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By which of the following systems electric power may be transmitted ?
Overhead system
Underground system
Both (a) and (b)
None of the above
Both (a) and (b)
Electric power transmission is achieved primarily through either overhead lines or underground cables, depending on voltage levels, environmental constraints, and economic factors. Overhead systems use bare conductors supported by insulators on poles or towers, while underground systems use insulated cables buried directly in the ground or laid in ducts.
Electric power transmission is achieved primarily through either overhead lines or underground cables, depending on voltage levels, environmental constraints, and economic factors. Overhead systems use bare conductors supported by insulators on poles or towers, while underground systems use insulated cables buried directly in the ground or laid in ducts.
C=ln(D/r)2╧А╧╡0тАЛтАЛ тАФ Capacitance per unit length of transmission line
PlossтАЛ=I2R тАФ Power loss in transmission conductors
The choice between systems is governed by factors like the surge impedance of the line, insulation requirement, and thermal dissipation. Overhead lines rely on air as the primary dielectric medium, making them cost-effective for long-distance transmission. Underground systems rely on solid/synthetic insulation (like XLPE), which is necessary in densely populated urban areas where overhead lines pose safety or aesthetic risks despite higher capacitive charging currents.
Overhead systems are standard for long-distance, extra-high voltage (EHV) transmission.
Underground cables have higher shunt capacitance due to the proximity of the sheath and conductor.
The surge impedance loading (SIL) is a critical parameter for determining load capacity.
Economic criteria typically favor overhead lines unless local regulations or congestion prohibit them.
Overhead: Lower initial cost, easier fault detection and repair, better heat dissipation.
Underground: Immune to weather-related faults, low aesthetic impact, high reliability in dense areas.
Overhead: Prone to environmental interference (lightning, wind, trees), radio interference.
Underground: High installation and insulation costs, difficult to locate and repair faults, high dielectric loss.
Overhead: Inter-state grid transmission, rural electrification.
Underground: Urban power distribution, sea crossings (HVDC submarine cables), airport surroundings.
| Feature | Overhead | Underground |
|---|---|---|
Initial Cost | Low | Very High |
Fault Localization | Easy (Visual) | Hard (Complex testing) |
Transmission voltages often exceed 132kV for overhead lines.
Option D is incorrect because transmission is universally accomplished via either aerial or buried paths.
C is correct тАФ Electric power can be transmitted via both overhead systems and underground systems based on specific project requirements.
Always remember that underground systems have much higher capacitive charging currents compared to overhead lines due to the small distance between the conductor and the metallic sheath.