Wind turbine loads during simulated thunderstorm microbursts
Authors: Nguyen Hieu Huy , Manuel Lance , Veers Paul S. 
Published: October 2011

The International Electrotechnical Commission (IEC) standard 61400-1 for the design of wind turbines does not explicitly address site-specific conditions associated with anomalous atmospheric events or conditions. Examples of off-standard atmospheric conditions include thunderstorm downbursts, hurricanes, tornadoes, low-level jets, etc. This study is focused on the simulation of thunderstorm downbursts using a deterministic-stochastic hybrid model and the prediction of wind turbine loads resulting from the simulated thunderstorm event’s wind field. The wind velocity field model for thunderstorm downburst simulation is first discussed; in this model, downburst winds are generated separately from non-turbulent and turbulent parts. The non-turbulent part is based on an available analytical model, while the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions adjusted by information on parameters such as the thunderstorm’s translation velocity. In an incremental manner, we address the chief influences of the wind velocity fields associated with downbursts—namely, large wind speeds and rapid direction changes during the storm—by simulating various velocity fields and studying associated turbine loads. The turbine loads are generated using stochastic simulation of the aeroelastic response for a model of the selected utility-scale 5 MW turbine. While we believe this study is likely the first one to directly address the influence of thunderstorm downbursts on turbine loads, we make some controls-related assumptions in this work—for one, we allow for significant yaw errors, during periods of rapid wind direction change, in computing loads; additionally, for brief periods when high winds are in excess of cut-out, the turbine is assumed to continue to operate with similar blade pitch control rates as for winds close to and below the cut-out speed. While these assumptions do influence the loads experienced, the various cases included in this study serve to illustrate how they do so. Moreover, the study highlights the need for enhancements to models for aerodynamic loads computation that can more accurately address large yaw error, yaw control, blade pitch control, and transitions from turbine operating to possibly parked states that are especially important in dealing with transient events such as thunderstorm downbursts. Finally, comparisons of the turbine response to downbursts with discrete events such as in the “extreme direction change” and “extreme coherent gust with direction change” load cases specified in the IEC standard are presented, and brief remarks are made about these comparisons. Such comparisons serve to indicate how turbine loads during thunderstorm downbursts can be quite different from those specified in the IEC standard’s design load cases. Simulation procedures, as outlined here for simulation of downburst-related inflow wind fields, are not difficult to include in site assessment for regions where thunderstorms occur frequently. They might also be considered in future standards-related design load case definitions.