Dive Brief:

• MidAmerican Energy says it is working with wind turbine manufacturer Vestas to understand the "root cause" of four blade failures that have forced it to temporarily idle some generation over the past year.
• Most recently, on Oct. 15, MidAmerican was forced to halt 46 turbines at its Beaver Creek wind farm in Greene County, Iowa, after a blade broke away and fell into a harvested field. It was the fourth blade failure the utility has experienced since last October, and the second in the last two months.
• All four of the failures have involved blades manufactured by Vestas. According to MidAmerican, the broken blades were "exposed to a direct or nearby lightning strike."

Dive Insight:

According to the American Wind Energy Association (AWEA), there are over 60,000 operating wind turbines in the United States today and no member of the public has ever been injured by a turbine.

"These sorts of failures are exceedingly rare," AWEA spokesperson Sam Brock said in an email.

Blade breakages are rare and turbines are safe, AWEA asserts, but MidAmerican says it is committed to finding the cause of the recent failures. Most of the Vestas turbines in MidAmerican's fleet have a nameplate capacity of 2 MW, meaning the most recent Iowa incident caused over 90 MW to be idled for some time.

Of the 46 turbines idled two weeks ago, all have since been inspected and "most have been returned to service after confirming there was no damage to the blades. Some remain offline for blade repair or replacement," MidAmerican spokesperson Geoff Greenwood said in an email.

"We are working with Vestas to monitor and manage this issue. We are also working to complete the root cause analyses of the most recent blade breaks and implement additional corrective measures to prevent these blade failures going forward," Greenwood said.

Vestas, in a statement, said it has agreed to "inspect a limited number of turbines that were within a certain proximity to significant lightning events."

The inspections, said the turbine maker, are being conducted in accordance with the company's "established lightning inspection protocol" that take into account the proximity and intensity of strikes.

"The vast majority of lightning strikes do not affect the operating wind turbine due to the protection provided by a lightning protection system," the company said. But it added that the risk of some damage, in the event of significant lightning events, "cannot be fully eliminated."

Vestas has installed roughly 75,000 wind turbines worldwide and says blade failures are "rare" but turbines "do occasionally incur damage."

MidAmerican operates more than 3,000 turbines — totaling more than 9,000 blades. The utility brought its first turbines online in Iowa in 2004, and last year delivered more than 60% renewable energy to customers in that state on an annual basis.

MidAmerican said it has an ongoing lightning inspection protocol in place and will temporarily pause certain Vestas turbines during and after lightning storms to ensure any blade damage is identified and addressed.

"Though a blade break is extremely rare, we are actively working with Vestas try to prevent any additional blade failures from occurring," said Greenwood. "That's why we've immediately enacted these additional precautions."