Effect of Pre-qualification Test on Properties of Semi-conductive Shielding of High Voltage XLPE AC Cable

To evaluate the performance stability and reliability of a domestic 220-kV cable shielding material, shielding samples cut from 220-kV high voltage cables that have passed the pre-qualification (PQ) test were examined, including a domestic shielding material and two imported shielding materials. The changes in electrical and mechanical properties of the material samples were investigated using the volume resistivity test and tensile test before and after the PQ test. Further, the influences of the PQ test on the structure of semi-conductive composite matrix resin and conductive carbon black were analyzed at the microscopic level using the gel content test, X-ray diffraction (XRD) analysis, differential scanning calorimetric (DSC) analysis, scanning electron microscopy (SEM) analysis and Raman spectroscopy analysis. The results show that before the PQ test, the degree of cross-linking in the domestic cable shield was low, the mechanical properties were poor, but the electrical properties were stable. After the PQ test, the electrical and mechanical properties of the three types of cable semi-conductive shielding layer still met the requirements of the national standard, which ensured the stable operation of the cable system. However, the material properties changed. The resistivity of the three shielding layers increased but its stability deteriorated. The elongation at break and fracture energy decreased, and the tensile strength increased after the PQ test. The reason for the deterioration of the electrical and mechanical properties of the composite materials was the re-aggregation of carbon black caused by high temperature during the PQ test and the decrease in material polarity, which led to diminished dispersion of carbon black in the system, and affected the formation of conductive network and the interface combination of carbon black and resin.