This paper focuses on the space charge and electric field characteristics in the extrusion molded joint (EMJ) for ±500 kV HVDC submarine cables. The double-layered XLPE samples are prepared to imitate the insulation interface in the EMJ, and the space charge characteristics are assessed utilizing the pulsed electro-acoustic (PEA) method. Then, a two-dimensional simulation model for space charge based on a modified BCT model containing the ionic charges is established. The results demonstrate that the simulation outcomes based on the modified BCT model exhibit consistency with the experimental observations by measuring the mobilities and trap characteristics of electrons and holes. The results of full-sized EMJ simulation present that the negative space charges mainly accumulate near the interface, resulting in the distortion of the field in the cable insulation. Meanwhile, the interface charges diminish the electric field value at the interface but intensify the angle and tangential component of the interface field. The electric field distortion is mitigated when the crosslinking temperature for the reinforcing insulation is adjusted to 180 °C. The simulation method of space charge and electric field in the EMJ insulation based on the modified BCT model can provide research means for the design and manufacture of the EMJ for HVDC submarine cables.

This article focuses on the effect of the polarity reversal on the space charge behavior and electric field evolution of ±500-kV HVDC cables under different temperature gradients. A modified bipolar electronic-ionic charge transport (BEICT) model of XLPE insulation considering the dissociation of impurities is established for full-sized cables. The results demonstrate that the increase in temperature gradient accelerates the injection and migration of homocharge in the vicinity of the inner semi-conductive layer under dc voltage, which leads to a higher electric field immediately after polarity reversal occurring on the interior side of the insulation. As the temperature gradient increases from 0 °C to 30 °C, the rate of charge variation increases more than tenfold, and the maximum electric field value increases by 15.8% during polarity reversal. Prolonging the polarity reversal period (PRP) results in the accumulation of negative space charge on the inner side of the insulation during polarity reversal, leading to a weakening of the maximum electric field. When the PRP increases from 300 ms to 120 s, the maximum electric field decreases by 9.8%. Moreover, a relaxation period of 600 s when the voltage drops to 0 kV will further reduce the maximum electric field by 16.1%. It is concluded that electron injection and carrier recombination dominate the charge behaviors on the interior side of the insulation during the polarity reversal, which are the primary factors affecting the electric field distortion.

This article presents the recent developments in the field of evaluation of the breakdown performance and remaining lifetime of XLPE insulation and analyzes the accuracy of existing lifetime prediction models through experiments. The effects of the crystalline morphology, cable thickness and sampling location of XLPE insulation on the evaluation of short-term breakdown performance are reviewed in the context of the experiments. The study reviews the application of the Ramu, Simoni, and Ramu multi-stress lifetime prediction models and explores the other remaining lifetime prediction models under the combined electrothermal stresses which are applicable to XLPE insulation. Finally, this paper recommends the most effective engineering evaluation methods and provides suggestions for improving the electrical performance of XLPE insulation for high-voltage cables.

This paper focuses on temperature-gradient-dependent space charge and electric field evolutions of ± 500 kV HVDC extruded cables with different insulation thicknesses. The modified bipolar electronic-ionic charge transport (BEICT) model for full-size cables, which considers both space charges generated by interface injection and ion dissociation, is established based on the finite element analysis (FEA) method. The results show that when a voltage of +500 kV is applied on the conductor, with the increase of temperature gradient (ΔT), the injection of homocharge near the inner semi-conductive layer accelerates, while the heteropolar ionic charge dominates on the low-temperature side. When ΔT reaches 30 °C, the Emax of the electric field is increased by about 53 % compared with 27.85 kV/mm when ΔT = 10 °C. And the Emax is located near the outer semi-conductive layer whether under the positive and negative polarities, which are 42.69 and 64.78 kV/mm respectively. The significant polarity effect is owing to the differences between the injection and migration parameters of positive and negative carriers. Furthermore, the increase in thickness will raise the ΔT within the insulation, aggravating the electric field uneven distribution. It is concluded an appropriate reduction of insulation thickness can effectively mitigate the electric field distortion problem caused by ΔT in ± 500 kV HVDC cables.