## 4 Analysis and Discussion
### Analysis of the Influence of the Manufacturing Process on the Resistance of XLPE to DC Tree  
Based on the analysis in Sections 2.1 and 2.2, the crosslinking process and cooling degassing process both impact the final physicochemical properties of XLPE material specimens in this study. During the high-temperature crosslinking stage of cable production, the crosslinking temperature theoretically ranges from 150 \({}^{\circ}\)C to near the degradation temperature of polyethylene, which is around 280 \({}^{\circ}\)C. However, excessively high temperatures can lead to over-crosslinking, negatively affecting the performance of XLPE [25].  
In practice, raising the crosslinking temperature increases the reaction rate, thereby enhancing production capacity, which is why cable manufacturers often opt for higher crosslinking temperatures. Moreover, for large-sized cables, due to the poor thermal conductivity of polyethylene, a temperature gradient tends to form during the crosslinking process. To ensure sufficient crosslinking in regions far from the heat source, cable manufacturers are compelled to select higher crosslinking temperatures.  
A study [26] shows that the crystalline and amorphous regions distribution of XLPE formed at the crosslinking temperature of 150-180 \({}^{\circ}\)C may be better and have better resistance to electrical trees. When comparing the crosslinking processes of the two types of XLPE materials, it is evident that the crosslinking temperature of 180 \({}^{\circ}\)C used for DC-XLEP in the laboratory is higher than that of Cable-XLEP and is more controlled and reasonable. Additionally, the 5 mm thickness of the DC-XLEP specimen allows for a more uniform temperature distribution during crosslinking. These factors result in DC-XLEP having a higher molecular weight and a more concentrated molecular weight distribution, contributing to its superior resistance to electrical treeing and partial discharge.  
The difference in degassing processes is also not negligible. Degassing removes impurities from XLPE materials, such as alkanols, acetophenone, and \(\alpha\)-methstyrene, which are crosslinking by-products. According to information obtained from the manufacturer, the insulation thickness of the Cable-XLEP is approximately 14 mm, and it underwent degassing in a single direction (radially outward) at 70 \({}^{\circ}\)C for three days. In contrast, the DC-XLEP, with a thickness of 5 mm, was degassed in both directions--at 90 \({}^{\circ}\)C for one day and at 70 \({}^{\circ}\)C for two days. Clearly, the degassing by-product removal rate is higher in DC-XLEP, resulting in fewer impurities, which also contributes to its enhanced resistance to electrical treeing and partial discharge.  
The research in this paper primarily compares and analyzes samples manufactured using two different processes to identify the differences in the electrical properties of the materials and provide recommendations for manufacturing DC cables with longer lifetimes and greater reliability. Future studies could further explore the impact of XLPE material structure on its electrical properties by considering the use of fractional calculation tools [27], multi-capacitor models, and other methods.