The metal shielding layer is an indispensable structure in medium voltage (3.6/6kV∽26/35kV) XLPE insulated power cables. The insulated cores of all cables should have metal shielding, which can be used on a single insulated core. A metal shield may be applied to several insulated cores. Scientifically designing the structure of the metal shield, accurately calculating the short-circuit current that the shielding layer bears, and formulating the shielding layer processing technology reasonably play a vital role in ensuring the quality of the cross-linked cable and the safety of the entire operating system.
The metal shielding methods of medium-voltage XLPE insulated power cables are mainly shielded with copper tape and shielded with a cover and shielded with sparsely wound copper wire.
According to the standards for cables with a rated voltage of 6kV to 35kV, the average overlapping rate of the copper tape in the copper tape shielding method is not less than 15% (nominal value) of the width of the copper tape, and the minimum value is not less than 5%. The copper tape thickness of single-core cable is ≥0.12mm, the average thickness of multi-core cable is ≥0.10mm, and the minimum thickness of copper tape is not less than 90% of the nominal value. Copper wire shielding is composed of loosely wound annealed copper wires, and its surface should be tightly bound by reversely wrapped copper wires or copper tapes. 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 electric field distribution. The role of metal shielding mainly has the following aspects:
1. When the cable is normally energized, the metal shielding layer passes the capacitive current, and the short-circuit current passes through the short-circuit fault.
2. The electromagnetic field caused when the cable is energized is shielded in the insulated core to reduce the electromagnetic interference to the outside world. The metal shielding layer also limits the influence of the external electromagnetic field on the inside.
3. The power station protection system requires the outer metal shield to have good lightning protection characteristics.
4. Homogenize the electric field to prevent axial discharge. Since the semiconductive layer has a certain resistance, when the metal shielding layer is not well grounded, the cable will be discharged along the surface due to uneven potential distribution in the axial direction of the cable.
