Critical voltage limit of transmission line is increased by

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Option A: Increasing the radius of conductors

Explanation & Concepts:
The critical disruptive voltage of a transmission line is the minimum phase voltage at which corona discharge commences. Increasing the radius of the conductor reduces the surface electric field intensity, thereby increasing the voltage required to initiate corona.

Working Principle: Corona is caused by the ionization of air surrounding a conductor when the electric field intensity exceeds the dielectric strength of air. By increasing the radius ($r$), the surface charge density distribution is spread over a larger area, resulting in a lower maximum electric field strength ($E_{max} = \frac{V}{r \ln(d/r)}$) for the same applied voltage, thus delaying the onset of ionization.

Key Formulas: $V_c = g_o \cdot r \cdot \ln(\frac{d}{r})$ — The disruptive critical voltage formula where $g_o$ is the dielectric strength of air.; $E_{max} = \frac{V}{r \ln(d/r)}$ — The maximum electric field intensity at the conductor surface.

Key Points: Corona loss is directly proportional to the frequency and $(V - V_c)^2$.; Increasing conductor radius is synonymous with using bundled conductors to reduce corona loss.; A smooth conductor surface significantly increases the disruptive voltage compared to a rough surface.

Advantages: Reduces power loss due to corona effect; Decreases radio interference and audible noise; Improves overall transmission line efficiency

Disadvantages: Increasing conductor radius increases the total weight and cost of the transmission line; Requires stronger tower structures to support increased mechanical load and wind pressure

Applications: High Voltage Direct Current (HVDC) transmission lines; Extra High Voltage (EHV) and Ultra High Voltage (UHV) AC lines; Bundled conductor design in modern power grids

Additional Info: The dielectric strength of air ($g_o$) is approximately 30 kV/cm (peak) under standard atmospheric conditions.. Option B and C: Increasing spacing ($d$) actually increases $V_c$, but the logarithmic dependency on $d$ is much weaker compared to the linear dependency on $r$ in the numerator, making radius adjustment the primary design lever.

Answer: A is correct — Increasing the radius of the conductor reduces the surface potential gradient, which requires a higher voltage to reach the air breakdown limit.

Exam Tip: Always remember that while increasing spacing increases $V_c$, changing the conductor radius (or using bundling) is the practical engineering approach to controlling corona in EHV systems.

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Option B: Increasing the spacing between conductors

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Option C: Reducing the spacing between conductors

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Option D: Reducing the radius of conductors

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