Main drawback of overhead system over underground system is

Question

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Option C: Surge problem

Explanation & Concepts:
The primary disadvantage of overhead transmission lines compared to underground cables is their susceptibility to atmospheric disturbances, particularly surges caused by lightning strikes. Because overhead lines are exposed to the atmosphere, they require extensive lightning protection, such as ground wires and surge arresters, to prevent insulation failure.

Working Principle: Overhead lines act as elevated conductors that intercept lightning strikes or experience induced voltages from nearby strikes, creating high-magnitude traveling waves (surges). These waves travel along the line toward terminals, posing a risk of flashover across insulators and damage to transformer windings due to their high $dv/dt$. In contrast, underground cables are shielded by the earth's surface, which acts as a natural lightning rod and grounds the potential, inherently protecting them from such surges.

Key Formulas: $V_{surge} = Z_c 	imes I_{surge}$ — where $Z_c$ is the surge impedance of the line.; $Z_c = \sqrt{\frac{L}{C}}$ — Surge impedance value for overhead lines is typically 400-600 $\Omega$.

Key Points: Overhead lines are prone to atmospheric voltage surges due to lightning.; Underground cables have low surge impedance and are naturally protected by the burial depth.; The surge impedance of underground cables is significantly lower (approx 30-50 $\Omega$) than overhead lines.; Overhead systems are cost-effective but less reliable in areas prone to severe storms.

Advantages: Lower initial installation cost; Easy to locate and repair faults; Higher current-carrying capacity due to air cooling

Disadvantages: High vulnerability to lightning surges; Requires larger clearances and right-of-way; Affected by wind, ice, and atmospheric pollution

Applications: Long-distance power transmission; Rural electrification; High-voltage bulk energy distribution

Additional Info: Option B (High charging current) is a characteristic disadvantage of underground cables, not overhead lines, due to higher capacitance between conductors and the sheath.. Option D (High initial cost) is a primary disadvantage of underground systems due to excavation and specialized insulation requirements.

Answer: C is correct — The overhead transmission system is highly susceptible to atmospheric surges caused by lightning, necessitating complex protective coordination compared to the inherently protected underground system.

Exam Tip: In competitive exams, always remember that 'High charging current' is associated with the high capacitance of Underground cables, while 'Surge problem' is synonymous with the high inductance and exposure of Overhead lines.

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Option A: Less flexible

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Option B: High charging current

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Option D: High initial cost

Feature	Overhead System	Underground System
Vulnerability to Surges	High	Negligible
Surge Impedance	400-600 $\Omega$	30-50 $\Omega$

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