Testing and system validation of large mechanical components: IWES increases the reliability of large wind turbine components

The focus here is on design suitable for both production and use, for example investigations into the specific load-bearing behavior of suction caissons as well as the development and validation of material models utilized to model the subsoil in the North Sea. The massive dimensions of these components mean that the design and production of a wind turbine support structure is always challenging. We offer customers support in onshore and offshore installations both above and below the Earth’s surface.

Support structures: Design at its limit

The wall thicknesses of towers and pile foundations are sometimes greater than 100 mm in current turbine models. The design of welded joints which are operationally stable is complex – the S-N curves usually employed are scarcely available for these dimensions, plus such dimensions are challenging under the load cases of a wind turbine from a process technology perspective.

We offer support in the qualification and optimization of types of welded joints with regard to fatigue, for example on the basis of optical methods for the determination of the micro- and macrogeometries. Our validated numerical models offer support in design suitable for both production and use as well as in the reduction of costs and technological risks.

Current focus: suction caissons 

Suction caissons represent an environmentally friendly and efficient method for offshore wind turbine foundations. In their application, however, there are always installation obstacles to be overcome: in some cases, it is not possible to install the structures at the planned depth and they are susceptible to buckling failure if overloaded during installation. IWES is thus developing experimentally validated numerical models to allow modeling of the complex behavior of the suction caissons during installation and offer wind farm planners a tool rendering the installation process more reliable overall.

Another focus of our activities is the investigation of the specific load-bearing behavior of suction caissons from a geotechnical perspective, which can offer potential for mass and cost optimization in the dynamic load range in particular.

Installation, geotechnics, and soil-structure interaction: water and sand

The subsoil in the North Sea is predominantly water-bearing sand with different compactness per region. This subsoil offers the opportunity to realize reliable foundations but is also a highly complex material with highly nonlinear behavior at the same time. The prediction of the ground’s mechanical properties required for the planning of a wind turbine – in particular considered over the turbine’s entire service life – is the subject of continuous intensive research. IWES develops and validates complex numerical and analytical material models, which can be employed to evaluate both the subgrade stiffness of the subsoil, which is important for the design, and the changes in the soil over the turbine’s service life. Consideration of the effects of the installation of the foundation itself is particularly important in this regard: the installation of a pile foundation, for example, via impact or vibratory driving changes the subsoil around the installation site and needs to be considered during dimensioning of the foundation. The analysis and evaluation of installation influences is a focus of work at IWES.

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The installation situation, the loads, the specific operating conditions, the lubricant employed, and a range of other factors all influence the reliability of these bulky but sensitive components. As a result of turbines constantly growing in size and the short times to market, it is not only the requirements but also the risks which are increasing. Fraunhofer IWES has thus been conducting research on the large rolling bearings of wind turbines since 2013 with the aim of boosting their reliability and paving the way for new calculation methods and designs. The LBL at the institute site in Hamburg bundles these activities and also expands them with experimental testing of bearings for the next generation of wind turbines.

Increasing reliability thanks to validation

With the LBL, IWES operates a unique test infrastructure in Hamburg which allows the investigation of fundamental influencing factors of the diverse damage patterns as well as the real-scale validation of large slewing bearings. The close networking with other departments allows the testing infrastructure of the entire institute to be utilized to respond to the requirements of different bearing positions. The LBL’s testing strategy is essentially divided into a functional test and a fatigue test. As part of the functional test, the dominant damage mechanisms in the bearing are determined, which, in turn, determine the setup of the subsequent accelerated endurance test. The test program is also prepared at the LBL on the basis of the time series of the load simulation. Comprehensive data analysis allows the test duration to be accelerated or shortened to runtimes acceptable within the development of a wind turbine. The accompanying simulation of the bearings supports the testing activities and includes individual contact simulations as well as global bearing models for FE and MBS analyses. The interfaces (rotor blade, screw connections, rotor hub, bearing housing, bearing seat, etc.) are also mapped in FE models. All simulation models are compared with measurement data and validated. From concept development to simulation, design, testing, and, finally, assessment, the IWES LBL portfolio thus covers the entire life cycle of a large rolling bearing.

Bespoke solutions for individual requirements

The associated requirements and challenges are as individual as the bearings themselves. With many years of experience gathered from tests on more than 300 bearings and the operation and development of more than eight test benches, the LBL team has a wealth of experience and knowledge at its disposal. We incorporate the continuous development of our methods and our focus on applied research into all of our projects. Together with our customers, we identify individual and specific requirements and develop customized solutions to ensure optimal validation of the entire blade pitch mechanism.

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Market launch difficult to plan with field tests

Grid compatibility testing for the certification of the electrical properties of new wind turbines or for post-certification of modifications to existing turbine types is often carried out using mobile test equipment in the field. The certification campaign generally lasts up to two years, is a significant cost factor in turbine development, and delays the market launch.

The demand for suitable locations for prototype certification is high, as is the number of turbines to be certified. Sufficient availability of wind and – due to increasing turbine size – higher requirements for the grid connection must be taken into consideration. Field tests are not reproducible, and results are only comparable to a limited extent, as wind and grid conditions are never absolutely identical. Waiting times render the market launch even more difficult to plan.

Advantage of DyNaLab: wind turbine nacelle testing with reproducible conditions

The Dynamic Nacelle Testing Laboratory (DyNaLab) has been offering a realistic test environment in the multimegawatt range for meaningful laboratory tests since 2015. Unique test services for the validation of prototypes are offered there with a drive power of 10 MW and the introduction of a nominal torque of 8.6 MNm. Grid and HiL wind load simulations make it possible to create different load scenarios under reproducible conditions. Wind turbine behavior during scenarios such as multi-dips in the grid during a storm, a network short circuit resulting from incorrect pitch control, and emergency stops is also tested.

The reproducibility of different operating scenarios makes it possible to shorten certification campaigns considerably. Operational management and control can also be optimized and model validations performed. This increases the reliability and availability of the turbines and cuts both maintenance and repair costs.

DyNaLab offers realistic testing conditions in the multimegawatt range, making it possible to validate and optimize existing and future wind turbine concepts. The use of a virtual grid with 44 MVA of installed converter capacity allows the simulation of typical grid faults such as voltage dips with a high repetition frequency.

Only test configuration of its kind in the world

The combination of mechanical tests with a grid emulator for testing wind turbines up to 10 MW is the only one of its kind in the world in this configuration. The prototype of the AD 8-180 was tested here, as was a superconducting generator from the EcoSwing research project, and Enercon, GE, and Siemens Gamesa Renewable Energy (SGRE) have also used the nacelle test stand for test campaigns.

Static tests can also be used to determine the active and reactive power output under different grid conditions. In addition, it is possible to simulate transient grid events affecting the entire nacelle system: tests of dynamic low-voltage ride-through (LVRT) and high-voltage ride-through (HVRT) events, as required by different grid codes, as well as dynamic changes in the grid frequency.

As the nacelle is tested on the test bench without the rotor and tower, its system properties are different than in the field. The loads and interactions occurring between the nacelle and rotor are calculated and impressed on the nacelle on the test bench in order to simulate the real conditions. Efficient real-time models and control algorithms ensure that the test bench including the DUT can be operated using the hardware-in-the-loop (HiL) method. Certification test campaigns can thus be precisely scheduled and defined on a manufacturer-specific basis.

Fraunhofer IWES provides support in comparing the results obtained on the test bench with field measurements. Synergies are developed through close cooperation with the Application Center for Wind Energy Field Measurements (AWF) in the field of metrology and sensor technology. The close internal link between field measurements and test operation makes it possible to carry out accredited load measurements in accordance with IEC 61400-13.

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To address this problem, we at Fraunhofer IWES developed the hybrid testing process and filed a patent application. It combines physical tests with innovative simulation methods to enable hybrid testing of drive trains even above the maximum load of the test bench.

For this purpose, the majority of the planned tests below the maximum capacity of the test bench can be carried out physically on the real test bench. The measurement data obtained is used to validate and optimize a simulation model of the DUT under partial load, which is then used to perform virtual tests above the physical capacity of the test bench. The combination of the physical measurement results with the simulation results up to the maximum load of the DUT produces a realistic simulation of a fully physical drive train test. The use of smaller and proven test benches saves investment and testing costs. 

Virtual tests and investigations of the drive train dynamics

A precise understanding of the dynamic loads and operating conditions is essential for increasing the reliability of wind turbine drive trains. Our simulation tools and models help with this. For example, by making it possible to investigate the influence of plain bearings in the gearbox on the system dynamics of the entire drive train and quantify it in more detail. The virtual tests support the design and development process of the drive trains and components and enable greater reliability and torque density as well as more lightweight designs among other things. In addition, any vibrations and resonance points that could lead to damage-relevant loads on the drive train are identified. Physical tests on a component or system test bench, accompanied by virtual tests, allow parameter variations and efficient expansion of the test results.

Employing our simulation models, we offer our customers improved test methods and customized test sequences. It is also possible to answer downstream questions – for example for the development of condition monitoring systems or the optimization of turbine control. The models also support real test campaigns, for example on the nacelle test bench or one of the component test benches. In addition, specific test scenarios can be tested and optimized in advance. During the test campaign, our simulation models help to verify the measurement results and help us to understand unexpected data or phenomena better.

Validation and adaptation of the simulation models

Our drive train simulations are based on detailed simulation models, often based on flexible multi-body simulation (MBS) or the finite element method (FEM). Our core competencies include the development and implementation of simulation models for predicting the system properties of the wind turbine drive train and its components. Validation using real measurement data is the decisive step in optimizing model accuracy. This includes both the comparison with physical data and the adaptation and optimization of the simulation model based on this data.

We provide support for targeted, efficient model validation, advise on the planning of tests for model validation, and recommend suitable test scenarios and the use of special sensors among other things. We also help to identify and adapt specific model parameters based on the measurement results.

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