1. The shielding layer has the function of uniform electric field and reducing the surface electric field intensity of the core.
The semi-conductive shielding layer on the core surface can uniformly distribute the non-uniform electric field on the core surface and reduce the maximum electric field intensity on the conductor surface due to the wire guide effect. For example, the insulation thickness of the cable is 5 mm at 10 kV and 25 mm load. When there is no semi-conductive shield layer, Emax is 24.24 kV/mm. When there is an inner shield layer, Emax is 3.16 kV/mm. The decrease of Emax can reduce the thickness of insulating layer and the timber of cable, and the decrease of the maximum electric field strength can make the cable run safer and more reliable. At the same time, the existence of shielding layer allows the designer to calculate the cable as a concentric cylinder, reducing the error caused by theoretical calculation.
2. The existence of shielding layer improves the initial discharge voltage of Cable Partial Discharge and reduces the possibility of partial discharge.
Because of the three layers extruded cable structure at the same time, the gap between shielding layer and insulating layer is prevented, so that the high and low voltage electrodes are tightly combined on the insulating surface, and the partial discharge is not easy to occur.
3. Suppression of Branch Growth
When the metal burrs on the conductor surface directly pierce the insulating layer, or when impurity particles, water and air gap exist in the insulating layer, these will cause high electric field and field emission at the tip and cause tree branches. For metal surface ICI burrs, semi-conductive shielding will effectively reduce the field intensity near the burrs and reduce field emission, thus improving the discharge characteristics of the resisting branch. If a substance capable of capturing moisture is added to the semi-conductive shielding material, the moisture introduced by the wire core can be effectively blocked from entering the insulating layer, thus preventing the formation of water tree branches in the insulation. Here in Shenzhen heat shrinkable pipe network Xiaobian in degree hints that the purchase of heat shrinkable pipes must choose a professional heat shrinkable pipe manufacturer.
IV. THERMAL BARRIER
As we have discussed before, the semi-conductive shielding layer has a certain thermal resistance. When the core temperature rises instantaneously, the cable has a thermal resistance of the semi-conductive shielding layer. The high temperature will not immediately impact the insulating layer, and the temperature rise on the insulating layer will decrease through the temperature separation of the thermal resistance.
V. External Semiconductive Shielding
Although the potential of the XLPE insulated cable is lower and the field strength is lower than that near the core, the insulating surface is subjected to tension when the cable is subjected to bending stress. If partial discharge occurs at this time, micro-cracks will occur due to the bending stress on the surface, which will lead to the formation of tree branches, or new cracks caused by partial discharge corrosion on the surface. Adverse reactions, so external shielding is indispensable. IEC stipulates that XLPE insulated cables above 3 kV shall be shielded by both inner and outer layers.