On the Road to Increased Transmission: Higher-Voltage Alternating Current
How Higher-Voltage Lines Over Longer Distances Can Tap Into More Wind and Solar Resources
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. Two previous articles explored dynamic line ratings and flexible alternating current transmission systems (FACTS). This brings us to our third option on the highway to improved transmission: increasing the voltage of high-voltage alternating current (HVAC) lines.
A Look at the Current Power Highways
When it comes to increasing the carrying capacity of U.S. transmission lines, there are a few key things to know.
Most importantly, the majority of the lines that make up the power grid are alternating current (AC) lines. This is largely because they are used over shorter distances and are lower cost for these situations.
"Because the vast majority of our transmission lines are designed to carry energy fairly short distances, AC lines have just made sense," said Paul Denholm, a senior research fellow at the National Renewable Energy Laboratory (NREL). "Traditionally, transmission lines were built to bring power from plants that were often less than a few hundred miles away from major population centers."
But as the grid diversifies and more renewables are introduced, it is becoming increasingly valuable to move power over longer distances. To do that, planners could look at ways to increase carrying capacity of the lines we already have.
"If you've got an existing line, there are two things that you can do to increase the carrying capacity," Denholm said. "One is you can put bigger wires or more wires on the line. The other is increasing the voltage, which is analogous to increasing the speed limit on a highway."
Raising the Speed Limit: Higher-Voltage Alternating Current Lines
Because so much of the grid is made up of AC lines, increasing the voltage of HVAC lines could make for a big increase in power with relative ease.
"If you double the voltage, you don't double the power you can put on the lines" Denholm said. "You actually quadruple the power. So, if we double the voltage, we can put four more times the power on that system."
Another advantage to increasing the voltage of current transmission lines is that more power can reach its destination over longer distances compared to lower-voltage lines. Raising the voltage requires less current to put the same amount of power through the lines, meaning the lines do not heat up as much and therefore lose less energy to heat.
Because each higher-voltage AC line can carry more power, fewer lines would be needed for long-range transmission. However, there is a trade-off with the higher-voltage lines: While they have much higher carrying capacities, they are much larger than the current transmission lines.
Navigating the Right of Way
Size plays a crucial part in determining where and what types of transmission lines can be built. The bigger the lines, the more land that is needed to support them—a concept known as right-of-way requirements. Building higher-voltage AC lines would mean the sizes of the towers (both how tall and how wide they are) increase for existing lines.
"The right of way does increase, but the amount of power that we can carry on that line increases by more than the increase in right of way," Denholm said. "So that ultimately provides an advantage."
The main reason building higher-voltage lines could have an advantage is simple: Fewer of them are needed to carry the same amount of power that could be carried by more, lower-voltage lines. Therefore, upping the voltage saves real estate when it comes to grid planning.
This option could be paired with flexible alternating current transmission system devices to steer power onto underutilized HVAC lines, rather than overloading smaller lines. Check out another article in this series to learn about this transmission upgrade.
The next article of this four-part series will discuss high-voltage direct current transmission lines. 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.