1. Overhead system 2. Underground system
1.1 Overhead System
In this system, the transmission of electrical power is by using
overhead transmission lines over long distances. In such system, the
appropriate spacing is provided between the conductors, at the supports
as well as at the intermediate points. This spacing provides insulation
which avoids an electric discharge to occur between the conductors. The
transmission by overhead system is much cheaper than the underground
system. The overhead transmission lines are subjected to the faults
occurring due to lightening, short circuits, breakage of line etc. but
overhead lines can be easily repaired compared to underground system. It
is also true that though such faults are rare, if occurred it is very
difficult to find exact point of fault as transmission lines are very
long. In the overhead system, the insulation must be provided between
the conductor and supporting structure. Hence the maximum stress exists
between conductor and earth.
1.2 Underground System
The cables are generally preferred in underground system. All
the conductors must be insulated from each other in the underground
system. As voltage level is high, insulation required is more. Hence due
to insulation difficulties, the voltage level used in underground
system is below 66 KV while the voltage level used in overhead
transmission lines can be as high as 400 KV. The maintenance cost of the
underground system is less compared to overhead system. In crowded
areas, overhead system using bare conductors is not practicable where
underground system using cables is preferred. The line surges are
suppressed by using the cables hence cable must be used for the last
part of the connection which can save transformers and generators from
the damage due to line surges.
In the underground system, the maximum stress exists on the insulation between the conductors.
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