Abstract: When a tunnel fire occurs, the high-voltage cables near the fire source are temporarily exposed to high temperatures. The degradation characteristics of cross-linked polyethylene (XLPE) cable insulation material after high temperature of fire need to be studied. Firstly, a simulation model is established to analyze the temperature variation curve of the cable insulation layer under typical fire conditions in this paper. Then the heat treatment platform is built. XLPE samples are processed according to the equivalent temperature rise curves with the highest temperatures of 300, 325, 350 ℃. Finally, the microstructure, functional groups, crystallinity, and breakdown field strength of the sample are tested. The results indicate that as the degradation temperature increases, the regularity of the sample surface gradually decreases. The polymer fragments and small molecular substances generated by XLPE molecule breakage aggregate together to form a granular structure. The crystallinity decreased from 28.67% to 16.47%. The carbonyl index increased from 0.022 to 0.344, and the hydroxyl index increased from 0.066 to 0.211. The breakdown field strength decreased from 74.16 kV/mm to 50.38 kV/mm. The research results can provide theoretical basis for the degradation assessment and maintenance replacement of cable insulation after fire accidents.

Key words: cable fire, insulation degradation, microscopic characteristics, breakdown field strength