A bundled conductor line compared to a single conductor line (with same conductor cross-sectional area and sam

Question

A bundled conductor line compared to a single conductor line (with same conductor cross-sectional area and same mean distance between conductors) has Self GMD, Mutual GMD and Inductance/phase

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Option C: Higher, nearly same and lower

Explanation & Concepts:
A bundled conductor increases the Geometric Mean Radius (GMR or Self-GMD) of the phase conductor, which significantly reduces the internal reactance. Since the physical spacing between the phases remains essentially unchanged, the Mutual GMD (GMD) remains nearly the same, resulting in an overall decrease in the line inductance per phase.

Working Principle: In bundled conductors, individual sub-conductors are placed in parallel at a distance $d$. The Self-GMD ($D_s$) of the bundle is $D_s = (D_{sb} 	imes d^{n-1})^{1/n}$, which is always higher than the Self-GMD of a single conductor of equivalent total area. Since Inductance $L = 2 	imes 10^{-7} \ln(\frac{GMD}{GMR})$, increasing the GMR (Self-GMD) leads to a reduction in the logarithmic term, thereby lowering the total inductance.

Key Formulas: $L = 2 	imes 10^{-7} \ln \left( \frac{D_{eq}}{D_{s}} ight)$ — Inductance formula per unit length; $GMR_{bundle} = \sqrt[n]{(r') 	imes d^{n-1}}$ — Effective GMR for an n-conductor bundle

Key Points: Bundling significantly reduces the surge impedance of the line.; It increases the effective radius, leading to a reduction in potential gradient and surface corona loss.; The Mutual GMD is based on the center-to-center distance between phases, which remains unchanged by bundling.; Lower inductance improves the power transfer capability of the transmission line.

Advantages: Reduced corona loss and radio interference; Increased power transfer capability; Improved stability limits

Disadvantages: Increased wind and ice loading on towers; Higher installation and maintenance costs

Applications: Extra High Voltage (EHV) transmission lines (400kV and above); UHV lines to control voltage gradients

Additional Info: The use of bundled conductors is a standard practice in lines above 220kV to mitigate corona effects.. Option A is incorrect because self-GMD increases, not decreases, with bundling.. Option B is incorrect because inductance decreases when the denominator (GMR) increases.

Answer: C is correct — Bundled conductors increase the GMR, which results in a lower inductance while keeping the mutual GMD nearly unchanged.

Exam Tip: Always remember: Anything that increases the Effective Radius (like bundling) reduces Inductance and increases Capacitance.

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Option A: Lower, nearly same and higher

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Option B: Higher, lower and nearly same

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Option D: Lower, higher and higher

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