To reduce corona effect usually

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Option C: Bundled conductors are used

Explanation & Concepts:
The corona effect is the electrical discharge caused by the ionization of air surrounding a conductor when the potential gradient exceeds the dielectric strength of air. Bundled conductors increase the effective radius of the conductor, which reduces the maximum surface potential gradient and consequently minimizes corona losses and radio interference.

Working Principle: The critical disruptive voltage ($V_c$) is directly proportional to the effective radius of the conductor. By using bundled conductors, we increase the geometric mean radius (GMR) of the phase, which lowers the surface electric stress (voltage gradient) at the conductor surface below the critical value required for air ionization.

Key Formulas: $g_{max} = \frac{V}{r \cdot \ln(D/r)}$ — Maximum surface potential gradient; $V_c = m_0 \cdot g_0 \cdot \delta \cdot r \cdot \ln\left(\frac{D}{r}\right)$ — Critical disruptive voltage

Key Points: Corona occurs when surface electric field exceeds air's breakdown strength (approx 30 kV/cm peak).; Bundling increases effective radius, thereby reducing surface field intensity.; Corona is more prominent at EHV (Extra High Voltage) levels (typically > 220 kV).; Factors influencing corona: atmospheric conditions, conductor surface condition, and voltage level.

Advantages: Reduces corona loss and radio interference; Increases transmission line capacity; Reduces line inductance and increases capacitance; Improves stability limit of long-distance transmission

Disadvantages: Increased wind pressure loading on towers; Higher cost of installation and complex hardware; Difficulty in maintaining uniform current distribution during fault conditions

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

Additional Info: Option A: Reducing distance increases the potential gradient, worsening corona.. Option B: Reducing diameter increases the potential gradient, causing severe corona.. Option D: Stranding affects skin effect and mechanical flexibility but does not significantly mitigate corona compared to bundling.

Answer: C is correct — Bundled conductors increase the effective radius of the phase conductor, which significantly reduces the maximum electric field strength on the conductor surface.

Exam Tip: Always remember: Corona is inversely proportional to the radius; larger effective diameter through bundling is the standard industry practice for EHV lines.

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Option A: Distance between the conductor is reduced

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Option B: Conductor diameter is reduced

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Option D: Stranded conductors are used

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