The role of micro-structure changes on space charge distribution of XLPE during thermo-oxidative ageing

In order to investigate the differences of the space charge distribution of XLPE aged at temperatures below and above the melting point, thermo-oxidative ageing was conducted on 110 kV cross-linked polyethylene (XLPE) cable insulation at 100 °C and much elevated temperature of 160 °C respectively. The XLPE insulation showed moderate changes on micro-structure and space charge distribution until the moment when the Oxidation Induction Time (OIT) dropped to zero. It was deduced by the results of Fourier Transform Infrared (FTIR) Spectroscopy, X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscope (SEM) tests that thermo-oxidative ageing could lead to a significant increase in carbonyl groups and a decrease in crystallinity. At 100 °C, the thermo-oxidative ageing mainly affected the amorphous regions because of the much denser structures of crystalline regions, which might make the oxygen diffusion more difficult. At 160 °C, the thermo-oxidative ageing affected the melted spherulites and leaded to obvious impact on lamellae, resulting in a significant decrease in crystallinity and the destruction of spherulites. The space charge distribution of aged XLPE samples was studied by Pulsed Electroacustic Method (PEA) test. Hetero-charges could be found in aged XLPE samples. These hetero-charges were derived from ionization of polar groups in XLPE generated during ageing. The surface trap parameters of XLPE samples were investigated by Isothermal Surface Potential Decay (ISPD) test. It was deduced that shallow traps with the energy level of about 0.95 eV and deep traps with the energy level of about 1.05 eV on the surface of XLPE were introduced by carbonyl groups and interfaces of crystal/amorphous respectively. Thermo-oxidative ageing at 160 °C dramatically decreased the deep trap density because of the destruction of spherulites, leading to enhanced electrode injection. As a result, compared with those of unaged samples and samples aged at 100 °C, increased amount of homo-charges were found in the XLPE samples aged at 160 °C.