Extreme wind fluctuations: joint statistics, extreme turbulence, and impact on wind turbine loads

Abstract

Abstract. For measurements taken over a decade at the coastal Danish site Høvsøre, we find the variance associated with wind speed events from the offshore direction to exceed the prescribed extreme turbulence model (ETM) of the International Electrotechnical Commission (IEC) 61400-1 Edition 3 standard for wind turbine safety. The variance of wind velocity fluctuations manifested during these events is not due to extreme turbulence; rather, it is primarily caused by ramp-like increases in wind speed associated with larger-scale meteorological processes. The measurements are both linearly detrended and high-pass filtered in order to investigate how these events – and such commonly used filtering – affect the estimated 50-year return period of turbulence levels. High-pass filtering the measurements with a cutoff frequency of 1∕300 Hz reduces the 50-year turbulence levels below that of IEC ETM class C, whereas linear detrending does not. This is seen as the high-pass filtering more effectively removes variance associated with the ramp-like events. The impact of the observed events on a wind turbine are investigated using aeroelastic simulations that are driven by constrained turbulence simulation fields. Relevant wind turbine component loads from the simulations are compared with the extreme turbulence load case prescribed by the IEC standard. The loads from the event simulations are on average lower for all considered load components, with one exception: ramp-like events at wind speeds between 8 and 16 m s−1, at which the wind speed rises to exceed rated wind speed, can lead to high thrust on the rotor, resulting in extreme tower-base fore–aft loads that exceed the extreme turbulence load case of the IEC standard.

Detection and characterization of extreme wind speed ramps

“…The first step in the selection, choosing high-variance events, is used for two purposes: first, to ensure that the selected ramp-like fluctuations are associated with scales that are large enough to cover any rotor of a multi-megawatt wind turbine. We have seen in a previous study that these fluctuations occur approximately simultaneously at two different measurement masts in Høvsøre that are separated by 400 m transverse to the mean wind direction (Hannesdóttir et al, 2019).…”

“…The data used in this study consist of 20 Hz measurements from March 2015 to February 2018 at 37, 103, and 175 m heights. More details on the site may be found in Hansen et al (2014).…”

“…The 10 min standard deviations are calculated using predefined windows (00:00-00:10, 00:10-00:20, etc.). A significant reduction in the 10 min standard deviation by high-pass filtering occurs when the measurement window includes a ramp-like fluctuation, as the latter causes a high observed standard deviation σ raw (Hannesdóttir et al, 2019). The filtering is performed with a second-order Butterworth filter where the cut-off frequency is chosen as…”

Comment on wes-2021-147

“…The wind measurements from the Høvsøre Test Centre for Large Wind Turbines in western Denmark (Dimitrov et al (2017), Hannesdóttir et al (2019)) are used in this study. The 10-minute high-frequency time series of three-dimensional wind velocities at a height of 100 m is selected.…”

“…The period of measurements is from 1 January, 2005 to 1 January, 2020, i.e. 15 years of measurement data (Hannesdóttir et al, 2019). Each 10-minute time series is used to calculate the u component mean wind speed u, and is linearly detrended to calculate the standard deviation σ u .…”