POWER October 2014 - 47

OPERATIONS & MAINTENANCE
Prepare Your Renewable Plant for
Cold Weather Operations
Although renewable generation is typically less affected by winter weather than
gas- and coal-fired plants, it is far from weatherproof. Understanding the
risks and preparing properly for them can avoid most problems.
Sonal Patel and Thomas W. Overton, JD
L
ast winter's polar vortex was a reminder
that, despite several years of mild winters,
colder months can still pack a wallop.
Decades of coal, gas, and nuclear plant
operations have taught plant operators the
dangers of bad weather, but the relative novelty
of utility-scale wind and solar generation
means many newer owners are unaware of
what can happen to performance when temperatures
plunge and the snow begins to fall.
While there may be no pipes to freeze in a solar
plant or wind turbine, for example, there
are still risks from severe winter weather.
Wind Farms
Though many wind turbines are installed in
regions of the world that see bitterly cold
winters, wind power isn't immune to extreme
cold events.
During the February 2011 Southwest cold
snap, an unprecedented 16% of wind units
within Texas grid operator Electric Reliability
Council of Texas' operation region reportedly
failed-709 MW due to blade icing and
another 1,237 MW because frigid temperatures
exceeded turbine limits.
And although PJM Interconnection lauded
wind power's " positive impact on supply "
during the January 2014 polar vortex, saying
it contributed to PJM's ability to maintain
reliability, data shows that wind energy generation
dropped precariously during demand
peaks during the Jan. 6-9 extreme weather
event. The Midcontinent Independent System
Operator's system also braced for the plunge
in wind output following Jan. 6, crediting accurate
wind forecasts. However, it records
that it saw 1,000 MW of wind power " cutouts "
as a result of the extreme cold on Jan. 6.
The Federal Energy Regulatory Commission
later speculated that the bulk of those wind
turbine failures were due to " models reaching
their minimum operating temperatures. "
Bracing for Low Temperatures. Wind
turbines are typically designed, per international
standards for turbine design, to operate
within ambient air temperatures of -10C
October 2014 | POWER
while operational and -20C at standstill. (For
offshore considerations, see the sidebar.) As
the North American Electric Reliability Corp.
(NERC) notes, turbines have an automatic
shutdown feature to protect components if
that range is exceeded.
Wind generators in places like Canada
typically install " cold weather packages " to
extend temperature ranges, using up to 200
kW to 300 kW of parasitic power per turbine
at conditions below -20C for heating components
such as the nacelle space, yaw drive
and pitch motors, and the gearbox, slip ring,
controller and control cabinet, and battery.
GE's 2010-introduced Cold Weather Extreme
package for its 2.5 x l turbine, for example,
ensures operations in temperatures to -30C
and a " survival mode " to -40C.
According to NERC, " it does not appear
that [cold weather packages] were used in
the Southwest " during the 2011 event. It
recommends that all entities investigate the
purchase of these packages in preparation for
extreme cold events.
Heating could also prevent another insidious
cause of turbine failure: When a turbine
is not running, oil that is stationary in radiator
passages can quickly cool, and its viscosity
can increase. Even if wind turbines are
not being used, an important lesson worth
learning is that the turbines should be cycled
online to provide flow of cooling oil, NERC
says. All cooling equipment for radiators on
wind turbines should also be disabled for
cold weather events.
Preventing Icing. Icing on wind turbines
affects three different aspects simultaneously:
the design (aerodynamics, load,
control system, and material), the safety
(ice throw, unbalanced rotor spinning, overpower,
and fatigue), and performance (annual
energy output, wind measurements,
and design life duration). But it can also
affect wind sensors-rendering ineffective
wind-measuring equipment-as well as increase
noise levels and generally decrease a
turbine's cost-effectiveness.
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A deep freeze could have long-term implications
for a wind turbine, warns Jeff
Nelson, a TransAlta wind engineer. Turbines
can eventually fault and shut down; in some
cases they have had up to 250 millimeters of
ice. That could mean an extended period of
downtime (if ambient temperatures drop significantly
and ice cannot be shed) of up to
three weeks.
For TransAlta, key takeaways from previous
icing debacles at its 96-MW Kent Hills wind
farm in Canada's northeastern province of New
Brunswick include the imperative to catch icing
events early, using instrumentation, ice detectors,
anemometer cameras, or/and blade accelerometers.
Twenty-four-hour monitoring of
icing conditions is also essential.
And here too cold weather packages can
prove valuable. Some offer, in addition to
technical solutions that widen operating
temperature ranges, active or passive deicing
or anti-icing systems for rotor blades.
Systems use a variety of methods, including
heat, water-resistant coatings, and controlled
blade acceleration/deceleration to shake the
ice off.
Last year, for example, Vestas introduced a
de-icing system that can detect an icing condition
(using algorithms measuring several
variables, including temperature, humidity,
wind speed, and turbine output), determine
whether it's worth shutting down the turbine
to perform de-icing, and initiate a de-icing
procedure if it is worthwhile. The outer third
of each turbine blade essentially includes a
small heating element and fan that uses up to
150 kW to run.
Hydroelectric Plants
Of all generation resources, hydroelectric
plants may be the least affected by nasty winter
weather. Indeed, many countries around
the world that rely heavily on hydropower,
such as Norway, Iceland, Canada, and Russia,
must also deal with severe winters. Though
reservoirs may freeze over, penstock intakes
are usually deep enough to withdraw water at
47
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POWER October 2014

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