The Power system is divided into three parts;
We have seen this in the overview of the power system.
The transmission lines are used to transmit the power for long-distance. There are two types of transmission lines;
- Overhead Transmission line
- 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.
What is the difference between the overhead transmission line and underground transmission line?
Comparison between Overhead Lines and Underground Cables
|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.|
|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 labor 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 the 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.|
|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.|
Now you clear about the comparison between Overhead Lines and Underground Cables. As we have seen, both systems have their own advantages and disadvantages. According to the requirement of the system, the transmission system decided. If cost is the main consideration, the overhead line is chosen. For this system, the non-economic factors like appearance, public safety, and traffic (space) are not considered.
Underground cables used when the cost is no consideration. But in this case also, large charging current at high voltage limits the use of the underground cable for long-distance.
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