Attachment of downward subsequent dart leadershas been recently proposed as a possible mechanism of lightningdamage of wind turbine blades. Since subsequent dart and dart-stepped leaders propagating after the first lightning discharge are one-to-two orders of magnitude faster than downward stepped leaders, the direct evaluation of the dart leader interception by upwardconnecting leaders from the turbine has not been attempted before. In this paper, the self-consistent leader inception and propagation model SLIM is used to evaluate the lightning attachmentprocess of subsequent dart leaders by accounting the rapid changingelectric fields produced by their fast descent toward the ground. For this, an improved evaluation of the charge per unit length requiredto thermalize the upward connecting leader is derived. The analysis considers upward connecting leaders propagating along the preheated channel of a prior discharge. Three study cases oflightning attachment of dart leaders and dart-stepped leader reported in rocket-triggered lightning experiments are evaluated. It is shown that reasonable predictions of the length, duration, andvelocity of positive upward connecting leaders can be obtainedwith SLIM in agreement with the experimental results. Furtherresearch on upward leader discharges necessary to improve themodeling of attachment of dart lightning leaders is discussed.

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 beenobserved in field studies. However, these damages inboard fromthe tip cannot be explained by the attachment of downwardstepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flashis investigated as a mechanism of strikes to inboard sections of the blade and the nacelle. Dart leaders in an upward lightning flashuse the channel previously ionized by the preceding stroke or thecontinuous current. The analysis is performed with the self-consistent leader inception and propagation model SLIM. A commercial wind turbine with 45 m long blades and hub height of 80 m 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 the 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. However, this mechanism cannot explain the lightning strikes observed in the close vicinity of theblade tip (in the region between 1.5 and 6 m from it). The modelling study here also shows that for the turbine under consideration, nacelle receptors intercept most of dart leaders,around 73%, even if the initial continuous currents flow to the blade tip. Overall, it is estimated that around 80% of strikes inupward lightning flashes attach to the tip receptor of the blade.

Field observations have shown that the frequency of dangerous lightning events to wind turbines, calculated according to the IEC standard 61400-24:2010, is grossly underestimated.This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to off shore wind power plants. Three different farms are used as case studies. The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis show that only a small fraction of damages observed in the analysed parks can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunderstorms in the area of the analysed parks can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can betriggered even under low thundercloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the casestudies.