Investigation of direct lightning strikes to wind turbine blades Nina Stipeti´ c, Božidar Filipovi´ c-Grˇ ci´ c, Ivo Ugleši´ c, Amir Toki´ c Abstract—Lightning discharge is a serious cause of damage to wind turbines. Because of their height, they are exposed to direct lightning strikes which can damage the blades, mechanical parts or the electrical and control systems. According to the lightning protection zone concept, only the external parts are subjected to direct lightning strikes. The internal parts are subjected to indirect effects of lightning, but they are sensible because of their low insulation level. This paper elaborates the wind turbine exposure to lightning strikes and presents the EMTP-RV simulation of a direct strike to a wind turbine blade, identifying the overvoltages at the generator and the low voltage side of the step-up transformer, the ground potential rise and the energy absorbed by the surge protective devices, depending on the grounding resistance. Keywords—lightning location system, direct lightning strike, wind turbine, surge protective device I. I NTRODUCTION W ITH a rapid growth of the wind power capacity in the world, the damages caused by lightning activity received more attention, especially in the regions with intense lightning activity. The statistic analysis of the reported wind turbine damages caused by lightning in the early growth in the wind turbine installations showed that the blade damage is the most expensive and damage of low voltage electrical equipment and control system within the tower was the most frequent [1]. Following the world’s trend, the wind power capacity in Croatia is in the ascent, with 576 MW currently installed and 11 % share in total power generation; and with plans to reach 1328 MW until 2021 and 2193 MW until 2028 [2]. As a Mediterranean country, Croatia has a relatively high keraunic level, especially on the coast and in the hilly regions close to the coast. This is exactly where the wind potential is significant and where all the Croatian Wind Power Plants (WPPs) are installed. Recent experiences of both blade damages and low voltage equipment failure in Croatia were the motivation for the research and analysis of the wind turbine exposure to lightning discharges and the transient phenomena due to a direct lightning strike to a wind turbine blade. This work was supported in part by the Croatian Science Foundation under the project number IP-2013-11-9299. N. Stipetic, B. Filipovi´ c-Grˇ ci´ c and I. Ugleši´ c are with University of Zagreb, Faculty of Electrical Engineering and Computing, Croatia (e-mail: nina.stipetic@fer.hr), A. Toki´ c is with University of Tuzla, Bosnia and Herzegovina Paper submitted to the International Conference on Power Systems Transients (IPST2019) in Perpignan, France June 17-20, 2019. A. Lightning activity observation at the Wind Power Plant location The data collected by the lightning location system (LLS) can be used to get the insight in the lightning activity at WPP locations. Such analysis was done in [3], where the authors analyzed the lightning data before and after the wind turbines’ installations, adapting the definition for the wind turbine attraction area from the IEC 61400-24 [4]. Since the wind turbines, as very tall, isolated structures attract lightning, the expected increase in the lightning strike density after the wind turbine installation was confirmed. The same analysis was done using the data from Croatian LLS. The lightning activity on the WPP micro-locations was observed before and after the wind turbine installations. The following example presents the lightning activity change on the location of two WPPs which consist of 14 wind turbines. The rated power of each wind turbine is 3 MW, the towers are 80 m high and the blades are 49 m long. Figure 1 shows the lightning strike density map of the WPP location before and after the installation of wind turbines in 2013. The map on the left considers lightning data recorded 2009-2012 and the map on the right the data recorded 2013-2017. The most significant change is observed in the impact area of wind turbine WT1-4, where the strike density after the wind turbine installation is 141.6 strikes/(km 2 year), including all first and subsequent strikes, which is a 5.25 times increase [5]. Due to the geographic characteristic of the location, the relative difference between the lowest and highest wind turbine position is 485 m. As expected, the most exposed WT1-4 is also the most elevated of all observed wind turbines. However, the data from the lightning location systems does not give a total insight into wind turbine exposure to lightning strikes. The reason are the upward strikes initiated from the blades, whose initiation mechanism is still not understood completely. Unlike in downward lightning, upward strikes always have the initial continuous current (ICC), which lasts relatively long and has low amplitudes (tens to hundreds of milliseconds and 100 A to a few kA) [6]. If there is no superimposed fast rising and high peak current impulse, the lightning is categorized as ICC only . ICC only events are not detected by the lightning location systems due to their very low frequency electromagnetic fields and their weak peak current amplitude [3], [6], [7]. According to measurements done in the world, approximately 50 % of upward lightning strikes are of the ICC only type [6], [7], [8]. The portion is not negligible, moreover the emphasis should be given to the