1 INTRODUCTION
This tutorial deals with the different models of drive train that can be done in S4WT. In this document two different architectures are presented, and some of their respective issues are investigated:
The Classical Drive train with a gear Box
The Direct Drive concept
Through these two examples, two modeling philosophies are also presented. With the first example (with Gear Box) the parametric models embedded in S4WT will be used. In the second example (Direct Drive) the user creates a complete new power train from user designed geometries.
These two methodologies illustrate the openness of the software which allows the user to model any kind of power train with the level of details desired by the user.
2 PART 1: GEARBOXES
“The wind energy industry continually evolves, and industry professionals have streamlined gearbox design to a consensus configuration. This configuration and its design iteration have existed for many years; consequently, design and manufacturing flaws have been minimized sequentially. Regardless of the maturity of the gearbox design and design process, however, most wind turbine downtime is attributed to gearbox-related issues.”
(F. Oyague, NREL, Gearbox Modeling and Load Simulation of a Baseline 750-kW Wind Turbine Using State-of-the-Art Simulation Codes, February 2009)
Several hypotheses have been offered to explain gearbox failure, among which we may cite for example: the absence of a number of load cases relevant to the design process; the transfer of non-torsional loads between the different components of the drivetrain; the lack of a uniform standardization of bearing-life analysis calculations; and poor communication between wind turbine designers, gearbox suppliers, and bearing manufacturers.
S4WT offers a complete new approach to simulate the gearboxes in a full flexible Wind Turbine context, taking into account the real dynamics.
This approach offers a better calculation of the dynamic forces (torsional and non-torsional) that transit through the gearboxes and a better understanding of the resonances in the gearbox itself and between the gearbox and other components. The capacity to address better this problem during the design phase allows the manufacturer to reduce the time to market of the wind turbine and leads to a more robust design.
The aim of the first analysis is to demonstrate the ability to perform local analysis on the gearbox (load distribution, modal behavior, detection of the potential resonances…). It should be pointed out that the methodology could be applied to any component of the wind turbine (Bedplate, blades, tower…).
In a second analysis, the gearbox is integrated in a complete model of wind turbine, under realistic load cases. A modal analysis is then performed on the full model to highlight the role played by the external components on the gearbox Eigen modes.
The steps to be achieved in the first part of this tutorial are :
Static calculation on a free gearbox design (Samtech property) – load distribution analysis
Modal behavior of the gearbox
Integration of the gearbox in a multi-megawatt WT model
Modal analysis of the full WT
Transient analysis under a turbulent wind.
2.1GearBox Design :
2.1.1General Description of the Model
The model corresponds to a gearbox with one planetary stage and 2 parallel stages and is pictured below, as it appears in the graphical window.
SAM GBX 1p2hB Ratio 70.7
The model used to create this gearbox is completely parametric. Any dimension, gear, bearing property (stiffness) can potentially be customized. S4WT’s help provides a detailed documentation of this model. Follow the path Models - Standard model - Gearboxes - Standard gearbox.
The parametric model used here and the distances parameters are presented on the picture below.
The next figure present the multi bodies elements used for the modeling of the gears and the bearings. In particular, the numbering illustrated is the one used inside S4WT and will be useful to understand the results code during the post-processing stage.
2.1.2General Data of the gearbox
The technical data is provided in the following table. It is given so that the user can have a clearer idea of what he is simulating.
2.2Static calculation on a free gearbox design (Samtech property) –load
distribution analysis
The first step of the tutorial consists in performing a “static” analysis of the gearbox subject to a linear torque. The analysis is qualified as “static” because the high speed shaft output point will be locked and the input shaft summited to a torque. As a consequence, no rotation of the gearbox is allowed. However, because of the non linearities of the system (clearances, backlashes, and nonlinear stiffness) the calculation itself is not strictly speaking static but rather dynamic. The resolution of the problem will be done dynamically.
The loadcase considered can be pictured as shown below.
The two torque arms points represent the fixation system of the gearbox on the bedplate of the wind turbine. Here, we simply clamp those two points, which means that any motion (translational or rotational) of the points is forbidden. In other words, all 6 DOF (degrees of freedom) are locked. Furthermore, as mentioned previously, the DOF that corresponds to the rotation of the output shaft is locked as well.
To load the model, open a new window of S4WT. Click on the open button and select the file [tutorial directory]/Gearbox/SAM_GBX_1p2hB_R70-7.czm. The model is loaded in the interface. Note that the wind turbine assembly contains only the gearbox.
To create the load case the following steps need to be achieved:
STEP 1: Creation of a Zero function. This function will be used to block the DOF of the torque Arms and the axial rotation of the high speed shaft. In the function tab, click on the “create function button”, and select “function defined by points” from the drop down menu. Fill the table of points in as shown in the picture below. You can set the units of the X and Y axis so that it appears on the graph, but it is not mandatory.
STEP 2: Creation of a Torque function. This function is used to load the input shaft. The first load case will be a linear torque increasing from 0 to 1e6N.m in 5 seconds. Proceed similarly to step 1.
STEP 3: Load case creation. In the loadcase tab, generate a new load case and select “advanced loadcase”. Only the gearbox is modeled so, in the global definition of the load case, we deactivate the pitch, the yaw, the generator, the brake. The analysis type is dynamic and the loads considered are Gravity and User loads.
STEP 4: To apply the user loads we use external connector of the gearbox. The connectors of the gearbox are listed in the EO Connectors of the SE Gearbox. The connectors used for this load case are: GBOX_MNSH_LINK_NODE, GBOX_HSCS_LINK_NODE, GBOX_YOKE_NEGY_LINK_NODE,
GBOX_YOKE_POSY_LINK_NODE. The names can be copied to be pasted in the user loads table of our loadcase.
STEP 5 : Paste the connector names in the user loads table. For each connector, we need to specify the type of load (loadType), its value (loadFunction), and the DOF (component) on which it is acting.
The following numbering is used for the component column1:
1.correspond to the X translation in the global frame
2.correspond to the Y translation in the global frame
3.correspond to the Z translation in the global frame
4.correspond to the X rotation in the global frame
5.correspond to the Y rotation in the global frame
6.correspond to the Z rotation in the global frame
0.correspond to the all the DOF
Fill the table as shown in the picture above.
The load type can be Acceleration, Displacement, Following force, Force, Position and speed. The following force corresponds to a force that will follow the rotation and displacement of the node it is attached with.
1 This is a general convention of SAMCEF.
STEP 6: Create the analysis from the selected load case. In the solver data, increase the amount of data produced by ticking the box “high”. In the Advanced mecano parameters, set the size of the maximum timestep to 1e-3. This will ensure that fast variation of the response will be captured. Finally, set the time of the analysis to 5 seconds and launch it.
v
STEP 7 : Results post-processing
Several results could be analyzed with this load case. Here is the list of some possible results to look at
If we look for example at the torque exerted by the first planet on the SUN Shaft Beam we can observe between 0 and 0.6 s some oscillations due to the clearance and the initial shocks generated by the increasing torque. Then the oscillations quickly stabilize and we can observe a linear slope, proportional to the torque load in the input shaft.
2.3Modal Analysis on the free gearbox model
2.3.1Methodology
The sequence of operations necessary for a modal analysis is the following :
1.Transient analysis under static torque.
2.Freeze the system and extract the tangent stiffness matrix. All the free degrees of freedom are
then locked for the eigen frequencies calculation
3.Calculation of the Eigen Modes of this “linearized” Matrix. By default the solvers computes all
the eigenmodes whose eigenfrequencies are comprised between 0 and 100 Hz.
In this example, we will consider the model in a state of loads that corresponds to the end of the previous calculation, that is to say, under an input torque of 1e6 N.m.
2.3.2How to do it in S4WT
Step 1 : Create a new analysis by clicking on the wind turbine assembly with the right button, and then on “Create Analysis”. Select “modal analysis”from the list of analysis proposed.
Step 2: In the applied load, select the torque load that was used previously.
Step 3: In the Solver Data, untick the case “Compute all modes in the range”,and set the upper bound of the frequency range to 1000 Hz. Hence, the solver will compute the first 200 eigenfrequencies which are comprised between 0 and 1000Hz.
Step 4: Launch the calculation
Step 5: In the results, besides a list of the computed eigen frequencies, it is possible to visualize the mode shape corresponding to each eigen frequency. In order to do this, click on the POST button with the modal analysis selected in the data tree. This will open a SAMCEF Field window where an assembly of beams corresponding to the shafts of the gearbox can be animated according to the eigen modes.
2.3.3Why it is important to perform modal analysis
The goal here is to detect some potential resonances with respect to the excitation existing in the system. The excitation could have several origins and depend strongly on the speed of rotation of the main shaft. Resonances have to be avoided at all cost around the rated speed, that is to say in the normal state of the machine.
For example if we imagine a rated speed of the rotor shaft set at 15 rpm. The potential excitations are listed below.
Some Eigen mode could be excited. The exercise here consist in :
Definition of the potential frequencies
Identification of the component responsibe for the mode and how they could be modified (Bearing stiffness, shaft diameter…)
Perform the modification in the parametric model
Verify the new design with a modal analysis
2.4 Integration of the gearbox in a multi-megawatt WT model
2.4.1 Creation of the WT model – 3MW Wind Turbine
The gearbox that has been analyzed up to now will be included in a global model of wind turbine whose main characteristics are presented in the following table.
Step1: Creation of the Path to the specific component created. The directory contains the Gearbox model and a specific controller for this machine.
1
3
2
Note: the controller has been specially optimized for this wind turbine. The generator curve that is defined for the control logic is presented in the figure below.
Step 2: In the wind turbine initialization window, select the WT component. Do not forget to select the connection table with nacelle.
Rated power Generator Curve
Analytical
Aerodynamic Torque
Step 3: Change the WT parameter. Select the wind turbine assembly in the data tree and the click on parameters. Set the rated power to 3MW.
2.4.2Modal analysis on the complete wind turbine
Step 1: Create a design loadcase with a constant wind of 15 m/s. Then, create a modal analysis on this loadcase, set the duration of the analysis to 30s (this is more or less the time it will take the system to stabilize under all the loads and for the transient vibrations to damp out). Compute all the frequencies in the band 0-1000Hz.
Step 2: Analysis of the mode shapes of the complete wind turbine. Check if there are no potential resonances of the Gearbox.
3 PART 2: DIRECT-DRIVE WIND TURBINES
There is a considerable interest nowadays in the direct-drive concept, in which the generator is directly connected to the rotor shaft. The main advantage is to avoid the use of the gearbox, which is a component simultaneously expensive and vulnerable on the long term. Direct-drive wind turbines are supposed to be more reliable and therefore more efficient. However, other types of problems may arise as will be explained later.
Here we will demonstrate how we can construct a model of direct drive wind turbine, using user-defined components. A set of analysis will then be performed and some attempts at improving the global design of the wind turbine will be made.
1.1Construction of the model:
Open a new window of S4WT and click on the initialization button. Because a direct-drive wind turbine is conceptually completely different than a geared one, we will need several user-defined components, created in SAMCEF Field.
In the dialog box select the following options:
o Standard segmented tower (90m)
o SFIELD Bedplate
o SFIELD Rotor shaft
o Advanced rotor 15 sections (Blade connections)
o SFIELD Generator
o Standard controller
All the other components may be left undefined. In particular, we do not need any gearbox, since the model is a direct drive, and we do not need any high speed shaft either, since the generator is directly connected to the low speed shaft.
In order to save time, the sfield models for the bedplate, Rotor shaft and Generator have been prepared beforehand; they can be found at the following location:
-[tutorial_directory]\Day2\DirectDrive\BEDPLATE_DIRECT_DRIVE\Bedplate_DD.sfield
-[tutorial_directory]\Day2\DirectDrive\MAINSHAFT_DIRECT_DRIVE\Mainshaft_DD.sfield
-[tutorial_directory]\Day2\DirectDrive\GENERATOR_DIRECT_DRIVE\Generator_DD.sfield.
For each of these, the path to the file model must be defined in the engineering object as shown on the picture below:
Then, update the connectors of each SFIELD component, by clicking on it with the right button of the mouse, and then clicking on “Update connectors”. This operation allows to reloads the connection point of the component. It is a necessary operation to be able to connect the component with the rest of the wind turbine model.
The models that we have just created are made of purely rigid bodies, except for the generator, which is made of flexible beams. The reason for this choice is that we wanted to limit the number of DOF as much as possible, to keep the computational time low. The main drawback is that effects such as bending of the main shaft or of the bedplate cannot be taken into account. A much more accurate approach would make use of super elements. At the end of the tutorial the user is invited to create the super elements of the Bedplate and the main shaft to see the influence of the flexibility in the dynamic loads.
The controller needs to be modified as well, because the standard controller is by default initialized for a geared wind turbine. A dll for the controller adapted to the current case is provided in the tutorial directory, along with a data file that specifies the parameters of the controller. To modify the controller data files, go to the SE “Controller”>EO “Control Data”. There, the path to the controller files has to be set to
-[tutorial_directory]\Day2\DirectDrive\Control\Control_DD.dll and
-[tutorial_directory]\Day2\DirectDrive\Control\Control_DD.dat,
respectively.
If the view is updated, you should observe that the components do not match, since they are not properly connected yet. As many components are user defined components, the name of the connection nodes is not the default one, and S4WT is not able to recognize the classical connectors. As a consequence, the connection table has to be built from the beginning.
To do this, select the wind turbine assembly in the data tree, and edit the connection. Add a couple of lines by clicking on the + button in the tool menu.