Capacitance Grading is a method of distributing the electrostatic stress across the insulation of cables by using materials with different dielectric strength. When a cable is live, the insulation around the conductor is in a state of electrostatic stress.
This stress is high near the centre and reduces towards the periphery. This uneven electrostatic stress can cause the failure of insulation. To prevent this kind of catastrophic failure, the insulation is graded. The permittivity of the insulating material is made to vary inversely as the distance from the centre.
It is not possible to have a single material which has a permittivity which varies depending on the distance. Hence, the insulation is made of a number of layers of insulation made from different materials.
Each of these materials has a different value of permittivity. This ensures that the insulation is approximately uniform throughout the cross section of the insulation.