Effect of Internal Defects or Impurities on Insulation Performance of Heat-shrinkable Pipe
The following are various defects in heat shrinkable tube insulation:
1. Pollutants (impurities) in heat shrinkable pipe insulation;
2. Outer shielding protuberance of heat shrinkable tube;
3. Electric branches;
4. Butterfly-shaped branches (water branches);
5. Internal shielding protrusion;
6. Micropore (often ring-shaped);
7. Poor contact of inner wall;
9. Inner wall voids;
10. Internal shielding gap;
11. Powder powder anti-aging agent.
These defects of thermal shrinkable tube insulation can generally lead to partial discharge of insulation and accelerate the electrical aging of insulation. There are 11 kinds of main defects, which can be divided into two categories. One is to cause electric field concentration in insulation and increase local electric field strength. Semiconductor shielding layer protruding into the insulator and conductive impurities in the insulator belong to this category. The other is to weaken the local insulation strength in the insulation. The gap or air gap in the insulator or or the interface between the shield layer and the insulator belongs to this kind of defect.
In the extrusion process of heat shrinkable tube insulation, the heating medium entering into the insulation material can form numerous small holes. The dielectric coefficient of the micro-holes is smaller than that of the insulation material. Therefore, under the action of electric field, the size of the micro-holes becomes larger and larger, which is prone to tree damage. Therefore, limiting the size and quantity of the micro-holes in insulation is the key to restrain dendrization. In the wet process of cross-linking in the past, water vapor will inevitably remain in the insulation and form water-bearing micropore. Dry cross-linking can reduce the micropore to less than 5 m. Therefore, when using cross-linked polyethylene heat-shrinkable pipes in the future, dry cross-linking products should be used as far as possible in the acceptable cost range, so as to ensure the safety of use.