Lightning strikes the United States about 25 million times a year, according to the National Weather Service. That’s an impressive figure, and one reason why wind turbine owners and operators breathe a sigh of relief when their equipment goes unscathed during electrical storms. However, even a “close call” can cause more harm than one might expect. According to new research, near lightning strikes can lead to serious turbine damage and downtime.
A quick search of the Internet for “turbine blade and lightning damage” generates multiple images of punctured and split blades. Most of these blades probably passed annual inspections, and may have already taken a lightning strike or two before being damaged to the point of requiring repair. In fact, these same blades likely experienced hundreds or thousands of “near strikes” that created microscopic levels of damage prior to the fatal lightning blow.
What is a near strike?
When a lightning storm passes near a wind turbine, the storm imposes a strong electric field on the turbine and blades. This electric field is amplified near the blade tips, causing the air by the tip to ionize and form energetic, high-voltage streamers and leaders—otherwise known as near strikes.
Researchers at the Polytechnic University of Catalonia in Barcelona recently recorded high-speed video of thunderstorms passing near a wind turbine facility. As can be seen in the researchers’ recordings and as described in their scientific article (watch the video here), electrical activity or near strikes occur at the blade tips. This continues for nearly every rotation of the blade for periods of a few minutes up to a few hours. The researchers also found that near strikes can occur even when a lightning storm is several kilometers away.
Not surprisingly, near strikes place considerable electrical stress on a blade’s composite structure. The electrical fields exceed the dielectric strength of the blade materials, creating permanent electrical fissures within the resin system. As near strikes occur over time, a wind blade’s structure becomes electrically porous, allowing electrical energy to pass through with ease.
In most cases, near-strike damage isn’t visible from the ground and can easily be missed during routine inspections. Even during up-close examination, the only visible effect will be minute burn marks or holes, which can often go unnoticed.
A wind turbine manufacturer’s standard blade lightning protection system relies on two interconnected factors: the dielectric strength of the blade structure, and the combined series of lightning receptors mounted on a blade’s exterior. As with most things, however, the dielectric strength of turbine blades naturally decreases with age. Add in any degradation caused by near strikes, and the dielectric strength can be reduced to nearly nil. As a result, a manufacturer’s lightning protection system becomes less effective over time, while the likelihood of significant lightning damage increases exponentially.
To extend the life of wind-turbine blades, receptors must be improved and the dielectric strength of the blades restored. Current repair options involve removing the blade tip to install a metallic tip or a new blade composite structure, which is an expensive and time-consuming process.
Segmented lightning diverters offer a simpler, more cost-effective solution. Composed of a series of small metal buttons (on a 0.3-mm thick by 10-mm wide laminate), segmented diverters increase the effectiveness of existing receptors and protect an already degraded blade structure. When in the presence of a near strike or a full blown lightning strike, an ionized channel forms over the metal buttons to direct lightning energy to the existing receptor system and away from the blade structure.
Recent technological advancements in diverter design also make it possible for some types of segmented diverters to withstand multiple IEC 61400 LPL1 lightning strikes. Years of in-service experience on wind-turbine blades and aircraft have confirmed that segmented diverters can be extremely durable in challenging environments.
With lightning strikes expected to increase by 50% due to climate change (according to a 2014 study led by University of California Berkeley, and published in the journal Science), protecting turbine blades is becoming even more essential. Preventing repetitive near-strike damage to blades is crucial to decreasing downtime and extending turbine performance.
Segmented lightning diverters provide an effective, cost-efficient means of protecting blades from near and direct lightning strikes, ensuring the full lifetime of a wind farm. WPE
Filed Under: O&M, Safety