Six Striking Facts about Lightning and Wind Turbines

Six Striking Facts about Lightning and Wind Turbines

Modern wind turbines are typically equipped with a lightning protection system (LPS). Internationally recognized standards require that the LPS should intercept and conduct the vast majority of strikes without damage to the turbine, yet insurance companies are reporting 20% of all wind project claims are due to lightning damage. Lightning has become a significant source of higher than expected costs and downtime in the operation of some wind farms. Though much is still unknown about lightning interaction with wind turbines, there are six facts worth knowing. 

Some damage is expected, even with LPL1

Most modern turbines are equipped with a LPS designed to Lightning Protection Level (LPL)1, the highest level of protection in the IEC 61400-24 standard(1). This does not mean the LPS should, or will, protect the turbine from all lightning damage. A LPS designed to LPL 1 is not expected to protect against extreme lightning events, which can be expected in about 1-10% of all lightning strikes depending on the lightning environment. Less extreme events should not cause function-impairing damage to the turbine, if the LPS is performing as expected, but are still expected to cause minor damage. 

A flash density map is not enough to characterize a lightning environment.

A flash density map will show an average number of lightning flashes per area (km2 or miles2) per year. However, local influences such as terrain, specific storm pathways, and presence of tall objects (such as wind turbines) will affect the localized lightning flash density. In some cases, using a flash density map to determine lightning flash density at a particular site could produce errors from 10-100%, or more. It is recommended that site-specific historical lightning data be used to characterize the lightning at a site, preferably over a period concurrent with wind turbine operation or, for pre-construction assessments, adjusted to account for the future presence of wind turbines.

Not all lightning can be detected by the NLDN

The National Lightning Detection Network (NLDN), which supplies the majority of lightning data in the U.S., reports a 95% detection efficiency (DE) throughout most of North America. DE is the fraction of flashes observed by a lightning location system (LLS) like the NLDN, and is a measure of performance of that system. There is more than one type of lightning; the reported DE is only relevant for downward lightning. Upward lightning is an important piece of the wind turbine lightning damage puzzle. Though the NLDN is one of the best-performing LLS in the world, it does not capture all upward lightning events. The actual DE for upward lightning is not well-understood, but one study(2) showed a DE of about 37% at a site in Austria. More research is required to fully understand how much upward lightning can be detected by the NLDN.

Turbine rotation may increase the prevalence of lightning strikes

Recent studies(3) indicate that the rotational motion of a turbine’s rotor makes it more attractive to lightning strikes. Lightning researchers commonly use rockets attached to a thin wire to trigger lightning during storms for studies(4). Similar to the rockets, blade tips travel at speeds of up to 200 mph toward the clouds during the upward sweep of the rotation. Rotor motion may incite lightning and increase the probability of a lightning strike to the blade. Though the research is preliminary, sites with significant lightning damage may want to experiment with shutting down select turbines during storms to limit lightning damage. 

Interception is not conduction

Conduction efficiency refers to the LPS’s ability to conduct current to ground, while interception efficiency refers to the LPS’s ability to cause a strike to attach to the turbine at an LPS receptor. These functions are separate. LPS testing and maintenance is often focused on conduction, because little can be done to field-test interception, and maintenance is limited to ensuring LPS components such as receptors, diverter strips, and metal caps or tips are in good condition. Nevertheless, most common types of lightning damage are delamination, disbonding, or incineration of the blade structure near the tip, which can be attributed to insufficient interception performance. Interception performance is primarily influenced by blade design and LPS design, including material selection, LPS component placement, and electrical insulation.

LPS design, carbon fiber, tip height, and terrain influence damage rates

The lightning environment and LPL are not the only considerations when estimating the frequency of function-impairing lightning damage at a particular site. LPS design, manufacturing quality, and installation are all influencing factors. Additional influences include presence of carbon fiber in the blades, tip height, and terrain. Turbines on the tops of hills attract more lightning than turbines on flat terrain. 

Lightning interaction with wind turbines is presently poorly understood but knowledge is evolving. While it is possible to calculate the risk of lightning damage(5), industry understanding of lightning interaction with wind turbines continues to increase. With that increased understanding, comes improvements in new LPS designs, as well as further evolution of governing standards, such as the IEC 61400-24, a new edition of which is currently in work. For wind projects that are operating now, engineering and installing effective retrofits for an under performing LPS is a challenging undertaking. 

As understanding of the interaction between lightning and wind turbines increases, LPS technology should progress and performance should improve. The current practice of writing off lightning damage as force majeure obfuscates responsibility and slows innovation in LPS design. Appropriate allocation of the costs of damage will help move LPS design forward, which would certainly strike anyone as positive progress for the wind industry.