Correlation between Terahertz Dielectric Properties of XLPE and Its Composition Structure during Electrothermal Aging

The electrothermal aging process of cross-linked polyethylene (XLPE) insulation has been an essential factor for the reliability of high-voltage (HV) cables, but the underlying mechanism remains a puzzle. In this paper, the dielectric properties in the terahertz domain of sliced XLPE samples from an HVDC cable are investigated at an electrothermal aging duration up to 1500 h. It is found that with the increase in aging time, the real part and imaginary part of the complex permittivity decrease and increase, respectively. To explain the dielectric phenomenon, chemical changes and crystallinity are characterized by Fourier transform infrared spectrum (FTIR) and differential scanning calorimetry (DSC), respectively. It is proven that declined crystallinity is positively correlated with the real part of complex permittivity by using effective medium theory. Furthermore, by comparing the terahertz dielectric response of unaged and aged XLPE samples in different temperatures, the appearance of small polar molecules has been proven to be responsible for the rise of the imaginary part of complex permittivity.