Undergrounding is the replacement of overhead cables providing electrical power or telecommunications, with underground cables. It demonstrates the higher technology in developed countries for fire prevention and to make the power lines less susceptible to outages during high wind thunderstorms or heavy snow or ice storms. An added benefit of undergrounding is the aesthetic quality of the landscape without the powerlines. Undergrounding can increase the initial costs of electric power transmission and distribution but may decrease operational costs over the lifetime of the cables.
The first uses of undergrounding had a basis in the detonation of mining explosives and undersea telegraph cables. Power cables were used in Russia to detonate mining explosives in 1812, and to carry telegraph signals across the English Channel in 1850.
With the spread of early electrical power systems, undergrounding began to increase as well. Thomas Edison used underground DC “street pipes” in his early distribution networks; they were insulated first with jute in 1880, before progressing to rubber insulation in 1882.
Subsequent developments occurred in both insulation and fabrication techniques:
- 1925: Pressurized paper insulation used on cables
- 1930: PVC insulation used on cables
- 1942: Polyethylene insulation first used on cables
- 1962: Ethylene propylene rubber-insulated cables become commercial available
- 1963: Preformed cable accessories become available
- 1970s: Shrinkable cable accessories become available
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 areas or areas where land is costly or environmentally or aesthetically sensitive. Underground and underwater crossings may be a practical alternative for crossing rivers.
- Less subject to damage from severe weather conditions (mainly lightning, hurricanes/cyclones/typhoons, tornados, other winds, and freezing)
- Decreased risk of fire. Overhead power lines can draw high fault currents from vegetation-to-conductor, conductor-to-conductor, or conductor-to-ground contact, which result in large, hot arcs.
- Reduced range of electromagnetic fields (EMF) emission, into the surrounding area. However, depending on the depth of the underground cable; greater EMF may be experienced on the surface. 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. See Electromagnetic radiation and health.
- Underground cables need a narrower surrounding strip of about 1–10 meters to install (up to 30 m for 400 kV 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.
- Underground cables have much less danger of conductor theft, illegal connections, sabotage, and damage from armed conflict.
- Burying utility lines makes room for more large trees on sidewalks, the trees conveying environmental benefits and increase of property values.
- 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. However, these calculations may neglect the cost of power interruptions. The lifetime cost difference is smaller for lower-voltage distribution networks, on the range of 12-28% higher than overhead lines of equivalent voltage.
- 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 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. Large charging currents arise due to the higher capacitance from underground power lines and thus limits how long an AC line can be. In order to avoid the capacitance issues when undergrounding long distance transmission lines, HVDC lines can be used as they do not suffer from the same issue.
- Whereas overhead lines can easily be uprated by modifying line clearances and power poles to carry more power, underground cables cannot be uprated and must be supplemented or replaced to increase capacity. Transmission and distribution companies generally future-proof underground lines by installing the highest-rated cables while being still cost-effective.
- Underground cables are more subject to damage by ground movement. The 2011 Christchurch earthquake in New Zealand caused damage to 360 kilometres (220 mi) of high voltage underground cables and subsequently cut power to large parts of Christchurch city, whereas only a few kilometres of overhead lines were damaged, largely due to pole foundations being compromised by liquefaction.
- An overloaded cable could melt its plastic jacket, causing flammable vapors to accumulate. The cable could then ignite the vapors.
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.
Methods of Undergrounding
- Horizontal Boring – This is a method in which one uses a drill bit to bore horizontal starting at one point on the surface of the ground and creating an arc underground to come back out of the surface. This method is used when minimal damage to the surface is preferred.
- Trench Undergrounding - The other method for undergrounding power lines is to dig trenches, lay power lines into the trench and cover them back up. This is done for the length of the power line.
Most electrical power in Japan is still distributed 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 2008.
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 national lottery.
All low and medium voltage electrical power (<50 kV) in the Netherlands is now supplied underground.
In Germany, 73% of the medium voltage cables are underground and 87% of low voltage cables are underground. The high fraction of underground cables contributes to the very high grid reliability (SAIDI < 20). In comparison, the SAIDI value (minutes without electricity per year) in the Netherlands is about 30, and in the UK it is about 70.
In the United States, the California Public Utilities Commission (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.
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.
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