On the Road to Increased Transmission: Dynamic Line Ratings

On the Road to Increased Transmission: Dynamic Line Ratings

How Dynamic Line Ratings Can Adapt to Changing Weather Conditions To Enhance Transmission

The current U.S. transmission system will need to be upgraded and expanded to make it possible to carry larger amounts of clean energy across longer distances. In a short video, NREL explains four options that can help do just that. This four-part series of articles dives deeper into each of those possibilities, starting with dynamic line ratings.

Adjusting the Speed Limit With Dynamic Line Ratings

Just like all major roads have speed limits, all transmission lines have "line ratings," which represent the maximum amount of power that is safe to transmit through a specific transmission line. Transmission lines heat up when power runs through them, causing the lines to stretch and sag.

"If a transmission line heats, stretches, and sags enough, it may be in danger of touching trees or other vegetation," said Paul Denholm, a senior research fellow at the National Renewable Energy Laboratory (NREL). "So to prevent the risk of fire, we put limits on the amount of power that is safely allowed to go through the lines."

And just as weather can impact the maximum speed at which it is safe to drive, it can also impact the maximum amount of power that it is safe to move on transmission lines. For instance, on a hot summer day, transmission lines will heat up and sag faster than on a breezy winter day.

"If you blow on a hot spoonful of soup, it cools down faster. The same thing happens when the wind blows on transmission lines," Denholm said.

This sparks the question: What if there was a way to change the line ratings as the weather changes?

Enter dynamic line ratings.

"As it turns out, we can easily add tools that monitor local weather conditions along various parts of a transmission line and say, 'You know what? It's cooler. Let's increase the rating of this line by 5% or 10%,'" Denholm said.

By retrofitting existing transmission lines with weather monitors, grid operators can adjust the line rating of a transmission line from hour to hour, boosting the carrying capacity when it is safe to do so.

Cutting Costs, Revving up Reliability

By maximizing the use of existing transmission infrastructure with dynamic line ratings, utilities can avoid expensive investments in new lines or equipment. The added efficiency translates to cost savings that can be passed on to consumers.

"If we can use dynamic line ratings to relieve a little bit of the congestion that builds up on our power grid, that would allow us to use more of the abundant, low-cost energy generated by renewable resources," Denholm said.

Dynamic line ratings can also increase grid reliability.

"In the United States, we've got a pretty darn reliable power transmission and generation system," Denholm said. "But dynamic line ratings could further boost reliability, especially for edge cases."

For instance, during rare blackouts resulting from cold weather extremes, dynamic line ratings could come to the rescue. On very cold winter nights, traditional power plants may fail to keep up with the high energy demand, but alternative power plants may not be able to generate at full output because there is simply not enough capacity in the transmission lines to move power to the right place.

"In a situation like this, dynamic line ratings could allow grid operators to increase line ratings that were determined during a typical, warm day to match the reality during periods of extreme cold weather," Denholm said. "This simple change could provide much-needed power to people that might not otherwise have it."

Another benefit of dynamic line ratings is that implementing them on the current transmission system is low risk. If, for any reason, a sensor or software enabling dynamic line ratings was to fail, it can simply be reverted to how the lines have always operated until it is fixed. "This means upgrading our current infrastructure with this technology is a very safe bet," Denholm said.

While there are a number of ongoing projects across the country already deploying dynamic line ratings, Denholm said this represents only a small fraction of lines that could be upgraded to help meet the nation's clean energy goals.

The next article of this four-part series will explore flexible alternating current transmission systems, or FACTS. In the meantime, be sure to watch NREL's short video summarizing four ways to increase transmission and subscribe to NREL's energy analysis emails to receive updates.