## 3 Results and Discussion
### Self-Healing Performance
#### 3.1.2 Repair Effect of Electrical Damage  
For high voltage and ultra-high voltage polyethylene cables, electrical tree is the main factor leading to insulation failure [41]. In the electric field, contaminants (impurities) and defective structures (such as protrusions and voids) in polyethylene cables can distort the electric field, resulting in electrical damage, such as PD and electrical tree. Electric tree is a degradation process, which can cause irreversible structural damage of microns and above, and directly threaten the insulation strength of polyethylene [41; 42]. Therefore, the self-healing of electrical tree can greatly improve the insulation performance and service life of polyethylene, but there are still few reports about it. This paper mainly studies the propagation stage of electrical tree. The experimental results are shown in Figure 4. The heating condition is 60 \({}^{\circ}\)C/30 min. This paper observes the panorama of electrical tree by a mosaic of multiple graphs to ensure clarity. In addition, there was no significant difference in the electrical tree of pure polyethylene prepared by the two processes. Furthermore, the emphasis of this paper is to study the effect of doping microcapsules on the typical performance of polyethylene. Therefore, this paper only gives the results of electrical tree in pure LDPE, with a modified process, as a comparison with the composite. Compared with pure polyethylene, there were three distinct changes in the morphology of electrical tree in the composite: 1. the size of electrical tree decreased significantly, which is similar to the result in [33]; 2. the electrical tree can develop into microcapsules and stop growing; 3. the electrical tree tended to branch near the microcapsule. It can be inferred that the local electric field can be affected by the microcapsule and the interface area between the microcapsule and matrix, which can attract the propagation of the electrical tree. When the electric tree develops into the microcapsule, it can break the wall structure of the microcapsule and consume some energy. The PD generated by the residual energy will turn into a new branch, thus the propagation of the electric tree is inhibited to a certain extent. Moreover, the microcapsule and interface area can destroy the continuity of the original structure in polyethylene, thus hindering the propagation of electrical tree. Therefore, the overall size of the electrical tree in the composite decreased and tended to develop towards the microcapsule area. In the appropriate conditions (60 \({}^{\circ}\)C/30 min), the composite exhibited an obvious self-healing ability for electrical damage defects. The morphology of electrical tree in the pure polyethylene had no obvious change. In the composite, except for the main branch of electric tree, the other branches disappeared. Moreover, the width of the main branch was clearly decreased. The electrical tree attracted by the microcapsule can break the wall of the microcapsule, while the core material of the microcapsule can flow into the tubules of electrical tree through capillary and infiltration action. The repair agent contacts and reacts with the catalyst to fill the damage defect. In addition, due to the density and dielectric constant of the reaction product being close to those of polyethylene, the compatibility between PDCPD and matrix is good. Thus, the low-density areas can be reduced. Figure 4: Morphology of electrical tree. (**a**) Unheated pure LDPE. (**b**) Heated pure LDPE. (**c**) Unheated composite. (**d**) Heated composite. Due to the large size of the electrical tree, the repair agent cannot completely repair the tubules of the electrical tree. However, the temperature and electric field coexist in the actual operation of polyethylene insulating material, and the propagation time of the actual electrical tree is longer [33; 41], which provides a good opportunity for the repair behavior of the microcapsule.