Extra-Low Frequency Dielectric Relaxation in HVDC Cable Insulation Material Caused by Cross-Linking Byproduct

The cross-linking byproducts affecting the conductivity of cross-linked polyethylene (XLPE) determine the electric field distribution and temperature stability of cable insulation, and thus the detection of residual cross-linking byproducts plays an important role in the reliability of High Voltage Direct Current (HVDC) cable system. This article employs the time-dependent polarization current measurement to investigate the dielectric relaxation in XLPE with different contents of cross-linking byproduct at extra-low frequency (ELF). The results show that two Debye-type dielectric relaxations appear at the frequency range of
$10^{-{4}}$
–
$10^{-{3}}$
Hz (defined as
$\alpha $
relaxation) and
$10^{-{3}}$
–
$10^{-{2}}$
Hz (defined as
$\beta $
relaxation) respectively, in which the associated static permittivity and the peak losses are proportional to the byproduct contents for the specimens with different degassing time. Further pulsed electro-acoustic (PEA) measurement shows that
$\beta $
relaxation time coincides with time length for accumulation of hetero-charge near electrodes caused by the ionization of byproducts. And the time-dependency of weight loss for the specimen suggests
$\alpha $
relaxation might be responsible for the dissipation of cross-linking byproduct during degassing. Our work may shed light on a promising non-destructive approach to detecting the level of residual cross-linking byproducts through dielectric relaxation measurement at ELF.