Use of bundled conductor reduces

Question

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Option D: All of above

Explanation & Concepts:
Bundled conductors, consisting of multiple sub-conductors per phase, effectively increase the Geometric Mean Radius (GMR) of the transmission line. This configuration reduces the surface voltage gradient and the overall electric field intensity around the conductor, which in turn minimizes corona loss, radio interference, and surge impedance.

Working Principle: Bundling increases the effective cross-sectional area and radius of the conductor group compared to a single conductor. According to the corona inception voltage formula, a higher effective radius reduces the maximum potential gradient at the conductor surface, thus suppressing corona discharge. Furthermore, the increase in GMR and decrease in Geometric Mean Distance (GMD) reduce line inductance ($L$) and increase shunt capacitance ($C$), leading to a lower surge impedance given by $Z_s = \sqrt{\frac{L}{C}}$.

Key Formulas: $E_{max} = \frac{V}{r \ln(D/r)}$ — Surface voltage gradient on a conductor; $Z_s = \sqrt{\frac{L}{C}}$ — Surge impedance of the transmission line

Key Points: Bundling increases the current-carrying capacity for the same voltage level.; It reduces transmission line inductance ($L$) and increases shunt capacitance ($C$).; Corona loss is proportional to $(V - V_c)^2$, where $V_c$ is the critical disruptive voltage; bundling increases $V_c$.; Radio interference is caused by high electric fields; bundling mitigates this by lowering the field strength.

Advantages: Reduces corona loss and radio interference; Lowers surge impedance, improving power transfer capability; Decreases line inductance, improving stability; Reduces surface electric field stress

Disadvantages: Increased wind and ice loading on towers; Complex installation and maintenance compared to single conductors

Applications: EHV (Extra High Voltage) lines (typically 400 kV and above); UHV (Ultra High Voltage) transmission systems

Additional Info: Bundled conductors are commonly used in transmission lines above 220 kV.. Common bundles include 2-conductor (duplex), 3-conductor (triplex), and 4-conductor (quadruplex) configurations.

Answer: D is correct — Bundled conductors reduce the surface electric field, which inherently lowers corona loss, radio interference, and surge impedance.

Exam Tip: Remember that bundling is a standard design choice to increase the effective diameter, which is the primary driver for both electrical (corona) and electromagnetic (surge impedance) performance optimizations.

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Option A: Power loss due to corona

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Option B: Surge impedance

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Option C: Radio interference

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