Effect of inhomogeneous ageing of insulation on electric field distribution in high voltage cable

Abstract

Polymeric insulation inherits outstanding electrical properties, rendering it highly suitable for high voltage applications. This inherent advantage places polymeric insulation in a superior position compared to paper-impregnated, oil-impregnated insulation and other type of insulations such as higher electrical and thermal stress withstand capability, easier manufacturing process and long life especially at design stress. With advent of high voltage AC cables, polymeric insulations performance in cable is expected to have paramount importance. The performance testing of power cables is aligned to the electric and thermal profiles of the dielectric during manufacturing and in-service conditions. CIGRÉ TB-496 and IEC-62067, provide guidelines for manufacturers as well as utilities for the testing of cables. However, testing standard for HVAC cables at 220kV is still under development stages and limited to laboratories and is at experimental stages. Therefore development of simulation tool not only for design and development but also for condition assessment is having paramount importance. When the electrical stress experienced by a cable increases, it necessitates a corresponding increase in the thickness of the cable insulation. This adjustment, while addressing the heightened electrical stress, introduces a notable challenge that distinguishes it from low voltage cables. The challenge at hand is multifaceted, primarily revolving around two significant aspects: heat dissipation along the radial direction and non-uniform ageing within the cable insulation. Firstly, the increased thickness of cable insulation results in challenges related to heat dissipation. As electrical stress rises, the cable encounters an elevated potential for heat generation. The thicker insulation layer may face difficulties in efficiently dissipating this heat along the radial direction. Efficient heat dissipation is crucial for maintaining the thermal stability of the cable and preventing adverse effects on its performance. Secondly, the non-uniform ageing within the cable insulation is another critical issue associated with heightened electrical stress and increased insulation thickness. The ageing process in insulation materials involves complex interactions influenced by factors such as temperature, electric field distribution, and material properties. In cables with thicker insulation subjected to higher electrical stress, the ageing may occur non-uniformly across the radial profile of the insulation. This non-uniform ageing can have implications for the long-term reliability and performance of the cable. In essence, the interplay between increased electrical stress, thicker insulation, and the resultant challenges in heat dissipation and ageing underscores the need for a comprehensive understanding of cable behavior under such conditions. Addressing these challenges is essential for designing robust high-voltage cables capable of withstanding the rigors of elevated electrical stress while ensuring prolonged and reliable operation. This work delves into the intricacies of inhomogeneous ageing observed in a 220 kV in-service HVAC cable, focusing on a systematic investigation of the XLPE insulation performance along the radial position. To facilitate this study, the XLPE layer is circumferentially peeled using a specialized lathe, and subsequent samples are subjected to material characterization tests. These tests include Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and dielectric spectroscopy. The outcomes of these tests reveal pronounced differences in the morphology of the XLPE layers, manifesting in notable variations in conductivity and permittivity measurements. To capture these local changes in material properties, mathematical expressions are formulated to depict their radial distribution. These expressions are then fitted to the experimental data, enabling a quantitative representation of the variations in permittivity and conductivity along the radial position Utilizing the fitted equations in finite element method (FEM) software to estimate the electric field distribution within the insulation bulk, for the first time, has been reported. The simulation results are not only verified analytically but also compared against the conventional model that assumes constant bulk conductivity and permittivity for DC. Significantly, the findings demonstrate a noteworthy enhancement in the electric field, particularly in the middle section of the cable insulation, challenging the conventional understanding for AC and much aggravated enhancement for DC. This comprehensive approach sheds light on the nuanced dynamics of inhomogeneous ageing, contributing valuable insights to the understanding of cable behaviour under real-world operating conditions.