Which of the following affects the corona least?

Question

Examoogle · Accepted Answer

Option B: Atmospheric temperature

Explanation & Concepts:
Corona discharge is primarily governed by the dielectric strength of air, which is significantly influenced by air density and impurities. Atmospheric temperature has a negligible effect on the corona loss compared to air density (pressure), humidity, and the presence of ions or dust particles.

Working Principle: Corona is the ionization of air surrounding a high-voltage conductor, which occurs when the surface voltage gradient exceeds the disruptive critical voltage of air. The breakdown strength is proportional to the air density factor $\delta = \frac{3.92p}{273+t}$. While temperature $t$ appears in this formula, its impact on the corona threshold and loss is far less significant than changes in barometric pressure ($p$) or the presence of conducting ions.

Key Formulas: $V_c = g_0 \delta r \ln(\frac{D}{r})$ — Critical disruptive voltage; $\delta = \frac{3.92p}{273+t}$ — Air density correction factor

Key Points: Corona loss is directly proportional to frequency and the square of the over-voltage ($V-V_c$).; Surface condition of the conductor (roughness, dust, ions) significantly increases corona loss.; Air density is the dominant meteorological factor affecting corona discharge.; Temperature changes have a minor role in altering air density, hence 'least' effect.

Advantages: Corona provides a natural discharge path for overvoltages.; Visual and audible corona act as early warning indicators for insulation stress.

Disadvantages: Power loss reducing overall transmission efficiency.; Generation of ozone causing corrosion of conductor materials.; Radio and television interference due to electromagnetic noise.

Applications: High Voltage (HV) transmission line design.; Electrostatic precipitators (industry air cleaning).; Ozone generators.

Additional Info: The air density factor $\delta$ varies with pressure ($p$) and temperature ($t$).. Ionization is triggered by electrons accelerating between collisions; mean free path is key.. Option D refers to the conductor surface condition and charge distribution, which heavily dictates local gradient.

Answer: B is correct — Atmospheric temperature has a minimal effect on the critical voltage and ionization process compared to pressure and conductor surface conditions.

Exam Tip: Always remember that in power systems, 'Air Density' is the primary atmospheric variable; temperature is only significant when it causes large fluctuations in density.

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Option A: Mean free length

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Option C: Number of ions

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Option D: Size and charge per iron

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