Analysis of Crystallization and Crosslinking Characteristics of Bilayer XLPE Insulation of Factory Joints

As the core component of long-distance transmission systems, the performance stability of high-voltage direct current (HVDC) cable factory joints directly affects the safety and efficiency of power transmission. In the complex structure of factory joints, the significant differences in preparation processes between the recovery insulation layer and the body insulation layer result in the bilayer insulation part becoming a weak link in the electrical performance of the entire HVDC cable. Crosslinked polyethylene (XLPE) is a key insulation material for HVDC cables, and its semi-crystalline characteristics have a decisive impact on the comprehensive properties of the material, such as mechanical strength, electrical performance, and heat resistance. Therefore, this article focuses on the crystal structure evolution of XLPE materials under different processing conditions, and systematically compares the differences in crystal morphology and crosslinking degree of four samples: monolayer XLPE, monolayer XLPE subjected to secondary hot-pressing treatment (Sec-XLPE), bilayer insulation X-X obtained by hotpressing XLPE and Sec-XLPE, and L-X, which simulates the actual factory joint bilayer insulation sample. The joint analysis of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) revealed that L-X formed a unique crystalline structure during the preparation process, exhibiting high crystallinity and the formation of new crystal structure. Further, by using scanning electron microscopy (SEM) techniques and successive self-nucleation annealing (SSA) analysis, it was confirmed that L-X has a large spheroidal size and a relatively thick lamellae, while the crosslinking degree test results indicated that its crosslinking degree was relatively low. This study provides important experimental basis and theoretical support for a deeper understanding of the performance evolution mechanism in bilayer insulation in factory joints, and has important guiding significance for optimizing the insulation structure design of HVDC cables and improving the overall performance of cable systems.