The method of equalizing the stress in the dielectric of the cable is called the
grading of cables.
Methods of grading.
(i) Capacitance grading
(ii) Intersheath grading
Capacitance grading is provided within gas insulated lightning arresters containing
stacked zinc oxide varistors by means of a grading ring electrically connected to the
line terminal or, for arresters of the higher voltage ratings, by means of a plurality of
telescoping external electrostatic shields. The shields are arranged so that the degree
of overlap between sequential shields decreases from the line end to the ground end of
the varistor stacks. The capacitance grading is provided by the degree of overlap
between the sequential shields and the ratio of the radii of the overlapping shields
Intersheath Grading
In this method of grading, the same insulating material is used throughout the cable,
but is divided into two or more layers by means of cylindrical screens or intersheaths .
These intersheaths are connected to tappings from the supply transformer, and the
potentials are maintained at such values that each layer of insulation takes its proper
share of the total voltage. The intersheaths are relatively flimsy, and are meant to
carry only the charging current. Since there is a definite potential difference between
the inner and outer radii of each sheath, we can treat each section separately as a
single core cable
gregorymasila answered the question on February 22, 2018 at 16:52
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