Modal Analysis of a 500 kW Wind Turbine

This report briefly describes the modal analysis of a 500 kW wind turbine carried out using finite element model. The wind turbine operates in turbulent windy atmosphere and experiences a wide range of dynamic loads. Flexible rotor blade and tower adopted requires design consideration to keep them out of resonance regime. The indigenously developed teetered two bladed downwind turbine has rotor of 45m diameter placed at the top of 60m tubular tower with four guy rope support. The modal analysis of the rotor, two blades and hub assembly, was performed with hinged boundary condition at the teeter axis. Modal frequencies of the parked rotor predicted would be used in conjunction with experimental data for validation of the FE model and that of spinning rotor for design. Tower modal frequencies were obtained using tapered beam elements for the tower and a lumped mass for the tower top mass. Guy rope stiffness was estimated using load elongation data obtained on a test specimen and the same was included as spring in the FE model. Campbell diagram were drawn for the rotor and the tower and the risk of resonance build-up by the potential excitations of rotor harmonics was explored. The Campbell diagrams indicated that the tower and rotor natural frequencies were well separated from the potential rotor excitations. Dynamic load monitored on the tower using strain gauge and the visible observation of rotor teeter at low wind speed during the run of prototype reinforces the results of the modal analysis. However dynamic loads during transients of wind turbine operation needs to be monitored and investigated for further improvement towards the smooth operation of the wind turbine as the first prototype was yet to be configured with yaw and teeter dampers