Undergrounding
Encyclopedia
Undergrounding refers to the replacement of overhead cable
s providing electrical power
or telecommunications, with underground cables. This is typically performed for aesthetic purposes, and increases distribution cost.
electricity
(e.g., 400 kV
) and are supported by large pylon
s are generally considered the least attractive feature of the countryside but are some of the most expensive to lay underground.
Underground cables can assist the transmission of power across:
Other advantages include:
Disadvantages include:
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.
, according to Japan's Construction and Transport Ministry, just 7.3 percent of cables were laid underground as of March 2005.
(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.
When undergrounding a powergrid for aesthetic reasons, it is a good idea also to replace open-air switchyards by indoor switchyards and especially pylon transformer
s by indoor transformers as transformers on pylons, which are only fed by underground cable look somewhat wierd.
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
s providing electrical power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
or telecommunications, with underground cables. This is typically performed for aesthetic purposes, and increases distribution cost.
Overview and Comparison
The aerial cables that carry high-voltageHigh voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
(e.g., 400 kV
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
) and are supported by large pylon
Electricity pylon
A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...
s are generally considered the least attractive feature of the countryside but are some of the most expensive to lay underground.
Underground cables can assist the transmission of power across:
- Densely populated urban areas
- Areas where land is unavailable or planning consent is difficult
- Rivers and other natural obstacles
- Land with outstanding natural or environmental heritage
- Areas of significant or prestigious infrastructural development
- Land whose value must be maintained for future urban expansion and rural development
Other advantages include:
- Less subject to damage from severe weather conditions (mainly lightningLightningLightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
, wind and freezing) - Greatly reduced emission, into the surrounding area, of electromagnetic fields (EMF). All electric currents generate EMF, but the shielding provided by the earth surrounding underground cables restricts their range and power. See section below, health concerns.
- Underground cables need a narrower surrounding strip of about 1–10 meters to install, 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, and are significantly safer as they pose no shock hazard (except to the unwary digger).
- 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. In highly urbanized areas the cost of underground transmission can be 10-14 times as expensive as overhead.
- Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks, and for this reason redundant lines are run.
- Underground power cables, due to their proximity to earth, cannot be maintained live, whereas overhead power cables can be.
- Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.
Asia
Most electrical power in Japan is still provided by aerial cables. In Tokyo's 23 wardsSpecial wards of Tokyo
The are 23 municipalities that together make up the core and the most populous part of Tokyo, Japan. Together, they occupy the land that was the city of Tokyo before it was abolished in 1943. The special wards' structure was established under the Japanese Local Autonomy Law and is unique to...
, 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 MarketsOffice of Gas and Electricity Markets
The Office of Gas and Electricity Markets , supporting the Gas and Electricity Markets Authority , is the government regulator for the electricity and downstream natural gas markets in Great Britain...
(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.
North America
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.When undergrounding a powergrid for aesthetic reasons, it is a good idea also to replace open-air switchyards by indoor switchyards and especially pylon transformer
Pylon transformer
A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. If mounted on a utility pole, they are called pole-mount transformers...
s by indoor transformers as transformers on pylons, which are only fed by underground cable look somewhat wierd.