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

The overhead conductor is more harmful to human and other animals than underground cables.

When a vehicle is parked under a high voltage transmission line an electrostatic field is developed in it. When a person who is grounded touches it a discharge current flows through the human being. In order to avoid this parking, lots are located below the transmission lines the recommended clearance is 17 m for 345 kV and 20 m for  400 kV lines.

EFFECT ON ANIMALS

Many researchers are studying the effect of the Electrostatic field on animals. In order to do so, they keep the cages of animals under a high Electrostatic field of about 30 kV/m. The results of these Experiments are shocking as animals (are kept below high Electrostatic field their body acquires a charge & when they try to drink water, a spark usually jumps from their nose to the grounded Pipe) like Hens are unable to pick up grain because of the chattering of their beaks which also affects their growth. 

EFFECT ON PLANTS

High power transmission lines affect the growth of plants.

Physiological parameter was primarily due to the effect of reduced cell division and cell enlargement.

From various practically study it was found that the response of the crop to EMF from 110 kV and 230 kV Power lines showed variations among themselves. Based on the results the growth characteristics like shoot length, root length, leaf area, leaf fresh weight, specific leaf weight, shoot/root ratio, total biomass content and total water content of the four crop plants were reduced significantly over the control plants.

SHORT TERM HEALTH PROBLEM

• Headaches.
• Fatigue.
• Insomnia.
• Prickling and/or burning skin.
• Rashes.
• Muscle pain

LONG TERM HEALTH PROBLEM

• Risk of damaging DNA.
• Risk of Cancer.
• Risk of Leukemia.
• Risk of Neurodegenerative Disease.
• Risk of Miscarriage.

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 the ground which high-quality insulation is required to withstand the very high voltage. Produced heat dissipation into the environment for underground cable conductor and their surrounding 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 the 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 a greenhouse gas and the 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 the rated current carrying capacity of cable thermally stable backfilling is required with properly graded sand. Space required 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. A cooling and ventilation system and rail-mounted access vehicle is required 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 the substation compounds needed 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 maybe 30m to 40m in length and 5m in width. The majority of the jointing accessory assembling is carried out on site. Sometimes cooling system pipe is laid alongside cable and pumping equipment and heat exchangers are required above ground and these must be sited which cause 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:

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