Certification Of Wind Turbines Research Articles

Deep learning predictions of unsteady aerodynamic loads on an airfoil model pitched over the entire operating range

For the design and certification of wind turbines, it is essential to provide fast and accurate unsteady aerodynamic load prediction models for the whole operational range of angle of attack, up to 180° for vertical-axis and 90° for horizontal-axis wind turbines. This work describes a computationally efficient unsteady forces prediction model based on a deep learning approach, namely the bidirectional long short-term memory (BiLSTM) algorithm, for an airfoil pitched over the full operational range of angles of attack up to 180°. No model has been developed to capture the unsteady forces at high angles of attack. Novel features based on operating conditions and the steady polars of the airfoil are used as inputs for the BiLSTM model. Direct measurements of steady and unsteady forces on a NACA 0021 airfoil model were conducted at reduced frequencies up to 0.075 and a Reynolds number of 120 000 in an open-jet wind tunnel for model learning and testing. The unsteady forces vary significantly from the steady values at high pitching amplitudes and post-stall angles, which, if not accounted for when simulating wind turbine performance, would result in inaccurate predictions. Furthermore, measurements revealed the effect of unsteady vorticity development and shedding on aerodynamic forces under forward and reverse flow conditions. The BiLSTM model is capable of capturing the underlying physics of unsteady aerodynamic forces under extreme operating conditions.

Pitfalls of testing wind turbine control algorithms on nacelle test benches

The validation and certification of wind turbines (WT) on nacelle test benches (NTB) is becoming increasingly important in the development process. While for certifiers the advantage lies in controlled test execution, for development departments it lies in testing as many system components as possible in a quasi-final prototype. However, the question arises which practical conditions must be fulfilled so that statements can also be made about WT control. In addition to the errors induced by a mechanical Hardware in the Loop (mHiL) system, the dynamic interactions between the WT controller and the NTB controller applying the mHiL concept are of interest. This analytical work based on simulations aims to systematically investigate how realistic the control behavior of a WT operated on a NTB is. For this purpose, the nominal behavior of a WT is compared with the operation on a NTB under realistic conditions and the resulting differences are subsequently reproduced in a synthetic load case. Finally, the differences are analyzed in terms of system theory. It is found that a frequency-dependent distorted behavior caused by operating the WT on a NTB is responsible for strong deviations compared to the WT operation in field. In the controller configuration studied, gain amplifications up to 5.17,text{dB} are identified. The distortion is not exclusively caused by the mHiL closed loop behavior, but results from the interaction of all subsystems in both control loops. Therefore, its behavior is identified as a function of the system and controller parameters of both the WT and the NTB.

Towards full electrical certification of wind turbines on test benches - experiences gained from the HiL-GridCoP project

In recent years, more and more wind turbine manufacturers have been using test benches, such as the Dynamic Nacelle Testing Laboratory at Fraunhofer IWES in Bremerhaven, to test and optimize their turbines. To further accelerate the test phases, Fraunhofer IWES, together with the project partners Nordex Energy SE & Co. KG and Vestas Wind Systems A/S, has built a new test bench for grid compliance testing of reduced systems, consisting of the generator, the converter, and the transformer plus control as well as protection system. The HiL-GridCoP project demonstrated the comparability of test bench measurements on a wind turbine equipped with a doubly-fed induction generator with field measurements performed using a fault ride-through container. The aim of this paper is to present some of the key findings related to the electrical commissioning and test execution. The focus is on the grid simulator and on its impedance simulation functionality. It is discussed which limitations may be caused by specific test bench components and which issues might affect the test results. Finally, some measurements of voltage dips are presented to verify the test bench's comparability with field tests so that the full electrical certification can be carried out next.

Impact of Multi-Physics HiL Test Benches on Wind Turbine Certification

Recently developed nacelle test benches for wind turbines, equipped with multi-physics Hardware-in-the-Loop (HiL) systems, enable advanced testing and even certification of next-generation wind turbines according to IEC61400-21. On the basis of three experiments carried out with a commercial 3.2 MW wind turbine, this paper shows to which extent test bench hardware and HiL systems influence certification results. For the crucial Fault-Ride-Through tests, all deviations were found to be below 1% compared to field and simulation results. For this test, the power HiL system and the accuracy of its impedance emulation are found to be of most relevance. The results for the test items Frequency Control and Synthetic Inertia were found to be more sensitive to shortcomings of the mechanical HiL with its control system. Based on these findings, the paper mentions general procedures to ensure the quality of test benches with HiL systems and, with that, ensure the quality of certification.

Estimation of Wear and Lifetime for an Improved Turbine Operation

Design, construction and certification of wind turbines (WT) generally target a lifetime of at least twenty years. Depending on the specified design criteria in comparison with the’real’ conditions on site lifetime may fall below or pass over the design lifetime targets. After installation and during operation WT continuously suffer from incremental damage due to loads, fatigue and the continuous exposure to the physical environment and operating conditions. The initially inherent wear reserve of the WT or its components permanently decreases until it finally reaches predefined safety margins or the occurrence of premature component failure. The objective of the work is to identify quantifiable wear-out metrics and to deduce information on lifetime issues in order to optimise WT operation. The method divides into a simplified approach explaining the general concept and its implementation and adaption from design to in-situ conditions. Furthermore, the methodology is transferred to a component-specific level, namely generator windings, in order to evaluate the general suitability for non-mechanical load exposures. Finally, economic issues including an economic optimisation strategy considering previously calculated wear-out figures complete the work.

Structural Evaluation of the DeepCWind Offshore Wind Foundation

Wind power technologies are being used as a mean of harvesting the energy from the wind with low carbon impact and with great potential to contribute to a new energy paradigm. Great benefits may arise from the use of offshore turbines, such as having a reduced visual impact, if far enough from the coast, or being able to produce more energy due to the use of bigger turbines. Current challenges for these technologies include producing a stable and profitable platform that allow turbines to be installed in deep waters, meaning that these solutions need to be of floating type. An example of such structure is the foundation developed by the DeepCWind consortium, which is evaluated for its structural integrity on this document. The evaluation consists of modal and static/transient analyses, according to a selected Design Loading Case, which is defined for the certification of wind turbines’ offshore foundations. The objective is to observe how this structure would perform when subjected to a simulated real life loading. The DeepCWind foundation exhibited great difficulties enduring the applied loads. Therefore, a structurally improved platform was design with the aim of successfully passing the load case.

Certification and testing technology for small vertical axis wind turbine in Taiwan

This study concerns the testing and certification of small vertical-axis wind turbines. Vertical-axis wind turbines have seldom been tested around the world. Additionally, Taiwan has few people with experience of the testing specifications of wind turbines, so manufacturers of small wind turbines in Taiwan must obtain product certifications from foreign test centers. For economic reasons and convenience, an international wind turbine test site must be built in Taiwan to support the small wind turbine (SWT) industry.Construction of the respective SWT test site in Taiwan has been completed, and all tests are conducted in accordance with Taiwan’s CNS 15176 series of standards, which was derived and modified from the international standards IEC 61400 and VDI 3834. The tests include power performance, durability, and strength and safety. This work demonstrates the test results for two VAWTs and proposes valuable suggestions to relative standards, test centers and manufacturers.

Influence of Icing on the Modal Behavior of Wind Turbine Blades

Wind turbines installed in cold climate sites accumulate ice on their structures. Icing of the rotor blades reduces turbine power output and increases loads, vibrations, noise, and safety risks due to the potential ice throw. Ice accumulation increases the mass distribution of the blade, while changes in the aerofoil shapes affect its aerodynamic behavior. Thus, the structural and aerodynamic changes due to icing affect the modal behavior of wind turbine blades. In this study, aeroelastic equations of the wind turbine blade vibrations are derived to analyze modal behavior of the Tjaereborg 2 MW wind turbine blade with ice. Structural vibrations of the blade are coupled with a Beddoes-Leishman unsteady attached flow aerodynamics model and the resulting aeroelastic equations are analyzed using the finite element method (FEM). A linearly increasing ice mass distribution is considered from the blade root to half-length and thereafter constant ice mass distribution to the blade tip, as defined by Germanischer Lloyd (GL) for the certification of wind turbines. Both structural and aerodynamic properties of the iced blades are evaluated and used to determine their influence on aeroelastic natural frequencies and damping factors. Blade natural frequencies reduce with ice mass and the amount of reduction in frequencies depends on how the ice mass is distributed along the blade length; but the reduction in damping factors depends on the ice shape. The variations in the natural frequencies of the iced blades with wind velocities are negligible; however, the damping factors change with wind velocity and become negative at some wind velocities. This study shows that the aerodynamic changes in the iced blade can cause violent vibrations within the operating wind velocity range of this turbine.

How to certify in offshore wind – Risk mitigation on a new level

AbstractThis paper introduces the new service specifications for project, type and component certification of wind turbines and power plants including a full overview on standards and recommended practices. The service specification applying to type certification of wind turbines as well as for component certification was published in summer 2016, the one for project certification of power plants at the end of 2015. The experiences gathered from certification projects in on‐ and offshore wind realised over the last years has been taken into account for these new service specifications. Correspondingly the project, type and component certification schemes have been adapted to meet market needs and expectations. But in course of the update of the whole scheme much more effort was taken to get to the next level of certification and standardization; beyond modules and elements of certification this includes all technical aspects laid down in standards and recommended practices.

Dynamic Models for Load Calculation Procedures of Offshore Wind Turbine Support Structures: Overview, Assessment, and Outlook

One of the main mechanisms for driving down the cost of offshore wind energy is to install ever larger wind turbines in larger wind farms. At the same time, these turbines are placed further offshore in deeper waters. As a result, traditional monopile foundations are not always feasible and multimembered foundations, such as jackets and tripods are required. Typically, thousands of load cases need to be simulated for the design and certification of offshore wind turbines (OWTs). As models of such foundations are significantly larger than their monopile counterparts, model reduction is often applied to limit the computational costs. Additionally, the foundation design is generally done by a specialized company, which bases its design on the results of the load simulations. Hence, an accurate estimation of the stresses in load simulation is essential to predict the integrity and the lifetime of different designs. The effect on the load accuracy of both the model reduction as well as the postprocessing method used by foundation designers (FDs) are investigated in this paper. A case study is performed on a jacket-based wind turbine model to verify and quantify the findings. First, it is observed that the effect of the reduced foundation model on the wind turbine loads is negligible. However, both the reduction method and the postprocessing method applied by the FD have a large influence on the fatigue loading in the jacket. It is shown that the popular Guyan reduction results in significant errors on the fatigue damage and that a static postprocessing analysis leads to serious underestimations of the fatigue loads. Finally, an outlook is given into future developments in the field of load calculations for OWT foundation design.

Fault Ride Through Certification of Wind Turbines Based on a Hardware in the Loop Setup

This publication presents a practical experience in implementing fault ride through investigations on a multiphysics power hardware in the loop platform for wind turbine nacelles. Next to the aim of doing full-scale system investigations with the nacelle, the use for system certification is included in the planning. While it is quite clear that such an approach would bring significant advantages compared with in-field testing, it is also important to understand if it is possible to overcome all the challenges from a practical and, in particular, measurement point of view that system level test bench certification implies. This question is analyzed with the support a unique platform developed by the authors in collaboration with other researchers of RWTH Aachen University. The platform is able to support test activities up to 1 MW.

Offshore wind turbine environment, loads, simulation, and design

In the design, certification, and optimization of offshore wind turbines, extensive loads simulation is inevitable to develop reliable and cost‐effective turbines. Description of the marine environment is based on a variety of techniques taking the stochastic nature of both, the wind and the water waves into account. The wind turbine is a highly dynamic system including effects of heavy rotating machinery and other significant nonlinearities leading to static, cyclic, transient, and stochastic loads. Due to the nature of the external loading, the system properties and the turbine design lifetime, offshore wind turbines are prone to fatigue‐driven failure. For loads assessment, aero‐hydro‐servo‐elastic tools are used including the coupled effects of the environment and the turbine to simulate the overall lifetime of the turbine in the harsh marine environment. These tools are constantly further developed and adapted to the needs of a fast‐growing industry and newly arising turbine concepts. Engineering approaches to allow for certification and reliable design are summarized in extensive guidelines and standards supporting engineers in daily design work.This article is categorized under: Wind Power > Science and Materials Wind Power > Systems and Infrastructure

Learning from wind energy policy in the EU: lessons from Denmark, Sweden and Spain

AbstractEnergy policy has recently become an important point on the political agenda of the EU, mainly due to the approaching ‘peak oil’ and climate change. This is increasing the focus on renewable energy sources, especially wind power, as an alternative to fossil fuels. In order to deduce the most efficient policies for promoting wind power, this paper compares the different national energy policies adopted by Denmark, Sweden and Spain, and relates them to wind energy outcomes. The successful promotion of Danish wind power in the last two decades of the 20th century is related to a number of factors including individual entrepreneurs, early official certification of wind turbines, systematic government support including favourable economic tariff schemes, and cooperative private ownership of wind turbines, which fostered broad public support. The analysis demonstrates the importance of long‐range national energy policies in creating and stabilizing the conditions required for the development of more sustainable energy systems. The lack or delayed development of such a supportive, stable environment explains the different patterns of wind development seen in Sweden and Spain, and points to the problems created by liberalized and short‐sighted commercial energy market, even for wind energy pioneers like Denmark. Copyright © 2007 John Wiley & Sons, Ltd and ERP Environment.

International Standards and Certification of Wind Turbines

International Standards and Certification of Wind Turbines