Wintertime Transmission: Bag the Sag

Polar vortexes and arctic blasts are winter conditions that bring dangerously cold temperatures to the region. But for TVA’s 16,000 miles of transmission lines, frosty weather carries an offsetting benefit—more capacity.

Transmission lines are metal conductors that carry power from one point to another. As power flows, it creates heat on the conductors.

“We deal with voltages of up to a half a million,” said TVA System Operations Center Manager Jerry Wynne. “The transfer of power creates heat so the colder it is outside, the more power you can flow through the conductors.”

But why?

Limitations of Heat

Imagine the three states of water—ice, liquid and steam. When water is frozen, the molecules forming an ice cube are tightly compacted. As the ice melts and turns to liquid, the water molecules move farther apart. If heated and the liquid is turned to steam, the distance between the water molecules becomes even greater.

The same is true for metal. The properties of thermal expansion force the molecules forming transmission line conductors to begin to move farther apart and stretch when heat is applied.

“When dealing with high-voltage power lines, we have to account for thermal expansion,” Wynne explains. “On hot summer days, we scale back the load on the lines to reduce heat and prevent them from sagging.”

That’s critical because if transmission lines sag too low, they can potentially come into contact with the ground, trees or other structures. In addition to being a safety hazard, low-sagging transmission lines would jeopardize the stability of the power grid.

When a transmission or sags too close to an object that is in contact with the ground—like a tree—power begins to flow through the object. This creates a line-to-ground condition that must be stopped immediately to protect the electrical grid.

Small sensors—called relays—are always monitoring TVA transmission lines around the clock. Should one of these relays see a problem, such as a line-to-ground, it automatically clears the fault. This means the relay kills the power flowing through the line. From start to finish, this protection process takes less than a second.

To prevent a transmission line from sagging too low and forcing a relay to interrupt the flow of power, TVA limits the amount of electricity flowing through the lines during the summer in order keep an adequate safety margin. With colder air temperatures the lines sag less, allowing the line to carry more power to millions of homes and businesses across TVA’s seven-state service area.

Experiment #1: Winter v. Summer

The Transmission Operations team carefully monitors the transmission system to ensure that power is always flowing to the millions TVA serves. They constantly balance the flow across an extensive web of lines, ensuring all transmission assets are protected, regardless of the time of year.

If you’re willing to wait six months to complete an experiment, you can actually see how heat-induced resistance affects a TVA transmission line.

Step 1: Go to your nearest high-voltage transmission line on a cold winter day and take a photo of the line stretched between the towers.

Step 2: Come back on a hot summer day and stand in the same spot, comparing your wintertime photo to what you see. The difference will be clear—summertime heat will cause the line between to towers to droop more toward the ground.

Experiment #2: Try It at Home

If six months sounds like too long to wait to try your transmission experiment, there is an easier way to learn about thermal expansion in just a few minutes. Wynne explains how you and a friend can safely replicate what happens to a high-voltage power line at your home or school.

Step 1: Cut a piece of yarn or thin rope in two 25-foot lengths. Make sure they are exactly the same. These will be your transmission lines.

Step 2: Soak one of the lines in water for a few minutes.

Step 3: Place the wet line in the microwave for about a minute. Be careful not to get the line too hot. It should only be warm to the touch.

 Step 4: Stretch both the warm line and the cooler line between you and your experiment partner, holding them both at chest level. Make sure the lines are stretched tight so they don’t touch the ground. Now, you and your partner are simulating TVA transmission towers. Just like a TVA transmission line that expands due to heat, your warm line easily stretches and sags lower than the cooler line.