Transmission 101: Exploring the Hidden Currents

By Joni Sliger, Energy Fellow

"Photoshop art created from two NREL-Image Gallery photos of
sunset view of electrical power towers combined with wind machines."
Credit: NREL and Raymond David (Photo Illustration)

Hailed as the world’s largest machine, our nation’s transmission and distribution infrastructure is a critical component of our electricity system. These power lines carry electricity from every electric power plant to every electricity consumer. Investing in this infrastructure is necessary for the transition to a clean energy future. This blog series explores the issues surrounding transmission, both nationally and in the Pacific Northwest, and considers various options for meeting our transmission needs in the 21^st century. This first post offers a basic introduction to transmission in the United States.

What is Transmission?

Transmission, in the electricity world, refers to the flow of electricity from its generation source, like a power plant or a wind farm, to a local substation, typically over long distances. The infrastructure that carries this electricity is the transmission system. At the substation, the transmission system connects to the distribution system, which carries electricity on to consumers. One may view a map of the nearly 200,000 circuit miles of transmission lines in the U.S. on the Energy Information Administration’s U.S. Energy Mapping System.

As electricity travels on a power line, some of it is lost due to inefficiencies in the system. For example, power lines may unintentionally produce heat, which uses up some of the electricity on the line. Heat losses are a common problem: consider the remarkably inefficient incandescent light bulb, which loses 90% of the electricity it draws to unintentionally producing heat. The energy lost in transmission or distribution is called a ‘line loss.’

In the U.S., 5 to 6% of the electricity generated is lost due to line losses nationally. Inside Energy provides an interactive graphic of the line losses in each state; Wyoming loses the least power at only 2.2%, while Idaho loses the most at 13.3%. Losses vary depending on whether a state has more transmission lines or more distribution lines. Transmission lines travel farther distances than distribution lines, but they have fewer line losses, because they operate at a higher voltage. High-voltage lines can carry more electricity to consumers, ensuring more sales. However, the voltage is too high for household use, so transformers reduce the voltage at the substation and along the distribution system.

To sum up, transmission and distribution lines carry power from a generating source to a user. Transmission lines carry power farther distances but are more efficient, because they operate at a higher voltage than distribution lines.

What are the Problems with Transmission?

With almost 200,000 miles of transmission lines in the U.S., one may be surprised to hear that a lack of transmission capacity is a major constraint to bringing renewable energy sources online. Unfortunately, our current transmission system suffers significant limitations.

Time plays two roles in affecting transmission systems. First, the passage of time decreases the efficiency of transmission systems. Over time, transmission lines suffer physical wear and tear and become less efficient. Additionally, new technologies are discovered, so the lines become outdated. Most transmission lines in the U.S. are based on 1950s technology. Replacing these lines and associated infrastructure with newer technologies is often referred to as ‘modernizing the grid.’

Second, time affects the demands on the transmission system. In the U.S., electricity must be kept operating at 60 Hertz, the frequency of choice for our systems. Grid operators constantly monitor the system to ensure its frequency is stable. If the frequency changes too much, the system crashes, causing brownouts or blackouts. As consumers draw power off the grid, electricity must be available to replace it, so that operators can balance the grid. If more electricity is available than consumers need, producers have to throw it away. Advances in energy storage technologies, like modern battery systems, may soon enable producers to save this excess energy and avoid the use of ‘peaking plants.’ Peaking plants operate only when demand is at a high, even if only for a few hours or days a year; with such limited use, the construction of peaking plants is very expensive relative to the energy they produce. Without energy storage, the transmission system cannot take advantage of changes in energy demands.

Transmission lines can only carry so much electricity at one time. Their capacity varies with the age and efficiency of the technology of the line; newer lines can operate at higher voltages, which allows them to carry more power, while suffering fewer line losses, as noted above. With only a limited amount of capacity available, generators have to compete to get their electricity to the market for sale. A congested transmission system, where too little transmission capacity is available, creates a bottleneck that constrains electricity from getting to where it is needed. Financing the deployment of new sources requires access to a market, so bottlenecks are a significant impediment to possible development.

Where transmission lines are located affects whether generators face a bottleneck or not. Unfortunately, most transmission lines are currently located far away from the best sites for renewable energy development. As my colleague, Amy Schlusser blogged previously, we primarily built our transmission system to carry power from large, fossil fuel-fired power plants to power-hungry urban centers. In rural areas, where the potential to develop renewable energy is often greatest, there is often a lack of available transmission or capacity. In short, our transmission lines are built in the wrong places for a future powered by renewable sources.

To bring more renewable energy sources online, the most obvious solution is to build more transmission lines. This is extremely expensive and faces challenges in siting. Concerns include conflicts with local private property owners (and NIMBYism) as well as conflicts with other needs, such as environmental regulations to preserve wilderness and protect wildlife, such as sage-grouse. Other options exist though, such as more distributed generation and offshore energy development, as I’ll discuss later on in this series. Before discussing possible solutions, however, this series will next look deeper in how the transmission system operates, particularly in the Pacific Northwest, and explore open-access transmission policies, pancaked rates, balancing areas, and more.