Hollow conductors are used in transmission lines to

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Option C: Reduce corona

Explanation & Concepts:
Hollow conductors are employed in extra-high voltage (EHV) transmission lines to increase the effective diameter of the conductor for a given cross-sectional area. This increase in radius reduces the surface voltage gradient, thereby mitigating the incidence of corona discharge.

Working Principle: Corona occurs when the electric field intensity at the surface of a conductor exceeds the dielectric strength of air. According to Peek's law, the disruptive critical voltage is directly proportional to the radius of the conductor. By using a hollow structure, the radius $r$ is increased without increasing the amount of conducting material, which lowers the surface electric field intensity $E = \frac{V}{r \ln(D/r)}$, thus reducing corona losses and radio interference.

Key Formulas: $E_{max} = \frac{V}{r \ln(\frac{D}{r})}$ — Electric field intensity at the surface of a conductor; $V_c = g_0 \delta r \ln(\frac{D}{r})$ — Disruptive critical voltage for corona

Key Points: Corona loss is proportional to $(f+25)\sqrt{\frac{r}{D}}(V-V_c)^2$.; Hollow conductors are typically made of copper or aluminum segments.; Larger effective diameter results in lower surface potential gradient.; Reduces radio interference and audible noise in high-voltage lines.

Advantages: Reduction in corona loss; Increased current carrying capacity due to better cooling surface area; Significant reduction in radio interference

Disadvantages: Higher structural complexity; Increased susceptibility to deformation compared to solid conductors; Higher manufacturing and handling costs

Applications: Extra High Voltage (EHV) Transmission lines; High Voltage Substations (as busbars)

Additional Info: Option A is incorrect because using hollow conductors often requires more mechanical support, which can increase overall project weight/cost despite using less conductive metal.. Option B is incorrect as stability is primarily governed by line reactance, not conductor geometry.. Option D is an indirect benefit due to reduced losses, but the primary design intent for selecting hollow profiles is corona control.

Answer: C is correct — Hollow conductors increase the effective radius, which reduces the surface voltage gradient and minimizes corona discharge.

Exam Tip: Always remember that corona is a function of voltage gradient; anything that increases the effective diameter (like bundling or hollow conductors) will reduce the gradient and thus the corona loss.

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Option A: Reduce weight of copper

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Option B: Improve stability

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Option D: Increase power transmission capacity

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