Underground Power Cable: Environment and Health Issues

Power Plant Cooling Tower

Impact of Power Cable Uses in Environmental and Human Health:

Technology changing every-day, every-moment! High voltage electricity transmission underground cable system also taking over instate of overhead power line. Though   underground high voltage electricity transmission more technically challenging and expensive than installing overhead lines; but to get enough space for overhead line, especially in urban area is very difficult now. So, it’s demand of time for underground cabling system. Of course, we have to keep in mind the potential impact on the environment and health of cable installations.

Insulating in Overhead Power Line and Underground Cable:

Conductors, no matter they are overhead or underground that transmit electricity must be electrically insulated. But there is one major difference between overhead lines and underground cables is the way they are insulated. Overhead lines are insulated by open air surrounding conductor whereas underground cable conductors are fabricated wrapped in layers of insulating material.  Air insulation is the simplest and free from nature for overhead current carrying conductor. Air also flow through the bare overhead conductors and remove the produced heat. On the other hand for cable system, conductors are buried into ground which high quality insulation is required to withstand the very high voltage. Produced heat dissipation into environment for underground cable conductor and its surround insulation in different installation method is a major issue for impact on nearby flora and fauna.

Technology for Cable Insulation System:

Fluid filled or Gas Insulated Cables: Mainly the first generation cables are fluid filled, in these cables insulation is provided by paper, impregnated with fluid or insulated oil under pressure, wrapped around the central copper conductor. Lead or Aluminium sheath uses to cover this paper layer and to prevent corrosion of the sheath PVC, MDPE or HDPE outer jacket is used.

Alternatively gas insulation lines are used for insulation where Aluminium conductors are supported by insulators contained within sealed tubes which are pressurized with a Nitrogen or Sulphur Hexafluoride (SF6) gas to provide the main insulation. The main advantage is that SF6 is greenhouse gas and system is cost benefit for sealing end system.

XLPE Insulated Cables: As a result of change and advance in cable technology fluid filled cables are moving back and XLPE cables are coming at the front everywhere. In these smart cables the core conductor is insulated by XLPE means Crossed Linked Polyethylene material. It requires less maintenance and no auxiliary fluid equipment to monitor and manage for XLPE cables.

Underground Cable Installation Methods, Requirement and Impact:

For high voltage or extra high voltage underground cable installation mainly three types of method are following:

Direct Buried;

Deep Tunnels;

Surface or Trench.

Direct Buried is the traditional method for high voltage cables installation system where trenches are approximately 1.5m wide and 1.2m deep for each circuit. To maintain rated current carrying capacity of cable thermally stable backfilling is required with proper graded sand. Space require for cable easement corridor: 20m – 40m, 55m width; Joint bays intervals:  500m – 800m; working space required for join bays, power roller for excavating cable trench, sand bed, backfilling material, handling cable drum in site etc.

Deep Tunnel is generally used in urban locations where direct bury installation would cause unacceptable disruption. Typically 4m and 12m for joint bay diameter depth of a tunnel around 25m to 30m and maintains proper slope to provide free drainage. Two head house buildings order of 16m x 16m x 7m high require for maintenance and for installation of the cables at each end. Cooling and ventilation system and rail mounted access vehicle is require for emergency exit, inspection, maintenance and repair. River or railway crossing can be made in this system.

Surface or Trench is excavated and is constructed concrete trenches where cables are laid directly within the trench which is capped with reinforced concrete covers laid flush with the ground surface. Surface or trench are used in urban situations or and within substation compounds where need to take reduced land.

Environmental Impact on Underground Cable Installation:

The major environmental issues associated with the installation of direct buried or cut and cover tunnel cables are the disruption to traffic, noise, vibration, visual intrusion and dust generation and deposition due to the excavation of trenches along the route. Heavy goods vehicle traffic will also be generated by the work, cable crossing major roads, removing spoil and bringing in plant and materials, including backfill, to trenches. About 30m x 80m land area required for each circuit of high voltage cable sealing end compound and a significant space required for a terminal tower construction.

Nearly all installations require joints at specific intervals along the route and joint bays may be 30m to 40m in length and 5m in width. The majority of the jointing accessory assembling is carried out on site. Some time cooling system pipe are laid alongside cable and pumping equipment and heat exchangers are required above ground and these must be sited which cause of noise on the locality. Direct buried cable produce heat which harmful for on and near vegetation and insects.

EMFs or Electric and Magnetic Field Effect of Underground Cable:

Conductor which energized or having presence of voltage there is electric field present surround it, when current flow continues magnetic field also present there surround the conductor, this mainly happen for overhead electric line. But underground cable able to eliminate its electric field totally as it is screened out by the sheath round the cable; but it still produces magnetic fields. EMFs field strength is stronger to the center of the field or near to conductor and gradually weakened at outer point from the center. In general sense overhead line is farther than underground cable to human, so it should overhead line is safer than underground cable. However, as the individual cables are installed much closer together than the overhead conductors which results in the magnetic field from cables falling more quickly with distance than the magnetic field from overhead lines. Overall result is that the cable produces a lower field than the overhead line; means EMFs effect is smaller for underground cable than overhead line.