Partial discharge (PD) can occur in cross-linked polyethylene (XLPE) insulation within high-voltage direct current (HVDC) cables with defects and the continuous development of PD will lead to the final breakdown of the cable. To study the PD behavior caused by defects in HVDC cable and provide effective guidance for the operation and maintenance of the cable, three types of defects were prepared in this paper, namely micro-void, protrusion, and interface fouling. For these three typical defects, experimental research on PD under stepped voltage boosting was conducted using the pulse current method at 30 ℃, 50 ℃, and 70 ℃, respectively. The research results indicate that the PD inception voltage (PDIV) of interface fouling is higher than that of micro-void and protrusion, while the discharge repetition rate is lower. The PD experimental results of the three types of defects all indicate that, when the defects do not develop significantly, the increase in temperature will reduce PDIV and increase their discharge frequency. The charge transfer model is employed to analyze the impact mechanisms of high temperature and electric field on the PD process, providing a rational explanation of the PD behavior during experiments. In addition, a discharge frequency evaluation method based on the equivalent circuit model of defects is proposed, which may assist in assessing defects’ destructiveness. The experimental findings and evaluation methods obtained in this study can serve as a reference for assessing the defect development process in DC cables under various operating conditions.

High-performance cross-linked polyethylene (XLPE) is currently employed in ultra-high-voltage direct current (UHVDC) cables, with the electrical tree being an important cause of DC cable breakdown. The comparison of XLPE samples under different manufacturing processes can provide a reference for the progress of cable production processes. This paper compares laboratory-prepared XLPE samples (DC-XLPE) with XLPE samples extracted from actual cables (Cable-XLPE) through electrical tree experiments, X-ray diffraction (XRD), and gel permeation chromatography (GPC). The experimental findings indicate that the breakdown time of DC-XLPE increased by nearly 50% compared to Cable-XLPE, with slower electrical tree growth and lower average discharge magnitude observed. Overall, DC-XLPE exhibited superior resistance to DC electrical tree and partial discharge. XRD and GPC analyses revealed minimal differences in crystallinity and grain size between the two types, with the primary distinction being DC-XLPE’s notably higher molecular weight and more concentrated molecular weight distribution. The differences in physicochemical properties may be attributed to more precise and uniform temperature control during the crosslinking process in laboratory settings, as well as a higher removal rate of crosslinking byproducts, ultimately leading to enhanced resistance to electrical tree and partial discharge in DC-XLPE.

This paper investigates the evolution of insulation performance in HVDC cross-linked polyethylene (XLPE) cables located in lightning-prone areas under frequent reclosing conditions. By simulating the reclosing process using a cyclic short-circuit grounding device, the aging mechanism of the XLPE under repeated short-circuit processes was characterized through four testing techniques. The experimental results demonstrate that the gradual temperature rise enhances molecular thermal motion and accelerates the charge carrier migration process. Simultaneously, this provides the necessary energy for microstructural degradation, leading to a reduction in crystallinity. Specifically, when the electric field increases from the working electric field (WEF) (18kV/mm) to the threshold electric field (TEF) (50kV/mm), the average shape parameter decreases by 33.79%. Physicochemical testing results reveal that the influence of the electric field on the main chain structure of polymer molecules at 30 or 50°C is more pronounced. Notably, crystallinity exhibits a systematic decrease at the TEF. Compared to 30°C, the crystallinity of XLPE decreases by 26.97% at 70°C. Finally, this paper examines the correlation analysis of the electric field and temperature on space charge accumulation. Moreover, by examining microstructural changes, the relationship between microscopic defects and localized electric field distortion is elucidated, advancing the understanding of XLPE aging mechanisms under frequent short-circuit conditions and providing critical insights into XLPE insulation degradation in complex aging scenarios.