New Blade Coating Enhances Existing Lightning Protection SystemsLightning damage stubbornly remains a major O&M expense for owner-operators in cost and frequency. Damage such as blade skin punctures, shell delamination, split trailing edges, and (less frequently) catastrophic damage to wind turbine blades is costly to repair and causes undesirable downtime. Even with the current mitigation systems in place, it is estimated that lightning damage costs the wind industry more than $ 100 million each year. Strikes are inevitable, and their frequency will only grow as turbines get taller, more onshore and offshore wind farms are developed, and our climate continues to change.
By Neal E. Fine, John A. Cooney and Christopher S. Szlatenyi, Arctura, Inc., USA
The technologists at Arctura have developed a novel wind turbine blade coating that reduces lightning damage to turbine blades by encouraging surface flashovers.
Wind Turbine Lightning Protection Systems
The most common lightning protection system (LPS) uses one or more surface-mounted receptors connected to a down conductor. A typical lightning strike begins when the strong electric field induced by a charged storm cloud causes upward streamers to emanate from these receptors. A direct strike results when one of the streamers from a surface receptor connects with a downward leader from the cloud, safely passing a large amount of electrical and thermal power in a short time through the turbine to ground.
The most common lightning protection system (LPS) uses one or more surface-mounted receptors connected to a down conductor. A typical lightning strike begins when the strong electric field induced by a charged storm cloud causes upward streamers to emanate from these receptors. A direct strike results when one of the streamers from a surface receptor connects with a downward leader from the cloud, safely passing a large amount of electrical and thermal power in a short time through the turbine to ground.
The Problem: Lightning Protection Systems Offer Incomplete ProtectionThe problem arises when streamers that originate from the down conductor or other metallic components inside the blade connect with the downward leaders from the cloud, bypassing the receptors. At best, these events lead to blade punctures, but more significant damage – from split trailing edges to total blade destruction – are often seen at wind farms where there is a lot of lightning activity. Many farms in the interior of the USA, for example, report that lightning damage is the number one or number two cause of turbine downtime and repair costs (the other being leading edge erosion). Even when the internal streamers do not lead to a direct strike, they can still penetrate the skin and weaken the structure. Over time, this can make the skin electrically porous, making it more susceptible to future lightning damage.
The Solution: A Blade Coating that Enhances the Effectiveness of the Lightning Receptors
Whenever there is lightning activity surrounding a wind turbine, there is competition between streamers that form inside the blade and those that form at the lightning receptors on the outside surface of the blade. The solution to the problem is to somehow make it more likely that the streamers that form at the receptors will ‘outcompete’ the interior streamers and be the first to connect with the lightning leaders.
Whenever there is lightning activity surrounding a wind turbine, there is competition between streamers that form inside the blade and those that form at the lightning receptors on the outside surface of the blade. The solution to the problem is to somehow make it more likely that the streamers that form at the receptors will ‘outcompete’ the interior streamers and be the first to connect with the lightning leaders.
This is the objective of a new blade coating, called ArcGuide, under development at Arctura. ArcGuide consists of a proprietary formulation of discrete conductive elements combined with a modern topcoat. Like other industry-standard topcoats, ArcGuide protects against environmental effects, including rain erosion, ultraviolet degradation, and particle impacts, while also enhancing the performance of the existing LPS. The coating can be easily applied as a retrofit solution. ArcGuide does not replace the existing LPS but rather works with it to enhance its effectiveness.
Promoting Surface Flashover
ArcGuide promotes the formation of a ‘surface flashover’ in the vicinity of the lightning receptors. A surface flashover is an ionised channel in the air adjacent to and above the surface. This ensures a safe passage of the lightning charge through the air to the lightning receptor and on to the ground via the down conductor. The coating does not directly conduct the strike energy (which would cause damage to the coating itself). Instead, the coating works by locally enhancing the electric field immediately above the blade’s surface. The local increase in the electric field strength then becomes the first to surpass the ionisation potential for air, resulting in regions of corona along the blade’s exterior surface. As the electric field strength increases, those regions of corona connect, resulting in a quicker surface flashover and a streamer that propagates away from the blade, thus reducing the field intensity around other conductors located inside the blade. In this way, it allows the streamers on the outside of the blade to ‘win the race’.
ArcGuide promotes the formation of a ‘surface flashover’ in the vicinity of the lightning receptors. A surface flashover is an ionised channel in the air adjacent to and above the surface. This ensures a safe passage of the lightning charge through the air to the lightning receptor and on to the ground via the down conductor. The coating does not directly conduct the strike energy (which would cause damage to the coating itself). Instead, the coating works by locally enhancing the electric field immediately above the blade’s surface. The local increase in the electric field strength then becomes the first to surpass the ionisation potential for air, resulting in regions of corona along the blade’s exterior surface. As the electric field strength increases, those regions of corona connect, resulting in a quicker surface flashover and a streamer that propagates away from the blade, thus reducing the field intensity around other conductors located inside the blade. In this way, it allows the streamers on the outside of the blade to ‘win the race’.
Alternative to Segmented DivertersThe physical principle underlying ArcGuide is similar to that of an existing product called a segmented diverter or a diverter strip. Originally developed for the aviation industry, the segmented diverter consists of a strip of conductive elements (e.g. copper or silver ‘dots’) separated by air gaps and arrayed on an adhesive substrate. During the pre-strike period when streamers form, the air between the conductive elements ionises, ultimately leading to a surface flashover above the diverter strip that provides a safe channel in the air to conduct the energy to the receptor. However, because the conductive elements are exposed to air, rain and particulates, the segments tend to erode over time. As a result, the diverters are less effective after the first year and can peel off the blade. They also increase aerodynamic drag, decreasing power generation. In contrast, ArcGuide integrates the same functionality with the outer protective coating on the blade skin. The added cost for ArcGuide is also much lower and there is no drag penalty.
ArcGuide Development and Testing
The ArcGuide coating was developed over a three-year period in coordination with NTS Lightning Technologies (Pittsfield, MA, USA) and with the help of a leading coatings manufacturer. The formula was developed through coupon testing involving more than 2,000 individual experiments and then refined through 700 additional tests using panels measuring 4 square feet. The company is now testing the most promising candidates with blade tips taken from operational turbines using the IEC lightning standard (IEC 61400-24).
The ArcGuide coating was developed over a three-year period in coordination with NTS Lightning Technologies (Pittsfield, MA, USA) and with the help of a leading coatings manufacturer. The formula was developed through coupon testing involving more than 2,000 individual experiments and then refined through 700 additional tests using panels measuring 4 square feet. The company is now testing the most promising candidates with blade tips taken from operational turbines using the IEC lightning standard (IEC 61400-24).
Winning the Race
One way to show that the ArcGuide coating promotes surface flashover is to measure flashover at a certified lightning test facility. In this test, Arctura uses fibreglass laminate panels measuring 4 square feet, coated with either a baseline topcoat or with ArcGuide. A high-voltage electrode is placed near the centre of one edge of the panel, and a grounded receptor is fixed at the centre of the panel. The electrode voltage is ramped linearly from zero to a peak of roughly 200kV. Near the peak voltage, the electric field exceeds the dielectric strength of the air, resulting in an electrical arc that shorts the circuit and terminates the test. The process, from the ramping up of the voltage to the formation of the surface flashover, takes about 250 microseconds.
One way to show that the ArcGuide coating promotes surface flashover is to measure flashover at a certified lightning test facility. In this test, Arctura uses fibreglass laminate panels measuring 4 square feet, coated with either a baseline topcoat or with ArcGuide. A high-voltage electrode is placed near the centre of one edge of the panel, and a grounded receptor is fixed at the centre of the panel. The electrode voltage is ramped linearly from zero to a peak of roughly 200kV. Near the peak voltage, the electric field exceeds the dielectric strength of the air, resulting in an electrical arc that shorts the circuit and terminates the test. The process, from the ramping up of the voltage to the formation of the surface flashover, takes about 250 microseconds.
If the ArcGuide coating is working as intended, achieving flashover will take less time with the ArcGuide coating in comparison with the same experiment using the baseline coating. Figure 4 shows the mean time to flashover for a set of 138 experiments like the one described in the previous paragraph. The significant reduction in time to flashover with the ArcGuide coating in comparison with the baseline coating is an indication that ArcGuide will promote the attachment of lightning at the receptors while diminishing the likelihood that lightning will puncture through the blade skin.Rain Erosion, Adhesion Strength, Colour and Other Frequently Asked Questions
The objective for ArcGuide is to provide all the existing benefits provided by the state-of-the-art topcoat with the added benefit of enhanced lightning protection. Mechanical and environmental effects testing to date show the potential for achieving this objective. The next steps planned for the validation of the technology include additional lightning testing in the laboratory and a series of pilot tests with industry partners.
The objective for ArcGuide is to provide all the existing benefits provided by the state-of-the-art topcoat with the added benefit of enhanced lightning protection. Mechanical and environmental effects testing to date show the potential for achieving this objective. The next steps planned for the validation of the technology include additional lightning testing in the laboratory and a series of pilot tests with industry partners.
Two field tests will be performed at existing wind farms in the interior of the USA, where lightning activity is high. In those planned field tests, the tips of eight wind turbines will be coated with ArcGuide at two wind farms (four on each wind farm). Lightning activity will be monitored, and differences in strike behaviour will be noted for the coated blades in comparison with nearby turbines that use the standard topcoat. The robustness of the coating with continuous exposure to the elements will also be evaluated. The pilot testing will cover two peak lightning seasons, beginning in the spring of 2022.
If successful, the ArcGuide coating will give wind farm owners and operators an elegant, inexpensive and easily applied technology to combat the costs of lightning strikes. Once validated, this technology can be applied as a retrofit at existing wind farms and it can be incorporated into new designs by the OEMs.
ArcGuide is a registered trademark of Arctura Inc.
Biography of the Authors
Neal E. Fine, PhD, is the CEO of Arctura. Dr Fine earned his PhD in Marine Engineering from the Massachusetts Institute of Technology, USA. He founded Arctura to develop and market technologies that will reduce the cost of wind energy.
Neal E. Fine, PhD, is the CEO of Arctura. Dr Fine earned his PhD in Marine Engineering from the Massachusetts Institute of Technology, USA. He founded Arctura to develop and market technologies that will reduce the cost of wind energy.
John A. Cooney, PhD, earned his PhD in Aerospace Engineering from the University of Notre Dame, IN, USA. As Arctura’s Chief Technology Officer, he leads all technology and product development activities at Arctura.
Christopher S. Szlatenyi, Senior Mechanical Engineer, leads the ArcGuide product development. He earned his bachelor’s degree in Mechanical Engineering from Worcester Polytechnic Institute, MA, USA.
