Corona losses are minimized when

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Option B: Smooth conductor is used

Explanation & Concepts:
Corona loss is minimized by using smooth conductors because they provide a uniform electric field distribution across the surface. Roughness, sharp points, or surface irregularities cause localized stress concentrations, leading to ionization of air at lower voltage thresholds.

Working Principle: Corona discharge occurs when the voltage gradient at the conductor surface exceeds the dielectric strength of air (approximately 30 kV/cm peak). Sharp points or uneven surfaces increase the local electric field intensity ($E = \frac{V}{r \ln(D/r)}$), triggering ionization at lower applied voltages. A smooth surface ensures a uniform $E$-field, preventing premature local breakdown.

Key Formulas: $E_{max} = \frac{V}{r \ln(\frac{D}{r})}$ — Represents the maximum electric field strength at the conductor surface; $P_{corona} \propto (f+25) \sqrt{\frac{r}{D}} (V - V_c)^2$ — Peek's law for power loss due to corona

Key Points: Corona is the breakdown of air molecules into ions in the presence of a strong electric field.; Smooth surfaces reduce electric stress concentration ($E_{max}$).; Increasing conductor radius ($r$) reduces corona loss by lowering the surface electric gradient.; Atmospheric conditions like humidity and rain significantly increase corona loss.

Advantages: Reduces power loss in transmission lines; Minimizes radio and television interference; Prevents ozone formation and conductor corrosion

Disadvantages: Large conductor diameter increases tower stress; High initial cost for bundled conductors or smooth high-gauge wire

Applications: High Voltage Direct Current (HVDC) transmission lines; Extra High Voltage (EHV) AC transmission lines

Additional Info: Corona loss is non-linear and becomes significant only when the supply voltage exceeds the critical disruptive voltage ($V_c$).. Option C is incorrect because sharp points increase surface charge density, leading to localized discharge.

Answer: B is correct — Smooth conductors ensure a uniform electric field distribution, which avoids localized ionization and minimizes corona loss.

Exam Tip: Always remember that bundling conductors is an effective way to increase the effective radius and reduce corona loss in EHV lines.

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Option A: Conductor size is reduced

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Option C: Sharp points are provided in line hardware

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Option D: Current density in conductors is reduced

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