Harm and mechanism analysis of thermal shrinkage of cross-linked polyethylene (XLPE) insulation

1. Hazard:

Cables with unqualified insulation heat shrinkage will be exposed to excessive insulation shrinkage over time during normal work and heating, which will cause exposed conductors, which are prone to leakage and electric shock hazards. In order to ensure the safety of cables, the standard The shrinkage of the insulation is specified.

2. Insulation shrinkage mechanism:

We know that cross-linked insulation is mainly divided into two categories: physical cross-linking and chemical cross-linking. Physical cross-linking is also called irradiation cross-linking. Chemical cross-linking is also divided into peroxide cross-linking and silane grafting cross-linking. Peroxide crosslinking is usually used for medium and high voltage cables. Compared with the radiation crosslinking process, the silane crosslinking process is simple and safe to operate. Low voltage power cable insulation.

There are two processes in the production of XLPE cables by silane crosslinking: grafting and crosslinking. At present, there are two commonly used process methods for producing silane XLPE cables in the cable industry: one-step method and two-step method. a. One-step method is to use PE insulating material directly added with cross-linking agent, grafting is carried out when extruding cable insulation, and then cross-link the insulating layer in warm water or steam; b. Two-step method is to use pre-grafted silane PE insulation material, extrusion cable insulation, and then in warm water or steam to cross-link the insulation. Since the one-step silane crosslinking process interacts with the crystallization process and the grafting process when extruding XLPE insulation, the thermal process should be controlled as much as possible during the extrusion process, so that the aggregated structure of the XLPE insulation material is in a reasonable state, so that it can Make XLPE insulation have excellent performance.

PE is a crystalline polymer, and its crystallization process is a process in which macromolecules or chain segments are rearranged through intermolecular interactions, changing from disorder to order. When cable insulation is extruded, the macromolecules in the viscous fluid state are subjected to shearing and pulling and stretching, so that the size of the grains of PE molecules increases along the stretching direction (longitudinal), decreases in the transverse direction, and improves the order, that is PE molecules are oriented, which easily induces many crystal embryos, increases the number of crystal nuclei, shortens the crystallization time, and increases the crystallinity. However, when the finished PE insulated cable is placed at room temperature, due to the internal stress (shrinkage stress) generated when the PE insulation is extruded, the crystallized PE molecules are easily disoriented (retraction tendency), which causes the heat loss of the PE insulation. shrinkage phenomenon.