AC and DC Transmission Lines

AC and DC Transmission Lines

After every hurricane, there is a hue and cry to put transmission lines underground.

Some people merely want less cluttered views, especially in scenic areas.

Interestingly, an AGW group, the Climate Institute, wants to convert AC transmission to DC transmission and put it underground to cut CO2 emissions.

It’s important to note that only a small percentage of outages, approximately 2%, are caused by transmission line failures. Distribution line failures are the main cause of outages.

For this reason, putting distribution lines underground can result in improved reliability while putting transmission lines underground has little effect on reliability.

The cost of putting distribution lines underground is far less than putting transmission lines underground, especially when new subdivisions are being built and the underground distribution system can be installed in conjunction with the building of roads and other facilities.

Putting AC Transmission Lines Underground

Costs are usually cited as the reason for not putting AC transmission lines underground, but there are some technical reasons also.

Two of the technical problems are: Insulation life and capacitance. 

  • Insulation failures can result in outages and costly repairs or replacements.
  • Capacitance can limit the length of an underground AC transmission line to as short as 25 miles, depending on the voltage. Capacitance creates a charging current that can equal the total current carrying capacity of the line. 

Overhead AC transmission lines have no insulation and rely on the distance between the cables, or cable to ground, to prevent flashovers. Underground cables require insulation.

For an overhead line, the distances between cables and cable to ground is maintained by the insulator strings attached to towers. Each insulator string has multiple insulators shaped like inverted saucers. Each skirt, or disc, shaped like an inverted saucer, forces electricity to flow over the skirt’s surface, with the length of the path to ground being dependent on the diameter of the saucer and the number of skirts on the string. An insulator on a pole for a 13,000-volt distribution line might have two skirts. A 345,000-volt transmission line might require a string of over 20 skirts.

Comparison of overhead and underground transmission lines

Costs are the main reason for not putting transmission lines underground. These costs vary depending on whether the lines are being built in rural areas or in urban areas since a large portion of the cost is for trenching and rights of way.

The voltage level also has a major effect on costs.

Overhead AC transmission lines. Photo by D. Dears

The accompanying photo shows how two, three-phase transmission lines are mounted on the same tower to reduce costs. It also shows how two rows of towers, each containing two, three-phase AC transmission lines can be sited on the same right of way to reduce right of way costs.

In this photo, four AC transmission lines are being accommodated in the same right of way.

According to the latest Edison Electric Institute report: 

  • The cost per mile for a single, new three-phase AC overhead transmission line can range from $174,000 in a rural area to $11 million in an urban area.
  • The cost per mile for a single, new three-phase AC underground line can range from $1.4 million in a rural area to $30 million in an urban area.

Roughly speaking, it costs three to ten times as much to build a new transmission line underground as overhead.

Converting an existing single, three-phase AC overhead transmission line to an underground line costs:

  • From $1.1 million to $6 million in a rural area
  • From $0.5 million to $12 million in urban areas

These per mile costs need to be applied to real situations.

For example, converting a single, three-phase  50-mile rural AC transmission line from above to underground could cost $300 million.

States that have studied the issue have determined that the cost of putting their transmission systems underground, which can amount to billions of dollars, far outweigh any benefits.

AC vs DC transmission

Accurate data is difficult to find, so some of the following comments are general in nature.

DC transmission lines can be used in place of AC. 

A major cost of DC transmission is the cost of converting AC to DC and then converting it back to AC again. As a result, the longer the transmission line, the lower the per mile cost.

There appears to be a breakeven length of approximately 400 miles, where below 400 miles AC transmission is less costly, while above 400 miles DC transmission is less costly.

Another important consideration is that line losses, I2 R losses, are approximately 30% lower with DC transmission.

AC transmission, however, is more flexible because DC transmission is point to point, while AC transmission lines can be tapped into most anywhere along the line.

As a result, generally speaking, for a single line: 

  • DC transmission is less costly when used to conduct electricity over long distances or under water.
  • AC transmission is less costly for shorter distances. It’s also more flexible.

The cost of putting DC transmission underground will be about the same as putting AC transmission underground, with costs in urban areas higher than in rural areas.

Using DC transmission to cut CO2 emissions

There have been at least two proposals to rebuild the US grid using DC transmission so as to cut CO2 emissions. The most recent proposal was by the Climate Institute in a Wall Street Journal article.

The next article will address the appropriateness of using DC transmission to cut CO2 emissions.

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10 Replies to “AC and DC Transmission Lines”

  1. I thought that we went with tesla’s ac instead of Edison ‘ s DC because of the inability to transmit DC long distance ?

    • That was essentially true until about the late 1950s. Prior to then, AC allowed lower voltages to be converted to higher voltages where transmission could be done with fewer losses. Around the 1950s ASEA developed a method for converting AC to DC and back again after it had been converted from a lower voltage, say 13,000 volts, to a higher voltage, say 600 KV. As noted in the article, the cost of converting AC to DC, and then back again, is very expensive. The initial motivation was to transmit power underwater. Beginning in the last third of the last century DC transmission was started to be used to transmit very large amounts of power for long distances. Itaipu in Brazil was the prime example until China started to build DC transmission lines from its hydropower installations to the big cities along the coast.

  2. Donn. Another excellent post on a subject few people know much about! Which makes them vulnerable to the claim that “we just have to build more transmission lines from the windy Western plains to the crowded East Coast!”

    In general, DC lines have been worthwhile when there are very big hydroprojects involved. For example, the Pacific DC Intertie can bring enough power to serve two to three million households in LA. The power comes from the Pacific Northwest, mainly the Columbia River. When you are moving power on that scale, a DC line makes sense.

    https://en.wikipedia.org/wiki/Pacific_DC_Intertie

    On the east coast, we have a similar situation, with DC bringing power from the HydroQuebec dams down to Boston.

    http://www.hydroquebec.com/international/en/exports/markets/new-england.html

    For smaller amounts of power and shorter distances, as I understand it, converting AC to DC and back makes DC not cost effective.

    • Thanks for your examples of DC transmission.
      My next article will examine DC transmission and will use examples from China and Brazil to demonstrate where it makes sense to use DC in comparison with AC transmission.
      As you indicate, the cost of converting AC to DC and back again, makes DC uncompetitive for shorter distances. The exception is for underwater transmission.
      I published today’s article for two reasons. First to establish why it doesn’t make economic sense to put transmission lines underground, and second, to provide the necessary background for my next article on DC transmission.

  3. Isn’t another big reason for migrating to DC the ease of interfacing with (DC) wind & solar that will soon ostensibly dominate electric generation?

    • My next article discusses this question. However, offshore wind installations will need to use DC transmission of they are more than 25 miles offshore, due to AC not working at that or longer distances.
      I’m glad you asked because many people make the assumption that DC transmission is needed to support wind and solar, which isn’t necessarily true.

  4. Just as a matter of curiosity , except for scale , is the ac/dc/ac conversion process the same as the inverter speed controllers electric motors ? Thanks for the great article and information .

    • I’m not sure I understand the question, and I am not current with motor controllers.
      Sorry, but I need to skip this one.

  5. A motor speed control will take ac , convert it to DC and as it changes it back to ac it varies the hertz below 60 to control speed . Thanks anyway , evidently different process .

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