Bundle conductor in a transmission line

Question

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Option B: Decreases radio interference

Explanation & Concepts:
Bundle conductors consist of multiple sub-conductors per phase, effectively increasing the geometric mean radius (GMR) of the conductor arrangement. This design significantly reduces the surface potential gradient and the corona discharge effect, which is the primary source of radio interference in transmission lines.

Working Principle: The surface electric field intensity $E$ is inversely proportional to the radius of the conductor. By using a bundle of sub-conductors, the effective radius $R_{eq}$ increases, which lowers the maximum voltage gradient ($E_{max}$) on the conductor surface. Since corona loss and radio interference are highly dependent on the voltage gradient exceeding the disruptive critical voltage, lowering this gradient effectively suppresses these phenomena.

Key Formulas: $E_{max} = \frac{V}{r \cdot \ln(\frac{D}{r})}$ — Electric field intensity for single conductor; $r_{eq} = (r \cdot d^{n-1})^{1/n}$ — Equivalent GMR for bundle of n conductors

Key Points: Bundling increases the effective radius of the conductor phase.; Higher effective radius reduces the surface voltage gradient.; Radio interference is primarily caused by corona discharges due to high localized electric fields.; Lower voltage gradient leads to lower corona loss and reduced ozone production.

Advantages: Reduced corona loss; Reduced radio interference; Increased transmission capacity due to lower line inductance; Improved stability limits

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

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

Additional Info: Radio interference is a result of high-frequency electromagnetic noise generated by local air ionization (corona).. For typical EHV lines, a bundle of 2, 3, or 4 conductors is used to manage the potential gradient.

Answer: B is correct — Bundle conductors reduce the surface voltage gradient, thereby minimizing corona discharges and the associated radio interference.

Exam Tip: In competitive exams, remember that bundle conductors reduce both inductance (improving power transfer capability) and corona effect (reducing interference and loss).

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Option A: Increases radio interference

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Option C: Does not affect radio interference

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Option D: Insufficient data

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