Thursday, July 11, 2013

Storm wind and fire – when wind turbines go wrong

An interesting article on New Scientist takes up the story of the Ardrossan wind turbine that spectacularly caught fire back in December 2011, in extreme weather conditions.
Infinis, the operator of the wind farm in question, has just published a report that claims the turbines should be able to withstand similar conditions if new safety measures are put in place.
The report states winds reached 176 kilometres per hour, and this forced the already-locked turbine blades grind against their brake pads. Apparently, this friction created extremely high temperatures…
Windfarm turbine fire Ardrossan 
When wind speeds reach 88 km/h turbine blades of wind turbines are usually twisted, or “feathered”, so that they no longer intercept airflow properly and they stop turning. This is to both protect people on site in the event of a blade loss and to protect the turbine from structural stress. But for this blade-stalling process to work properly the turbine head must be rotated horizontally, or “yawed”, and pointed into the wind.
The report details how a Vestas Wind Systems turbine at the Ardrossan wind farm, turbine T8, suffered two major heat-producing problems that contributed to its structural ignition. First, yaw control on the turbine was lost owing to a gear failure, so the feathered blades could not be pointed into the wind. This meant the turbine head swung back and forth in the wind, generating extreme frictional heat and sparking a fire in the generator enclosure.
Second, the report reveals that the turbine was configured to apply a brake to the turbine blades when no power is available to run its electronic systems. So when the wind brought down power lines, the brakes were automatically applied to fix the blades in a stationary position. But the Atlantic storm’s winds; proved too strong and the wind forced the blades to turn regardless, dragging the brake pads around a metal disc, generating heat and causing a second flashpoint, possibly through ignition of hydraulic oil.
Video footage captured the head of T8 “swinging wildly” says the report, burning brightly and belching smoke, while at the same time the blades were turning with their hub alight. At one point one of its blades shed its carbon-fibre skin downwind, leaving a bare metal spoke. The turbine burned and sent debris flying across a wide area.
Much of the evidence was burned, and Infinis and Vestas disagree on which was the key initial cause of the destructive fire: Infinis believes it was the loss of yaw control, while Vestas thinks brake drag more the root cause. While Vestas has produced its own report, an expert was not available to discuss its findings with New Scientist.
Vestas has since fixed the brake problem. In future, the feathered rotor will not have the brake applied in high winds; it will be free to turn if it needs to. “Vestas no longer do this and have modified all turbines at Ardrossan to prevent application of the parking brake, which is now only applied during maintenance,” says Infinis spokesman Andrew Dowler. And a slip clutch should ensure that any future loss of yaw control will not generate excessive heat.
Given the risk from fire that is above the reach of firefighters, Infinis’s report recommends that turbine-makers improve fire detection and prevention. It urges more use of fire retardant materials in turbine construction, the fitting of “more comprehensive” fire-detection systems and the development of automatic fire extinguishing systems for retro-fitting to older turbines as well as an option in new ones. “Vestas have confirmed that they are investigating such options as are other wind turbine manufacturers,” the report says.
Although no one was injured in the Ardrossan event, it is still being scrutinised by the UK government’s Health and Safety Executive, which polices workplace safety.