Fraunhofer IWES Research Articles

Framework for Design Validation of Control Algorithms used on Mechanical Hardware-in-the-Loop Nacelle Test Rigs

Full scale multi-MW Wind Energy Converter (WEC) nacelles including the mechanical and electrical components as well as the main and converter control systems, are tested at the Dynamic Nacelle Testing Laboratory (DyNaLab) at Fraunhofer IWES. A Hardware-in-the-Loop environment emulates the missing rotor and accurately applies torque to the nacelle via a motor and the test rig drivetrain. In a current nacelle test campaign, several structural changes have been made to reduce the necessary adaptor tolerances of the test rig interface and to include the nacelle hub. These changes can potentially impact the system’s dynamic response and controller performance. This paper presents the current research with a focus on improving the control algorithms for accurate application of the rotor torque by implementing a simulative validation framework. The objectives of this framework include addressing the impact of modifications to the test rig drivetrain, variations in the nacelle adaptor, and communication delays between real-time simulation models, the test rig, and the WEC controller prior to performing tests on the real system. The methodology shown therefore involves three steps: controller design using a simplified multi-mass torsional model of all components necessary for testing, validation with a high-fidelity virtual test bench model and testing the resulting controller performance during the commissioning phase for the actual test. Overall, this approach is expected to reduce the commissioning duration, enhance the performance of the test rig, and identify the limitations in achieving stable torque control before the application to the real wind turbine nacelle on the test rig.

Preface

This Special Issue presents selected contributions from the WindEurope Annual Event 2024 Conference that took place 20-22 March in Bilbao. A total of 23 papers are included, addressing one (or more) of the conference themes:• Resource assessment• Turbine technology• Wind farm: installation and operations• Floating offshore wind• Digitalisation• Environmental sustainability and social acceptance• Grid and system integration• Renewable hydrogen and Power-to-X. Conference Committee for the Scientific Program • Amir R. Nejad, Professor, NTNU• Nicolaos Cutululis, Professor, Technical University of Denmark (DTU)• Simon Watson, Professor, Delft University of Technology• Julia Gottschall, Chief Scientist, Fraunhofer IWES• A. Mehrad Kazemi Amiri, Research Associate, Strathclyde University• Jan Helsen, Professor, Vrije Universiteit BrusselWe thank all, more than 30, reviewers for their engagement and effort, without which these proceedings would not have been possible. As with previous WindEurope events, the European Academy of Wind Energy (EAWE) has been responsible for organizing the review process for scientific contributions and has contributed to the development of the conference sessions. WindEurope has been responsible for the organization and logistics of the conference, and we thank their highly professional staff and their associates for the excellent collaboration. Proceedings editor Amir R. Nejad, Professor, Norwegian University of Science and Technology (NTNU)

Virtual Model Development of the Load Application System of a Wind Turbine Nacelle Test Bench for Hybrid Test Applications

Abstract The existing nacelle testing methods require continuous improvements to satisfy the ever-increasing demands for testing modern wind turbines. One way to achieve this goal is to use advanced simulation techniques to undertake hybrid testing, in which experiments and simulations are combined to push the boundaries of nacelle testing even further. To do so requires the development of a virtual model of the test bench featuring the true test bench dynamics and functionalities. This contribution presents the development of a virtual model of the complete nontorque load application system of a nacelle test bench at Fraunhofer IWES. The model development methodology is explained and the impact of different levels of modeling depth of the hydraulic system model is investigated. It is concluded that modeling of friction and valve dynamics is necessary as they have significant influence on the generated loads. These findings can help in the development of virtual models of nacelle test benches and pave the way for performing hybrid testing for wind turbine nacelles.

An investigation of spatial wind direction variability and its consideration in engineering models

Abstract. We propose that considering mesoscale wind direction changes in the computation of wind farm cluster wakes could reduce the uncertainty of engineering wake modeling tools. The relevance of mesoscale wind direction changes is investigated using a wind climatology of the German Bight area covering 30 years, derived from the New European Wind Atlas (NEWA). Furthermore, we present a new solution for engineering modeling tools that accounts for the effect of such changes on the propagation of cluster wakes. The mesoscale wind direction changes relevant to the operation of wind farm clusters in the German Bight are found to exceed 11∘ in 50 % of all cases. Particularly in the lower partial load range, which is associated with strong wake formation, the wind direction changes are the most pronounced, with quartiles reaching up to 20∘. Especially on a horizontal scale of several tens of kilometers to 100 km, wind direction changes are relevant. Both the temporal and spatial scales at which large wind direction changes occur depend on the presence of synoptic pressure systems. Furthermore, atmospheric conditions which promote far-reaching wakes were found to align with a strong turning in 14.6 % of the cases. In order to capture these mesoscale wind direction changes in engineering model tools, a wake propagation model was implemented in the Fraunhofer IWES wind farm and wake modeling software flappy (Farm Layout Program in Python). The propagation model derives streamlines from the horizontal velocity field and forces the single turbine wakes along these streamlines. This model has been qualitatively evaluated by simulating the flow around wind farm clusters in the German Bight with data from the mesoscale atlas of the NEWA and comparing the results to synthetic aperture radar (SAR) measurements for selected situations. The comparison reveals that the flow patterns are in good agreement if the underlying mesoscale data capture the velocity field well. For such cases, the new model provides an improvement compared to the baseline approach of engineering models, which assumes a straight-line propagation of wakes. The streamline and the baseline models have been further compared in terms of their quantitative effect on the energy yield. Simulating two neighboring wind farm clusters over a time period of 10 years, it is found that there are no significant differences across the models when computing the total energy yield of both clusters. However, extracting the wake effect of one cluster on the other, the two models show a difference of about 1 %. Even greater differences are commonly observed when comparing single situations. Therefore, we claim that the model has the potential to reduce uncertainty in applications such as site assessment and short-term power forecasting.

On a new methodology for testing full load responses of wind turbine drivetrains on a test bench

Today’s nacelle test benches are facing several challenges regarding meeting the increasing demands of modern wind turbines, which have been growing rapidly in operational range, size, and complexity. These challenges include reproducing the demanded extreme loads, dynamic load bandwidth, power capacity, and cost of testing. This contribution presents a new testing approach to tackle some of the aforementioned challenges faced by existing nacelle test benches. The method is demonstrated in a case study involving experimental measurements and simulations of a multi-megawatt wind turbine drivetrain recently tested at the DyNaLab of Fraunhofer IWES. By combining high-fidelity simulation models and partial load tests, the proposed approach has shown high potential for representing the full load response of a wind turbine nacelle. The proposed testing methodology has potential for resolving some of the challenges being faced by modern test benches in terms of obtaining full load responses on a nacelle test bench with a possibility of reducing the cost of testing.

Stability information derived from a floating lidar system using bulk Richardson formulation

In this contribution we introduce a Fraunhofer IWES floating lidar system (FLS) equipped with an integrated pulsed lidar device, motion sensors, near-surface atmospheric variable sensors (air temperature, relative humidity and air pressure) and a sea-surface temperature sensor. All measures were compared and validated with the nearby offshore meteorological mast FINO3 in the German North Sea. Atmospheric stability information was derived using the bulk Richardson formulation from the FINO3 station at both 30-m and 100-m as well as at the FLS at a 2-m height. The stability parameter was then extrapolated to the higher heights from the FLS using the mean wind speed measured from the lidar. The FLS was able to recreate the stability conditions calculated at the 30-m height but could not effectively match that of the 100-m calculations. This is mainly attributed to the significant changes with height of the atmospheric variables and their importance in the stability calculations.

First comparison of the electrical properties of two grid emulators for UVRT test against field measurement

The technical rules for connecting turbines to the medium, high or extra-high voltage grid in Germany require the certification of the UVRT characteristics of wind turbines. The state-of-art voltage divider-based test equipment, also named UVRT-Container, is well equipped for executing UVRT tests in field. To conduct the UVRT in field the full wind turbine should be already installed. A second option to perform UVRT tests are system level test benches. They enable the testing of the nacelle. The components that are not actually present, such as the turbine tower or the blades, are emulated via a mechanical hardware in the loop (HiL) system. In this work, for the first time, the performance of two different grid simulators installed at the DyNaLab at Fraunhofer IWES and at the CWD at RWTH Aachen University is compared with a field measurement of the same type of wind turbine. Thus, not only a system test bench measurement is compared to a field measurement. Rather, two system test benches with individual technical approaches are additionally compared with each other. The focus of this work is to investigate the characteristics of the grid simulators within the steady-state range of the UVRT tests to replicate identical fault shapes on the test benches and in the field.

Implementation and Experimental Validation of a Dynamic Model of a 10 MW Nacelle Test Bench Load Application System

The 10 MW Dynamic Nacelle Testing Laboratory of Fraunhofer IWES provides a controlled environment for performing electrical and mechanical tests on a wind turbine nacelle. Apart from physical testing, system level simulations are another paradigm in the framework of nacelle testing. In this contribution, the development of a dynamic model of the load application system for the 10 MW nacelle test bench is presented. The test bench load application system controls are integrated in the model via co-simulation in Simulink. The model is evaluated using experimental results. By utilizing the modal strain recovery method, a direct comparison of the strain results of the model with the experimental results is achieved. Moreover, it is shown that the actual applied loads on the device under test can be estimated by analysing the strain readings. The developed model provides a platform for developing a high fidelity virtual nacelle test bench.

Development and Verification of an Aero-Hydro-Servo-Elastic Coupled Model of Dynamics for FOWT, Based on the MoWiT Library

The complexity of floating offshore wind turbine (FOWT) systems, with their coupled motions, aero-hydro-servo-elastic dynamics, as well as non-linear system behavior and components, makes modeling and simulation indispensable. To ensure the correct implementation of the multi-physics, the engineering models and codes have to be verified and, subsequently, validated for proving the realistic representation of the real system behavior. Within the IEA (International Energy Agency) Wind Task 23 Subtask 2 offshore code-to-code comparisons have been performed. Based on these studies, using the OC3 phase IV spar-buoy FOWT system, the Modelica for Wind Turbines (MoWiT) library, developed at Fraunhofer IWES, is verified. MoWiT is capable of fully-coupled aero-hydro-servo-elastic simulations of wind turbine systems, onshore, offshore bottom-fixed, or even offshore floating. The hierarchical programing and multibody approach in the object-oriented and equation-based modeling language Modelica have the advantage (over some other simulation tools) of component-based modeling and, hence, easily modifying the implemented system model. The code-to-code comparisons with the results from the OC3 studies show, apart from expected differences due to required assumptions in consequence of missing data and incomplete information, good agreement and, consequently, substantiate the capability of MoWiT for fully-coupled aero-hydro-servo-elastic simulations of FOWT systems.

Measuring MNm torques as part of a prototype testing campaign of a high-temperature superconducting generator for wind turbine application in the scope of the Ecoswing project

Modern wind turbines have a driving torque in the range of up to 10 MNm. The measurement of such high torques poses challenges, as there is no standard measurement equipment for this torque level available today. During the EU funded EcoSwing project, the world’s first superconducting low-cost and lightweight multi-megawatt wind turbine generator has been designed and tested on the DyNaLab nacelle test rig of Fraunhofer IWES. For this test campaign, a specifically designed torque measurement system has been developed to measure the torque directly at the flange of the device under test in order to evaluate the efficiency of the power train.

A study of mechanical torque measurement on the wind turbine drive train—ways and feasibilities

AbstractThe signal of the drive train mechanical torque can be a very informative input for new technologies on the wind turbine, helping further reducing the cost of energy. However, measuring the mechanical torque is not an easy task even during a test campaign on a prototype. In case of the long‐term measurement for operational purpose of use on the turbine, it is often considered as technically and economically not feasible. This paper discusses possible ways of the torque measurement and presents a test campaign where two different methods of torque measurement are conducted. One of the methods measures the shear strain signal using strain gauges at one section of the turbine main shaft. The other method measures the torsional deformation between two positions of the drive train, with an incremental encoder measuring the angular position of the drive train at each position. The tests are conducted on an 8‐MW wind turbine drive train under tests on the nacelle test bench “DyNaLab” of Fraunhofer IWES. During the tests, different load steps of torque and other load components are applied to the drive train. The measurement results from both measuring methods are presented and analysed. In the end, technical and economical feasibilities are discussed for both methods of torque measurement; some suggestions are also given respectively.

Modelling the failure behaviour of wind turbines

Modelling the failure behaviour of wind turbines is an essential part of offshore wind farm simulation software as it leads to optimized decision making when specifying the necessary resources for the operation and maintenance of wind farms. In order to optimize O&M strategies, a thorough understanding of a wind turbine's failure behaviour is vital and is therefore being developed at Fraunhofer IWES. Within this article, first the failure models of existing offshore O&M tools are presented to show the state of the art and strengths and weaknesses of the respective models are briefly discussed. Then a conceptual framework for modelling different failure mechanisms of wind turbines is being presented. This framework takes into account the different wind turbine subsystems and structures as well as the failure modes of a component by applying several influencing factors representing wear and break failure mechanisms. A failure function is being set up for the rotor blade as exemplary component and simulation results have been compared to a constant failure rate and to empirical wind turbine fleet data as a reference. The comparison and the breakdown of specific failure categories demonstrate the overall plausibility of the model.

Assessment of Storage Options for Reduction of Yield Losses in a Region with 100% Renewable Electricity

Abstract In the federal state Schleswig-Holstein (SH) the installed capacity of distributed generation has been growing faster than the transport capacity of power networks. Currently, this causes network congestions and requires corrective measures as curtailment. In 2012, around 3.5% of renewable generation in SH was curtailed. As one possible option for reducing these yield losses energy storage has been suggested. In a project for the state government of SH Ecofys and Fraunhofer IWES investigated potential benefits of storage technologies in electricity networks in this region. The analysis was carried out separately for the transmission and distribution levels. Based on hourly simulations of future renewable generation and load time series of a 2025 scenario, congestion effects on the transmission grid level were evaluated by using residual load analysis. By matching situations of negative residual load against current and future transfer capacities in SH it becomes obvious, that an appreciable amount of surplus situations for a potential utilization of energy storage only occurs with current transfer capacities. It diminishes with the planned expansion of transmission grid. On distribution grid level, Ecofys assessed the characteristics of curtailment applied during recent years with detailed empirical data. The analysis of congestion management actions showed that the storage's cycling intensity related to curtailment will be low whereas power intensity is potentially high. The economic benefits of reducing RES-E losses do not offset the costs of storage. Hence, application of storage for reducing curtailment losses is viably only if part of the investment costs are socialised in one way or another. Finally, the paper identifies a number of arguments supporting the development of storage technologies and describes key framework conditions which could be changed to support storage projects.

Results and Conclusions of a Floating-lidar Offshore Test

In this contribution we introduce the Fraunhofer IWES Wind Lidar Buoy, a floating-lidar prototype comprising a proven marine buoy design and a pulsed lidar device integrated in it. A software correction algorithm was developed to remove the effects of sea conditions resulting in system motions from the recorded lidar data. The final performance of the system was verified in a measurement campaign next to the offshore meteorological mast FINO1 in the German North Sea. Both the results of an accuracy assessment and a more detailed sensitivity analysis of the floating-lidar performance with respect to different external parameters were performed.For the recorded horizontal mean wind speeds a very good measurement performance with an R2-value of 0.996 is achieved even when no motion correction is applied. To use the turbulence intensity and wind direction data from the system, the application of a motion correction is necessary. The overall System Availability for the nine-weeks-measurement period was derived as 98%.