## 4 Conclusions  
In this paper, the preparation method and typical electrical performances of the novel self-healing polyethylene insulating composite were explored exploitatively and creatively. On the basis of proving the self-healing ability of the composite, the effect mechanisms of the doping microcapsules on the important electrical performances of polyethylene were studied emphatically. The application advantages of self-healing insulating composites were proven by linking the micro-mechanism with the macro-performance. The main conclusions are as follows:  
(1) The microcapsule/polyethylene insulating composite prepared in this paper has obvious self-healing abilities against structural damage. Moreover, according to the repair results of breakdown strength, the appropriate repair conditions selected in this paper were 60 \({}^{\circ}\)C/30 min. In these conditions, the performances of the composite hardly changed, but the self-healing effect was remarkable. The composite can not only fill the structural defect caused by mechanical damage (such as a scratch) and electrical damage (such as an electrical tree), but also homogenize the local high electric field in the defect area, restoring its insulation strength;(2) Doping microcapsules can directly change the basic physicochemical properties (such as crystallinity) of polyethylene, and then affect the electrical performance. Microcapsules increase the crystallization source of polyethylene by the heterogeneous nucleation effect of the interface between the microcapsule and matrix. However, the microcapsule cannot change the size of the original crystal region in matrix, and the modified process for doping microcapsules (reducing pressure and prolonging heating time) can decrease the number and size of crystal grains in the polyethylene;  
(3) The effect of doping microcapsules on the electrical performance of polyethylene, such as breakdown strength, volumetric resistivity, dielectric properties, and space charge characteristics, are mainly related to impurity (i.e., contaminant) and the interface. The polarization and ionization of the impurities introduced by the microcapsule system can reduce the electrical performance of the polyethylene. However, the interface between the microcapsule and matrix can not only improve the microstructure of the matrix (such as the heterogeneous nucleation effect increasing the number of crystal regions, and the anchoring effect enhancing the stability of the amorphous regions), but also increase the space charge trap. Moreover, the microstructure and charge trap can affect the characteristics of carrier transport, material polarization, and space charge accumulation, thus improving the electrical performance of polyethylene to a certain extent.  
In addition, the typical electrical performance of the self-healing polyethylene composite in this paper can meet the basic application requirements of insulating material. It can also be predicted that the composite has high application value in the appropriate concentration range of the microcapsules.  
Conceptualization, Y.W. and Y.L.; Methodology, Y.L.; Validation, Y.W., Y.L. and Z.Z.; Data curation, Y.Z. and Z.Z.; Formal analysis, Y.W. and Y.L.; Writing--original draft preparation, Y.L.; Writing--review and editing, Y.W.; Visualization, Y.L. and Y.W.; Project administration, Y.W.  
This research was funded by National Natural Science Foundation of China (51777018).  
The authors declare that they have no conflict of interest.