The work presented in this thesis aims at investigating the attachment of lightning flashes to wind turbines. Modern wind turbines are highly exposed to lightning strikes, due to the increase of their height and the rotation of the blades. Upward lightning is the dominant mechanism of lightning strikes to them. Therefore, this study evaluates the initiation of the initial upward leader discharge and the process of lightning attachment of dart leaders taking place prior to the first return stroke in upward flashes. This work extends the self-consistent leader inception and propagation model (SLIM) to evaluate the lightning attachment of dart and dart-stepped leaders to grounded objects. SLIM was originally proposed to evaluate the lightning attachment of stepped leaders. Unlike the well-studied lightning attachment of stepped leaders, upward connecting leaders initiated in response to dart and dart-stepped leaders develop under a significantly faster change of the ambient electric field. Additionally, these connecting leaders could develop in warm air pre-conditioned by the previous strokes in the same flash. An analytical expression to evaluate the charge required to thermalize the connecting leader per unit length is also developed in the extended model. This model is validated through the analysis of three attachment events recorded in rocket-triggered lightning experiments. Good agreement between the predicted properties of the upward leaders and the measurements has been found. The model is utilized to evaluate the different conditions where connecting leaders can develop prior to the return strokes in upward lightning. The extended model of SLIM is also applied to study the interception of lightning dart leaders by upward connecting leaders initiated from wind turbines. The evaluation considers the influence of the return stroke peak current, the blade rotation and wind on the attachment of lightning dart leaders to wind turbines. The probability of lightning strikes to the receptors along the blade and on the nacelle is calculated for upward lightning flashes. It is shown that the lightning attachment of dart leaders is a mechanism that can explain the lightning damages to the inboard region of the blades (more than 10 meters from the tip) and the nacelle of wind turbines. Furthermore, the critical stabilization electric field required to initiate upward lightning from wind turbines is evaluated for both ‘self-initiated’ and ‘other-triggered’ upward flashes. The calculation shows that the stabilization electric field of an operating wind turbine periodically changes due to the rotation of its blades.  The initiation of upward lightning is greatly facilitated by the electric field change produced by nearby lightning events. However, the rate of rise of the electric field only has a weak impact on the stabilization electric field. The evaluation of the stabilization electric field provides essential information needed for the estimation of the incidence of upward lightning to wind turbines. / <p>QC 20161201</p>

Field observations has shown that wind turbinesare especially exposed to lightning strikes. The probability forlightning strikes to offshore wind turbines has been analysed ina previous article. In this project the probability for upwardself-initiated lightning strikes to onshore wind turbines anda lightning protection tower was analysed. This was done bycollecting elevation data and recreating the site topographyin COMSOL Mutliphysics 5.5, and also by collecting weatherdata which were analysed in MATLAB. The probability for thecritical electrostatic field was then calculated and analysed. Theresult shows that the risk of lightning strike is correlated to thetopography and cloud height. / Fältobservationer har visat att vindkraftverk är särskilt utsatta för blixtar. En tidigare studie har analyserat sannolikheten för blixtar på vindkraftverk belägna ute till havs. I det här projektet analyserades sannolikheten för blixtar på vindkraftverk och en vädermast på land. Detta gjordes genom att samla väder och topografidata från de undersökta områdena, som sedan modellerades i COMSOL Multiphysics 5.5. Sannolikheten att ett kritiskt elektriskt fält uppstår beräknades med hjälp av MATLAB. Resultatet visar att risken för blixtar är korrelerat med topografin och molnhöjden. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm