The terminal wire shielding layer is a thin semiconducting layer with very low resistivity, which is divided into conductor shielding layer and insulation shielding layer. It can improve the distribution of power lines in the cable insulation and improve the cable insulation level; The so-called "shielding" on the terminal line structure is actually a measure to improve the electric field distribution. The terminal wire conductor is twisted by several guide wires, which is easy to form an air gap between it and the insulation layer. The conductor surface is not smooth, which will cause electric field concentration. Add a shield layer of semi conductive material on the surface of the conductor, which is equipotential with the shielded conductor and in good contact with the insulation layer, so as to avoid partial discharge between the conductor and the insulation layer. This layer of shielding is also called the inner shielding layer.
There may also be gaps between the terminal wire at the contact between the insulation surface and the sheath. When the terminal wire is bent, cracks are easily caused on the insulation surface of the oil paper cable, which are all factors that cause partial discharge. A shielding layer of conductive material is added on the surface of the insulating layer. It has good contact with the shielded insulating layer and is equipotential with the metal sheath, so as to avoid partial discharge between the insulating layer and the sheath.
The material of the shielding layer is semiconducting material, and its volume resistivity is 103-106 n · m. The shielding layer of the oil paper terminal wire is semiconducting paper. This paper is made of ordinary paper with a proper amount of colloidal carbon black particles added. The semiconducting paper can also adsorb ions, which is conducive to improving the electrical properties of insulation. The shielding layer material of extruded insulated terminal wire is a polymer added with carbon black particles. For extruded insulated cables without metal sheath, in addition to the semi conductive shielding layer, the metal shielding layer wrapped with copper tape or copper wire shall be added. This metal shielding layer is used to pass capacitive current during normal operation. When the system is short circuited, it acts as a channel for short circuit current and also shields the electric field. In the cable structure design, corresponding requirements shall be put forward for the sectional area of the metal shielding layer according to the size of the system short-circuit current.
