Do you know what shielded cable is?
Shielded cable has the word "shielded" in it. As the name implies, it is a cable with the ability to resist external electromagnetic interference formed by adding a shielding layer to the transmission cable. The so-called "shielding" in the cable structure is also a measure to improve the distribution of the electric field. The cable conductor is made of multiple strands of wires twisted together, and it is easy to form an air gap between it and the insulation layer. The surface of the conductor is not smooth, which will cause the electric field concentration. Then let's talk about the knowledge of shielded cables in detail.
1. Cable shielding layer
1. Add a shielding layer of semi-conductive material on the surface of the conductor. It has the same potential as the shielded conductor and has good contact with the insulation layer, thus avoiding local discharge between the conductor and the insulation layer. This layer of shielding is also called the inner shielding layer. There may also be gaps at the contact point between the insulation surface and the sheath. When the cable is bent, cracks are easily formed on the insulation surface of the oil-paper cable. These are factors that cause local discharge.
2. Add a shielding layer of semi-conductive material on the surface of the insulation layer. It has good contact with the shielded insulation layer and has the same potential as the metal sheath, thus avoiding local discharge between the insulation layer and the sheath.
In order to make the conductive core and the insulation electric field uniform, medium and high voltage power cables of 6kV and above generally have conductor shielding layers and insulation shielding layers, and some low-voltage cables do not have shielding layers. There are two types of shielding layers: semi-conductive shielding and metal shielding.
2. Shielded cable
The shielding layer of this type of cable is mostly made of metal wires woven into a mesh or metal film, with a variety of different ways of single shielding and multi-shielding. Single shielding refers to a single shielding net or shielding film, which can wrap one or more wires. Multi-shielding is a method where multiple shielding nets and shielding films are coexisting in one cable. Some are used to isolate electromagnetic interference between wires, and some are double-layer shielding used to enhance the shielding effect. The mechanism of shielding is to ground the shielding layer to isolate the inductive interference voltage of the external connection to the wire.
1. Semi-conductive shielding
The semi-conductive shielding layer is usually arranged on the outer surface of the conductive wire core and the outer surface of the insulating layer, respectively called the inner semi-conductive shielding layer and the outer semi-conductive shielding layer. The semi-conductive shielding layer is composed of a semi-conductive material with very low resistivity and thin thickness. The inner semi-conductive shielding layer is to uniformly form the electric field on the outer surface of the wire core to avoid partial discharge of the conductor and insulation due to the uneven surface of the conductor and the air gap caused by the twisting of the wire core. The outer semi-conductive shielding layer has good contact with the outer surface of the insulating layer and is at the same potential as the metal sheath to avoid partial discharge with the metal sheath due to defects such as cracks on the cable insulation surface.
2. Metal shielding
For medium and low voltage power cables without metal sheath, in addition to the semi-conductive shielding layer, a metal shielding layer must be added. The metal shielding layer is usually wrapped with copper tape or copper wire, and mainly plays the role of shielding the electric field.
Moreover, because the current passing through the power cable is relatively large, a magnetic field will be generated around the current. In order not to affect other components, a shielding layer can be added to shield this electromagnetic field inside the cable. Moreover, the cable shielding layer can play a certain role in grounding protection. If the cable core wire is damaged, the leaked current can flow along the shielding layer to the grounding grid, playing a role in safety protection. This shows that the role of the cable shielding layer is still very large.
