Overview of lightning interaction and damages to wind turbines

It is well known that tall structures such as telecommunication towers can be exposed to numerous lightning incidence. Wind turbines are in this group, but two particular characteristics distinguish wind turbines from the rest. The first peculiarity is that a substantial part of the structure is rotating. That corresponds to the rotor blades. The second, is also related to the rotor blades and corresponds to the intensive use of composite materials. Due to these two factors, protection against lightning requests very special attention.Postprint (author's final draft

Understanding lightning leaders

a lightning flash is defined by the initiation and multiple development of leaders. But lightning leaders are still poorly understood. Since most of my atmospheric electricity has been focused on lightning that means that it has been devoted to understand lightning leaders. In this work, I summarize my investigations related to different aspects of lightning leaders. That includes high-speed video observations of lightning leaders, investigations of bidirectional development of leaders by means of radio mapping systems, research on the high energy emissions produced by lightning leaders and the study of upward leader initiation from wind turbines and development. In the second part, the need of an improved lightning leader model is exposed and proposed as future work. Lightning leader models can be approached by different ways depending on the purpose but agreement between all the approaches is still not complete.Postprint (published version

Lightning interaction and damages to wind turbines

Multi-megawatt wind turbines are a special type of tall structures. Two characteristics stand out from other tall objects such as buildings and telecommunication towers. The first particularity is that a large part of the structure is rotating, and the second, is the extensive use of composite materials in the rotor blades. Due to these two factors, the protection against lightning requests very special attention. Wind turbines installed all over the world have shown how a particular design of their lightning protection system (LPS) can perform very different in areas with different thunderstorm climatology. Up to now, the most severe damages are reported in those areas affected by winter thunderstorms such as Japan and Europe (e.g. Honjo, 2015). These type of storms pose the most favorable conditions for upward lightning initiation from tall structures. That is because the heights of charge regions of winter thunderstorms are significantly low (e.g. Montanyà et al., 2007). On the other hand, most wind turbine blades are built wind composite materials such as glass fibers (GF) and carbon reinforced plastics (CFRP). Blades are typically composed of two shells enclosing a spar. The lighting protection system consists of one or several air terminals connected to a down conductor that brings lightning currents to ground (e.g. IEC 61400-24, 2010). However, the insulation provided by the shells is not perfect and it is possible to experience lightning attachments to non-desired locations. Some blades are equipped with electrically conductive carbon fiber reinforced plastics (CFRP), these components shall be bonded to the lightning protection system (IEC 61400-24, 2010). CFRP laminates are poor conductive presenting high anisotropy. Moreover, electrical connections to CFRP can be very complex because in most of the cases it is not possible to use bolted connections. In the first part of this work lightning interactions to wind turbines are described according to the types of thunderstorms. The effects of the different type of interactions to the calculation of risk assessment is discussed. In the second part, lightning damages are treated. This include a review of the mechanisms of damages and statistics. The case of incidences to blades and protection to CFRP components are highlighted.Postprint (published version

X-Rays from long laboratory sparks in air

This collective book provides a review of research concentrated on runaway electron beams and X-rays in an inhomogeneous electric field with different gases at increased pressure. Attention is also given to supershort avalanche electron beam (SAEB) in the optimal conditions. New experimental techniques and equipment, including those with picosecond time resolutions, were required for diagnostics of electrical and optical signals. The book consists of twenty-four chapters, some of which were written jointly by researchers of different teams. Some chapters consider the range of SAEB applications.Postprint (published version

Electromagnetic radiation of simulated mid-gap streamers

Electromagnetic field from simulated double-headed mid-gap streamers is computed. The effect of the applied electric field is studied by considering electric fields higher than the conventional breakdown threshold (‘overvoltage’). The obtained results show a 20 dB decay at 3.8 GHz for conventional breakdown electric field, 35 GHz for ambient electric fields five times the conventional breakdown and 170 GHz for fields ten times the conventional breakdown electric field. The limitation in nature of the intense electric fields suggest further experiments using a radio receiver at frequencies of more than 10 GHz in order to investigate the origin of the detected x-rays in laboratory sparks.Preprin

A closer look at lightning reveals needle-like structures

Structural features have been identified on positively charged lightning channels that are not present on negatively charged ones. The discovery could explain why these two types of channel have different behaviours.Peer ReviewedPostprint (author's final draft

Statistics and variability of the altitude of elves

From June 2008 to January 2016 nearly 800 elves have been recorded by a low-light camera in northeastern Spain. Elves occur in this region mainly over the lower-topped cold airmass maritime thunderstorms, peaking from November to January. Cloud-to-ground strokes still produce elves when maritime winter storms are carried inland, suggesting the cold season thunderstorm charge configuration favors strokes with large electromagnetic pulses. Altitudes of 389 elves were determined using optical data combined with a lightning location network. The overall median altitude was 87.1 km, near the typical OH airglow height, but average heights during individual nights ranged between 83 and 93 km. The lower elve nights (~84 km) occurred during slightly elevated geomagnetic conditions (Kp>3-, ap-index >10). Elve altitude often shifts by several kilometers during the night, apparently in response to changing background conditions in the upper mesosphere.Postprint (author's final draft

The 2018 revision of the standard IEC 61400-24 : lightning protection of wind turbines

The first edition of the standard IEC 61400-24, Wind Generator Systems-Part 24 Lightning Protection, was issued in June 2010, and the scope was to reflect the experiences and technical understanding of lightning protection of wind turbines by manufacturers, certification organizations, research institutes and universities. It presented background statistical information on lightning damage to wind turbines, and it provided guidance on lightning protection best practices. Since then, the wind power industry has further developed towards even larger onshore and offshore wind turbines and into a mature industry. This is the background for the current draft of the 2018 revision of the IEC 61400-24, which transforms the 2010 edition into an evolution of the previous issued standard based on improved technical experience and expertise.Peer ReviewedPostprint (published version

Turbines shoot upside-down lightning

Wind turbines emit lightning flashes upwards, producing these electrical discharges at regular intervals relative to the turbine’s rotation, and can do so tens of kilometres away from an active thunderstorm area.Peer ReviewedPostprint (published version

High-speed videos of laboratory leaders emerging from wind turbine blade tips

This paper presents some observation results of the attachment test of a wind turbine blade as described in the upgrade of the IEC-61400-24. The optical observations were performed by means of high-speed video and UV-sensitive photo camera. This study investigates the effects on the leaders emerging from the blade comparing three different air-terminals: discrete receptors, discrete receptors with diverter strips and metallic tip. In the first two types, salt mist pollution was applied to the blade. For non polluted tips positive leaders appeared from receptors towards the ground avoiding the blade surface. When the blade was polluted additional leaders emerged moving horizontally. Moreover, in this situation the leader progressed on the blade surface. Pollution also produced the inception of more leaders when the blade was equipped with discrete receptors and diverter strips.Preprin