Influence on Aggregation Structure Changes of Retired Cross-linked Polyethylene (XLPE) Cable Insulation under Pre-qualification Test

In order to investigate the cross-linked polyethylene (XLPE) insulation characteristics of a retired high-voltage cable that has been in operation for 16 years and to evaluate the reliability of reusing it for practical operation, a 180-day pre-qualification test method was applied to a section of this cable to inspect the changes of aggregation structure. In this paper, the electric field and the thermal field of the inner, medium and outer positions of the insulation under the accelerated aging test were analyzed and converted to the corresponding equivalent condition at the surface of the conductor core. It can be found that equivalent testing conditions of the medium and outer positions are close to the cable practical operation condition. The diagnostic measurements of fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and X-ray diffraction (X-ray) were conducted to analyse the aggregation structure of the samples. The results showed that the molecular chain of the inner position of the cable insulation was damaged and the crystalline structure was slightly degraded under high-strength test condition. On the contrary, the crystalline structure of medium and outer positions were both improved after the accelerated aging test because of the reduction of the impurities and the annealing effect. So it can be expected that the rest of this retired cables still have the potential to reuse in long-term actual operation condition.