Presentations

Abstract

We present observations of a gravity wave event impacting an offshore wind farm. The Borssele 1&2 wind farm is in Dutch water 23 kilometres from the coast and consists of 94 turbines each with a capacity of 8 MW. The gravity wave substantially impacts the power production over the span of an hour before the wave train exits the wind farm. The observations highlight the significant short-term influence that atmospheric gravity waves can have on wind farm power production. We discuss the implications for short-term power forecasting error and grid stability and highlight the effect on the turbine control system and loads. By analysing high-frequency SCADA data we utilize the turbines as a distributed sensor network to reveal the spatiotemporal structure of the gravity wave train. The wave pattern causes turbine-level power oscillations with an amplitude exceeding 2 MW and a period of 5 minutes. Note that with the typical 10-minute averaging of turbine data the oscillations associated with the gravity wave would be unresolved.   The gravity wave has a wavelength of 2 km and passes through the wind farm from the north towards the south in the span of an hour. The wave train interacts with the flow pattern from the wind turbine wakes, which complicates the structure and may play a role in the eventual breakup of the wave pattern.   We observe the gravity wave event with a broad range of sensors in the wind farm. A profiling Doppler lidar on the offshore substation Borssele Beta, providing wind observations up to a height of 300 m, shows that the gravity wave results in oscillations in the horizontal wind speed and wind direction, as well as oscillations in the vertical wind speed up to 2 m/s. The oscillations in the horizontal wind speed can be detected from the lowest lidar measurement height, which is below the turbine hub height. This provides a clear link between the atmospheric perturbation and the power oscillations.   These observations indicate a horizontally propagating gravity wave, trapped in a stable layer between the sea surface and an inversion well above 300 m. A ceilometer on the offshore substation Borssele Alpha detects oscillations in the backscatter signal, indicating the vertical motion of aerosols by the gravity wave up to a height of 1 km. Other meteorological observations, such as temperature and humidity, show oscillations resulting from the rising and sinking of air parcels. Finally, comparison between the air temperature and sea surface temperature indicates stable atmospheric conditions near the surface prior to and during the gravity wave event.   The gravity wave event coincides with a wind speed ramp, due to the passage of a cold front coming from the north, which is probably also the source of the gravity waves. Hindcast weather data are analysed to understand the synoptic situation around the event, including vertical profiles of the potential temperature.