Extra-low frequency dielectric relaxation: Characterization of crosslinking byproducts in XLPE HVDC cable insulation

Brochure 852 (2021) specifies destructive quantification methods for residual byproducts using TGA and chromatography, requiring deviation from type test samples to remain within 25%. However, these methods are destructive and highly dependent on sampling location, highlighting the need for a reliable, non-destructive evaluation approach.

In this work, we propose a non-destructive method based on extra low frequency (ELF) dielectric spectroscopy to assess crosslinking byproduct content in XLPE-insulated HVDC cables. By measuring charging/discharging currents in the frequency range of 10⁻⁴–10⁻² Hz, followed by filtering, exponential fitting, and Fourier transformation, two relaxation processes—α (10⁻⁴–10⁻³ Hz) and β (10⁻³–10⁻² Hz)—were observed. The relative permittivity and peak magnitude of both processes correlate strongly with byproduct content. The method was also validated on a full-size cable.

In summary, the proposed non-destructive method utilizes characteristic parameters and their variation trends in the ELF dielectric spectrum to evaluate residual byproduct levels, showing strong potential for assessing the degassing process in EHVDC cable systems.