Difference Between Overhead Lines and Underground Cables

The overhead lines and underground cables are methods used for the transmission and distribution of power in an electrical power system. The selection of overhead line or underground cable depends upon various factors like operating voltage, amount of distance the power to be transmitted, initial cost, maintenance cost, condition of land through which the line is to be passed, etc.

By considering these factors either overhead lines or underground cables are selected for transmitting the power. Let us see the differences between overhead and underground systems used for the transmission and distribution of electrical power.

Comparison Between Overhead Lines and Underground Cables :

Characteristic	Overhead lines	Underground Cables
Initial cost	Less	For the same power transfer, the cost is double as compared to overhead lines
Public safety	Less	More
Maintenance cost	More	Less
Occurrences of faults	More	Less
Frequency of accidents	More	Very less
Appearance	Not good	Good looking
Identification of faults	Easy	Very difficult
Interference to communication lines	Affected	No effect
Voltage drop	More	Low
Damage due to lightning and thunderstorms	Affected	Free from lightning and thunderstorms
Jointing of wire/cables	Easy	Very difficult
Operating voltage	Used for any range i.e., 756kV or even higher	Up to 132kV
Charging current	Less	More (since the spacing between conductors is less)
Surge effect	More	Less

Overhead Lines :

The overhead transmission and distribution line system is the most widely used method of transmitting and distributing electrical power from one place to another place.

It is the only method used for transmitting a large quantity of electrical power over longer distances i.e., power transmission between power station and substation, or between substations. The various components of overhead lines include supports, cross-arms, insulators, conductors, stays, guys, lightning arrestors, fuses, isolating switches, earth wire, guard wires, etc.

Since there is no insulation coating is used over conductors, the overhead system for transmission and distribution is very cheap compared to the underground system.

But the use of overhead lines includes drawbacks like they are liable to hazards from lightning discharges, public safety is less, problems due to interference with neighboring communication lines, difficulty to use near submarine crossings, etc.

Underground Cables :

Electrical power can be transmitted and distributed either by the overhead system or by underground cables. The underground cables have several advantages such as rugged construction, greater service reliability, increased safety, lesser chances of faults, low maintenance cost, better appearance, and lesser interference from external disturbance like storms, lightning, ice, trees, etc, as compared to overhead system.

However, the use of underground cables have a greater installation cost, also insulation problem arises at high voltages with cables. Hence, power transfer throw cables are used where it is impracticable to use overhead lines.

Earlier days underground cables were mainly used in thickly populated areas and that to these are limited for low and medium voltages only, but nowadays due to requirements even Extra High Voltages for longer distances.

The use of underground cables for power transmission reduces (mostly eliminates) the possibility of supply interruption due to lightning. For long-distance power transmission, cables can not be used due to their large charging currents.

Key Differences Between Overhead Lines and Underground Cables :

• For a particular amount of power transferred at a given voltage the underground cable system costs almost double the cost of the overhead system. Hence the underground system is more expensive.
• The underground cable system is safer compared to the overhead transmission system.
• The maintenance work of underground cables is very complex while it is very simple in the case of overhead lines.
• Because the cables are laid underground hence the failure or faults are less as compared to the overhead system.
• The chances of accidents in the underground systems are very low as compared to overhead systems.
• The appearance of the underground cable system is good and neat because no wiring is visible as compared to overhead lines.
• Fault location and repairs are difficult and expensive in the case of underground cables as compared to the overhead lines.
• It is difficult to join underground cables in case of conductor breakage, whereas the conductors in the overhead system can be easily joined.
• In the case of an underground cabling system, there is no interference to communication lines as compared to overhead transmission lines.
• The underground cable requires a very high degree of insulation, hence it can not be operated above 132 kV. Whereas, the overhead transmission lines have capable of transmitting power at 765 kV or even higher.
• The underground cable system is free from lighting and thunderstorm as compared to the overhead transmission system.
• The voltage drop is low in the case of underground cables as compared to an overhead line.
• Because of less spacing between conductors in the cable, it causes much capacitance, hence it has more charging current as compared to overhead lines.
• In underground cables, the surge effect is smoothened down as surge energy is absorbed by the sheath.