Simulation of Inflow Velocity Fields and Wind Turbine Loads during Thunderstorm Downbursts

Abstract

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 associated large wind speeds and direction changes during the storm—by first simulating several different simpler 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 5MW turbine. While we believe this study is likely the first one to directly address a transient event such as a thunderstorm downburst and its effect on turbine loads; for that same reason, there are clearly gross over-simplifications that are made here: (i) large yaw errors result from large wind direction change and limited or no yaw control during the thunderstorm—such large yaw errors are likely out of the range of validity of the loads module implemented in the simulations; and (ii) even when hub-height wind speeds are greatly in excess of the cut-out wind speed, blade pitch control and associated pitch angle changes and associated loads are computed as if the turbine continues to operate. Despite these limitations, this study serves to directly address an important transient event—i.e., a thunderstorm downburst—and what its effects may be on turbine loads. 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 in thunderstorms. 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.