Transmission and distribution of electric power can be carried out by overhead as well as underground systems. Comparison between the two is given below:
1.Public Safety: Underground system is more safer than overhead system.
2.Initial Cost: Underground system is more expensive. For a particular amount of power to be transmitted at a given voltage the underground system costs almost double the cost of overhead system.
3.Flexibility: Overhead system is more flexible than underground system. In overhead system new conductors can be laid along the existing ones for load expansion. In case of underground system new conductors are to be laid in new channels.
4.Working Voltage: The underground system cannot be operated above 66 kV because of insulation difficulties but overhead system can be designed for operation upto 400 kV or higher even.
5.Maintenance Cost: Maintenance cost of underground system is very low in comparison with that of overhead system.
6.Frequency of Faults or Failures: As the cables are laid underground, so these are not easily accessible. The insulation is also better, so there are very few chances of power failures or fault as compared to overhead system.
7.Frequency of Accidents:The chances of accidents in underground system are very low as compared to overhead system.
8.Voltage Drop: In underground system because of less spacing between the conductors inductance is very low as compared to overhead lines, therefore, voltage drop is low in underground system.
9.Appearance: Underground system of distribution or transmission is good looking because no wiring is visible. Due to its good looking, inspite of its higher cost it in adopted in modern cities like Chandigarh(Punjab) Faridabad.Ahmedabad.(Gujarat)
10.Fault Location and Repairs: Though there are very rare chances of occurring fault in underground system, but if occurs it is very difficult to locate that fault and it repair is difficult and expensive.
11.Charging Current: On account of less spacing between the conductors the cables have much capacitance, so draw higher charging current.
12.Jointing: Jointing of underground cables is difficult so tapping for loads and service mains is not conveniently possible in underground system.
13.Damage Due to Lightning and Thunder Storm: Underground system is free from interruption of service on account of thunder storm, lightning and objects falling across the wires.
14.Surge Effect: In underground system surge effect is smoothened down as surge energy is absorbed by the sheath.
15.Interference to Communication Circuits: In underground system there is no interference to communication circuits.
Mostly the high voltage transmission is carried out by overhead system due to low cost. However, distribution in congested areas and in modern cities, like Chandigarh, Ahemdabad is carried out by underground cables.
The overhead line as a mean of transmitting electrical power over long distances is chep and efficient (a 400 kV double-circuit line may transmit 2,000 MW over a distance of 160km in either direction with an efficiency of 98%).
Reference from.
Transmission and distribution of electrical power by- J.B.Gupta.
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