A data-driven reassessment of the extreme coherent gust with direction change

Abstract. The International Electrotechnical Commission (IEC) standard Wind Turbines – Part 1: Design Requirements defines an external condition known as the extreme coherent gust with direction change (ECD). This forms part of a design load case in which wind speed increases by 15 ms^-1 and wind direction shifts by more than 60° over a 10-second period. Because of these abrupt changes, the ECD strongly influences the design of many wind turbine components, particularly the tower. As turbines have grown substantially in size over recent decades, the need to update these design standards has become increasingly pressing. In particular, questions have arisen over whether the ECD accurately captures the most extreme conditions experienced by modern turbines. This study therefore reassesses the ECD at a regional level, using data from 319 Vestas turbines operating across the globe. By treating each wind turbine as an anemometer, we find that wind speed and direction ramps are caused by thunderstorm outflows, also known as cold pools. While other mechanisms also cause ramps, cold pools most often show wind direction changes greater than 30°. Of the 75 events identified, none matched the ECD's defined characteristics. Furthermore, these events were concentrated in specific regions, notably the United States of America, Germany, Australia, and Argentina. Applying a probabilistic gust model to these four regions, we estimate the ECD's return period to be greater than 200 years – well above the 50-year return period intended by the standard. Comparing extreme loads from representative 50-year events in these regions to those produced by the ECD, we find a 28 % reduction in the resultant bending moments at the bottom of the tower. These findings underscore an urgent need to redefine the ECD, and we believe this work provides a step in that direction.