Saturday, 29 June 2019

What is Overhead and Underground Transmission Lines

What is Overhead and Underground Transmission Lines


About Overhead and Underground Transmission system.


       
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 System


An Overhead Power line is a structure used in Electric Power Transmission and distribution to transmit electrical energy along large distance.

It consists of one or more suspended by towers or poles.

Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy,

Overhead Transmission Lines are cheaper as the insulation cost is lesser and the conductor material cost is lesser and the conductor material cost is lesser too. They also have better heat dissipation.
                 

In this system transmission of electrical power by using overhead transmission line over a long distance.


In such system the appropriate spacing is provided between the conductor, at the supports as well as at intermediate points.

This spacing provides insulation which avoids and adds Electrical discharge to occur between the conductors. It is cheaper than the UG system and lines are subjected to the false occuring to lightning, short circuit and breakage of lines etc.,


but it can be easily repaired compared to UG cable system. In this system the insulation must be provided between the conductors and Supporting Structures.

Hence the maximum stress exists between conductor and earth.


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.



Underground Transmission System


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 obstruction 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.

Thus, 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.
             
The cables are generally preferred in Underground System.

All conductors must be insulated from each other in the underground system.

As the voltage level is high, insulation required is more hence due to insulation difficulties the voltage level used in Underground system is below 66KV, while the voltage level used in overhead transmission lines can be as high as 400KV.

             
The maintenance cost of the underground system is less compared to overhead system.

In case of crowded area where overhead system using bare conductor is not practicable where I cable is preferred.


The lines surges are suppressed by using UG cables. Hence UG cables must be used for the last path for the connection which saves transformer and generators from the damage due to line surges.







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