Undergrounding
Undergrounding refers to the replacement of overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed for aesthetic purposes but increases the costs of electric power transmission and distribution.
Comparison
The aerial cables that carry high-voltage electricity and are supported by large pylons are generally considered an unattractive feature of the countryside. Underground cables can transmit power across densely populated or areas where land is costly or environmentally or esthetically sensitive. Underground and underwater crossings may be a practical alternative for crossing rivers.
Other advantages include:
- Less subject to damage from severe weather conditions (mainly lightning, wind and freezing)
- Greatly reduced emission, into the surrounding area, of electromagnetic fields (EMF). The electric current in the cable conductor produces a magnetic field, but the closer grouping of underground power cables reduces the resultant external magnetic field and further magnetic shielding may be provided at additional cost. See Electromagnetic radiation and health.
- Underground cables need a narrower surrounding strip of about 1–10 meters to install (up to 30m for 400kV cables during construction), whereas an overhead line requires a surrounding strip of about 20–200 meters wide to be kept permanently clear for safety, maintenance and repair.
- Underground cables pose no hazard to low flying aircraft or to wildlife.
- Much less subject to conductor theft, illegal connections, sabotage, and damage from armed conflict.
Disadvantages include:
- Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line. Above ground lines cost around $10 per foot and underground lines cost in the range of $20 to $40 per foot.[1] In highly urbanized areas the cost of underground transmission can be 10-14 times as expensive as overhead.[2]
- Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks,[3] and for this reason redundant lines are run.
- Underground cable locations are not always obvious, which can lead to unwary diggers damaging cables or being electrocuted.
- Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.[citation needed]
- Underground cables are more subject to damage by ground movement and earthquakes.
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.
Japan
Most electrical power in Japan is still provided by aerial cables. In Tokyo's 23 wards, according to Japan's Construction and Transport Ministry, just 7.3 percent of cables were laid underground as of March 2005.
Europe
The UK regulator Office of Gas and Electricity Markets (OFGEM) permits transmission companies to recoup the cost of some undergrounding in their prices to consumers. The undergrounding must be in National Parks or designated Areas of Outstanding Natural Beauty (AONB) to qualify. The most visually intrusive overhead cables of the core transmission network are excluded from the scheme. Some undergrounding projects are funded by the proceeds of the national lottery.
All low and medium voltage electrical power (<50 kV) in the Netherlands is now supplied underground. Other EU countries such as the UK and Germany are undergrounding a proportion of these cables each year.
California
In the United States, the California Public Utilities Commission's (CPUC) Rule 20 permits the undergrounding of electrical power cables under certain situations. Rule 20A projects are paid for by all customers of the utility companies. Rule 20B projects are partially funded this way and cover the cost of an equivalent overhead system. Rule 20C projects enable property owners to fund the undergrounding.
Variants
A compromise between undergrounding and using overhead lines is installing air cables. Aerial cables are insulated cables spun between poles and used for power transmission or telecommunication services. An advantage of aerial cables is that their insulation removes the danger of electric shock (unless the cables are damaged). Another advantage is that they forgo the costs - particularly high in rocky areas - of burying. The disadvantages of aerial cables are that they have the same aesthetic issues as standard overhead lines and that they can be affected by storms. However if the insulation is not destroyed during pylon failure or when hit by a tree, there is no interruption of service. Electrical hazards are minimised and re-hanging the cables may be possible without power interruption.
See also
References
- ^ Edison Electric Institute - Underground Vs. Overhead Distribution Wires: Issues to Consider
- ^ Los Angeles Department of Water & Power System Development Division cost estimate data
- ^ Should Power Lines be Underground?