10-35KV medium voltage cable semi-conducting inner shie […]
10-35KV medium voltage cable semi-conducting inner shielding, insulating shielding, copper tape shielding, what role do they play, what are the differences
The shielding is mainly a uniform electric field and transmitting unbalanced current. The metal shielding is mainly in the neutral line of three-phase four-wire. When the cable is short-circuited, it can be used as a loop and can discharge the axial discharge of the cable. The main function is to equalize the electric field. .Copper tape shield also plays the role of grounding and bear short circuit current. The shielding on the cable structure is a measure to improve the electric field distribution.
Cable and cable conductors are formed by twisting multiple conductors. It is easy to form an air gap between the insulation layer and the conductor surface is not smooth, which will cause electric field concentration. Add a shielding layer of semi-conductive material on the surface of the conductor, which has the same potential as the shielded conductor and good contact with the insulating layer, so as to avoid partial discharge between the conductor and the insulating layer. This shielding is the inner shielding layer; Similarly, there may be a gap between the insulating surface and the sheath contact, which is a factor that causes partial discharge.
Therefore, add a shielding layer of semi-conductive material on the surface of the insulating layer, which has good contact with the shielded insulating layer and protects the metal Set the equipotential to avoid partial discharge between the insulating layer and the sheath. This layer of shielding is the outer shielding layer; the extruded insulated cable without a metal sheath, in addition to the semi-conductive shielding layer, copper tape should be added Or a metal shield layer wrapped by copper wire. This metal shield layer passes the capacitor 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 the copper tape shield do not exist in the cable, the possibility of insulation breakdown between the cores of the three-core cable is very high.
The role of the metal shield is
1. Do electrostatic shielding to limit the effect of electric field in cable insulation
2. As a neutral line of a three-phase four-wire system, conduct unbalanced current.
3. The power station protection system requires good lightning protection characteristics with external metal shielding.
4. Under normal circumstances, the capacitor current flows and acts as a fault current loop during short circuit. 5. Prevent axial surface discharge.