The bundling of conductors is done primarily to

Question

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Option A: Reduce reactance

Explanation & Concepts:
Bundling of conductors refers to using two or more conductors per phase instead of a single large conductor. This technique effectively increases the Geometric Mean Radius (GMR) of the phase, which directly decreases the inductance and increases the capacitance of the transmission line.

Working Principle: The inductance of a transmission line is proportional to $\ln(D_{eq} / D_s)$, where $D_s$ is the GMR. By splitting one large conductor into a bundle of sub-conductors, the equivalent GMR ($D_s^b$) increases significantly. Since the inductance $L = 2 	imes 10^{-7} \ln(D_{eq} / D_s^b)$, an increase in $D_s^b$ results in a reduction of the total line reactance $X_L = 2\pi f L$. Additionally, the increased surface area reduces the voltage gradient, thereby mitigating corona discharge and radio interference.

Key Formulas: $L = 2 	imes 10^{-7} \ln(\frac{D_{eq}}{D_s^b})$ — Inductance per unit length for a bundled conductor; $X_L = 2 \pi f L$ — Inductive reactance formula

Key Points: Bundling increases the GMR of the conductor system.; Higher GMR reduces the internal and external magnetic flux linkages.; Bundling is primarily used in Extra High Voltage (EHV) lines (above 220 kV).; It also helps in reducing the corona loss by decreasing the surface electric field intensity.

Advantages: Reduced inductive reactance; Increased transmission capacity (SIL); Reduced corona loss and radio interference; Lower voltage gradient on the conductor surface

Disadvantages: Higher cost of support structures due to increased wind load; Increased ice and snow loading in cold climates; Complex installation and maintenance of spacers

Applications: EHV and UHV transmission lines; Power lines requiring high surge impedance loading

Additional Info: Option D is also a consequence of bundling, but it is considered a secondary benefit compared to the primary objective of managing line impedance parameters.. The surge impedance $Z_0 = \sqrt{L/C}$ is also reduced, which increases the surge impedance loading (SIL) capacity of the line.

Answer: A is correct — Bundling increases the GMR of the phase conductor, which logarithmically reduces the inductance and consequently the inductive reactance of the line.

Exam Tip: Always remember that bundling increases capacitance ($C$) while decreasing inductance ($L$), which helps improve the stability and power transfer capability of long EHV transmission lines.

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Option B: Increase reactance

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Option C: Increase ratio interference

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Option D: Reduce radio interference.

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