Wind Turbine Structural Loads Research Articles

A time‐varying formulation of the curled wake model within the FAST.Farm framework

AbstractIn this article, we present a time‐varying formulation of the curled wake model that we implemented in FAST.Farm. The curled wake model, originally developed for steady‐state conditions, is used to produce realistic wake profiles behind a wind turbine in yawed (or skewed) conditions. We begin by introducing the key elements of the FAST.Farm framework. Then, after briefly summarizing the original wake dynamics formulation of FAST.Farm based on a polar wake profile, we present the new time‐varying formulation of the curled wake model, compare the two, and highlight the differences with the original curled wake model. After discussing some implementation details, we present different applications with increasing levels of complexity: single turbine with uniform and turbulent inflow, fixed and transient yaw, and multiple turbines. We verify our results using the original FAST.Farm implementation and large‐eddy simulations. The results with the new curled wake model are improved compared to the original implementation, as they include cross‐flow velocities and wake asymmetry. Yet, large‐eddy simulation results show a more pronounced lateral convection of the wake and a stronger concentration of vorticity at the top vortex. The new curled wake implementation in FAST.Farm should enable the calculation of not only generator power but also wind turbine structural loads for applications involving intentional or unintentional skewed flow and wind‐farm control involving wake steering.

A novel multi-dimensional reliability approach for floating wind turbines under power production conditions

Floating offshore wind turbines (FOWT) generate green renewable energy and are a vital part of the modern offshore wind energy industry. Robust predicting extreme offshore loads during FOWT operations is an important safety concern. Excessive structural bending moments may occur during certain sea conditions, posing an operational risk of structural damage. This paper uses the FAST code to analyze offshore wind turbine structural loads due to environmental loads acting on a specific FOWT under actual local environmental conditions. The work proposes a unique Gaidai-Fu-Xing structural reliability approach that is probably best suited for multi-dimensional structural responses that have been simulated or measured over a long period to produce relatively large ergodic time series. In the context of numerical simulation, unlike existing reliability approaches, the novel methodology does not need to re-start simulation again each time the system fails. As shown in this work, an accurate forecast of the probability of system failure can be made using measured structural response. Furthermore, traditional reliability techniques cannot effectively deal with large dimensionality systems and cross-correction across multiple dimensions. The paper aims to establish a state-of-the-art method for extracting essential information concerning extreme responses of the FOWT through simulated time-history data. Three key components of structural loads are analyzed, including the blade-root out-of-plane bending moment, tower fore-aft bending moment, and mooring line tension. The approach suggested in this study allows predicting failure probability efficiently for a non-linear multi-dimensional dynamic system as a whole.

A coupled numerical framework for hybrid floating offshore wind turbine and oscillating water column wave energy converters

Integrating floating offshore wind turbines with oscillating-water-column wave energy converters has been seen as a promising solution for hybrid offshore renewable energy production, as the cost-effective wave energy devices could possibly help increase the overall power absorption, reduce platform dynamic responses, and mitigate loads for critical wind turbine structures etc. As most existing research works on dynamic analysis of these hybrid concepts are based on frequency-domain simulations or scale model experiments, this work focuses on establishing an aero-hydro-elastic-servo-mooring coupled numerical framework for integrated time-domain dynamic analysis. In particular, the water column dynamics are characterised based on an equivalent virtual oscillating body approach so that the time-domain analysis capability for oscillating-water-columns with power take-off control is enabled. For validation, a novel combined concept is designed, and its time-domain numerical results under various environmental conditions have been compared against the 1:50 scale model wave basin test data. Good agreement has been observed between the numerical and experimental results, demonstrating the feasibility of the proposed numerical framework. Furthermore, different power take-off control strategies for the oscillating-water-column wave energy converters have been proposed, and it is found that the designed gain-scheduling control schemes are more beneficial for mitigating the platform motion responses and wind turbine structural loads compared with traditional linear damping control, resulting in 15% platform pitch motion mitigation and 6% tower base fatigue load reduction. Further studies on multi-objective optimal power take-off control design regarding both load reduction and power maximisation could be conducted for hybrid energy platforms based on the established numerical framework.

Individual Pitch Control of Wind Turbines Based on SVM Load Estimation and LIDAR Measurement

With the development of wind turbines to the large scale with high rated powers, how to reduce fatigue loads is the key factor for modern wind turbine control. Aiming at reducing the wind turbine structural fatigue loads, an individual pitch control (IPC) method based on SVM estimation of blade root loads was proposed in this paper. The blade root loads were dynamically estimated by a SVM model according to light detection and ranging (LIDAR) wind speed data. Two approximate decoupling systems for yaw moments and pitch moments were then obtained through Coleman transform. Meanwhile, two individual pitch controllers were designed to reduce the loads and a wind speed feedforward controller was adopted to improve the pitch control performance. The proposed method was verified by a FAST, LIDAR and RT-LAB semi-physical real-time combined simulation system. On this basis, a master control unit was developed and the overall unit was further verified by a CRRC wind turbine master control and pitch joint test platform. Compared with collective pitch control (CPC) and the IPC based on azimuth, the proposed method is effective on reducing fatigue loads and vibration.

Simulation of an airfoil with a deformable flap applicable in wind turbine structural load reduction

Flow over an airfoil equipped with Deformable Trailing Edge Flap (DTEF) has been numerically studied in a two-dimensional steady-state condition with various angles of attack. The airfoil is NACA 64-418, and the flap angle is defined by changing camber-line geometry at 10% chord length from the trailing edge. It has been shown that the direction of the flap deflection has significant impacts on aerodynamic behaviors, which provides an extra means to adjust wind turbine structural loads. Simulations have been conducted with aerodynamic-aeroelastic FAST code in the form of an open-loop control scheme to determine the DTEF's performance in a wind turbine. The wind turbine behavior has been plotted and compared for various flap angles. The load-variation ranges of the wind turbine have been identified, which help determine their sensitivity to flap changes. Finally, an open-loop control circuit is aimed at reducing the amplitude of oscillations of the blade root flapwise bending moment.

DTUWEC: an open-source DTU Wind Energy Controller with advanced industrial features

An open-source wind turbine controller, DTU Wind Energy Controller (DTUWEC), developed by Technical University of Denmark (DTU), is presented. The DTUWEC includes many advanced features and it is publicly available to the wind energy community. It is envisioned that this research work converts the old version of Basic DTU Wind Energy Controller into a generic, powerful and robust open-source wind turbine controller with a large number of developers and users in both research institutions and industries. DTUWEC is implemented in Fortran and offers the DNV-GL Bladed/OpenFAST style interface. Because of its open-source and modular characters, researchers are able to couple DTUWEC with different aero-elastic simulation codes to investigate the effect of different controllers on the wind turbine structural loads and power output, and to collaborate for future potential improvements.

Platform stabilization and load reduction of floating offshore wind turbines with tension‐leg platform using dynamic vibration absorbers

AbstractFloating offshore wind turbines (FOWT) are subject to significant increases in structural loads due to the platform motion under turbulent wind and wave. The under‐actuation challenge in FOWT control demands for development of extra actuators for platform stabilization. For FOWT with tension‐leg platform (TLP), this paper presents a comprehensive study on design and control simulation for realizing active mooring line control via the deployment of vertically operated dynamic vibration absorbers (DVAs) at the spokes of TLP structure. The DVA is designed based on the suppression of the primary modes of platform pitch and roll motion. In addition to the enhancement of FAST‐based simulation module, an 11 degrees‐of‐freedom (DOFs) control‐oriented model is derived for the TLP‐FOWT‐DVA system. Based on the control‐oriented model, a linear quadratic regulator (LQR) controller is designed. Simulations are performed for 9 m/s and 18 m/s turbulent winds with different wind and wave directions. The wind turbine performance, platform motions, and structural fatigue loads are evaluated. The results show that the platform motion and tower loads in the lateral direction are significantly reduced, while the tower load in the fore‐aft direction can be moderately reduced. Also, significant reduction in the mooring line tension loads is observed. For achieving the performance in platform motion stabilization and load reduction, the average power consumption of the DVA actuators is less than 0.27% of the wind turbine power generated during the simulated periods. The figures of merits promise significant potential for the feasibility of DVA based control for TLP‐FOWT.

A novel method to quantify atmospheric stability

SummaryAtmospheric stability significantly influences the characteristics of a wind resource and strongly affects wind turbine power production and structural loads. Stability is governed by the thermal structure of the atmospheric boundary layer (ABL). Unstable ABLs are convective and increase turbulence, but reduce vertical wind shear, while stable ABLs reduce turbulence and increase wind shear. In neutral ABLs mechanical effects of terrain and roughness dominate.The Monin-Obukhov length (MOL) and Richardson numbers are the recommended methods to quantify atmospheric stability. These methods require advanced and expensive measurements of temperature differences or 3D-sonic covariances, not installed on standard wind power masts.This study presents a novel methodology to quantify stability based only on standard wind measurements. At high wind speeds turbulence and wind shear converge to the neutral ABL response in a given direction while the relative deviations of shear and turbulence at lower wind speeds strongly correlate with stability. When normalized by the neutral shear and turbulence, these deviations directly quantify stability. Here we present a novel method to estimate MOL directly from the ratio of normalized shear and turbulence.The proposed method to quantify stability from standard wind measurements, provides an important input to advanced wind flow modelling of stability effects to improve the accuracy of predicted power production and structural loads.

Improved inertial control for permanent magnet synchronous generator wind turbine generators

With increasing integrations of large-scale systems based on permanent magnet synchronous generator wind turbine generators (PMSG-WTGs), the overall inertial response of a power system will tend to deteriorate as a result of the decoupling of rotor speed and grid frequency through the power converter as well as the scheduled retirement of conventional synchronous generators. Thus, PMSG-WTGs can provide value to an electric grid by contributing to the system's inertial response through the inherent kinetic energy stored in their rotating masses and fast power converter control. In this study, an improved inertial control method based on the maximum power point tracking operation curve is introduced to enhance the overall frequency support capability of PMSG-WTGs in the case of large supply–demand imbalances. Moreover, this method is implemented in the CART2-PMSG integrated model in MATLAB/Simulink to investigate its impact on the wind turbine's structural loads during the inertial response process. Simulation results indicate that the proposed method can effectively reduce the frequency nadir, arrest the rate of change of frequency, and alleviate the secondary frequency dip while imposing no negative impact on the major mechanical components of the wind turbine.

Advanced control algorithms for reduction of wind turbine structural loads

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

Unbalanced voltage faults: the impact on structural loads of doubly fed asynchronous generator wind turbines

This paper investigates the impact that unbalanced voltage faults have on wind turbine structural loads. In such cases, electromagnetic torque oscillations occur at two times the supply voltage frequency. The objectives of this work are to quantify wind turbine structural loads induced by unbalanced voltage faults relative to those during normal operation; and to evaluate the potential for reducing structural loads with the control of the generator. The method applied is integrated dynamic analysis. Namely, dynamic analysis with models that consider the most important aeroelastic, electrical, and control dynamics in an integrated simulation environment based on an aeroelastic code (HAWC2) and software for control design (Matlab/Simulink). In the present analysis, 1 Hz equivalent loads are used to compare fatigue loads, whereas maximum–minimum values are used to compare extreme loads. A control concept based on resonant filters demonstrates reduction of the structural loads (shaft torsion and tower top side-to-side moment) induced by an unbalanced voltage fault.Copyright © 2013 John Wiley & Sons, Ltd.

Impact of fault ride‐through requirements on fixed‐speed wind turbine structural loads

AbstractThe emphasis in this article is on the impact of fault ride‐through requirements on wind turbines structural loads. Nowadays, this aspect is a matter of high priority as wind turbines are required more and more to act as active components in the grid, i.e. to support the grid even during grid faults.This article proposes a computer approach for the quantification of the wind turbines structural loads caused by the fault ride‐through grid requirements. This approach, exemplified for the case of a 2MW active stall wind turbine, relies on the combination of knowledge from complimentary simulation tools, which have expertise in different specialized wind turbines design areas. Two complimentary simulation tools are considered i.e. the detailed power system simulation tool PowerFactory from DIgSILENT and the advanced aeroelastic computer code HAWC2, in order to assess of the dynamic response of wind turbines to grid faults. These two tools are coupled sequently in an offline approach, in order to achieve a thorough insight both into the structural as well as the electrical wind turbine response during grid faults.The impact of grid requirements on wind turbines structural loads is quantified by performing a rainflow and a statistical analysis for fatigue and ultimate structural loads, respectively. Two cases are compared i.e. one where the turbine is immediately disconnected from the grid when a grid fault occurs and one where the turbine is equipped with a fault ride‐through controller and therefore it is able to remain connected to the grid during the grid fault. Copyright copy; 2010 John Wiley & Sons, Ltd.