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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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Inter sheath in cables are used to
Provide proper stress distribution
Minimize the stress
Use inferior insulation
Provide protection against current and voltage surges
Provide proper stress distribution
Inter-sheaths are metallic layers inserted within the insulation of high-voltage underground cables. They are used to improve the dielectric stress distribution across the insulation layer by dividing the total potential gradient.
Inter-sheaths are metallic layers inserted within the insulation of high-voltage underground cables. They are used to improve the dielectric stress distribution across the insulation layer by dividing the total potential gradient.
gmaxтАЛ=rln(rRтАЛ)VтАЛ тАФ Electric stress at the conductor surface
gxтАЛ=xln(rRтАЛ)VтАЛ тАФ Electric stress at a distance x from the center
In a single-core cable, the electric stress is maximum at the conductor surface and decreases as we move toward the sheath. By placing metallic inter-sheaths at intermediate radii and maintaining them at specific potentials, the cable behaves like a series combination of capacitors. This forces the potential gradient to be more uniform, preventing the insulation near the conductor from being overstressed.
Inter-sheaths allow for the use of thinner insulation layers for a given voltage rating.
The inter-sheaths are maintained at potentials proportional to their radii to ensure uniform stress.
They are primarily used in extra-high voltage (EHV) cables.
They make the dielectric design more economical but increase the complexity of cable construction.
Uniform stress distribution reduces the risk of insulation breakdown.
Allows for reduction in the total thickness of dielectric material.
Complex cable construction and handling.
Difficulty in maintaining intermediate potentials due to charging currents.
Increases cost and probability of cable failure due to additional layers.
High-voltage underground transmission lines.
Extra-high voltage (EHV) cable systems.
Due to the difficulty in maintaining correct intermediate potentials and the risks of charging currents, inter-sheaths are rarely used in modern HV cable designs, which favor graded insulation or synthetic materials.
Option B (Minimize stress) is partially true but less precise than 'Proper stress distribution' because the goal is to manage the gradient rather than just reduce total magnitude.
A is correct тАФ Inter-sheaths are specifically employed to achieve a uniform dielectric stress distribution throughout the cable insulation.
Always remember that cable grading (capacitance or inter-sheath) is designed to make gmaxтАЛ approach gminтАЛ, aiming for a more efficient utilization of the insulation material.