Multiple conductor lines have

Question

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Option A: Lower surge impedance than single line conductor

Explanation & Concepts:
Multiple conductor lines (bundled conductors) exhibit lower surge impedance than single-conductor lines because the bundle configuration increases the Geometric Mean Radius (GMR), which effectively reduces the line inductance.

Working Principle: The surge impedance ($Z_c$) of a transmission line is given by $Z_c = \sqrt{\frac{L}{C}}$. By using bundled conductors, the effective inductance ($L$) decreases due to an increase in the self-GMD, while the shunt capacitance ($C$) increases. Since $L$ decreases and $C$ increases, the ratio $L/C$ decreases significantly, leading to a reduction in surge impedance.

Key Formulas: $Z_c = \sqrt{\frac{L}{C}}$ — Surge impedance of a transmission line; $L = 2 	imes 10^{-7} \ln(\frac{GMD}{GMR})$ — Inductance per unit length

Key Points: Bundling increases the effective cross-sectional area and reduces the surface voltage gradient.; Reduction in $Z_c$ increases the Surge Impedance Loading (SIL) of the line ($SIL = \frac{V^2}{Z_c}$).; Bundled conductors help in minimizing corona loss and radio interference due to lower electric field stress.

Advantages: Increased Power Transfer Capability (higher SIL); Reduced Corona loss and Radio Interference; Improved stability limits

Disadvantages: Increased wind and ice loading on towers; Higher cost of installation and complex spacers; Increased charging current

Applications: Extra High Voltage (EHV) transmission lines; Ultra High Voltage (UHV) lines; Systems where corona suppression is critical

Additional Info: Typical surge impedance for overhead lines is about 400 Ω for single conductors, which reduces to 250-300 Ω with bundling.. Option B is incorrect because bundling reduces the self-inductance of the phase path, thereby decreasing the surge impedance rather than increasing it.

Answer: A is correct — Multiple conductor lines reduce the inductance and increase the capacitance, resulting in a lower surge impedance compared to single-conductor lines.

Exam Tip: Remember that SIL is inversely proportional to $Z_c$. Therefore, bundling conductors increases SIL, allowing for higher power capacity over longer distances.

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Option B: Higher surge impedance than single line conductor

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Option C: Higher surge impedance than single line conductor for short length

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Option D: Higher surge impedance than single line conductor for medium length

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