By
Ed Hillier, Associate Director, Natural Resources, Charles Taylor Adjusting with Inputs from Morten Handberg, Chief Blade Specialist, Wind Power LAB
Lightning is an ever-present natural phenomenon. Indeed the destructive power of a lightning strike, and the resulting wildfires it can cause, is actually key to the lifecycle of some species of flora. However, the increased use of composite materials in aviation in the last half century has led to a renewed interest in this topic as increasing numbers of structural aircraft components are composite in nature which can potentially be more susceptible to damage from lightning strikes. On this basis, sophisticated lightning protection systems incorporating numerous materials are now a key safety feature in all aircrafts. In short this is a well-researched and understood science within the aviation industry.
Conversely the insurance market has experienced considerable losses as a result of damage caused by lightning strikes to Wind Turbine Generators (WTG), especially damage to the blades. Unattributed comments by the Royal Canadian Airforce suggests that a commercial aircraft is struck by lightning approximately every 1,000 to 3,000 flying hours although there is often little damage, if any.
Why should this be so?
he first point to consider is the construction of a WTG blade which is essentially Fibre-Reinforced Plastic (FRP) shaped to form the desired aerofoil shape. The former material (fibres) is an electrical insulator whilst the bonding resins are an electrical conductor although when amalgamated to form a blade the structure is a less effective conductor than a comparable unit made from aluminium or carbon fibre. This is a major difference to aircraft which are in general excellent conductors of electricity as a result of their high aluminium content meaning that the free-flowing current has to be channelled to a dissipation point: typically trailing edges of control surfaces. Conversely WTG blades need to have specific metallic components engineered into them for the express purpose of creating a conductive medium as part of a path to earth.
Returning to WTGs, the net result is that despite being relatively unattractive as an electrical conductor, their blades are still struck on a regular basis due to their elevation and the relative lack of taller, more conductive structures in the vicinity. If there was no Lightning Protection System (LPS) built into these blades, they would be unable to quickly dissipate the huge and sudden energy release which would likely result in catastrophic damage. In order to overcome this, a WTG is fitted with a series of components that comprise the LPS which are typically as follows:
• Blade receptors
• Down conductor
• Earth termination point
In a recent study1 it was identified that 90% of all blade damage occurs in the first four metres of the blade travelling from the tip toward the root. In order to mitigate this, the most popular LPS system design integrates a tip receptor and a number of metal receptors along the length of the blade that are connected to a down connector. However, it is also important to recognise that the blade area is much larger than the area of the receptors, but WTG manufacturers have traditionally weighed this up against the statistical likelihood of a damaging strike. Whilst all commercial LPS are designed around the standards as set out in IEC 61400-24 which was first published in 2002, an updated 2018 version addresses the lightning risk of the increasing size of WTGs and their associated technology. That said, from anecdotal claims information there would appear to be an increase in damaging lightning strikes to blades which would indicate that the progress in LPS development is not always in line with the development of increasingly larger WTGs. This is something that the 2018 edition of IEC 61400-24 seeks to rectify. Given the increasing number of deployed WTG especially in the onshore market, that were designed before these new standards were adopted, the commercial reality of the cost of retrofitting these fleets versus their remaining operational life and income is a challenge many operators may need to consider in the coming years.
So where does this leave the Insurer who is facing an increasing number of lightning related losses and a correspondingly disappointing loss ratio?
The answer to this in terms of onshore WTG may not be a palatable one but will centre largely on the nature of the WTG generating assets they insure, their operational life, and the willingness or otherwise of the Insured to invest in preventative measures. That said, there is only so much that can realistically be done to improve the LPS of an existing asset and so perhaps prudent risk selection and policy wordings can assist with a more equitable risk transfer. In contrast, in the offshore space where the majority of the technological advances are being made and the majority of larger capacity WTG units exist, damaging lightning strikes rarely seem to occur.
In summary, lightning has, is, and always will be a natural phenomenon that humanity and its engineers can best hope to mitigate rather than tame. Through increased knowledge will come resilience. In the meantime, insurers will be called upon to fill this gap with suitable risk transfer products and by working with all interested parties, a better-quality risk will result.
1 Garolera, A.C.; Madsen, S.F.; Nissim, M.; Myers, J.D.; Holboell, J. Lightning Damage to Wind Turbine Blades From Wind Farms in the U.S. IEEE Trans. Power Deliv. 2016, 31, 1043–1049
