It was a picture that warmed the hearts of opponents of wind farms the
world over. In December 2011, a wind turbine at the coastal town of
Ardrossan in the UK spectacularly exploded during a storm. Pictures of the
flaming debris shower flashed across global media, triggering claims that
turbines cannot cope in extreme weather.

But Infinis, the operator of the wind farm, claims in a report into the
incident just published that the turbines should be able to withstand such
conditions if new safety measures are put in place. The report says winds
that day reached 176 kilometres per hour, forcing the turbine blades –
locked in position because of the hurricane-force winds – to turn against
their brake pads. The friction this created resulted in extremely high
temperatures.

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.

We have ignition

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.

Burn the evidence

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. “We’re in the middle of an investigation to see
whether a criminal offence has been committed,” says Karl Turner, spokesman
for the HSE. “We’ll obviously take the [Infinis and Vestas] reports into
consideration, but we’ve got our own assessments to make and these can take
time. There’s no timescale for the completion of the investigation.”


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