A failure in the system that controls the blades on a Huron County wind turbine led to “overspeed conditions” and the ultimate collapse of the 396-foot, 485,000-pound structure.
Kristen Otterness, a spokeswoman for Exelon, said a failure of seals on the cylinders in the windmill’s pitch system caused the system to malfunction.
Pitch is the angle of the wind turbine blades to the wind. Automatic controls set the blades to a steep angle in low winds for maximum power output. At high wind speeds, the blades are moved to a shallow angle so that they bite less air and don’t spin too fast.
Otterness said the seal failure caused a loss of hydraulic pressure that would have held the blades at the proper pitch.
“As designed the turbine had put the blades in a locked position because of the high wind speed,” Otterness said. But because of the loss of pressure, the high winds caused the blades to continue spinning at high rates of speed.
“It resulted in an overspeed condition,” Otterness said. “Because of the load that was placed on the tower … it led to it falling over.”
Otterness said the rotor was spinning at 18 rpm when it failed. The turbine was designed to spin at a maximum of 14 rpm.
The National Weather Service has said the wind speed Feb. 25 was estimated to be between 40 and 45 mph, with isolated gusts of 50 mph.
The 396-foot windmill fell between 5 a.m. and 8 a.m. Feb. 25 in a field in Elkton. The $1.5 million turbine was about eight years old and part of Exelon’s Harvest I wind project.
No one was injured in the collapse. Otterness said setbacks between occupied structures and wind turbines vary, but they generally are more than twice the tip height of the turbine. After the collapse, Exelon stopped all turbines in the Harvest I wind project for a short time to perform external visual inspections.
Michael Jury, environmental manager in the Bay City District office of the Michigan Department of Environmental Quality, said the DEQ also was notified of an oil spill resulting from the collapse. Jury said the collapse spilled what was initially estimated to be about 30 gallons of oil.
Otterness said Exelon has remediated the property after the collapse and oil spill. She said wind turbines can hold up to 400 gallons of oil at a time.
Otterness said, since the collapse, the company has put in early detection, notification and diagnostics to monitor for the specific drop in hydraulic pressure.
“If we get any kind of alert that happens, there are additional elements in the turbine that will activate and prevent any continued drop in pressure,” Otterness said.
“This monitoring gives us really good confidence that the initiating conditions that led to the Harvest 1 failure can be detected with sufficient time to avoid a repeat event.”
Otterness said Exelon has not determined whether the windmill will be replaced.
The Elkton turbine collapse came about a week after a blade broke on a DTE Energy wind turbine Feb. 19 in Sigel Township, also located in Huron County.
The blade bent and wrapped around the nacelle of the turbine, flinging a 12-foot piece of blade about 120 yards from the base.
A DTE spokeswoman said the cause of the break remains under investigation.
The Exelon collapse came at a time when some residents in Sanilac County’s Bridgehampton and Marion townships are fighting against a planned Exelon development of 68 wind turbines in Bridgehampton, Marion and Custer townships.
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