Today in the wind turbine global analysis codes such as Hawc2  or FAST , the entire gear train is modelled by one degree of freedom constant stiffness torsional spring. This is because the focus in the global analysis programs lies mainly on the aerodynamic loads and the dynamic behaviour of structural members. For the small size gear trains, since the internal natural frequencies are expected in a frequency range above the overall wind turbine harmonics, this approach can be justified. However, as the industry trend is toward the larger drivetrains in offshore developments, the internal dynamic of gear trains are required to be modelled more accurately. Moreover, the development in generator technology with low, medium and high speed options has brought a variety of gear train design options with specific dynamic behaviour.
In this paper the natural modes and internal dynamic excitations of high ratio wind turbine gear trains is investigated. Case study gear trains of 0.6, 2, 5 and 10 MW are modelled by pure torsional elements in a Multi Body Simulation (MBS) program; Simpack , where the natural modes are obtained and possible excitation are evaluated. The results show the resonance trend in various size wind turbine gear trains.