Lightning damage to wind-turbine blades is one of the most frustrating and expensive in-service issues confronting operators today. Damaged blades lead to costly and time-consuming repairs and extended periods of down-time. Over the last several years, the wind community and lightning researchers have studied lightning effects on wind turbines and have added segmented lightning diverters to improve the lightning protection of blades.
Historically, glass-fiber reinforced plastic (GFRP) blades have used heavy-duty lightning receptors spaced along the blade’s span with a large down conductor located through the center of the blade, providing a low resistance path from the receptors to the nacelle. The receptor and down conductor approach has shown success at conducting lightning energy. However, lightning punctures of this blade design are, unfortunately, still a common occurrence.
Lightning punctures of the GFRP blades occur when the receptors are not ideally located or if the dielectric strength of the blade structure is insufficient to prevent lightning from penetrating the blade. Typical strikes of 5 to 20 kiloamperes that puncture the blade can delaminate the GFRP structure (typically in 25 to 75 mm diameters) or disbond small lengths of the blade. Larger lightning punctures can structurally degrade the blade to the point of failure.
As the wind industry has grown, so has the number of lightning-punctured blades. These damaged blades affect the bottom line operational costs of a turbine, and the industry is now driving to improve the lightning protection of blades.
Recent lightning research has found that lightning strikes tend to occur at the blade tip and rarely attach further than 4 or 5 meters from the tip. This conclusion follows common logic that lightning will tend to strike the highest object (Ben Franklin’s lightning rods are a good example of this.) In fact, most lightning attachments occur within the first one to two meters of the blade’s tip. As turbines and blades grow taller, they will initiate more lightning events, showing increased damage near the tip.
Blade design engineers are now focusing on the use of segmented lightning diverters to enhance the lightning protection of both existing and new blade designs. Segmented lightning diverters were originally developed for aerospace use, and for over 40 years they have protected the GFRP nose cones on aircraft. These specifically designed devices consist of a line of small metal segments on a flexible substrate that, when exposed to lightning conditions, form an ionized channel above their surface to direct the lightning energy toward grounded metal structure. Segmented diverters are approximately 0.3 mm thick and 10 mm wide and can conform to the complex curvatures of the blade. Their function is illustrated Segmented lightning diverter.
A typical installation of segmented diverters to an in-service GFRP blade, the following image, positions three segmented diverters from the outermost receptor to the tip, leading edge, and trailing edge of the blade. A standard two-part epoxy or double-sided tape is commonly used in the field to bond the diverters to the blade surface. Capable of conducting multiple lightning strikes, these segmented diverters force the lightning energy to the existing receptor. This simple modification can significantly reduce lightning punctures to the tip of the blade thereby keeping it in-service longer.
Improved lightning protection on blades results in significant savings in operational costs. Segmented lightning diverters, added to existing blade protection systems, are an integral component to these savings. WPE
Filed Under: News, O&M, Obstruction lighting