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By Lennart Wern and Mikael Magnusson, Swedish Meteorological and Hydrological Institute (SMHI), Norrköping, Sweden
View the pdf of this article
The weather has a significant influence
on the capacity of high-voltage
power grids. As the lines carry more
power, they dissipate more heat, and the
excess heat causes the cables to stretch
and sag. For safety and security reasons,
strict rules have been established that set
guidelines for the minimum distance that
power lines can be above the ground. In
August, 2003, overloaded cables that
sagged and came into contact with trees
initiated a widespread power failure in
the northeastern United States. If the
meteorological conditions are favorable,
with cool temperatures and adequate
wind, excess heat from overloaded
cables can be removed more easily from
the lines. Knowledge of the local weather
conditions can therefore help utility
companies plan for occasional power
surges or increased demands. It can also
help in the utilization of the power grid
as a whole, as different sectors experience
different needs at any given time.
The demand for reliable power has been
continually on the increase, despite the
fact that the construction of new lines or
new production facilities has not.
Optimizing the capacity of the existing
grid is therefore a cost effective way to
improve the performance of the system.
Height data around the powerline
Wind speed 13 m above ground
The Swedish research company, STRI,
has been working with the Swedish
Meteorological and Hydrological
Institute (SMHI) to evaluate how much
the power transmission can be increased
in some Scandinavian locations. The utility
companies Statnett (Norway) and
Svenska Kraftnät (Sweden) are sponsoring
the work. At present, most electricity
transmissions are limited by assumed
weather conditions of +30°C and calm
winds, which rarely occur. Engineers at
STRI suggest that by removing this
restriction, an increase of 10-30% might
be realized for certain meteorological
conditions.
Wind profile inside and outside a 20 m high forest (blue)
and in open terrain (red)
Wind speed along the powerline
Using FLUENT, the small-scale wind
patterns along a 30 km stretch of high
voltage power line south of Oslo have
been studied. A 35 km square section of
terrain was simulated to a height of 5
km. The topographical data has 50 m of
horizontal resolution and 1m of vertical
resolution. A 1.9 million cell mesh was
created for the turbulent calculation.
Detailed land use data, taken from satellite
images with 25 m resolution, was
imported into FLUENT to set the varying
roughness at ground level. Water, forest,
open, built-up, and bog areas are examples
of different land use classes, each
requiring a different wall roughness factor.
This information is very important for
extracting high-resolution wind and turbulence
information along the power
line. The flow patterns throughout the
area have been studied for a number of
different wind directions. Results from
these calculations have been used
together with a weather forecast model
to supply STRI with detailed weather
information along the power line. The
results of the project, which is still ongoing,
are very promising.
More information:
www.smhi.se/cfd
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