Wind Ramp Events at Turbine Height – Spatial Consistency and Causes at two Iowa Wind Farms

Wind ramp events observed at an 80-m meteorological tower in Pomeroy, IA from 29 May 2008 to 12 November 2009 and 26 June 2010-8 September 2010 and from nacelle data at six 80-m wind turbines in central Iowa from 26 June-8 September 2010 and 28 June-16 August 2011 were examined to better understand spatial coverage and causes of the ramps. These sites are approximately 100 miles apart. A ramp event was defined as a change in wind power of 50% or more of total capacity in either four or two hours or less, and was estimated using wind observations and a power curve. 834 ramps were observed at Pomeroy, IA and 1324 ramps were observed in central Iowa among the six turbines. The study focused on (i) consistency of ramp events between the two sites during the roughly 10 week period when data were available at both locations, (ii) dependence of ramp occurrence at nacelle sensors on wind direction and the orientation of the turbines auses for ramp events, (iii) consistency of ramps between 80 m and nearby 10 m AWOS wind observations, (iv) causes of ramps, and (v) skill of 1 and 2 day persistence forecasts for these ramps.

It was found that forty percent of ramps in central Iowa occurred within 6 hours of ramps at the meteorological tower near Pomeroy, IA. Twenty percent occurred simultaneously. These results suggest that in general ramps are very localized in nature, and knowledge of a ramp event at an upstream site may not be helpful in predicting ramp occurrence. With 100 miles separating the two sites, however, it is interesting that in 20% of events, ramps occurred simultaneously at the two sites. In addition, in some cases, ramps were not observed at all six turbines in the central Iowa farm. Due to their east-west orientation and the “B” line being directly south of the “A” line of turbines, it is likely that wind direction determines what all turbines experience a ramp. Analysis of wind directions for these cases is ongoing. In addition, we will examine 10 m wind observations from nearby AWOS sites to determine if ramp behavior at 10 m is similar to what occurs at 80 m elevation.

Atmospheric phenomena have also been examined to determine the cause of ramp events at both locations. In Pomeroy, IA, thunderstorms or a strong pressure gradient are often present during ramps. Analysis is ongoing for the central Iowa ramps. We will also study the pressure gradients present during ramp events.

Finally, the skill of one and two-day ramp persistence forecasts was evaluated and compared to skill from 6 different WRF simulations using different planetary boundary layer schemes. Threat scores were typically between 15% and 25% for both day 1 and day 2 persistence forecasts, values a bit less than the WRF simulations for day 1 but comparable for day 2.