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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced rotor technologies' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

José Javier Gil Soto Acciona Windpower, Spain
Co-authors:
José Javier Gil Soto (1) F P Sergio Gárriz Sanz (1)
(1) Acciona Windpower, Imarcoain, Spain

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Abstract

DEVELOPING MORE RELIABLE SPECIFICATIONS FOR THE VIBRATION TEST OF LARGE WIND TURBINE GENERATORS

Introduction

Vibration test for rotating machines as wind turbine generators can be specified according to international standard [1]. According to this standard, only rms values measured at the drive-end and non-drive-end bearings are relevant. Moreover, the vibration test is performed in a no-load condition under standardized boundary conditions which in most cases do not match the real ones.
Considering the current market trends, where generator systems tend to be larger and support structures tend to be lighter, the need to find a new approach to the above mentioned vibration test is justified.


Approach

The main objective of this work is to develop a test bench comprising the support structure, the mountings and the generator to be installed in the generator supplier vibration test facilities. The dynamic behavior of the former assembly has to be comparable to that exhibited by the same components in the nacelle vibration test to be held in Acciona Windpower facilities. In the case of the nacelle vibration test, the principal components of the drive train are present. In both cases the test is performed under no-load conditions, meaning that the system is driven by the generator.
The approach to the former development can be divided into the following stages:
• Design a preliminary version of the test bench to be installed in the supplier facilities.
• Analyze the main differences in the dynamic behavior of the assemblies to be compared (test bench and full drivetrain assembly respectively).
• Correct the possible deficiencies of the preliminary design of the test bench.
• Establish the acceptance criteria for the vibration levels to be obtained in the test bench. These values must be in accordance to those obtained for the same components in the full drivetrain assembly. In this case, threshold values are established for both, the generator and the generator frame.
In order all these stages to be accomplished, a combination of different methodologies such as Finite Element Modeling, Experimental Modal Analysis [2] and Statistical data Analysis [3] is used. Once the theoretical models are validated, they can be used to assist the design process of the wind turbine drive-train.

Main body of abstract

The main objective of this paper is to present a general approach that can be used to define an augmented vibration test specification for large wind turbine generators.
In this case, an advanced vibration test bench comprising a support structure, the mountings and the generator itself is presented. This test bench has been installed in the generator supplier vibration test facilities.
The vibration test bench specification takes into account the following aspects:
• Dynamic interactions between the generator and its support structure may occur in a range of pre-defined operational frequencies.
• In the case of using rigid mounting, the stiffening effect of the generator housing on the support structure must be taken into account.
• The vibration results obtained in the test bench must be comparable to those obtained in the nacelle vibration test, where the main components of the drivetrain are present.
At the end of the day, the results of the presented development include:
• A detailed description of the test bench, including the list of materials required to build it.
• A technical specification which describes the vibration test in terms of type of sensors and measurement locations.
• An acceptance criterion for the vibration levels obtained in the vibration test. Acceptance values must be in accordance to those obtained in the full drivetrain vibration test.
As a first stage of this development, a preliminary version of the test bench is evaluated from dynamical point of view. In order to do that, Finite Element models are created and further analyzed in order to determine the natural frequencies and mode shapes of the system in a specific frequency range.
The preliminary system comprises the support structure, a specific silent-block configuration and a specific generator model. Also in this stage, Experimental Modal Analysis techniques [2] are applied to update the Finite Element models. Figure 01 shows the correlation between the theoretical Finite Element Model of the Generator Frame and the corresponding Experimental Modal Model for a specific mode of the structure.

Figure 01: Theoretical Model and Experimental Modal Model of the generator support structure
The second stage analyses the boundary conditions of the support structure compared to those find in the full drivetrain assembly.
For such a large system, the equivalent stiffness of the bolted joint connecting the main frame and the generator frame clearly influences the modal behavior of the generator frame. Therefore, this stage is essential to obtain a good correlation between both experiments. Figure 02 shows the Finite Element Model of the resulting vibration test bench.

Figure 02: Finite Element Model of the vibration test bench
The third stage is devoted to generate an augmented vibration test specification paying special attention to the relevant control points monitored in the full drivetrain test. Considering that both tests are performed under no-load conditions, a complete statistical data analysis is carried out in order to establish a correlation between both operational systems. In this case, only a selected range of operational regimes is analyzed.
Once the methodology is validated, the vibration test specification guarantees a successful integration of the tested generators in the full drive-train assembly.


Conclusion

In this paper, the development of an advanced test bench where more reliable vibration tests specifications can be performed. The proposed development combines Finite Element methodologies with Experimental Modal Analysis and Statistical Data Analysis methods in order to obtain optimal correlation between the test bench, installed in the supplier facilities, and the full drive-train, assembled in the manufacturing plant of Acciona Windpower.
The results of the proposed development can be summarized as:
• A detailed description of the test bench, including the required list of materials.
• A technical specification describing the vibration test in terms of type of sensors and measurement locations.
• An acceptance criterion for the vibration levels obtained in the vibration test. The proposed acceptance values are in accordance to those obtained for the nacelle vibration test carried out in the facilities of Acciona Windpower. In this case, the augmented specification is used to detect potential problems in the generators before the drive-train assembly takes place.
In addition to the former results, the test bench can be used to assess the following subjects:
• Optimal selection and dynamic characterization of the generator silent-block configuration, including rigid mounting. Note that operational frequency ranges may change depending on the grid frequency (e.g. 50 or 60 Hz generators).
• Evaluate the influence of small changes (e.g. inertial properties of the generator) in the dynamic behavior of the whole assembly.
It should be noted that none of the above mentioned vibration test is representative of the real behavior of the wind turbine, where all components operate in a full load condition and reference vibration levels are much higher as stated in [4].



Learning objectives
Take advantage of the Combination of different methods such as Finite Element Analysis, Experimental Modal Analysis and Statistical Data Analysis in order to solve complex correlation problems.


References
[1] IEC 60034-14: Mechanical vibration of certain machines with shaft heights 56 mm and higher – Measurement, evaluation and limits of vibration severity (2007)
[2] D.J. Ewins. Modal Testing: Theory and Practice. Wiley & Sons (1995)
[3] D. A. Freedman. Statistical Models: Theory and Practice. Cambridge University Press (2009)
[4] VDI 3834: Measurement and evaluation of the mechanical vibration of wind energy turbines and their components (2009)