When lightning strikes: managing impacts on wind turbines 

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fter recording extraordinary warranty costs of €175m in the second quarter of 2020, Vestas’ CEO Henrik Andersen said “high-intensity lightning” was responsible.

Elaborating slightly, he added that a "limited number of models" had issues but that a “considerable number of blades that are already installed” had been impacted, though he refused to divulge more.

The specifics of the incidents may be opaque, but the impact was stark. That quarter, the company recorded a €94m fall in earnings before interest and tax, compared with the same period in the previous year.

Vestas’s story highlights the hidden costs and challenges that lightning strikes can pose to the wind power sector. What’s more, experts say that it’s a problem that is set to worsen as turbines get taller and blades are increasingly made of carbon.

Tall structures are attractive to lightning, especially when located on flat plains with nothing much else around, as wind turbines often are. However, while experts can't say with any certainty how much lightning damage is occurring across the industry, it is a well-known threat.

Søren F. Madsen, head of simulation and modelling at global lightning protection services company Polytech, has worked in the field of wind turbine lightning strikes for 15 years and says that, on average, a blade will receive around 20 strikes during its lifetime, but the number will largely depend on the geographical location of a wind farm.

Two types of lightning can occur, he says: one that starts in a thunderstorm and then propagates downwards, attaching to the turbine tip - called downward initiated lightning – and another, ‘upward lighting’, that happens when the turbine gets very tall and itself starts to generate lightning that wouldn't otherwise occur.

“Initially, in early discussions on the risk of lightning strikes to turbines, the latter was not understood, which led to an underestimation of how many events a turbine will actually experience because the risk assessment was based on the lightning environment before the turbine was erected,” explains Madsen.

The upwards lightning effect starts to happen when a turbine exceeds 100m in height. Modern structures can easily exceed 200m in tip height. Madsen says that around 70% of all the strikes measured in turbines are actually starting on the blade and triggered by the turbine.

Though turbines and their blades are designed to withstand a certain severity of lightning, this can sometimes be exceeded causing harm to the structure. When damage occurs, it can be extremely costly to fix, as the Vestas case study shows.

This is because the turbine will need to be shut down and the damage repaired, which can involve heavy machinery such as cranes. The challenges increase as the structures get bigger and more complex, and of course, due to access issues, damage to offshore wind turbines is significantly more expensive to address than onshore.

Furthermore, when blades get bigger and longer, they need to be lighter, for which carbon is an ideal material and now routinely used. Carbon, however, has the electrical downside of conducting current, creating new challenges for turbine design.

For traditional glass fibre blades, a single down conductor is required, which is very easy to implement. Whereas carbon is produced in individual planks that are insulated from each other, making the design effort more complex.

“Some design effort must be made to allow the current to enter and then be guided into the carbon without damaging it,” says Madsen.

“The biggest challenge is mechanical engineers who design blades fundamentally don't like the concept of having lightning current in the structural component, but they must realise this will happen whether they like it or not. It's better to control the current rather than letting Mother Nature decide how it will happen, which can be catastrophic.”

Alex Byrne, wind energy engineer specialising in operational excellence at DNV, has also encountered "challenges" with carbon blades.

“In the LPS [lightning prevention system] design, they need to consider how the electricity is going to flow in the blade. Usually that's achieved by electrically bonding the LPS to the carbon, however, this is not always done,” says Byrne.

Some models are also more susceptible to lightning strikes than others. Byrne says that, although they don’t have a "complete picture" of the industry, they tend to get calls to lightning incidents happening to repeat models. She declines to say which models are more affected, saying only that it is LPS design specific, and there's likely some bias due to the unique lightning environment.

“You could put the same wind turbine model in West Texas and in North Dakota and they could have very different lightning damage experience; probably even the OEMs [original equipment manufacturers] wouldn’t feel comfortable saying exactly how a certain LPS performs, because they’d need to remove out the influence of the environment to get a good understanding,” she explains.

When it comes to mitigating lightning-induced turbine damage, Byrne says that the design of LPS is by far the most important factor, which is largely in the hands of OEMs.

“Once it's built in the blade, there isn't much that can be done, blades can’t be opened-up and the LPS easily modified,” she adds.

Retrofits have been done, such as modifications to the tip of blades or column diverter strips attached on top of the blade surface. However, Byrne says that while these actions provide "some benefits", it's not significant.

When designing LPS, there are two important things to consider, says Madsen: to make sure the blade gets struck in the right place and when lightning strikes, to ensure the current travels down to the other end of the blade without causing damage along the way. For this purpose, a down conductor is inserted into a blade, along with physical metal conductors.

“If things are designed and tested properly, then damages should not be expected in the field,” she says.

However, due to the labour-intensive nature of wind turbine manufacturing, defects can lead to an inadequate system.

“It’s important that designs are made so robust that even an unskilled manufacturer can't mess it up,” she explains.

The industry needs to put pressure on OEMs to better address LPS design and encourage them to understand the impact of lightning by putting sensors onto the turbines themselves to accrue more relevant data, says Byrne.

“Once we get that data, not only will it be more beneficial for assigning responsibility for damage, but it'll also help the industry understand just exactly what the lightning looks like that's causing this damage,” says Byrne.

Madsen says that Polytech is already doing this today. It can install lightning sensors on the blade to determine the severity of a strike and where it struck the blade. Mechanical sensors can also measure strain and vibration to detect if the structural response of the blade has been affected by a strike.

“In the future, there will be even more sensors that will help foresee changes on the LPS system before being struck, and to explain to the operator exactly what has happened during a strike,” he explains. “Ultimately, this will be ‘everything is good, stick to the maintenance schedule’ or ‘stop the turbine and inspect before the minor damage escalates into a complete blade failure’.”

Preventing damage before it occurs will be key to keeping costs, such as those incurred by Vestas last year, to a minimum.

“If we can detect something before it becomes too expensive to fix, it is better; therefore, there will be much more development on the condition management side,” he concludes.