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 on the reliability of High Voltage Direct Current (HVDC) cable system. This paper 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 α relaxation) and 10 ^-3 ~10 ^-2 Hz (defined as β 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 measurement (PEA) shows that β 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 α 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 extra-low frequency.