Bundled conductors are mainly used

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Option D: To decrease series reactance

Explanation & Concepts:
Bundled conductors, consisting of two or more conductors connected in parallel per phase, are primarily employed in EHV and UHV transmission lines to reduce series reactance and suppress corona discharge. By effectively increasing the Geometric Mean Radius (GMR) of the conductor bundle, the inductive reactance of the line is significantly lowered.

Working Principle: The inductive reactance of a transmission line is given by $X_L = 2\pi fL$, where inductance $L$ is proportional to $\ln(1/GMR)$. When conductors are bundled, the GMR increases, which causes the effective inductance ($L$) to decrease. Consequently, the reduction in inductance leads to a lower series reactance ($X_L$), improving the power transfer capability of the transmission line.

Key Formulas: $L = 2 	imes 10^{-7} \ln\left(\frac{GMD}{GMR}ight)$ — Inductance per unit length; $X_L = 2\pi f L$ — Series inductive reactance

Key Points: Bundled conductors reduce corona loss and radio interference by lowering the surface voltage gradient.; The surge impedance loading (SIL) of the line increases as reactance decreases.; Bundled conductors lead to an increase in shunt capacitance ($C$), not a decrease, which helps in voltage regulation.; The number of conductors per bundle is chosen based on the voltage level (e.g., 2 for 400kV, 4 for 765kV).

Advantages: Reduced line inductance (lower $X_L$); Increased line capacitance (higher $C$); Reduced corona effect and power loss; Increased surge impedance loading

Disadvantages: Increased wind loading on towers; Higher mechanical complexity and cost; Requirement for spacers and dampers

Applications: Extra High Voltage (EHV) transmission lines; Ultra High Voltage (UHV) transmission lines; Lines where surge impedance loading needs to be maximized

Additional Info: Option A is incorrect because bundling actually increases shunt capacitance due to the increased effective surface area.. Option B is incorrect because capacitance is proportional to $\ln(1/r)$, and the increase in effective radius during bundling increases capacitance.. The reduction in series reactance enhances the steady-state stability limit of the power system.

Answer: D is correct — Bundled conductors effectively increase the Geometric Mean Radius (GMR) of the line, which significantly reduces the line inductance and, consequently, the series inductive reactance.

Exam Tip: Always remember: Bundling increases the effective radius, which increases Capacitance ($C$) and decreases Inductance ($L$). Since $X_L = \omega L$, $X_L$ decreases; since $X_C = 1/(\omega C)$, $X_C$ also decreases.

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Option A: To increase shunt capacitance

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Option B: To decrease shunt capacitance

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Option C: To increase series reactance

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