The electrostatic stress in a single core cable has maximum value (gmax) at the conductor surface and it goes on decreasing as we move towards the sheath. The unequal stress distribution in a cable is undesirable. The term grading of cables refers to the process of achieving uniform electrostatic stress in the dielectric of cables.
The maximum voltage that can be safely provided to a cable depends upon electrostatic stress at the conductor surface. In order to achieve safe working of a cable having homogeneous dielectric, the dielectric strength must be more than gmax.
If we use a dielectric of high strength for a cable, it is beneficial only near the conductor where stress is maximum. However, as we move away from the conductor, the electrostatic stress decreases and the dielectric will be unnecessarily over-strong. There are two major reasons for which the unequal stress distribution in a cable is unwanted.
- More insulation is required that in turn is uneconomical and increase cable size thereby raising issues while doing installation and laying of cables.
- It can lead to insulation breakdown
To avoid the above-mentioned issues, it is vital to have a uniform stress distribution in cables. The grading of cables refers to the process of distributing the stress in such a way that its value is increased in the outer layers of dielectric.
Two types of grading is in practice: Capacitance grading and Intersheath grading.
See Complete Lecture on Grading of Cables below