Question

The "Corona Effect" in a transmission line is most likely to occur during

• A. High-voltage transmission in fair weather
• B. Low-voltage transmission in humid weather
• C. High-voltage transmission in rainy or foggy weather
• D. Low-voltage transmission in dry weather

Hint:

Corona loss is proportional to frequency and voltage:
\[ P_c \propto (f + 25)(V - V_d)^2 \] Since wet/foggy weather reduces \(V_d\), the difference \((V - V_d)\) increases, causing corona losses to rise significantly during wet weather.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the combination of operating voltage and weather conditions under which the Corona effect is most likely to occur on overhead transmission lines.

Step 2: Detailed Explanation:

• Corona is the phenomenon of self-sustained ionization of air surrounding a conductor when the localized electric field intensity (voltage gradient) exceeds the breakdown strength of air.

• It is characterized by a faint violet glow, a hissing sound, ozone gas liberation, and power loss.

• For corona to occur, the operating voltage must exceed the critical disruptive voltage (\(V_d\)):
\[ V_d = m_0 \cdot g_0 \cdot \delta \cdot r \cdot \ln\left(\frac{d}{r}\right) \] where \(g_0\) is the breakdown strength of air, and \(\delta\) is the air density correction factor.

• Under normal fair-weather conditions, the breakdown strength of dry air is high (about 30 kV/cm peak).

• In rainy, humid, or foggy weather, water droplets and moisture accumulate on the conductor surface, forming sharp points that concentrate the electric field.

• Additionally, the moisture in the air lowers its effective breakdown strength (\(g_0\)) and reduces the air density factor (\(\delta\)), lowering the critical disruptive voltage \(V_d\).

• As a result, high-voltage transmission lines are highly susceptible to Corona discharge during rainy or foggy weather.

Step 3: Final Answer:
The Corona effect is most likely to occur during high-voltage transmission in rainy or foggy weather.