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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Surge impedance values for cables are
Higher than those of overhead lines
Lower than those of overhead lines
In a same order is those of overhead line
None of above
Lower than those of overhead lines
The surge impedance (or characteristic impedance) of a transmission line is defined as Zc=CL. Cables have significantly higher capacitance (C) 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.
The surge impedance (or characteristic impedance) of a transmission line is defined as Zc=CL. Cables have significantly higher capacitance (C) 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.
Zc=CL — Surge impedance of a lossless transmission line
v=LC1 — Velocity of electromagnetic wave propagation
In an overhead line, conductors are separated by air (dielectric constant ≈1), 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 Zc∝C1, the increase in C dominates, causing Zc to drop typically from 400-600 Ω for overhead lines to 30-50 Ω for cables.
Overhead line surge impedance is typically 400−600 Ω.
Cable surge impedance is typically 30−50 Ω.
Higher capacitance in cables is due to the small distance between the conductor and the grounded sheath.
Lower surge impedance in cables reduces the magnitude of voltage surges entering connected equipment.
Higher capacitance aids in maintaining voltage stability over shorter distances.
High shunt capacitance leads to charging currents that limit the effective transmission length of cables.
Increased cable cost compared to overhead lines due to insulation requirements.
Underground power distribution in urban areas
Submarine power transmission
| Feature | Overhead Lines | Underground Cables |
|---|---|---|
Surge Impedance (Zc) | 400 - 600 Ω | 30 - 50 Ω |
The permittivity of insulation materials (ϵr) in cables is significantly higher than that of air (ϵr≈1), which is the primary reason for the increased capacitance (C=ln(D/d)2πϵ).
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.
B is correct — Surge impedance of cables is significantly lower than overhead lines due to high shunt capacitance.
Remember that Zc is inversely proportional to C; since cables are designed to have high capacitance per meter, they inherently possess lower surge impedance.