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In capacitance grading of cables, the dielectric used is
Composite
Homogeneous
Hygroscopic
Porous
Composite
In capacitance grading of cables, a composite dielectric is used consisting of layers of different insulating materials with varying permittivity. This technique ensures that the stress throughout the dielectric is uniform, which allows for a more efficient design of the cable insulation.
In capacitance grading of cables, a composite dielectric is used consisting of layers of different insulating materials with varying permittivity. This technique ensures that the stress throughout the dielectric is uniform, which allows for a more efficient design of the cable insulation.
g=rln(R/r)VтАЛ тАФ Electric stress at radius r
╧╡1тАЛg1тАЛ=╧╡2тАЛg2тАЛ тАФ Condition for uniform stress in grading
The principle relies on making the product of permittivity (╧╡)and potential gradient (g) constant across the dielectric layers, such that ╧╡_1 g1тАЛ = ╧╡_2 g2тАЛ = \dots = ╧╡_n gnтАЛ. By selecting layers of different dielectric constants (where the inner layer has the highest permittivity and the outer has the lowest), the electric stress is redistributed to avoid exceeding the breakdown limit of any single layer.
Capacitance grading aims to make the electrostatic stress uniform throughout the cable insulation.
The material with the highest permittivity is placed nearest to the conductor.
Grading allows the use of thinner insulation for the same operating voltage.
It is primarily used in high-voltage DC or specialized high-voltage cable designs.
Uniform stress distribution across the dielectric
Allows for a smaller overall cable diameter
Improved dielectric utilization
Complex manufacturing process for multiple dielectric layers
Higher risk of interface breakdown between layers
Increased cost compared to homogeneous insulation
High-voltage underground power cables
Specialized insulation designs for high-voltage bushings
Homogeneous dielectrics (Option B) result in non-uniform stress, where the stress is highest at the conductor surface.
Hygroscopic and Porous materials are generally undesirable in cable insulation as they absorb moisture, leading to dielectric failure.
A is correct тАФ Capacitance grading utilizes a composite dielectric with multiple layers of varying permittivity to achieve uniform stress distribution.
Remember that for capacitance grading, the inner layer always requires the highest permittivity (╧╡)because the electric stress is naturally highest at the conductor surface.