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ElectricalPower System
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Surge impedance values for cables are

A

Higher than those of overhead lines

B

Lower than those of overhead lines

C

In a same order is those of overhead line

D

None of above

Correct Answer

тЪЩя╕П TE тАв Technical Concept & PrincipleElectricalPower System
Option B

Lower than those of overhead lines

Quick Summary:

The surge impedance (or characteristic impedance) of a transmission line is defined as Zc=LCZ_c = \sqrt{\frac{L}{C}}ZcтАЛ=CLтАЛтАЛ. Cables have significantly higher capacitance (CCC) due to the close proximity of conductors separated by thin dielectric insulation compared to the wide spacing of overhead lines, leading to a much lower surge impedance value.

тЪЩя╕ПTETechnical SolutionConcept & Principle
ЁЯТб Explanation

The surge impedance (or characteristic impedance) of a transmission line is defined as Zc=LCZ_c = \sqrt{\frac{L}{C}}ZcтАЛ=CLтАЛтАЛ. Cables have significantly higher capacitance (CCC) due to the close proximity of conductors separated by thin dielectric insulation compared to the wide spacing of overhead lines, leading to a much lower surge impedance value.

ЁЯФв Key Formulas

Zc=LCZ_c = \sqrt{\frac{L}{C}}ZcтАЛ=CLтАЛтАЛ тАФ Surge impedance of a lossless transmission line

v=1LCv = \frac{1}{\sqrt{LC}}v=LCтАЛ1тАЛ тАФ Velocity of electromagnetic wave propagation

тЪЩя╕П Working Principle

In an overhead line, conductors are separated by air (dielectric constant тЙИ1\approx 1тЙИ1), resulting in low capacitance and high inductance per unit length. In contrast, cables utilize solid or liquid insulation with high permittivity, increasing capacitance significantly while reducing inductance. Since ZcтИЭ1CZ_c \propto \sqrt{\frac{1}{C}}ZcтАЛтИЭC1тАЛтАЛ, the increase in CCC dominates, causing ZcZ_cZcтАЛ to drop typically from 400-600 ╬й\Omega╬й for overhead lines to 30-50 ╬й\Omega╬й for cables.

ЁЯУМ Key Points
  • тЦ╕

    Overhead line surge impedance is typically 400тИТ600┬а╬й400 - 600 \ \Omega400тИТ600┬а╬й.

  • тЦ╕

    Cable surge impedance is typically 30тИТ50┬а╬й30 - 50 \ \Omega30тИТ50┬а╬й.

  • тЦ╕

    Higher capacitance in cables is due to the small distance between the conductor and the grounded sheath.

тЬЕ Advantages
  • тЦ╕

    Lower surge impedance in cables reduces the magnitude of voltage surges entering connected equipment.

  • тЦ╕

    Higher capacitance aids in maintaining voltage stability over shorter distances.

тЭМ Disadvantages / Limitations
  • тЦ╕

    High shunt capacitance leads to charging currents that limit the effective transmission length of cables.

  • тЦ╕

    Increased cable cost compared to overhead lines due to insulation requirements.

ЁЯЫая╕П Applications / Uses
  • тЦ╕

    Underground power distribution in urban areas

  • тЦ╕

    Submarine power transmission

ЁЯФД Comparison Table
FeatureOverhead LinesUnderground Cables

Surge Impedance (ZcZ_cZcтАЛ)

400 - 600 ╬й\Omega╬й

30 - 50 ╬й\Omega╬й

ЁЯУД Additional Information
  • тЦ╕

    The permittivity of insulation materials (╧╡r\epsilon_r╧╡rтАЛ) in cables is significantly higher than that of air (╧╡rтЙИ1\epsilon_r \approx 1╧╡rтАЛтЙИ1), which is the primary reason for the increased capacitance (C=2╧А╧╡lnтБб(D/d)C = \frac{2\pi\epsilon}{\ln(D/d)}C=ln(D/d)2╧А╧╡тАЛ).

  • тЦ╕

    Option A is incorrect as cables have much higher capacitance, reducing impedance. Option C is incorrect as the values differ by an order of magnitude.

ЁЯУК Diagram / Illustration
Surge Impedance Formula
Zc=LZ_c = \sqrt{L}ZcтАЛ=LтАЛ
CCables: High C u2192 Low Z_c
тЬЕ

B is correct тАФ Surge impedance of cables is significantly lower than overhead lines due to high shunt capacitance.

Core Concepts Used
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Transmission Line Parameters Surge Impedance Loading (SIL) Dielectric Constants
ЁЯТб EXAM TIP

Remember that ZcZ_cZcтАЛ is inversely proportional to C\sqrt{C}CтАЛ; since cables are designed to have high capacitance per meter, they inherently possess lower surge impedance.

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