The so-called "shielding" on the cable structure is essentially a measure to improve the electric field distribution. The cable conductor is formed by stranding a plurality of guide wires, and an air gap is easily formed between the cable conductor and the insulating layer, and the surface of the conductor is not smooth, which causes electric field concentration. A layer of semiconducting material is applied to the surface of the conductor, which is equipotential to the shielded conductor and in good contact with the insulating layer to avoid partial discharge between the conductor and the insulating layer. This layer of shielding, also known as the inner shield.
There may also be gaps at the contact between the insulating surface and the sheath. When the cable is bent, the insulating surface of the oil-paper cable is liable to cause cracks, which are factors that cause partial discharge. A layer of conductive material is applied to the surface of the insulating layer, which has good contact with the shielded insulating layer and equipotential with the metal sheath to avoid partial discharge between the insulating layer and the sheath. The material of the shielding layer is semi-conductive material, and its volume resistivity is 103-106n·m. The shielding layer of the oil-paper cable is semi-conductive paper. This paper is added to the appropriate amount of colloidal carbon black particles in the ordinary paper, and the semi-conductive paper is also It has the function of adsorbing ions, which is beneficial to improve the electrical properties of the insulation. The shielding material of the extruded insulated cable is a polymer with carbon black particles added, and a sheathed insulated cable without a metal sheath, in addition to the semi-conductive shielding layer, a metal shielding layer wrapped with copper or copper wire is added. The function of this metal shielding layer is to pass the capacitor current during normal operation. When the system is short-circuited, it acts as a short-circuit current channel and also acts as a shielding electric field. In the cable structure design, according to the size of the system short-circuit current, the corresponding requirements for the cross-sectional area of the metal shielding layer are put forward.