Correlation Between Polarization Mechanisms and Material Characteristics in Thermally Aged 320-kV HVDC Cable Insulation

In the present work, degradation analysis associated with the thermal stress in a 320-kV cross-linked polyethylene (XLPE) insulated cable insulation has been carried out. Accelerated thermal aging at 140 °C for 30 days is done for XLPE insulation to understand the material degradation. Physicochemical analyses such as Fourier transform infrared (FTIR) Spectroscopy, differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses have been performed to understand the material degradation due to thermal stress. These material investigations show interesting features regarding changes in characteristic peaks and formation of carbonyl group, microstructural changes, and decreased degree of crystallinity with thermal aging as evident by FTIR, XRD, and DSC, respectively. Measurements of polarization and depolarization current (PDC) have been carried out in the thermally aged insulation samples. It is observed that with thermal aging the polarization and depolarization current magnitude seem to increase. A novel technique employing relaxation frequency distribution (RFD) has been used to get a deeper insight into the thermal degradation of the material by understanding the relaxation mechanism of the dipoles with the thermal aging. RFD spectrum shows a dominant slow polarization as the material undergoes thermal aging. Higher dielectric loss with thermal aging of the samples has been observed from the RFD spectrum. This higher dielectric loss is also validated using dielectric dissipation factor ( $\tan \delta $ ) measurement. Moreover, a correlation between RFD spectrum and physicochemical analyses has been established.