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 offshore 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 shows that only a small fraction of damages observed in the analysed farms 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 farms can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can be triggered 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 case studies.

Attachment of downward subsequent dart leaders has been recently proposed as a possible mechanism of lightning damage 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 upward connecting 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 attachment process of subsequent dart leaders by accounting the rapid changing electric fields produced by their fast descent toward the ground. For this, an improved evaluation of the charge per unit length required to 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 of lightning 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, and velocity of positive upward connecting leaders can be obtained with SLIM in agreement with the experimental results. Further research on upward leader discharges necessary to improve the modeling 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 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 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 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 6 m from it).

This paper presents the numerical evaluation of the propagation of positive upward connecting leaders under the influence of lightning dart leaders. The simulation is performed with the self-consistent leader inception and propagation model - SLIM-. An analytical expression is derived for calculating the charge per unit length required to thermalize a new upward leader segment. The simulation is validated with two dart leader attachment events in a lightning triggering experiment reported in the literature. Good agreement between the estimations and the measurements of dart leader interception in length, duration and velocity of upward leader propagation has been found. Further analysis is carried out on dart lightning leader interception.

Wind farms are an important part of renewable energy system but they are frequently damaged by lightning strikes, especially to the blades of the turbine. Recent field observations have shown lightning damages several meters inboard away from the blade tip, which cannot be explained fully by the interception of stepped leaders or swept strokes. This paper proposes a possible additional mechanism to explain the lightning attachment points in inboard area of blade due to dart lightning leaders. This mechanism is verified through the Self-consistent Leader Inception and Propagation Model -SLIM-which is used here to dynamically evaluate the upward connecting leader propagation under the influence of both stepped and dart leaders.

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.

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.