Shielding plays an important role in medium voltage cross-linked polyethylene. It can improve the uniformity of electric field distribution. According to the standard understanding, shielding is divided into non-metallic shielding (conductor shielding, insulation shielding) and metal shielding (copper tape, copper wire).

The medium voltage cross-linked cable now uses a tight circular conductor structure. On the one hand, the electric field on the surface of the conductor, on the other hand, I understand that it plays a role in blocking water. However, even if such a tight pressing method is adopted, there are still tiny gaps between the conductor monofilaments, which will inevitably lead to the uniformity of the electric field distribution and the fine burrs generated during the conductor stranding, which will lead to the increase of partial discharge and serious breakdown. The cross-linked conductor shield then contributes to this aspect of the defect and enables close contact between the insulation. The function of the insulation shield is also to reduce the air gap between the insulation and the copper strip, and to uniformly insulate the surface electric field.

The volume resistivity of the cross-linked insulation shield exceeds the standard requirements, which is easy to cause the increase of the discharge capacity, so the control of the volume resistivity of the raw material inspection is very important. The role of copper tape shielding is to be used as a channel for leakage current when a short circuit occurs, and to work through capacitive current and lightning protection.

During the partial discharge withstand voltage test of the finished product, a section of the insulation shield should also be stripped off. Generally, the end of 8.7/10 kV cable is rarely broken down. Why is the breakdown rate of 26/35 kV cable end very high? And the breakdown points are all 2-3mm below the semi-conductive port. The analysis is:

1. As the voltage increases, the electric field strength increases gradually.

2. The shielding stripping length is not enough and the semi-conductive port is not smooth.

3. The installation of stress cone is not in place. These aspects are the main causes of terminal breakdown. The treatment method is to increase the stripping length of the insulation shield. According to experience, it is advisable to not less than 70cm. The screw of the stress cone is installed 2mm above the semi-conductive port. For non-smooth treatment, the glass can be gently scraped flat, but the insulation cannot be damaged.

Domestic indoor and outdoor terminal accessories are now mainly cold shrinkage, the technology is basically no problem, the key is the treatment of shielding.

If the treatment is not good, it is easy to cause terminal breakdown. The key is the treatment of the outer shielding opening. According to relevant data, the accessory breakdown caused by improper control of the semi-conductive opening is higher than that of other reasons, because the electric field here is relatively concentrated and the treatment of the semi-conductive opening should be smooth. For example, 35 kV cross-linked cables now mostly use non-strippable outer shielding materials.

Only by stripping and cutting the semi-conductive with a stripping knife, the closing may not be very smooth, and thin glass can be used to scrape a slope in the arc of the semi-conductive opening, which is beneficial to the improvement of electric field uniformity.

The application or improvement of shielding technology in cross-linked cables will continue to improve with the development of the information society. Only by continuously exploring and discovering, will many problems be discovered, and continue to verify through practice, so as to improve product quality.