Browsing by Author "Cheng, Po Wen"

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Open Access
Acoustic and seismic emissions from wind turbines
(2017) Calarco, Francesca; Cheng, Po Wen; Zieger, Toni; Ritter, Joachim
With regards to the interdisciplinary “TremAc” Project funded by the German Federal Ministry for Economic Affairs and Energy, this paper examines acoustic and seismic emissions generated by wind turbines with the aim of identifying a better understanding of their interaction. Measurement campaigns will be carried out in the field around a single wind turbine plant and results in terms of acoustic and seismic signals will be correlated and then evaluated in relation to environmental factors such as wind speed, wind direction and temperature as well as to data related to the wind turbines-specifications (e.g. rotation speed).
Open Access
An adaptive data processing technique for lidar-assisted control to bridge the gap between lidar systems and wind turbines
(2015) Schlipf, David; Fleming, Paul; Raach, Steffen; Scholbrock, Andrew; Haizmann, Florian; Krishnamurthy, Raghu; Boquet, Matthieu; Cheng, Po Wen
This paper presents first steps toward an adaptive lidar data processing technique crucial for lidar-assisted control in wind turbines. The prediction time and the quality of the wind preview from lidar measurements depend on several factors and are not constant. If the data processing is not continually adjusted, the benefit of lidar-assisted control cannot be fully exploited or can even result in harmful control action. An online analysis of the lidar and turbine data is necessary to continually reassess the prediction time and lidar data quality. In this work, a structured process to develop an analysis tool for the prediction time and a new hardware setup for lidar-assisted control are presented. The tool consists of an online estimation of the rotor effective wind speed from lidar and turbine data and the implementation of an online cross-correlation to determine the time shift between both signals. Further, we present initial results from an ongoing campaign in which this system was employed for providing lidar preview for feedforward pitch control.
Open Access
Adaptive Vorsteuerung für Windenergieanlagen
(2013) Schlipf, David; Cheng, Po Wen
Der Beitrag beschreibt, wie Windmessungen mit LIDAR in einer Vorsteuerung verwendet werden können, um die Drehzahlschwankungen und damit die Belastungen von Windenergieanlagen zu reduzieren. Kernstück dieser Vorsteuerung ist ein Filter, der adaptiv auf die aktuellen Messungen eingestellt werden muss, da sich die Prädiktionszeit und die Korrelation zwischen Vorhersage und Anlagenverhalten kontinuierlich ändern. Die Ergebnisse werden mit Messdaten einer 5MW Anlage validiert.
Open Access
Analysis of floating offshore wind turbine hydrodynamics using coupled CFD and multibody methods
(2013) Beyer, Friedemann; Arnold, Matthias; Cheng, Po Wen
The focus of this study is the application of a higher order hydrodynamic modeling technique for the analysis of Floating Offshore Wind Turbine dynamics. This approach is based on a coupling between Multibody and Computational Fluid Dynamics methods. Results of the translational and rotational platform displacement are presented for a basic free-decay simulation in surge direction in still water. A comparison to linear hydrodynamics is presented. Additional, pressure mapping is demonstrated.
Open Access
Application of a Monte Carlo procedure for probabilistic fatigue design of floating offshore wind turbines
(2018) Müller, Kolja; Cheng, Po Wen
Fatigue load assessment of floating offshore wind turbines poses new challenges on the feasibility of numerical procedures. Due to the increased sensitivity of the considered system with respect to the environmental conditions from wind and ocean, the application of common procedures used for fixed-bottom structures results in either inaccurate simulation results or hard-to-quantify conservatism in the system design. Monte Carlo-based sampling procedures provide a more realistic approach to deal with the large variation in the environmental conditions, although basic randomization has shown slow convergence. Specialized sampling methods allow efficient coverage of the complete design space, resulting in faster convergence and hence a reduced number of required simulations. In this study, a quasi-random sampling approach based on Sobol sequences is applied to select representative events for the determination of the lifetime damage. This is calculated applying Monte Carlo integration, using subsets of a resulting total of 16 200 coupled time-domain simulations performed with the simulation code FAST. The considered system is the Danmarks Tekniske Universitet (DTU) 10 MW reference turbine installed on the LIFES50+ OO-Star Wind Floater Semi 10 MW floating platform. Statistical properties of the considered environmental parameters (i.e., wind speed, wave height and wave period) are determined based on the measurement data from the Gulf of Maine, USA. Convergence analyses show that it is sufficient to perform around 200 simulations in order to reach less than 10 % uncertainty of lifetime fatigue damage-equivalent loading. Complementary in-depth investigation is performed, focusing on the load sensitivity and the impact of outliers (i.e., values far away from the mean). Recommendations for the implementation of the proposed methodology in the design process are also provided.
Open Access
Collective pitch feedforward control of floating wind turbines using lidar
(2015) Schlipf, David; Simley, Eric; Lemmer, Frank; Pao, Lucy; Cheng, Po Wen
In this work a collective pitch feedforward controller for floating wind turbines is presented. The feedforward controller provides a pitch rate update to a conventional feedback controller based on a wind speed preview. The controller is designed similar to the one for onshore turbines, which has proven its capability to improve wind turbine control performance in field tests. In a first design step, perfect wind preview and a calm sea is assumed. Under these assumptions the feedforward controller is able to compensate almost perfectly the effect of changing wind speed to the rotor speed of a full nonlinear model over the entire full load region. In a second step, a nacelle-based lidar is simulated scanning the same wind field which is used also for the aero-hydro-servo-elastic simulation. With model-based wind field reconstruction methods, the rotor effective wind speed is estimated from the raw lidar data and is used in the feedforward controller after filtering out the uncorrelated frequencies. Simulation results show that even with a more realistic wind preview, the feedforward controller is able to significantly reduce rotor speed and power variations. Furthermore, structural loads on the tower, rotor shaft, and blades are decreased. A comparison to a theoretical investigation shows that the reduction in rotor speed regulation is close to the optimum.
Open Access
Comparison of feedforward and model predictive control of wind turbines using LIDAR
(2012) Schlipf, David; Pao, Lucy Y.; Cheng, Po Wen
LIDAR systems are able to provide preview information of wind disturbances at various distances in front of wind turbines. This technology paves the way for new control concepts such as feedforward control and model predictive control. This paper compares a nonlinear model predictive controller and a feedforward controller to a baseline controller. Realistic wind "measurements" are obtained using a detailed simulation of a LIDAR system. A full lifetime comparison shows the advantages of using the wind predictions to reduce wind turbine fatigue loads on the tower and blades as well as to limit the blade pitch rates. The results illustrate that the feedforward controller can be combined with a tower feedback controller to yield similar load reductions as the model predictive controller.
Open Access
Comparison of linear and nonlinear model predictive control of wind turbines using LIDAR
(2014) Schlipf, David; Grau, Patrick; Raach, Steffen; Duraiski, Ricardo; Trierweiler, Jorge; Cheng, Po Wen
Recent developments in remote sensing are offering a promising opportunity to rethink conventional control strategies of wind turbines. With technologies such as LIDAR, the information about the incoming wind field - the main disturbance to the system - can be made available ahead of time. Feedforward control can be easily combined with traditional collective pitch feedback controllers and has been successfully tested on real systems. Nonlinear model predictive controllers adjusting both collective pitch and generator torque can further reduce structural loads in simulations but have higher computational times compared to feedforward or linear model predictive controller. This paper compares a linear and a commercial nonlinear model predictive controller to a baseline controller. On the one hand simulations show that both controller have significant improvements if used along with the preview of the rotor effective wind speed. On the other hand the nonlinear model predictive controller can achieve better results compared to the linear model close to the rated wind speed.
Open Access
Comparison of measured and simulated structural loads of an offshore wind turbine at Alpha Ventus
(2016) Müller, Kolja; Reiber, Mario; Cheng, Po Wen
A comparison of fatigue and extreme loads from simulations with full-scale measurements collected over a period of ten months in the offshore test field, Alpha Ventus, is presented in this paper. There are two goals of this study: (1) to check if the measured range of fatigue and extreme loads can be captured correctly by simulations when the variations of relevant environmental parameters are taken into account; and (2) to investigate if measured extreme loads can be reproduced by simulations when ten-minute averages of the environmental parameters are used. The results show a good overall match of loads when the variation of environmental parameters is considered but an insufficient match when the events of maximum load occurrence are compared.
Open Access
Conceptual design of floating wind turbines
(2013) Sandner, Frank; Cheng, Po Wen
The need for different numerical models with varying degrees of simplification for the conceptual design of a floating offshore wind turbine is the focus of this paper. While parts on the component level can be designed apart from the others the overall dynamics on the system level have to be assessed from the beginning. Starting with very simple models and identifying the significant contributions to the system behavior while going step by step to more detailed ones makes a successful dimensioning possible. The significant effect of the blade pitch controller on the system dynamics is analysed and preliminarily designed with a simple 1-degree of freedom (dof) model. Further on the section forces at tower base and the distributed platform loads are calculated with a 9-dof multibody system with simplified aerodynamics and Morison equation allowing a pre-dimensioning of the structure.
Open Access
Control design methods for floating wind turbines for optimal disturbance rejection
(2016) Lemmer, Frank; Schlipf, David; Cheng, Po Wen
An analysis of the floating wind turbine as a multi-input-multi-output system investigating the effect of the control inputs on the system outputs is shown. These effects are compared to the ones of the disturbances from wind and waves in order to give insights for the selection of the control layout. The frequencies with the largest impact on the outputs due to the limited effect of the controlled variables are identified. Finally, an optimal controller is designed as a benchmark and compared to a conventional~PI-controller using only the rotor speed as input. Here, the previously found system properties, especially the difficulties to damp responses to wave excitation, are confirmed and verified through a spectral analysis with realistic environmental conditions. This comparison also assesses the quality of the employed simplified linear simulation model compared to the nonlinear model and shows that such an efficient frequency-domain evaluation for control design is feasible.
Open Access
Correlation-model of rotor-effective wind shears and wind speed for lidar-based individual pitch control
(2015) Haizmann, Florian; Schlipf, David; Cheng, Po Wen
In this work the spectra based model of the correlation between lidar systems and wind turbines is extended from rotor-effective wind speed only, to rotor-effective wind speed and linear horizontal and vertical shear components. This is achieved by the incorporation of a model based wind field reconstruction method solving a set of linear equations with the least-squares method. The model allows to optimize a lidar system’s measurement configuration for a specific wind turbine a-priori by means of direct and fast spectra calculations. Furthermore, it allows to assess the filter parameters to be expected and needed for the application of lidar-assisted control. By extending the model to rotor-effective linear shears, the results can be used for lidar-assisted individual pitch control.
Open Access
Coupled MBS-CFD simulation of the IDEOL floating offshore wind turbine foundation compared to wave tank model test data
(2015) Beyer, Friedemann; Choisnet, Thomas; Kretschmer, Matthias; Cheng, Po Wen
A two MW floating offshore wind turbine is currently developed within the EU-FP7 project FLOATGEN. A wave tank test of the floater model at 1/32th scale has been performed in extreme wave conditions. In the present study numerical calculations of the floating foundation with regular waves using coupled MBS-CFD methods are compared to experimental data enabling a validation. Results of the wave elevation, floater motion and mooring line tension show a very good correlation. Flow phenomena like vortex shedding at the hull of the floater are shown. The presented methodology provides detailed knowledge allowing analysis of wave impact and resulting load assessment of floating offshore structures.
Open Access
Design and evaluation of a lidar-based feedforward controller for the INNWIND.EU 10 MW wind turbine
(2015) Fürst, Holger; Schlipf, David; Iribas Latour, Mikel; Cheng, Po Wen
For the development of the next generation of multi megawatt wind turbines, advanced control concepts are one of the major tasks. Reduction of fatigue and extreme loading could help to improve the overall design process and make plants more cost effective. This work deals with the application of the promising methodology of feedforward control using nacelle-based lidar sensor measurements on a 10 MW wind turbine concept. After lidar data processing has been described, the feedforward controller is designed such that disturbances from the changing wind speed to the generator speed are compensated by adding an update to the collective pitch rate signal of the normal feedback controller. The evaluation of the feedforward controller is done in two steps: Firstly, simulations using perfect lidar data measurements are applied to check the robustness of the controller against model uncertainties. After that, simulations with realistic lidar measurements are investigated. To improve control performance, the scanning configuration of the used lidar system is optimized. Over all it can be shown that lidar-assisted control leads to significant load reductions, especially in the full load region of the 10 MW turbine.
Open Access
Detection of wind evolution and lidar trajectory optimization for lidar-assisted wind turbine control
(2015) Schlipf, David; Haizmann, Florian; Cosack, Nicolai; Siebers, Tom; Cheng, Po Wen
In this work a collective pitch feedforward controller for floating wind turbines is presented. The feedforward controller provides a pitch rate update to a conventional feedback controller based on a wind speed preview. The controller is designed similar to the one for onshore turbines, which has proven its capability to improve wind turbine control performance in field tests. In a first design step, perfect wind preview and a calm sea is assumed. Under these assumptions the feedforward controller is able to compensate almost perfectly the effect of changing wind speed to the rotor speed of a full nonlinear model over the entire full load region. In a second step, a nacelle-based lidar is simulated scanning the same wind field which is used also for the aero-hydro-servo-elastic simulation. With model-based wind field reconstruction methods, the rotor effective wind speed is estimated from the raw lidar data and is used in the feedforward controller after filtering out the uncorrelated frequencies. Simulation results show that even with a more realistic wind preview, the feedforward controller is able to significantly reduce rotor speed and power variations. Furthermore, structural loads on the tower, rotor shaft, and blades are decreased. A comparison to a theoretical investigation shows that the reduction in rotor speed regulation is close to the optimum.
Open Access
Determination of stationary and dynamical power curves using a nacelle-based lidar system
(2012) Würth, Ines; Rettenmeier, Andreas; Schlipf, David; Cheng, Po Wen; Wächter, Matthias; Rinn, Philip; Peinke, Joachim
This paper investigates the determination of stationary and dynamical power curves using a nacelle-based lidar system. Wind speed measurements on one of the REpower 5MW turbines at the German offshore test site "alpha ventus" were carried out with a pulsed lidar system that is capable of measuring the wind field at different measurement planes over the rotor swept area. The results show that the stationary lidar-based power curve has a small scatter but is shifted towards lower wind speeds compared to a conventional power curve measured with a cup anemometer from a met mast. The new approach of calculating dynamical power curves shows short-time dynamics of the turbine and allows a quick detection of changes such as the icing of an anemometer or the reduction in the maximum power output of the wind turbine.
Open Access
Direct speed control using LIDAR and turbine data
(2013) Schlipf, David; Fleming, Paul; Kapp, Stefan; Scholbrock, Andrew; Haizmann, Florian; Belen, Fred; Wright, Alan; Cheng, Po Wen
LIDAR systems are able to provide preview information of the wind speed in front of wind turbines. One proposed use of this information is to increase the energy capture of the turbine by adjusting the rotor speed directly to maintain operation at the optimal tip-speed ratio, a technique referred to as Direct Speed Control (DSC). Previous work has indicated that for large turbines the marginal benefit of the direct speed controller in terms of increased power does not compensate for the increase of the shaft loads. However, the technique has not yet been adequately tested to make this determination conclusively. Further, it is possible that applying DSC to smaller turbines could be worthwhile because of the higher rotor speed fluctuations and the small rotor inertia. This paper extends the previous work on direct speed controllers. A DSC is developed for a 600 kW experimental turbine and is evaluated theoretically and in simulation. Because the actual turbine has a mounted LIDAR, data collected from the turbine and LIDAR during operation are used to perform a hybrid simulation. This technique allows a realistic simulation to be performed, which provides good agreement with theoretical predictions.
Open Access
Down-regulation and individual blade control as lifetime extension enablers
(2018) Pettas, Vasilis; Cheng, Po Wen
As more and more wind turbines are coming close to the end of their design lifetime, evaluation of end of life strategies is becoming highly relevant. Moreover, as turbine technology matures and wind farms grow larger, lifetime extension becomes a financially attractive option compared to re-powering and decommissioning. Present work suggests control strategies, namely down-regulation and individual blade control, as lifetime extension enablers. The concept of using them as retrofit control implementations is explained. Their individual and combined potential in fatigue load reduction is evaluated, along with their effect on other performance and pitch system metrics. Finally, the possible period of extension, beyond the nominal 20 years, is evaluated in an example case where the retrofit control strategy is applied after 15 years of baseline operation. The aeroelastic simulations are performed with a 10 MW reference wind turbine, according to load certification standards. Results show that the two methods complement each other in load alleviation. The pitch actuator demands are also significantly decreased when the two methods are combined.
Open Access
FAST.Farm load validation for single wake situations at alpha ventus
(2021) Kretschmer, Matthias; Jonkman, Jason; Pettas, Vasilis; Cheng, Po Wen
The main objective of the presented work is the validation of the simulation tool FAST.Farm for the calculation of power and structural loads in single wake situations; the basis for the validation is the measurement database of the operating offshore wind farm alpha ventus. The approach is described in detail and covers the calibration of the aeroelastic turbine model, transfer of environmental conditions to simulations, and comparison between simulations and adequately filtered measurements. It is shown that FAST.Farm accurately predicts power and structural load distributions over wind direction with discrepancies of less than 10 % for most of the cases compared to the measurements. Additionally, the frequency response of the structure is investigated, and it is calculated by FAST.Farm in good agreement with the measurements. In general, the calculation of fatigue loads is improved with a wake-added turbulence model added to FAST.Farm in the course of this study.
Open Access
Field testing of feedforward collective pitch control on the CART2 using a nacelle-based lidar scanner
(2012) Schlipf, David; Fleming, Paul; Haizmann, Florian; Scholbrock, Andrew; Hofsäß, Martin; Wright, Alan; Cheng, Po Wen
This work presents the first results from a field test to proof the concept of LIDAR assisted collective pitch control using a scanning LIDAR device installed on the nacelle of a research turbine. The purpose of the campaign was to show that a reduction of rotor speed variation is feasible with a feedforward update without changing the feedback controller. Although only a small amount of data could be collected, positive effects can be observed not only on the rotor speed but also on tower, blade and shaft loads in the case that the correlation of the wind preview and the turbine reaction is taken into account.