The conductors are bundled primarily to

Question

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Option D: Both (a) and (b)

Explanation & Concepts:
Bundling conductors involves using multiple cables per phase instead of a single large conductor to increase the effective surface area. This increase in the geometric mean radius (GMR) significantly reduces the electric field intensity at the surface, thereby suppressing corona discharge and associated radio interference.

Working Principle: Bundling increases the Equivalent Self Geometric Mean Distance (GMD) or GMR, which effectively reduces the shunt capacitance and the surface voltage gradient. By reducing the surface voltage gradient, the potential gradient drops below the critical disruptive voltage of air, minimizing the ionization of surrounding air molecules. Since corona is the primary source of radio noise in high-voltage transmission lines, its reduction naturally mitigates radio interference.

Key Formulas: $E_{max} = \frac{V}{r \cdot \ln(\frac{D}{r})}$ — Electric field intensity at the conductor surface; $GMR_{bundle} = \sqrt[n]{r' \cdot d^{n-1}}$ — Geometric Mean Radius of a bundle with $n$ subconductors

Key Points: Bundling increases the effective radius of the conductor phase.; Higher effective radius reduces the maximum electric stress at the conductor surface.; Corona loss is non-linear and depends heavily on the voltage gradient.; Bundling also reduces the series inductive reactance of the line.

Advantages: Reduced corona loss and radio interference; Increased power transfer capability due to lower line reactance; Improved stability limits

Disadvantages: Higher wind and ice loading on towers; Increased cost due to more hardware and subconductors; Complex mechanical design for spacers

Applications: Extra High Voltage (EHV) transmission lines (above 220 kV); Ultra High Voltage (UHV) transmission lines

Additional Info: A standard EHV line typically uses 2, 3, or 4 subconductors in a bundle.. Option C is technically true regarding line reactance, but bundling is *primarily* implemented to mitigate the detrimental effects of corona at high voltages.

Answer: D is correct — Bundling conductors increases the effective radius, which simultaneously lowers the surface voltage gradient to reduce corona and minimizes radio interference caused by corona discharges.

Exam Tip: Always remember: Bundling affects both shunt parameters (capacitance/corona) and series parameters (inductance/reactance) of a transmission line.

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Option A: Reduce corona

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Option B: Reduce radio interference

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Option C: Reduce line reactance

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