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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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In a three core extra high voltage cable, a metallic screen around each core insulation is provided to
Facilitate heat dissipation
Give mechanical strength
Obtain radial electric stress
Obtain longitudinal electric stress
Obtain radial electric stress
In high-voltage cables, the potential distribution between the conductor and the sheath is non-uniform, leading to tangential electric stresses that can cause insulation breakdown. By placing a metallic screen around each core, the cable forces the electric field to be purely radial, ensuring the insulation is uniformly stressed.
In high-voltage cables, the potential distribution between the conductor and the sheath is non-uniform, leading to tangential electric stresses that can cause insulation breakdown. By placing a metallic screen around each core, the cable forces the electric field to be purely radial, ensuring the insulation is uniformly stressed.
ErтАЛ=xln(R/r)VтАЛ тАФ Radial electric stress at distance x from the center of the conductor
gmaxтАЛ=rln(R/r)VтАЛ тАФ Maximum stress occurring at the conductor surface
Without a metallic screen, the electric field lines possess both radial and tangential components due to the geometric configuration of the three-core cable. Introducing a metallic screen makes each core act like a single-core cable with a concentric sheath, effectively eliminating tangential components and confining the electric stress to the radial direction (ErтАЛ=rln(R/r)VтАЛ).
Metallic screens are mandatory for cables rated above 11 kV to manage dielectric stress.
The screen prevents the formation of tangential stresses that could cause surface tracking or partial discharge.
It maintains a uniform equipotential surface around the core insulation.
Prevents internal ionization and insulation degradation
Provides a path for charging currents
Ensures uniform distribution of potential
Increases the overall cable diameter and cost
Introduces screen losses due to induced currents
Extra High Voltage (EHV) underground power transmission
Submarine power cables
High-voltage distribution networks
Option A: Heat dissipation is primarily handled by the bedding and serving layers, not the metallic screen.
Option B: Mechanical strength is provided by the steel wire/tape armor, not the thin metallic screen.
Option D: Longitudinal stress is actually what the screen is designed to eliminate, not obtain.
C is correct тАФ The metallic screen forces the electric field lines to be purely radial, which prevents the development of damaging tangential electric stresses within the cable insulation.
Always remember that metallic screens in cables are about controlling the electric field geometry; for mechanical protection, look for 'Armoring' instead.