BURIED ALTERNATIVE: CABLE IN CONDUIT
Aggressive wind assumptions, peak loads and still air may create sag problems.
- Published in Southwire's T&D Update newsletter in February 1998
- Reprint permission granted
Overhead transmission loads only seem to grow. Where
do you find extra capacity without major expenditures.
"One route to increased capacity is reevaluate the
original rating assumptions,:" says Ridley Thrash, Southwire
chief engineer for overhead conductors. "A key rating
assumption is conductor cooling due to wind."
Are your assumptions conservative?
The thermal limit (maximum ampacity) for most transmission
lines was based on a perpendicular cross wind averaging two
feet per second (fps). That's not much air movement. Is it
conservative? There is a high probability that you'll get at
least this much air movement coincident with your peak load.
What if you assume an average of three, four or even six fps?
Cooling - and load capacity - increase significantly. But what's
the probability that the wind speed will actually be that high
when your load is at its peak?
"There's a good chance that calm air in early morning
or early evening may occur when your load is at maximum," says
Thrash. High loads and calm air could be a recipe for disaster.
High conductor temperatures when the wind is less than predicted
could create enough sag in critical spans to exceed National
Electrical Safety Code (NESC) clearance limits.
"There's nothing wrong with looking at new ways
to rate your line. But be aware of average local weather conditions
and terrain, and be realistic in your probabilistic assumptions," says
Thrash.
If the terrain shields part of' the line from the wind,
or the angle of the line varies, you've got different conditions
in those places. You might want to choose several different
rating scenarios that apply at different times of day and different
seasons.
Other Thermal Rating Variables
There are other wind-related variables to consider. One
of them is wind direction. if the wind is blowing parallel
to a conductor, rather than perpendicular to it, you lose up
to 00 percent of the cooling effect. Distance from the weather
station that reports wind speed history, and differences in
terrain and tree cover can make a difference also, especially
at low wind speeds.
The reported wind velocities themselves may be open to
question. Standard cup-type anemometers aren't very accurate
at wind speeds below five mph. Accurate measurement of low
wind speeds usually requires a hot-wire anemometer.
Southwire supplies a DOS-based thermal rating program
called SWRATE with every copy of the Southwire Overhead Conductor
Manual. SWRATE uses the same ampacity calculation method that
IEEE suggests in their Standard 733. SWRATE can help you evaluate
different wind-speed and temperature scenarios.
"It's important to remember that when we're talking
about line capacity, pure current flow isn't the limit," Thrash
observes. "We're talking about thermal capacity. The better
the cooling, the higher the capacity."
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