Lightning has long posed operational risks to wind farms all over the world. Recently, Reece Enderson and Meagan Santos of Natural Power, in conjunction with James Madson of Wells Fargo, presented on the topic at the CLEANPOWER 2021 virtual conference. Their presentation discussed lightning risk at a project site, lightning commercial and contractual risks, and inspection methods to help mitigate future issues from both an Independent Engineer (IE) and tax equity perspective. You can find their presentation here.
Lightning and wind turbines
Lightning is an awe-inspiring natural phenomenon. Lightning can be defined as an electrical discharge caused by charge imbalances between storm cloud and the ground, or within the clouds themselves. This electrical discharge releases an extreme amount of energy in a fraction of a second. For cloud-to-ground strikes, the lightning current aims to reach the ground by following the shortest and most conductive path possible. Because of this, tall objects such as trees, buildings, and wind turbines are more likely to be struck by lightning than the surrounding area.
Wind farms are designed to harvest the free-flowing wind and as such are typically positioned away from tall trees or buildings that would act as obstructions. Onshore wind turbines are reaching record tip heights of up to 200 meters, while offshore turbines can reach heights of roughly double that size. Therefore, wind turbines are typically the tallest structures in their surrounding area and are naturally at greater risk of being struck by lightning.
When lightning strikes a wind turbine it will usually strike the blade. The blades are typically made of fiberglass composite or carbon fiber material. While fiberglass is not a conductive material, carbon fiber blades and the lightning receptors and cable wiring inside the blade are conductive, increasing the probability of a strike. The turbine tower and internal framework and bedplate of the nacelle are made of conductive metal material that could attract lightning strikes as well. When lightning does strike a wind turbine, it can produce catastrophic damage. Blades can explode upon impact, the turbines can catch fire from the extreme heat transfer, and electrical equipment can be damaged or destroyed from the sudden increase in voltage.
Figure 1: A lightning strike may melt or crack the blade tip or leading edge.
Source: https://www.windpowerengineering.com/the-importance-of-testing-wind-turbine-lightning-protection/
Wind turbine lightning protection systems
Wind turbine lightning protection systems are designed to reduce physical damage from lightning strikes by providing an alternative path for the strike to pass through the turbine and safely into the ground. Not all lightning protection systems are designed to the same standard. The International Electrotechnical Commission (IEC) categorizes lighting protection levels (LPLs) into four different classifications, LPL I through LPL IV. LPL I provides the highest level of protection. Although LPL I is the highest standard, this does not address lightning strikes outside of its parameters nor is it intended to provide 100% protection from lightning strikes within the LPL I criteria.
Blade strikes are the most common strike on wind turbines, with damage ranging from minor discoloration to catastrophic failure. Blade damage is classified into four different categories ranging from (I) to (IV). Surface scorching is an example of a category (I) minor event not needing immediate repair. The categories increase in severity up to category (IV), which is a catastrophic incident such as blade rupturing and falling and causing possible injury and/or death.
The severity of the damage depends on where on the blade the lightning strikes and how deep and wide the structural damage is to the blade. The category of the blade damage will also indicate how to approach the damage. This could mean inspecting and recording the damage while keeping the turbine operational; inspecting and repairing the blade at the next available opportunity; or taking the turbine out of operation until the repair can be made. In some cases, the blade may need to be completely replaced.
Inspection protocols
Natural Power recommends that annual inspections should take place as part of the typical operation and maintenance (O&M) agreement. Additionally, annual visual inspections of lightning protection system (LPS) receptors on blades should be incorporated into the O&M agreement. With the advancement of drone technology, many of these visual inspections can be done safely without crews having to leave the ground. Since not all damage can be seen externally, Natural Power recommends internal inspections of the blades themselves take place at lightning prone wind farms.
When lightning strikes the turbine or the immediate surrounding area, an inspection of the turbine may be warranted. Inspections should occur if a lightning strike has been detected within 300 meters of the turbine and meets the following criteria:
• Peak current >1 kA for positive strikes.
• Peak current <-20 kA for negative strikes.
• Strikes longer than 2 milliseconds in duration, also known as a continuing current strike.
For continuing current strikes a 1,000-meter radius from the turbine is used because the larger amount of total electrical charge being transferred from the cloud to the turbine or ground could result in more damage.
The reaction time of the maintenance crew and the quality of the inspection play a major role in the severity of lightning damage. Once a strike has been detected, it can take several days for a maintenance team to go to the turbine and inspect the damage, during which time the damage from an initial strike has the potential to propagate. Also, tiny fractures in the fiberglass can go undetected and lead to intensified blade degradation over time. Documenting these inspections can prove to be invaluable when dealing with any warranty claims and O&M disputes.
Lightning risk
How is lightning risk analyzed? Turbine original equipment manufacturers do not include a standard lightning risk assessment as part of a suitability review. Project sponsors can provide high-level lightning information on projects within relative proximity to the project area or a project using the same turbine type or model. On repowering projects, project sponsors can provide historical lightning damage reporting based on the pre-repower wind farm. IEs can provide a high-level risk assessment based on known issues with specific turbine types, and models and county level lightning maps and can provide an in-depth site-specific analysis upon request.
The IEC 61400-24 Site-Specific Analysis outlines a method for estimating annual lightning frequency and damage rates given the ambient climatological strike density, site geographical parameters, and the turbine layout/total structure height.
By contrast, one of Natural Power’s approaches to a site-specific lightning analysis takes a deeper dive into historical lightning data at the site. The basis of this analysis is a five-year historical lightning dataset that contains detailed information about individual strikes occurring in the project area, including location, time, amplitude of the lightning current and more. The analysis sorts strikes into those that are within and outside of the LPL I thresholds. The number of historical strikes that should have triggered an inspection is also estimated, which is useful in providing an estimate of inspection workload based on the recommended inspection criteria. This analysis can be further expanded to include a 10-year historical lightning dataset, which includes continuing current parameters. These categories can be expanded to provide monthly statistics as well as interannual variability of the strike.
Lightning risk from a commercial perspective
Lightning risk is a vitally important issue due to its potential to cause significant damage at wind farms. Unfortunately, owners and operators that have had negative experiences are more likely to be ahead of the curve on the topics such as inspection protocols, contract negotiations, warranties and insurance claims. This can leave owners and operators who have not taken a deeper look into their lightning risk plans and protocols vulnerable about the topic.
It should be a requirement regardless of wind farm location to at least have a lightning protection inspection protocol. Detailed documentation of inspections will aid in being prepared for any warranty and/or serial defect claims. A third-party subject matter expert review should be conducted when repairing or retrofitting a lightning protection system. Furthermore, adapting protocols over time to reflect any current or ongoing issues seen within the industry should be best practice.
During the contract review stage, lightning strikes and damage need to be thoroughly covered. Typical warranty, force majeure and serial defect contracts can be deliberately vague with regards to lightning damage. Furthermore, O&M contracts should be reviewed to confirm if lightning damage is covered under unplanned maintenance and to confirm if downtime due to lightning is excluded from the availability guarantee. Ensuring these contracts cover specific lightning parameters will ease the mind of investors and lower potential risk with the project.
Determining risk at your project
When developing or repowering wind farms, lightning risk should not be left out of the discussion.
Getting detailed historical and site-specific lightning data supports the project in identifying the potential for lightning strikes. Identifying turbine manufacturers and models with previous or current lightning issues will aid in turbine selection process. Moreover, establishing contract language to include lightning parameters will assure risk is mitigated if lightning damage occurs.
Natural Power has the capacity to assess potential lightning risk with historical site-specific lightning analysis and consultation on current best practices and lightning inspection protocols. For more information or to discuss your needs, contact Evan Osler, commercial manager - advisory, at evano@naturalpower.com.