Overhead Versus Underground Power Transmission

The transmission lines are used to transmit the power for long-distance. There are two types of transmission lines;

1. Overhead Transmission line
2. Underground Transmission line

Overhead transmission line uses bare conductors. These conductors placed at a height from the ground. To maintain clearance between the conductors and ground, supporting towers are used. The voltage of the transmission line decides the height of the tower. The insulators used to provide insulation between the conductor and the tower. As the transmission voltage level increases, the height of the tower increases to provide more clearance between the ground and conductors.

In the underground transmission system, the number of conductors bunched together with proper insulation. The underground cable provided with lead sheet and armoring. These provide protection against moisture and mechanical injury. As the voltage level increase, the thickness of insulation increases.

	Overhead Line	Underground cable
Fault location	As the overhead line is  visible, it is easy to find the location of the fault.	As the underground cable is invisible, it is very difficult to find the location of the fault.
Initial cost	There is no requirement of digging, manholes, and trench. So, the overhead line system is cheaper than the underground system.	The initial cost of the underground transmission system is more compared to the overhead line because it needs digging, trenching, etc.
Chance of fault	As overhead line exposed to the environment, the chances of faults are more.	The cables are not exposed to the environment, there is less chance of fault.
Safety	This system is less safe as the conductors placed on the towers.	This system is safer as the cables placed underground.
Useful life	In this system, useful life is approximately 20 to 25 years.	Useful life is approximately 40 to 50 years.
Appearance	The general appearance of this system is not good because of all lines are visible.	The general appearance of this system is good because of all lines are invisible
Maintenance cost	In this system, no need to dig at the time of maintenance. Hence, for the same number of faults, the maintenance cost is less.	In this system, to find the fault, digging is compulsory. It increases labour cost. Hence, for the same number of faults, the maintenance cost is more.
Flexibility	This system is more flexible. Because the expansion of the system is easily possible.	This system is not flexible. The expansion cost is nearly equal to the new erection of the system.
Conductor size	The conductors placed in atmosphere. So, the heat dissipation is better. Therefore the size of the conductor is small compared to the underground system.	Because of the poor heat dissipation, the size of the cables is more.
Interference with communication line	The communication lines are run along the transmission line. In this case, it is possible to cause electromagnetic interference.	In this case, there is no chance of interference with communication lines.
Proximity effect	The distance between the conductor is very high. So, proximity effect does not affect.	As the distance between cables is very less, the proximity effect is very high.
Application	The cost of this system is low. Therefor overhead lines used in the long transmission system and in rural areas for the distribution system.	Because of the high cost, it uses in the short distance and in populated areas. Where space is a major problem for the overhead transmission line.