The metal shielding layer is an indispensable structure in the intermodal polyethylene insulated power cable, GB/T12706.2-2008 and GB/T12706.3-2008 Part 7 of the cross-linked polyethylene insulated power cable. There should be metal shielding on the insulation line. You can cover the metal shielding on the single insulating wire core or several insulating wire cores. Scientific design of the structure of metal shielding, accurately calculating the short-circuit current undertaken by the shielding layer, and reasonably formulated the shielding layer processing process. It plays a vital role in ensuring the quality of the cross-linked cable and the safety of the entire operating system.
1 The method and role of metal shielding
The method of medium-voltage cross-linked polyethylene insulation electric cable metal shielding is mainly two ways: copper belt covering bag shielding and sparse copper shielding.
According to the standards of GB/T12706-2008 rated voltage of 6kV to 35kV cables, the average capacity of the copper belt in the copper belt shielding method is not less than 15%(nominal value) of the copper bandwidth, and the minimum value is not less than 5%. The thickness of the copper belt of the single-core cable is ≥0.12mm, the average thickness of the multi-core cable is ≥0.10mm, and the minimum thickness of the copper belt is not less than 90%of the nominal value. A copper shield is composed of sparse soft copper wires. The surface should be tightened by the copper wire or copper band of the reverse wrap, 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 electric fields. The role of metal shielding mainly has the following aspects:
1. When the cable is normal, the metal shielding layer passes through the capacitor current, and the short-circuit current is passed during short-circuit failure.
2. Block the electromagnetic field caused by the cable to power into the insulation line to reduce the electromagnetic interference generated by the outside world. The metal shielding layer also has the impact of restricting external electromagnetic fields on the internal.
3. Power station protection system requires that external metal shielding has good lightning protection characteristics.
4. Broadcasting electric field to prevent axial discharge. Because the semi-conductive layer has a certain resistance, when the metal shielding layer is poorly grounded, the axis of the cable is discharged along the surface due to the uneven distribution of the potential.
Understanding the role of the metal shielding layer has important guiding significance for the control of the pressure cable in production. The complete shielding manufacturing process and the reasonable design and calculation of the shielded cross-section are significant means to ensure the quality of the cross-linked cable metal shielding.