The way and role of metal shielding in medinum voltage power cable
The metal shielding methods of medium-voltage XLPE insulated power cables are mainly composed of copper tape overlap wrapping shielding and sparse copper wire shielding.
According to the GB/T12706-2008 standard for cables with rated voltage from 6kV to 35kV, the average overlap ratio of copper tape in the copper tape shielding method is not less than 15% of the width of the copper tape (nominal value), and the minimum value is not less than 5%. The thickness of copper tape for single-core cables is ≥ 0. 12mm, the average thickness of multi-core cables is ≥ 0. 10mm, and the minimum thickness of copper tape is not less than
90% of the nominal value. The copper wire shield consists of loosely wound soft copper wires, the surface of which should be fastened by reversely wound copper wires or copper tapes, and the average gap between adjacent copper wires should not be greater than 4mm.
The shielding on the cable structure is a measure to improve the distribution of the electric field. The role of the metal shielding mainly has the following aspects:
1. When the cable is normally energized, the metal shielding layer passes the capacitive current, and when the short-circuit fault occurs, the short-circuit current passes.
2. The electromagnetic field caused when the cable is energized is shielded in the insulated core to reduce electromagnetic interference to the outside world, and the metal shielding layer also limits the impact of the external electromagnetic field on the interior.
3. The power station protection system requires the outer metal shield to have better lightning protection properties.
4. Homogenize the electric field to prevent axial discharge. Since the semi-conductive layer has a certain resistance, when the metal shielding layer is poorly grounded, the cable will discharge along the surface due to uneven potential distribution in the cable axis.