Comparison of overhead and underground transmission lines

1. N:Ahmed Usama Saleh
G:E501

2.  An overhead cable is a cable for the
transmission of information, laid on utility
poles. Overhead telephone and cable TV
lines are common in North America.
Elsewhere, overhead cables are laid mainly
for telephone connections of remote
buildings and temporary mechanisms.
 as for example building sites. The same
poles sometimes carry overhead power
lines for the supply of electric power. Power
supply companies may also use them for an
in-house communication network.
 Sometimes these cables are integrated in
the ground or power conductor. Otherwise
an additional line is strung on the masts.

3.  Undergrounding is the replacement
of overhead cables providing electrical
power or telecommunications, with
underground cables. This is typically
performed for aesthetic purposes, but also
serves the additional significant purpose of
making the power lines less susceptible to
outages during high wind thunderstorms or
heavy snow or ice storms. Undergrounding
can increase the initial costs of electric power
transmission and distribution but may
decrease operational costs over the lifetime
of the cables.

4.  Advantages:
 The main advantage is that they are easier to repair if
damaged (Usually just go up a ladder and do the
necessary work)
 They are not restricted by the landscape that they are
in e.g. you can easily suspend them over a motorway
or river
 Also they have a less chance of electrocuting people
as they are so high up
 Lose less energy then if they were underground as
they do not have any good conductors near them to
transfer energy away as easily
 Cheaper to set up and maintain than if they were
underground

5.  Disadvantages:
 Main problem is that they
visually pollute the areas
they are set up in
 Suffer from problems like
vandalism, assisting
terrorism (Easy access),
and lightning strikes
 Birds and low flying
aircrafts can sometimes
fly into them

6.  The main advantages of underground cables are:
 • Reduced visual impact.
 • Reduction or elimination of electrical and
magnetic fields above ground, and hence
elimination of potential health concerns.
 • Reduced transmission losses.
 • Reduced planning delays.
 • Increased security of supply, with elimination
of disruption due to extreme weather.
 • Road side developments can be made without
Section 37 application.

7.  Cost of Underground Cables: The cost of HVDC
underground cables has been quoted by SHETL as
being approximately three times that of a 400kV
overhead line. Underground cables carrying 400kV AC
are more expensive, but the very high costs quoted by
SHETL have been challenged by an independent report
by Jacobs Babtie (2005). This report dealt specifically
with the conditions found on the Beauly to Denny line,
with cost ratios quoted in the range 6 to 12 times that
of an overhead line. Unit costs increase with a
reduction in length of line placed underground, and
also with the use of fluid filled cable. The Jacobs
Babtie report does not give detailed costing for
underground cables in reinforced concrete troughs,
but the additional cost associated with this technology
will be offset by the use of the cheaper XLPE cables.

8.  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.[4] In highly urbanized areas the cost of underground
transmission can be 10-14 times as expensive as overhead.
 Whereas finding and repairing overhead wire breaks can be accomplished in hours,
underground repairs can take days or weeks,[6] 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.[citation
needed]
 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.

9.  Underground cables offer an affordable and
justifiable solution for critical parts, and in
some cases the entire length, of overhead
high voltage power lines. With appropriate
technology used in appropriate places, the
environmental impact of underground cables
can be minimized.

11.  Electric Power needs to be carried over long distances from the point
of generation to the point of consumption. This Transmission is
done either through overhead lines or underground cables. Each of
these two methods of transmission has its own advantages and
disadvantages.
Overhead Transmission lines are cheaper as the insulation cost is
lesser and the conductor material cost is lesser too. They also have
better heat dissipation.
However, they have significant disadvantages. Overhead lines are
vulnerable to lightning strikes which can cause
interruption. Overhead lines use bare conductors and can cause
damage if they break. They are considered to be unsightly as they
mar the scenery of the landscape. The maintenance cost of
overhead lines is more and the voltage drop in overhead lines is
more.

12.  Underground transmission due to cables is costlier than overhead
transmission as the ground needs to be excavated. This can be
difficult when passing though geographic obstructions such as hills,
marshes and rivers. Special trenches need to be constructed when
passing through loose soil. Besides, heat dissipation in
underground cables is an issue. Hence, the conductors have to be
thicker. The insulation required for the cables is expensive. Hence,
it is difficult to use underground cables for voltages at HV levels (>
33 kV).
 Underground cables may have to be rerouted to accommodate other
underground structures such as pipelines, sewage lines, etc. It is
necessary that the routes of underground lines are clearly marked
with sign boards to prevent accidents when excavations are carried
out for other reasons at a later date.