train. Thus a target mass of front mounted superconducting direct drive generators is found to be between 183-272 tons.