Compared with a solid conductor of the same radius, corona appears on a standard conductor at a lower voltage

Question

Compared with a solid conductor of the same radius, corona appears on a standard conductor at a lower voltage because stranding

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Option D: Produces surface of smaller radius

Explanation & Concepts:
Corona inception is heavily dependent on the surface electric field intensity, which is higher for conductors with smaller radii. Stranded conductors, while having a larger overall diameter, possess individual strands with smaller radii of curvature; these strands cause local field enhancement, leading to ionization at lower voltages than a solid conductor of equivalent overall radius.

Working Principle: The critical disruptive voltage $V_c$ is directly proportional to the surface irregularity factor $m_0$. Because a stranded conductor presents a non-smooth surface with smaller radii of curvature at the individual strand contacts, the local electric field gradient $E$ at these points is significantly higher. Since $E = \frac{V}{r \ln(D/r)}$, a smaller effective local radius $r$ results in an earlier attainment of the dielectric strength of air, triggering corona discharge.

Key Formulas: $E = \frac{V}{r \ln(\frac{D}{r})}$ — Electric field intensity at the surface of the conductor; $V_c = m_0 g_0 \delta r \ln(\frac{D}{r})$ — Disruptive critical voltage formula

Key Points: Corona is a localized discharge occurring when the electric field strength exceeds the dielectric strength of air (~30 kV/cm peak).; Stranded conductors are used for flexibility, but their surface is inherently rougher than a solid conductor.; The irregularity factor $m_0$ accounts for surface conditions, being 1.0 for smooth and 0.82-0.87 for stranded conductors.

Advantages: Increased flexibility during transport and installation; Reduces the cost of manufacturing and handling long cable spans

Disadvantages: Lower critical disruptive voltage compared to smooth conductors; Higher susceptibility to corona loss and interference due to surface roughness

Applications: High Voltage AC Transmission Lines; Extra High Voltage (EHV) Power Distribution

Additional Info: The actual radius used in the corona formula for a stranded conductor is the radius of the individual strand, not the overall conductor radius.. Option A is incorrect because stranding does not promote ionization; rather, the geometry causes it.. Option B and C describe the physical configuration of the conductor, but they are not the primary reason for lower corona inception voltage.

Answer: D is correct — The individual strands of a stranded conductor have a smaller radius of curvature than a solid conductor, which increases the local electric field intensity and triggers ionization at a lower applied voltage.

Exam Tip: Always remember that corona inception is a local phenomenon; even if the 'apparent' diameter is large, look for local curvature at the surface to predict field intensity.

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Option A: Assists in ionization

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Option B: Make the current flow spiral about axis of conductor

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Option C: Produces oblique section to a plane perpendicular to axis of conductor

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