On the attachment of dart lightning leaders to wind turbines

Abstract

Wind turbines are prone to damages due to lightning strikes and the blades are one of the most vulnerable components. Even though the blade tip is usually protected in standard designs, lightning damages several meters away from it have also been observed in some field studies. However, these damages inboard from the tip cannot be explained by the attachment of downward stepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flash is investigated as a mechanism of strikes to inboard sections of the blade and the nacelle of large wind turbines. Dart leaders in an upward lightning flash use the channel previously ionized by the preceding stroke or the continuous current. The analysis is performed with the self-consistent leader inception and propagation model (SLIM). A commercial large wind turbine with 45m long blades and hub height of 80m is analysed as a case study. The impact of the prospective return stroke peak current, the rotation angle of the blade and the wind on the location of lightning strikes on this mechanism is analysed. The probability of lightning attachment of dart leaders along the blade for the case study is also calculated. It is shown that this damage mechanism could create a new strike point only when the blade of a wind turbine rotates sufficiently from its initial position (at the inception of the initial upward leader) until the start of the dart leader approach. Thus, dart leader attachment is a mechanism that can explain lightning strikes to the nacelle and to the inboard region several meters away from the blade tip in large wind turbines. However, dart leader attachment cannot explain the lightning strikes observed in the close vicinity of the blade tip (in the region between 1.5 and 6m from it).