By Michelle Froese, Senior editor
Windpower Engineering & Development
The average energy released in a lightning strike is 55 kWh. Understandably, most wind owners and operators breathe a sigh of relief when a major storm passes their wind farm without damage to a turbine. However, measurements have shown that lightning strikes may hold more power than initially calculated, and that large strikes can be multiples of the average. This means a powerful lightning strike could be 20 times that of an average one.
“Lightning is a serious concern for wind-farm owners,” shares Daniel J. Sylawa, Business Development Manager with Phoenix Contact. “But it is rarely discussed in detail or at length because lightning protection and management is something that’s generally considered an OEM responsibility.”
Sylawa says manufacturers typically equip wind turbines with some form of basic lightning protection that uses grounding down conductors in the blades and grounding systems in the turbine. “Depending on the OEM, you’ll find different types of lightning receptors on the turbine blades, which are then connected to the nacelle via brushes or a spark gap that lets lightning conduct to a good firm ground.”
In addition to lightning protection, surge suppression is a critical turbine safeguard that mitigates lightning or static effects. “Even without the risk of a direct strike, turbine blades rotate through the air to capture and generate energy,” he says. “Essentially turbines are large static machines, so surge protection is essential and should be found on the pitch control, tower electronics, inverter, and control system to protect against component failure.”
Most wind-farm owners are aware of the benefits of a quality surge and lightning protection system. However, lightning risks may vary greatly from one turbine to another at a wind farm.
“Unfortunately, these assets are often purchased en masse,” says Sylawa. “So unlike buying a new car where you may pick the model and specific features, buying a wind farm typically involves a less detailed selection. The developer agrees to buy a project under a set PPA with a specific number of turbines. And those may very well come from several different manufacturers.”
This scenario means a wind-farm operator may face challenges in optimizing a fleet. “To optimize production and ROI, a wind farm has to run as a unit. It is an electric power plant. But if one part of that plant is generating at less than full capacity or facing downtime, that puts the whole plant at risk of lost production.”
There are tools available that may offer protection to a wind farm. For example, weather-measurement systems use metrological data to predict the probability of a lightning strike in a given area. Lightning sensors installed at wind sites perform a similar function but use local measurements to determine the location of a strike.
These systems may provide an extra measure of support, but they are unable to tell if a turbine is directly impacted by lightning. While a direct strike may be obvious to an O&M team, near strikes are a different story. Near strikes are indirect hits that can occur from several miles away. In most cases, near-strike damage is invisible from the ground and may go unnoticed during routine ground-level inspections.
“Lightning would be somewhat easier to manage if all strikes were centered directly on a target, but Mother Nature is much less predictable than that,” says Sylawa. “Near strikes are offshoots of lightning that can lead to serious problems — problems that may go undetected or simple not present as such for some time after striking.”
One example is damage to a turbine’s blade material. A near strike may cause a small fault in the blade’s substrate material, which may go unnoticed until rain seeps in and eventually results in water damage. “When winter hits, a freeze-thaw cycle can also expand and degrade the under-grading material and lead to blade failure,” he adds.
Another example is secondary lightning effects, which may cause electromagnetic pulses or surges that can damage electronics inside a turbine or substation.
“I was recently in talks with one turbine manufacturer who is working to increase the requirements for surge protection for their electrical system and components because of damage from secondary lightning effects. So OEMs are certainly cognizant of these effects and the need for high-quality surge and lightning protection.”
Setting up safeguards
Wind turbine or blade damage can occur for many reasons: wear, debris, precipitation, operational errors, manufacturing defects, lightning strikes, and others. Early detection and mitigation techniques are necessary to avoid or reduce damage.
However, what happens when a near strike causes delayed onset damage such as nick in a blade that only becomes detectable over time?
“This can be a big warranty issue,” says Sylawa. “If an area had been hit by a storm, experts may try to decipher whether damage is from lightning or from workmanship and material defects. In some cases, near-strike damage cause may prove challenging to substantiate.”
What can a wind-farm operator do to protect their assets and warranty? Sylawa has some suggestions.
Recognize the risk
“Most wind-farm operators opt for some type of weather service, or have some form of lightning detection.” He says the most common are direct-measurement systems such as strike counters, which are surge-counting devices, and card sensors. The card sensors attach to a turbine’s down conductor and measures the peak current that traveled down that conductor during a strike. “Ideally, an effective lightning measurement system detects a variety of risks.
Inspect, inspect, and inspect some more
A quality surge and lightning protection system is one key to a profitable wind farm. An excellent O&M plan is another. “There are inherent risks in operating a wind plant even with the best protection. The number one thing a wind-farm operator can do is implement high-level inspections because, at the end of the day, you cannot fully know what the effects of lightning are on turbines.”
Sylawa adds that it is important to look beyond the surface of the turbine during O&M calls. “After a lightning strike, a wind tech may not find noticeable damage to the tower or blades and think everything is fine. But it’s extremely important to go through and test the electrical system. A turbine may have a tripped or blown surge suppressor that’s impossible to notice with an inspection.”
Condition monitoring for blades
Lightning detection sensors are not new, but advanced turbine and blade lighting detection systems that connect remotely to data centers can measure and provide greater insight. For example, they can provide asset monitoring, predict maintenance issues, and send event notification — such as strike warnings.
“Wind power is moving toward a smarter, more data-driven industry,” says Sylawa. “Sensors can now record information for a wind owner or operator remotely, and in seconds they’ll have access to a full analysis of what’s going on for a specific turbine or blade in that turbine.” Such data could potentially also support warranty claims.
“Wind owners who have access to big data, which helps determine the health and status of their assets, will be ahead of the O&M game.”