Observations of Corona Discharges from Wind Turbines

Abstract

In the summer of 2013 we participated in a field program to study lightning attachments to wind turbines. As part of this program, we deployed a ten-station compact Lightning Mapping Array (LMA) in the vicinity of a wind farm in Kansas. Because of the small size of the LMA network (25 km diameter) and the low environmental RF noise levels, the LMA was very sensitive, being able to locate sources with radiated powers as low as 10 milliwatts. In addition to the LMA, two continuously-recording electric field mills were located near two of the wind turbines in the wind farm. When the local electric fields were high in the vicinity of the wind farm (|E| > 1 kV/m), some of the turbines would emit corona discharges with sufficiently strong radiated power to be located by the LMA. During active periods, a turbine would emit several hundred detectable discharges per minute. The emissions were not continuous, but were emitted in periodic groups, with tens of discharges emitted in about one second, followed by a few seconds of no emissions. The periodicity of the groups was the same as the rotation period of the turbine rotor. The fact that group periodicity is the same of the rotation rate indicates that only one of the three blades emitted detectable corona discharges. Also, while some turbines emitted LMA-detectable corona, other nearby turbines did not, indicating that corona emissions possibly were due to variations in the blades of the different turbines. One turbine was located about 50 m from an LMA station. Because of the close LMA station, horizontal and altitude location errors from discharges produced by that turbine were about 20 m, smaller than turbine blade length. The LMA was able to track the positions of the VHF radiation from the corona discharges relative to positions of the turbine blade tips. The data show that the source of the radiation is near the middle of the blade. While the actual discharge is confined to a small region of the turbine (probably at the spark gap in the turbine hub, at the tip of the blade, or at an imperfection along the blade), the lightning conductor in the blade acts as an antenna, so the centroid of radiation is near the mid-point of the blade. Preliminary analysis suggests that there may be a correlation between turbine blades which produce the largest amount of detectable discharges, and the probability of lightning damage to that turbine. However, more analysis is needed to confirm this. The wind turbines were not the only tall structures in the area. In addition to numerous wind turbines, there were five tall communications towers located near the center of the LMA. While the turbines emitted detectable corona whenever fields in the vicinity were strong, the taller communications tower rarely emitted locatable corona discharges. During the time the LMA was deployed (26 April through 3 September), there was only one short period when four of the five towers emitted detectable discharges. This indicates that the corona discharges from the turbines are not associated with the turbines' heights, but are due to the rotation of the turbine blades.