…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.
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.
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.
. . .