Which of the following statements is true regarding corona?

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Option A: It takes place at a voltage lower than breakdown voltage

Explanation & Concepts:
Corona is a phenomenon of self-sustained discharge that occurs when the potential gradient exceeds the dielectric strength of air. It manifests as a luminous glow, hissing noise, and ozone production, occurring well before the complete breakdown of the insulation (sparkover).

Working Principle: When the voltage across a transmission line reaches a value called the 'Critical Disruptive Voltage', the electric field strength at the conductor surface exceeds the breakdown strength of air (approx. 30 kV/cm peak). This ionizes the surrounding air molecules, turning the air into a conductor locally. Because the field intensity is non-uniform and highest at the surface, this ionization remains confined to the immediate vicinity of the conductor, resulting in corona discharge.

Key Formulas: $V_c = g_0 \cdot \delta \cdot m \cdot r \cdot \ln(\frac{D}{r})$ — Critical disruptive voltage formula; $P_c \propto (f+25) \cdot \sqrt{\frac{r}{D}} \cdot (V-V_c)^2$ — Power loss due to corona

Key Points: Corona is a localized discharge phenomenon.; It causes power loss, radio interference, and ozone formation.; It is highly dependent on the conductor surface condition and atmospheric factors.; Occurs significantly when the operating voltage is higher than the critical disruptive voltage.

Disadvantages: Power loss in the transmission line; Radio and television interference; Chemical corrosion of conductors due to ozone formation

Additional Info: Breakdown voltage corresponds to the voltage at which a complete arc or flashover occurs between conductors, which is much higher than the voltage at which corona starts.. Option C is incorrect because corona is a power-loss phenomenon (energy dissipation).. Option D is incorrect because corona causes power loss, effectively decreasing transmission line efficiency.

Answer: A is correct — Corona discharge initiates at the critical disruptive voltage, which is significantly lower than the air breakdown voltage required for a complete arc-over.

Exam Tip: Always remember that corona is a surface phenomenon associated with high electric field intensity; increasing conductor radius 'r' is the most effective way to reduce corona loss.

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Option B: It takes place at a voltage higher than breakdown voltage

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Option C: It is a current phenomenon

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Option D: It increases the transmission line efficiency

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