The influence of earthquake ground motion on wind turbine loads
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The design of wind turbines installed in various regions of the world where earthquakes are likely must take into account loads imposed on the turbine due to ground shaking. Currently, design standards such as the International Electrotechnical Commission’s standard, IEC 61400-1, do not provide detailed guidelines for assessing loads on wind turbines due to seismic input excitation. In regions of high seismic hazard, it is extremely important to perform a thorough seismic analysis. Various simplified and full-system wind turbine models have been published and used for seismic analysis of turbine loads in recent years. Among these models, the open-source software, FAST, allows for full-system simulation of the response of wind turbines subjected to earthquake ground motion along with other sources of loading such as from the mean wind field and turbulence. This study employs this open-source software to simulate seismic loads and presents statistical and spectral summaries resulting from extensive analyses undertaken by simulating turbine response to various input motions from Western U.S. earthquakes. A total of 150 different earthquake ground motion records with varying magnitude and distance from fault rupture are selected and normalized/scaled to selected target levels prior to response simulation using a utility-scale 5-MW wind turbine model. The records selected are divided into six groups of 25 records each; the groups consist of different magnitude and distance-to-rupture values. The records in each bin are scaled to have similar demand levels as the average of the demand of the unscaled records in that bin. Two different normalization options are considered—in one, the scaling is at the rotor rotation rate (or the once-per-rev or 1P frequency); in the other, the scaling is done at the tower fore-aft first mode frequency. A study of various turbine load measures is conducted. It is found that turbine tower loads, in particular, are especially influenced by the earthquake excitation.