The transmission and distribution of an electrical power can be with
the help of overhead transmission lines or by underground cables. It has
been mentioned that in thickly populated are like towns and cities, the
use of overhead lines is not practicable. In such cases electrical
energy is transmitted and distributed with the help of underground
cables. In its basic form, an underground cable is a conductor provided
with proper insulation. As the voltage level increases, the cost of the
insulation increases rapidly and thus the use of underground cables is
restricted to low and medium voltage distribution.
1.2 Comparison of Underground cables and Overhead lines
Compared to overhead lines, the underground cables have the following advantages,
1.
It ensures non-interrupted continuity of supply. The possible supply
interruptions due to lighting, storms and other weather conditions are
eliminated because of underground cables.
2. It requires less maintenance.
3. The accidents caused due to breakage of overhead line conductors are eliminated due to use of underground cables.
4. The voltage drop in the underground cables is less.
5. The life of underground cables is long compared to overhead lines.
6- The beauty of cities and town get maintained due to underground network of cables.
7-
The overhead lines use bars conductors which is unsafe in thickly
populated area. Hence from safety point of view, the underground cables
are more advantageous.
The only drawbacks of underground cables are the extremely high initial
cost and insulation problems at high voltages. In India, the big cities
have adopted the system of underground cables for the transmission and
distribution.
Thus the use of underground cables is mainly for the distribution of an
electrical power at low and medium voltages. Its use is almost
compulsory at the location where use of overhead lines is not
practicable due to the safety reasons such as congested urban area,
crossing of wide roads, near gas plants and refineries, near substation
etc.
Still the overhead lines also have some advantages compared to the underground cables which are,
1- Long distance transmission is possibly by the overhead lines.
2- The conductors in overhead lines is less expensive.
3- The size of the conductor in overhead lines is less than underground cables due to good heat dissipation in overhead lines.
4-
The insulation cost is very less as the air itself acts as an
insulation between the conductors. The gas or oil is not required for
overhead lines. For high voltage levels, spacing in air can be easily
adjusted in case of overhead lines to obtain proper insulation.
5- The erection cost is much less for the overhead lines. The underground cable laying is difficult and complicated.
Depending upon the situation and the requirements, the underground cables or overhead transmission system can be used.
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