Thermal and mechanical properties analysis for EHV XLPE cables with different operating years

Analysis of the properties of cable materials operated for different years is beneficial for more accurate modeling of life-expectancy and asset management. The aggregation state, thermal stability, and mechanical properties of insulation materials were researched for 110 kV and 220 kV cross-link polyethylene (XLPE) cables out of service in China. The slices cut from the cable were analyzed through the Differential scanning calorimetry (DSC), Thermogravimetry (TG), Fourier transform infrared spectrum (FTIR), and mechanical property analysis. Compared with new cable, the initial melting temperature of melting peak for cables operated for more than 10 years were shifted by approximately 10 oC to the lower temperature direction as obtained from the DSC results. The crystallinity, crystal size and its distribution have changed. The crystallization rate of the operated samples increased with operation year, which means that some bonds of the crosslink points were broken. From the thermal stability analysis by TG and FTIR, only some samples presented a certain extent of thermal oxidation aging. The elasticity modulus and fracture energy showed a slight decrease of the operated cables. However, the other mechanical parameters such as elongation at break, yield strength and fracture strength showed no evident difference between the new and the operated cables.