Dynamic Analysis of a Wind Turbine Gearbox Towards Prediction of Mechanical Tonalities

This dissertation describes the development of a methodology and modelling approach to lower the mechanical noise of the drive train of a modern wind turbine with a strong focus on the wind turbine gearbox. Although this mechanical noise is not the main noise source from a wind turbine, it could - especially when it contains audible tonal components - result in non-conformity to local noise regulations. This becomes more stringent when wind turbines are installed closer to urbanised areas. This research is motivated by inefficiencies in a cost and time consuming reactive trial and error approach to reduce or remove audible mechanical tonalities from the wind turbine noise. This dissertation focusses on the mechanical tonalities linked with the wind turbine gearbox. This mechanical noise originates from the interaction between the gears inside the gearbox, and then propagates directly airborne through the air or indirectly structure-born through the other wind turbine components to the outside. Two fundamental approaches exist to reduce or to remove a mechanical tonality. The first approach attempts to reduce the noise source by optimising the gears for low-noise. The second approach modifies the propagation path, also called transfer path from the source to the listener, such that the noise originating from the noise source is not amplified or even attenuated when it reaches the listener. The methodology proposed in this dissertation focusses on the second approach: using virtual prototyping models to assess and to optimise these transfer paths. The development of this methodology consists of 3 main parts: firstly individual components of the wind turbine gearbox are investigated in detail and if necessary experimentally validated. Secondly these individual components are assembled into a wind turbine gearbox model. The impact of these individual components on the global eigenmodes of the gearbox are investigated, and the model of the complete gearbox is experimentally validated by performing an experimental modal analysis. Lastly a model of two gearboxes on the end-of-line test rig is generated, investigated and also experimentally validated. This step-by-step approach, including the numerous experimental validation cases, resulted in a significant increase in both insight and confidence in the dynamic behaviour predicted by these virtual prototyping models. This dissertation demonstrates this methodology and modelling approach by performing two optimisation cases. The first optimisation case focusses on the impact of the bearings on the resulting vibrations. The second optimisation case modifies the flexible gearbox housing, which is considered the most dominant component, to shift an important eigenmode out of the operating range of the wind turbine. Both these optimisation cases clearly illustrate the potential of pro-actively using virtual simulation models to optimise the noise and vibration behaviour of the wind turbine gearbox during its design.Abstract Acronyms Contents List of Figures List of Tables 1 Introduction 1.1 Research objectives 1.2 Scope definition 1.3 Research facilitators 1.4 Research strategy 1.4.1 Modelling 1.4.2 Experimental validation 1.4.3 Used software 1.5 Main contributions 1.6 Overview of the dissertation 2 Introduction to wind turbines and wind turbine noise 2.1 Introduction 2.2 Climate change and renewable energy 2.3 Wind turbines 2.3.1 Layout 2.3.2 Operating range 2.4 Wind turbine noise 2.4.1 Wind turbine noise characteristics 2.4.2 Wind turbine noise sources 2.4.3 Wind turbine noise perception and annoyance 2.4.4 Regulations on wind turbine noise 2.5 Wind turbine farm planning 2.5.1 Site location 2.5.2 Site layout 2.6 Wind turbine gearboxes 2.6.1 Design 2.6.2 Excitation sources 2.7 Wind turbine noise transfer paths 2.8 Conclusions 3 Wind turbine drive train modelling, an overview 3.1 Wind turbine drive train design load models 3.2 Noise and vibration models 3.2.1 Non wind turbine gearboxes 3.2.2 Wind turbine drive train noise and vibration modelling 3.3 Limitations and shortcomings of current models to predict the NV behaviour 4 Modelling and experimental validation of individual wind turbine gearbox components 4.1 Introduction 4.1.1 Multibody modelling 4.2 Gearbox housing 4.2.1 Individual components 4.2.2 Assembly of individual components 4.2.3 Usage in MB model 4.3 Planet carriers 4.3.1 Modelling 4.3.2 Experimental validation 4.3.3 Conclusions 4.4 Bearings 4.4.1 Bearing stiffness determination 4.4.2 Bearing force introduction 4.5 Shafts and gears 4.5.1 Shafts 4.5.2 Gears 4.6 Conclusions 5 Modelling and experimental validation of a wind turbine gearbox 5.1 Introduction 5.2 HSH and MC 5.2.1 Model 5.2.2 Measurements 5.2.3 Correlation 5.2.4 Conclusions 5.3 Full gearbox - Structural model 5.3.1 Model 5.3.2 Measurements 5.3.3 Correlation 5.3.4 Conclusions 5.4 Full gearbox - Acoustic model 5.4.1 Need for acoustic modelling 5.4.2 Acoustic model 5.4.3 Structural - acoustic linking 5.4.4 Acoustic runup simulation 5.5 Conclusions 6 Modelling and experimental validation of two wind turbine gearboxes on the end of line test rig 6.1 Modelling 6.1.1 Impact of the flexible supporting structure on the global dynamics 6.1.2 Impact of load on the global dynamics 6.1.3 Sensitivity study 6.2 Measurements 6.3 Correlation 6.4 Conclusions 7 Virtual prototyping methodology to pro-actively avoid tonal wind turbine noise 7.1 Modelling method 7.1.1 Mechanical noise sources 7.1.2 Mechanical transfer paths 7.1.3 Acoustic radiation 7.2 Required analysis 7.3 Conclusions 8 Optimisation cases 8.1 Optimisation case 1: Impact of bearings on the resulting wind turbine gearbox vibration amplitudes 8.1.1 Using transfer path analysis to gain insight in the transmission of gear excitation through the different bearing positions 8.1.2 Effect of bearing stiffness & damping values on the transmission of gear excitation 8.1.3 Effect of bearing position on the transmission of gear excitation 8.1.4 Conclusions 8.2 Optimisation case 2: Wind turbine gearbox housing bending modes 8.2.1 Conclusions 8.3 Conclusions 9 Conclusions 9.1 Overview and main contributions 9.2 Recommendations for future research 9.2.1 Sources 9.2.2 Transfer paths 9.2.3 Noise radiation A Femtools A.1 Simpack model reader A.2 Simpack model writer A.3 Simpack solver & results reader B Roller based bearing model B.1 Distance calculation B.2 Roller force calculation C Overview on experimental validation campaigns C.1 Individual wind turbine gearbox components C.1.1 Torque arm C.1.2 Low speed ring wheel C.1.3 Intermediate bearing housing, low speed side C.1.4 High speed housing and main cover C.1.5 Assembled gearbox housing C.2 Assembled gearbox C.2.1 High speed housing and main cover C.2.2 Complete gearbox Bibliography List of publicationsnrpages: 266status: publishe

Comparison of multibody simulations and measurements of wind turbine gearboxes at Hansen’s 13 MW test facility

Continuous up-scaling of wind turbine size into the multi-megawatt class, together with developments for off-shore installation are calling for new wind turbine configurations and technologies. High product reliability is a key factor in these developments, cascaded down to each component manufacturer in the supply chain. Increasing the reliability of wind turbine drive trains for wind turbines with ever increasing size requires dedicated simulation models which can provide more insight in the internal gearbox dynamics in the early stages of the design process. However, simulation models can only add value to the design process if their results prove to be representative and reliable. Therefore validation based on measurements on real multi-megawatt wind turbine gearboxes is an absolute necessity. For this reason, Hansen Transmissions developed a back-to-back 13MW test rig set-up capable of dynamically testing gearboxes and validating their dynamical models. This validation is based on three foundations: validation of model parameters, validation in the frequency domain and validation in the time domain. The initial results of the validation of model parameters, validation in the frequency domain and validation in the time domain already demonstrate the added value of multibody gearbox models. Therefore the confidence level in the applicability of multibody simulation models in gearbox design is increased.status: publishe

Dynamic Behavior of a multi-megawatt Wind Turbine Drivetrain under Voltage Dips using a Coupled Flexible Multibody Approach

High turbine reliability is of utmost importance to keep the cost of wind energy to a minimum. A considerable problem in this regard is that of premature drivetrain failures, which have plagued the wind turbine industry since its inception. Accurate prediction of the loads encountered by the drivetrain components during their lifetime is essential for reliable wind turbine design. Of particular interest are transient load events, which are expected to have a detrimental effect on the lifetime of drivetrain components, especially when they give rise to torque reversals. At the electrical side of the wind turbine, transient events worth investigating include grid faults, emergency stops and grid loss. Unlike previous research on the impact of these events, which typically uses simplified gearbox representations, this paper investigates the dynamic behavior of wind turbine drivetrains during grid faults using a coupled simulation of a flexible multibody model of a commercial multi-megawatt wind turbine drivetrain and a Simulink model of a doubly fed induction generator (DFIG) and its controller. The mathematical modeling of the DFIG as well as the flexible multibody modeling of the drivetrain are described. Both gear and bearing forces on several components of the gearbox are examined during a symmetrical and asymmetrical voltage dip, and the influence of gearbox flexibility on these loads is assessed.status: publishe

A robust approach for substructure decoupling

As noise and vibration requirements become more and more stringent in many of today’s engineering applications, the need for robust identification methods increases proportionally. In some cases, the identification of a component is greatly complicated by the presence of a (supporting) substructure that cannot be removed during the identification. This paper will investigate what is necessary to make the experimental decoupling of a complex industrial component (e.g. a gearbox) from a supporting structure (e.g. a test rig) feasible on an experimental level. The dual approach for substructure decoupling will be verified on a finite element model representative for a gearbox and extended to a more generalized decoupling approach. Also the necessity of measuring the rotational degrees of freedom at the coupling points, and by extension, the necessity of measuring the coupling degrees of freedom will be discussed. Finally the influence of the conditioning of the matrices used for the substructuring is discussed together with a detailed analysis of the influence of several measurement error types on the quality of the results.status: publishe

Using transfer path analysis to assess the influence of bearings on structural vibrations of a wind turbine gearbox

Noise and vibration issues can be dealt with using several approaches. Using the source - transfer path - receiver approach, a vibration issue could be solved by attenuating the source, modifying the transfer path or by influencing the receiver. Applying this approach on a wind turbine gearbox would respectively correspond with lowering the gear excitation levels, modifying the gearbox housing or by trying to isolate the gearbox from the rest of the wind turbine. This paper uses a combination of multi-body modelling and typical transfer path analysis (TPA) to investigate the impact of bearings on the total transfer path and the resulting vibration levels. Structural vibrations are calculated using a flexible multi-body model of a 3 stage wind turbine gearbox. Because the high speed mesh is often the main source of vibrations, focus is put on the 4 bearings of this gear stage. The TPA method using structural vibration simulation results shows which bearing position is responsible for transmitting the highest excitation levels from the gears to the gearbox housing structure. Influences of bearing stiffness values and bearing damping values on the resulting vibration levels are investigated by means of a parameter sensitivity study and are confirmed with the results from the TPA. Because both the TPA and the parameter sensitivity analysis revealed a big influence on radial stiffness for a certain bearing this was investigated in more detail and showed the big importance of correct axial bearing position. Main conclusions of this paper are that the total vibration behaviour of a wind turbine gearbox can be altered significantly by changing both bearing properties such as stiffness, damping and position; and bearing support stiffness.status: publishe

Some trends and challenges in wind turbine upscaling

The wind turbine industry is continuously trying to reduce the cost of wind energy. A trend in this regard is the upscaling trend which tries to increase the power output of wind parks by increasing the power produced by each individual turbine. However bigger wind turbines impose higher loads on the wind turbine drive train. Good design of the drive train is indispensable for the reliable and optimal operation of the turbine. This paper discusses some of the trends related to the drive train design for these upscaled machines.status: publishe

Holmes, Laura J.

Laura J. Holmes - Plant and soil Lab Research Technician.https://digitalcommons.library.umaine.edu/univ_photos/2597/thumbnail.jp

Operational Modal Analysis for Estimating the Dynamic Properties of a Stadium Structure during a Football Game

During a football game, the ambient vibrations at the roof of a football stadium were recorded. A very large data set consisting of 4 hours of data, sampled at 80 Hz, is available. By a data reduction procedure, the complete data set could be analysed at once in a very short time. The data set was also split in shorter segments corresponding to certain events before, during and after the game to investigate the influence of varying operational conditions on the dynamic properties

Combining multibody and acoustic simulation models for wind turbine gearbox NVH optimisation

This paper investigates the structure-borne and airborne noise radiated from a wind turbine gearbox. A flexible multibody model is used to calculate the structural deformations of the gearbox during a 120 sec RPM runup going from zero to nominal RPM. An acoustic FE model is set up to compute the acoustic response during this RPM runup, based on the structural deformations. Emphasis has been put on the accuracy of both multibody and acoustic models. For the multibody model, it is shown that flexibility of the major gearbox components is required to obtain reliable results. On the gearbox housing, which is the largest flexible component of the gearbox, a mesh convergence study is performed and the influences of 6 large bolted connections on the modal behaviour of the gearbox housing are investigated. The need for an advanced acoustic model is shown by calculating the radiation efficiency of the gearbox housing. It also shows how an acoustic model can be efficiently combined with a flexible multibody model allowing to calculate the acoustic response from a time simulation. Using these models, an initial acoustic simulation is performed and presented.status: publishe

Multibody modelling of varying complexity for modal behaviour analysis of wind turbine gearboxes

In the currently booming market of wind turbines, a clear focus is put on the design of reliable and cost-effective subsystems, such as the gearbox. A requirement for reliable gearbox design calculations is sufficient insight in the dynamics of the entire wind turbine drive train. Since traditional wind turbine design codes reduce the drive train to just a few degrees of freedom, considerable research effort is spent in advanced modelling and simulation techniques to gain more insights in the dynamics at hand. This work focusses on the gearbox modal behaviour assessment by means of three more complex modelling techniques of varying complexity: the purely torsional-, rigid six degree of freedom with discrete flexibility- and flexible multibody technique. Both simulation and experimental results are discussed. Typical mode categories for traditional wind turbine gearboxes are defined. Moreover the challenge of the definition of an accurate approach to condense finite element models for representing the flexible components in the flexible multibody models is overcome. Furthermore the interaction between the structural modes of the planet carrier and planetary ring flexibility with the overall gearbox modes is investigated, resulting in the definition of two new mode categories: the planet carrier modes and planetary ring modes.status: publishe