BAD AXE – What’s it take to topple a 400-foot, 485,000-pound wind turbine?
Mechanical failures, an imbalance and the ensuing “violent act.”
And a dose of irony.
Or as officials call it, an “overspeed event.”
That’s the determination wind energy developer Exelon Wind Generation, turbine manufacturer Vestas and a third party have made after a months-long investigation into the root cause of a turbine’s collapse onto an Oliver Township farm field at the site of the state’s first utility-scale wind energy project. It happened on Feb. 25.
The turbines in Harvest 1 Wind Farm are eight years old. On the turbine that fell, near Berne and Gagetown roads, seals failed on all three cylinders in its pitch system, which controls how the turbine’s blades move back and forth, officials said.
The turbine exceeded designed rotation speeds – 14.4 rpm (revolutions per minute) is nominal, and at that speed, blade tips are moving at about 200 mph, according to Exelon. It created an imbalance as blades picked up speed. At the time of collapse, rotation speeds reached 18 rpm, officials said.
“At that high of an rpm, the thing just basically shook itself apart,” said Chris Higgins, an Exelon operations manager.
“It was a pretty violent act going on at that speed with an imbalance in the blades.”
Mark Van Diepenbeek, senior operations manager at Vestas, headquartered in Denmark, told county planners Wednesday the seal was worn on all three cylinders in the turbine.
“And it would take all three cylinders to be worn to cause this event,” Van Diepenbeek said.
Exelon says the pitch cylinder seals leaked internally and weren’t able to hold the blades out of the wind. A hydraulic pump also shut off, and oil pressure couldn’t be maintained, officials said.
It happened during an eight-hour period, according to Exelon.
“It was there, we just didn’t see it,” Higgins said. “We weren’t monitoring it at the time.”
Checking for such cylinder wear was not part of normal maintenance, officials said.
“We are finding wear and anything that’s showing wear will be replaced,” Van Diepenbeek said, adding recent inspections found wear on no more than one cylinder.
“May that become part of normal maintenance?” Planning Chair Clark Brock asked.
“Yes,” Van Diepenbeek said.
The company will now have remote, 24/7 monitoring of turbines, and that maintenance will be added, he said.
Adding to the irony, officials say a contributing factor to the collapse was wind speeds near 45 mph.
“Any lower than that, the winds wouldn’t have had enough force to push the blades into that position which created an overspeed,” Van Diepenbeek said.
However, the explanation left county planners with some uncertainty.
Member John Nugent asked Van Diepenbeek whether a turbine that exceeds designed rotation will always result in catastrophic failure.
“I can’t answer that question,” Van Diepenbeek said.
“Typically – do you expect it to?” Nugent said. “That’s a pretty thin line.”
Exelon Spokesperson Kristen Otterness says the company is fully committed to ensuring an event that happened in the Harvest Wind Farm “doesn’t happen anywhere else across our fleet,” and that Exelon has “shared this information with the industry.”
“Worldwide, this is the first (failure)” for the V82 model turbine,” Van Diepenbeek said.
The turbine was valued at $1.5 million, Exelon says. The company said in March it hadn’t made a decision on whether it will be replaced.
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