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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.
underground electric line is very much costly!
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