[Music]
Narrator: One major concern in transmitting electricity is power-line sag.
A combination of hot weather and increased electricity demand can cause high-voltage
transmission lines to heat up, stretch and sag.
Line sag can create dangerous situations.
An overloaded line can sag into a tree or structure, and in these cases the current
can be discharged to the ground, causing a short circuit and sometimes triggering a major
power outage.
Transmission line sag triggered two major U.S. blackouts in both 1996 and 2003.
Today, we’ll see how iron conductor responds to heat.
Michael (Mick) Johnson, Mechanical Engineer, Bonneville Power Administration: So we’re
ready?
For this demonstration, we’ve set up a sixty-foot span of iron conductor.
In the early days they would have used iron wire for telegraph, for telephone and for
power.
But as we try to put more electricity – more current through the lines – it causes them
to heat up and they [lines] sag.
And right now it’s reading zero on our marker.
We’re going to energize this with power from the high-current test set.
So let’s run it up, Darryl.
Darryl Fujita, electrical-mechanical test & development craftsman trainee, Bonneville
Power Administration: Okay.
Mick Johnson: So as you put power through conductors, especially iron conductor, the
current causes it to heat up.
As the wire heats, it also starts to sag.
That is, the thermal expansion causes the wire to grow longer and that allows the conductor
to hang closer to the ground.
On our system we want to avoid that because as the wire gets closer to the ground it causes
more potential danger for our people under the lines.
And if the lines get low enough they can sag into trees or structures and cause shorts
on the system.
As we see our wire right now, it’s down about an inch closer to the floor than it
was just a few moments ago when we started.
The temperature in here is in the high 60s.
And right now our wire is up to 120 degrees Fahrenheit.
On our system we use aluminum wire and we allow it to go to about 160 or 170 degrees.
So we’re approaching the temperature that we would allow our aluminum conductors to
go to.
Although their spans would be much longer than 60 feet, and so the amount that it would
sag would be more pronounced.
And one of the tests that we do here is to determine what temperature a conductor will
operate at if you just leave the same current on it for a long time.
So that is, what will its stable temperature be or on our lines that’s what we call rated
current — the amount of current that it can carry at the maximum temperature that
we will allow the lines to operate at.
Narrator: As you can see, under higher current this iron wire steadily sags as it heats up.
You can imagine the same effect on high-voltage power lines during a hot summer day when homeowners
and businesses are cranking their air conditioning.
Mick Johnson: Alright, take it down.
Narrator: Conductor like this iron wire could permanently elongate if it gets too hot.
But in most cases the line returns to its original shape and length once the current
goes back down and the conductor cools.
[What can be done about it?]
Sagging power lines getting too close or coming in contact with trees or other structures
can create a dangerous and sometimes lethal situation, so electric utilities closely monitor
buffer zones between growing vegetation and high-voltage power lines.
These vegetation management practices, combined with operating the lines at or below their
rating ensure public safety and reduce the risk of unplanned outages.