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| The Effect of Gearbox |
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In order to specify a wind turbine gearbox, simulations of the wind turbine behaviour areundertaken under a wide range of varying conditions. These simulations usually incorporate asimple gearbox model which just represents a constant gear ratio. From the simulation data aload duration distribution (LDD) is calculated which specifies the loads encountered and thetotal amount of time (or number of revolutions) for which each load level is experienced. Thisinformation is used to rate the performance and life of the gearbox.
With a simple constant ratio gearbox model, the boundary conditions, and hence LDDs, at theinput and output shafts will have a very simple relationship: the torque at the output will be thatat the input divided by the gearbox ratio, and the speed at the output will be that at the inputmultiplied by the gearbox ratio.
To design the gearbox, the designer will usually use quasi-static methods. These methods lookat quasi-static single point solutions of the gearbox for a given set of input forces. Due to thequasi-static nature of the analysis, the input and output boundary conditions will match those ofthe simple constant ratio gearboxes model (apart from any small rotation between the input andoutput shafts which may be ignored in the analysis) i.e. the output torque will always be theinput torque divided by the gearbox ratio.
Including Gearbox Dynamics in the design processWhen gearbox dynamics are included, either in the analysis of the wind turbine as a whole or aspart of the analysis of the gearbox, the above situation changes.
If the gearbox dynamics are included in the wind turbine simulation then the input and outputboundary conditions, and hence the LDDs, for the gearbox will become slightly different, due tothe dynamics of the gearbox. The simulation is therefore inconsistent with the quasi-staticmethod of analysis, and there is no longer a single LDD to represent the load distributionsthroughout the gearbox. In fact, separate LDDs can now be defined for any of the internal partsof the gearbox.
If simulation is used as part of the gearbox design, but not as part of the wind turbine simulationthen again the boundary conditions will be different. In fact, if dynamics are used in both it isvery difficult to keep the boundary conditions consistent as the gearbox design itself affectsthose conditions. The question then becomes, is it important to keep the boundary conditionsconsistent, and what are the best boundary conditions to use.
Including dynamics in the analysis of a gearbox is far from straight forward. One possible routeforward is to design the gearbox as with current methods but to check the gearboxes design byincluding it in the drive train model for the complete wind turbine simulation. This model canthen be run, exactly as for the initial generation of the LDD for the gearbox, but instead ofgenerating one LDD, several LDDs are generated for each key component in the gearbox.
These LDDs are then used to rate the individual components (not used in complete gearboxes model) to check that the design is satisfactory.
Analysis methodThe analysis in this paper brings together two commercial software packages:
GH Bladed is a widely-used and market-leading wind turbine simulation software package usedfor design and certification of wind turbines. It incorporates detailed models of the rotoraerodynamics, structural deformations, electrical and control systems etc., but the gearbox is generally represented simply as a gear ratio. The package includes standard models of threedimensionalwind turbulence, and is designed for compatibility with the international standardsused for design and certification of wind turbines.
RomaxDesigner is a package used for detailed gearbox design. It accurately models thedynamics of the gearbox, and includes detailed models of bearings and gear tooth interactions.
GH Bladed has recently been extended to allow the possibility to link the wind turbine simulationto a separate gearbox simulation, using a standard interface. The gearbox dynamics are codedseparately into a dynamic link library (DLL) and the interface between GH Bladed and thegearbox DLL is published in a precise and well-documented specification [1]. This interfacespecification was used to extend the RomaxDesigner software, adding the capability to export asuitable DLL directly for use in GH Bladed. The interface specification ensures that the turbineand gearbox dynamics are fully coupled to each other.
AnalysisIn order to complete some example analyses a generic 2.0 MW wind turbine model has beenused, and an appropriate gearbox design has been developed for it.
The wind turbine is fictitious, but based on typical commercial turbines of this class. It is a 75mdiameter three-bladed upwind turbine suitable for shallow offshore conditions, operating atvariable speed with full-span pitch regulation. The gear ratio is 84.24, and the overall torsionalstiffness of the drive train was assumed to be 2.4x108 Nm/rad.
This wind turbine model was used with the GH Bladed software package to run a set ofturbulent wind simulations representative of the complete wind turbine lifetime:
10-minute simulations with IEC turbulence Idling at low and high wind speeds Power production at 2m/s intervals from cut-in to cut-outUsing a long-term Weibull distribution of hourly mean wind speeds, the simulations wereweighted according to the probability of occurrence of the wind conditions, and a LDD wascomputed for the gearbox.
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