Shielded cable, as the name suggests, is a cable with anti-external electromagnetic interference capability formed in the form of a transmission cable with a shielding layer. The so-called “shielding” on the cable structure is also a measure to improve the distribution of electric fields. The conductor of the cable is composed of multiple strands of wire, which is easy to form an air gap between it and the insulation layer, and the conductor surface is not smooth, which will cause the concentration of electric field.
1.Cable shielding layer
(1). Add a shielding 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 known as the inner shielding layer. There may also be gaps in the contact between the insulation surface and the sheath, and when the cable is bent, the oil-paper cable insulation surface is easy to cause cracks, which are factors that cause partial discharge.
(2). Add a shielding layer of semi-conductive material on the surface of the insulation layer, which has good contact with the shielded insulation layer and equal potential with the metal sheath, so as to avoid partial discharge between the insulation layer and the sheath.
In order to evenly conduct the core and insulate the electric field, 6kV and above medium and high voltage power cables generally have a conductor shield layer and an insulating shield layer, and some low-voltage cables do not have a shield layer. There are two kinds of shielding layers: semi-conductive shielding and metal shielding.
2. Shielded cable
The shielding layer of this cable is mostly braided into a network of metal wires or a metal film, and there are a variety of different ways of single shielding and multiple shielding. Single shield refers to a single shield net or shield film, which can wrap one or more wires. The multi-shielding mode is a plurality of shielding networks, and the shielding film is in one cable. Some are used to isolate electromagnetic interference between wires, and some are double-layer shielding used to strengthen the shielding effect. The mechanism of shielding is to ground the shielding layer to isolate the induced interference voltage of the external wire.
(1).Semi-conductive shield
Semi-conductive shielding layer is usually arranged on the outer surface of the conductive wire core and the outer surface of the insulation 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 a thin thickness. The inner semi-conductive shielding layer is designed to uniform the electric field on the outer surface of the conductor core and avoid partial discharge of the conductor and insulation due to the uneven surface of the conductor and the air gap caused by the stranded core. The external semi-conductive shield is in good contact with the outer surface of the insulation layer, and is equipotential with the metal sheath to avoid partial discharge with the metal sheath due to defects such as cracks on the cable insulation surface.
(2). Metal shield
For medium and low voltage power cables without metal sheaths, in addition to setting a semi-conductive shield layer, but also add a metal shield layer. The metal shield layer is usually wrapped by copper tape or copper wire, which mainly plays the role of shielding the electric field.
Because the current through the power cable is relatively large, the magnetic field will be generated around the current, in order not to affect other components, so the shielding layer can shield this electromagnetic field in the cable. In addition, the cable shielding layer can play a certain role in grounding protection. If the cable core is damaged, the leaked current can flow along the shielding laminar flow, such as the grounding network, to play a role in safety protection. It can be seen that the role of the cable shield layer is still very large.
Post time: Nov-14-2024