What is the function of inner and outer shielding of cable medium voltage and high voltage cables?
The so-called "shielding" on the cable structure is essentially a measure to improve the electric field distribution. The cable conductor is made up of multiple wires twisted, and it is easy to form an air gap between it and the insulating layer. The surface of the conductor is not smooth, which will cause the electric field to concentrate.
Add a layer of shielding layer of semi-conductive material on the surface of the conductor, which has the same potential as the shielded conductor and is in good contact with the insulating layer, so as to avoid partial discharge between the conductor and the insulating layer. This layer of shielding is the inner shielding layer.
Also, there may be gaps at the contact between the insulating surface and the sheath, which is a factor that causes partial discharge. Therefore, a layer of semi-conductive material shielding layer is added on the surface of the insulating layer, which has good contact with the shielded insulating layer and is in contact with the metal sheath. Sheath equipotential, so as to avoid partial discharge between the insulation layer and the sheath, this layer of shielding is the outer shielding layer.
For extruded insulated cables without metal sheath, in addition to the semi-conductive shielding layer, a metal shielding layer wrapped with copper tape or copper wire should be added. The function of this metal shielding layer is to pass capacitive current during normal operation; When the system is short-circuited, it acts as a channel for short-circuit current and also plays a role in shielding the electric field.
It can be seen that if the outer semiconductor layer and copper shielding in the cable do not exist, the possibility of insulation breakdown between the core and the core of the three-core cable is very high.