Damage to blades has become a new headache in the wind turbine industry.
This is evident from Ingeniøren’s new request for access to Danish Energy Agency’s documents containing information on wind turbine failures over 10 years.
Damage to blades was recorded 35 times and is the single largest cause of failures. The experts who inspect wind turbines on a daily basis are of the same opinion.
“As the rotor diameter constantly increases, so does the blade tip speed. And we have now reached a speed where even raindrops cause erosion of the leading edge of the blade,” says Steen Buss, who has for the past 14 years worked as a technical consultant at Nordic Wind Consultants (formerly Danish Wind Turbine Owners’ Association). Before that, he worked for several big wind turbine manufacturers, such as Siemens Wind Power and NEG Micon.
According to the consultant, new calculation methods and data collection enable wind turbine manufacturers to build the blades with optimized material consumption and as close to the safety limits as possible.
His says that lightning is the main cause of damage to blades in Denmark. His colleague Lasse Mathiasen, technical consultant at Nordic Wind Consultants, shares his opinion.
“We occasionally see wind turbines with significant erosion of the leading edge or damage to the blade tip due to lightning. All in all, blades often need to be repaired specifically,” Lasse Mathiasen says.
Figures from the Danish Energy Agency show that, on average, one wind turbine per year fails due to lightning strikes. And from 2011 to 2021, Vestas reported nine failures as a result of lightning—mainly damage to blades, but also fire and broken-off parts.
In the past, it was typically wind turbines’ gearboxes that failed and had to be replaced so frequently that they reminded of fuses in old homes.
Gearbox failures were the norm in the 90s and 00s because the gears struggled with the fluctuating loads that are the name of the game when dealing with wind.
“We replaced an incredible number of gearboxes in the past. But that has changed. The manufacturers can now make more accurate load calculations, and they have become better at monitoring the gears. So even though we still see gearbox failures, it’s not at all as common as before.”
However, gearbox issues have not completely disappeared, and it is especially bearing damage and indentations that pose challenges for wind turbine manufacturers.
Researchers from DTU Wind Energy and DTU Mechanical Engineering have previously described issues with the phenomenon of white etch cracking (WEC) in gearbox bearings, causing cracks under their surface, in Ingeniøren’s articles.
However, according to the wind turbine inspectors, manufacturers have become better at building the gearboxes, and on the whole, Steen Buss believes that the industry is now finally getting a handle on the forces that threaten gearboxes.
“After all, it also took car manufacturers decades to build a good gearbox, and in the same way, wind turbine manufacturers are only now getting a handle on the rather strong forces that threaten wind turbines,” he says.
“Remember that wind can change within just a few seconds, and the large and heavy wind turbines have to react to that. The engineering calculations of loads have become much better and, in combination with sensors and monitoring, it is easier to manage it than 10–20 years ago,” Steen Buss says.
In the early days of wind turbines, the first sensors were simple devices in the nacelle, where a ball of metal the size of a table tennis ball was placed in a kind of cup. If the ball had rolled out of the cup, it meant that the maximum permissible vibration level in the wind turbine was exceeded, and in some cases this triggered an automatic shutdown.
“Today, sensors can register all vibrations, even changes in oscillations from blade to blade,” Lasse Mathiasen says.
Today, more and more wind turbine owners are upgrading their monitoring technology, as Ingeniøren wrote in several other articles.
For example, Hofor will install sensors in the Bonus turbines at the Middelgrunden Wind Farm outside Copenhagen to monitor the natural frequency and be notified if there are changes in the pattern.
Ørsted also monitors its Siemens turbines at the Nysted Wind Farm southwest of Gedser, where a 110-meter-high wind turbine overturned back in January. A review of the data showed that it had an aberrant vibration pattern.
Are there some manufacturers or types of wind turbines that fail more frequently than others?
“No, not anymore. Those with teething problems and errors have simply been pushed out of the market, which is very competitive. If you can’t deliver a top-quality professional product today, then you’re out,” Steen Buss says.
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