DESIGN GUIDE FOR GLASS FIBER REINFORCED PLASTIC (GFRP) WIND TURBINE BLADES.

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1 DESIGN GUIDE FOR GLASS FIBER REINFORCED PLASTIC (GFRP) WIND TURBINE BLADES

2 WXGuard is the new generation of segmented diverters, featuring excellent lightning protection, increased survivability against rain erosion, and stronger adhesion properties than traditional diverters. This guide provides recommendations on where to install WXGuard diverters on GFRP blades to enhance the existing metal lightning receptors and down conductor cable. The recommendations are based on the engineering knowledge available, and are not to be used as a guarantee of lightning protection. Lightning protection of GFRP blades depends on many variables, including the location and number of lightning receptors and the dielectric strength of the blade material. However, these recommendations should provide a baseline for improving the lightning protection of GFRP blades. Further knowledge regarding the lightning protection ability of a particular blade design can be obtained through lightning testing that follows the guidelines of IEC Global Lightning Protection Services ( and Lightning Technologies ( provide lightning testing that follows the IEC standards. If you have questions about WXGuard segmented lightning diverters or how to install the diverters on your blades, please contact Shine Wire at or wxguard@shinewire.com. 2

3 GFRP DESIGN GUIDE Lightning damage to the blades is one of the most frustrating inservice issues confronting wind turbine 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 WXGuard 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 receptors 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. When a thunderstorm approaches, the electric field created by the storm generates a high potential on the lightning receptors and the metal components inside the blade (the internal down conductor and the portion of the metal receptors inside the blade). High voltage streamers will form at these metal components and begin to travel towards the storm. As seen in Figures 1 and 2, if the high voltage streamers from the internal components are energetic and penetrate a weak spot in the GFRP blade surface, then the lightning energy can enter inside of the blade. 3

4 Figure 1 GFRP Blade During Lightning Conditions Figure 2 Lightning 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 diameter areas) 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 lightningpunctured 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. 4

5 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 using WXGuard segmented lightning diverters to enhance the lightning protection of both existing and new blade designs. WXGuard diverters consist of a line of small metal segments on a flexible substrate that, when exposed to lightning conditions, form a conductive plasma channel above their surface to direct the lightning energy toward grounded metal structure. WXGuard 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 in Figure 3. Figure 3 WXGuard Directing Current to Lightning Receptor 5

6 A recommended installation of segmented diverters to a GFRP blade positions three segmented diverters from the outermost receptor to the tip, leading edge, and trailing edge of the blade (see Figures 4 6). Additional segmented diverters may be placed on the inboard receptors, if desired, but most lightning strikes will occur to the outermost receptor. Figure 4 Typical Segmented Lightning Diverter Configuration Figure 5 Span-wise Cross Section of Diverter Configuration 6

7 Figure 6 Cord-wise Cross Section of Diverter Configuration WXGuard diverters are available with a double-sided tape for a simple installation, or the diverter can be adhered with standard two-part epoxy. For instructions for applying WXGuard segmented diverters to the blade surface, please refer to our FREE installation guide, available at WXGuard segmented lightning diverters should be an integral component to your lightning protection system. If you have any questions about how to protect your blades, please contact Shine Wire or wxguard@shinewire.com. 7