In order to reduce the inductive interference in parallel communication circuit to the minimum

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Option D: Either (a) or (b)

Explanation & Concepts:
Inductive interference in parallel communication circuits occurs due to electromagnetic induction from high-voltage power lines. Transposition balances the phase currents/voltages to cancel induced EMFs, while bundled conductors reduce the overall magnetic flux linkage, thereby minimizing the mutual inductance.

Working Principle: The principle relies on the neutralization of induced voltages. Transposition cycles conductors through different spatial positions, ensuring that the net voltage induced over a full cycle in the adjacent communication circuit is near zero. Bundled conductors reduce the effective geometric mean radius (GMR), which decreases the line inductance and external magnetic flux, thereby mitigating the interference potential on nearby lines.

Key Formulas: $M = \frac{\mu_0}{2\pi} \ln(\frac{D_{12}}{D_{12}'})$ — Mutual inductance between parallel lines; $E_{ind} = -M \frac{di}{dt}$ — Induced interference voltage

Key Points: Inductive interference is primarily caused by mutual magnetic coupling between power and communication lines.; Transposition is the process of physically swapping the positions of power conductors at regular intervals.; Bundled conductors reduce the GMR, which decreases the inductance per unit length of the power line.; The total interference is proportional to the mutual inductance, which is reduced by both physical separation and balanced conductor configurations.

Advantages: Minimizes crosstalk and noise in communication circuits.; Improves signal-to-noise ratio in data and voice transmissions.

Disadvantages: Transposition adds structural complexity and costs to transmission tower designs.; Bundling increases wind loading and mechanical stress on insulators.

Applications: Long-distance HVDC/HVAC transmission lines.; Telecommunication cables running parallel to high-voltage substations.

Additional Info: Transposition is mandatory for lines exceeding 100 km to ensure balanced phase impedances.. Option C is incorrect because merely using thicker conductors does not provide the phase-cancellation benefits of transposition or the magnetic flux reduction of bundle configurations.

Answer: D is correct — Both transposition of conductors and the use of bundled conductors effectively minimize the mutual inductive coupling that causes interference.

Exam Tip: Always remember that 'balanced' conditions in 3-phase systems are key to suppressing inductive interference in neighboring metallic circuits.

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Option A: Conductors are transposed at regular intervals

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Option B: Bundled conductors are used

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Option C: Thick conductors are used

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