The so-called shielding in the cable structure is essentially a measure of improving the electric field distribution.
The conductor of the cable is twisted by a plurality of wires, and the air gap is easy to form between the conductor and the insulating layer. The surface of the conductor is not smooth, and the electric field can be concentrated.
The shielding layer with a layer of semi conductive material is added on the surface of conductor, It has the same electric potential as the shielded conductor and in good contact with the insulation layer, so as to avoid partial discharge occurs between the conductor and the insulating layer. This layer of shielding, also known as the inner shielding layer.
There may also be gaps in the contact between the insulating surface and the sheath, cable bending, oil paper cable insulation surface cracks easily caused, these are caused by partial discharge factors
A shield layer of semi conductive material is added on the surface of the insulating layer, which has good contact with the shielded insulating layer and the same electric potential as the metal sheath so as to avoid partial discharge between the insulating layer and the sheath.