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- Part III.. the long term benefits of no power outages and therefore adding to zero maintenance.
As of today, some power lines are located underground, mostly in downtown areas as a convenience but also a number of distribution lines to neighborhoods. As you might have guessed, it is more costly to bury lines underground, due to the amount of trenching and shielding needed for the cable.
A more interesting proposal is to eventually place all power lines underground, including high voltage lines, a subject of scrutiny addressed in previous articles.
When compared to overhead lines, the benefits of underground power cables include:
1. Eliminating the electrical field through shielding, and reducing the magnetic field through phase
2. Reducing the negative health impacts of overhead electromagnetic fields (EMFs) to almost zero.
3. Safer because they can’t electrocute people or animals.
4. Will not negatively affect livestock or crop production.
5. Won’t fall over in high winds, tornadoes, hurricanes, or ice storms.
6. Eliminates power outages.
7. Safer for helicopters, airplanes, and hot air balloons who can’t crash into them.
8. Not an eyesore.
However, there are number of considerations in placing conductors underground. Since they are unable to dissipate heat as well as overhead lines, underground cables need to be made thicker. For a 400kV circuit, it may require dividing 12 separate lines into 4 trenches in order to dissipate the heat. The work space needed to construct such lines could be up to 40m wide.
A typical underground cable installation (National Grid)
There are different types of cables. One is called XLPE, or a Cross Linked Polyethylene Extruded cable, which can achieve power ratings from 33kV to 400kV. After this, it is recommended to go to a type of gas insulated line, or GIL, which contains aluminum-copper conductors within a sealed tube pressurized with Nitrogen-Sulphur Hexafluoride gas (SF6) to provide the main insulation. Although appearing effective and achieving a high power rating, it is an emerging technology and SF6 is considered a greenhouse gas.
Another type of cable are fluid-filled cables (FFC) which are copper conductors wrapped with paper insulation impregnated with fluid under pressure. A lead or aluminum sheath surrounds this followed by a further plastic sheath to prevent corrosion. A majority of cables of this type are implemented in a network operated by National Grid (Great Britain).
The XLPE type cable employs advanced technology of solid materials and does not require fluid. The central conductor is insulated by a cross-linked polyethylene material and is preferred over fluid-filled cables whenever possible. Its relative simplicity allows it to be installed in most areas such as tunnels, ducts and troughs, and buried directly.
A standard conductor from an overhead line is shown on the right. To its left is a solid dielectric underground cable. The aluminum can is shown for comparison purposes.
There are discussions over the increase of costs for underground power cables, some estimates claiming many times that of overhead lines. There are other groups, however, who have pressed the issue realistically towards the long term benefits of no power outages and therefore adding to zero maintenance. For example, a 40-kilometre-long 500kV line buried in Tokyo in year 2000 has had zero maintenance issues.
A Canadian organization by the name of RETA has actually derived the costs of underground cables to be less than overhead lines. This is based on the reasoning that underground cables cannot be knocked out by high wind storms of tornadoes, or ice storms, and are generally immune to weather deterioration. The transmission loss of electricity into heat (resistance loss) will also be less in buried cables and can add up substantially over time periods (around 60 years). These are combined with the health and property devaluation costs of overhead lines in summarizing a proposal to save money by placing power lines underground.