High‐voltage direct current (HVDC) and high‐voltage alternating current (HVAC) cables are the most important equipment for high‐voltage, large‐capacity and long‐distance power transmission. Electrical tree is a pre‐breakdown phenomenon leading to failure of insulation materials, and it is the major issue that threatens the safe and stable operation of HVDC and HVAC cable systems. This study summarises and analyses the achievements in the research of electrical tree for HVDC and HVAC cables. The initiation mechanisms of the electrical tree, including Maxwell electro‐mechanical stress, charge injection–extraction, charge trapping and electroluminescence theories, are elaborated for fully understanding the electrical degradation process in insulation materials. Then, the influences of the high electric field, high temperature and mechanical stress on electrical tree behaviours are discussed, and the relationship between charge transport and the electrical tree is analysed and illustrated. The suppression methods of the electrical tree are put forward by introducing inorganic and organic additives into insulation materials, and the suppression mechanisms are presented from the viewpoints of the structure‐property and microscale–macroscale relationships. Recently, the electrical tree research studies are focused on the high‐precision of initiation models, high‐dependence of multi‐physical fields and high‐efficiency of suppression methods. The achievements provide theoretical support for improving the electrical performance of insulation materials, while it is a practical problem to explore their application feasibility in HVDC and HVAC cable.

The role of trap characteristics in modulating charge transport properties is attracting much attentions in electrical and electronic engineering, which has an important effect on the electrical properties of dielectrics. This paper focuses on the electrical properties of Low-density Polyethylene (LDPE)/graphene nanocomposites (NCs), as well as the corresponding trap level characteristics. The dc conductivity, breakdown strength and space charge behaviors of NCs with the filler content of 0 wt%, 0.005 wt%, 0.01 wt%, 0.1 wt% and 0.5 wt% are studied, and their trap level distributions are characterized by isothermal discharge current (IDC) tests. The experimental results show that the 0.005 wt% LDPE/graphene NCs have a lower dc conductivity, a higher breakdown strength and a much smaller amount of space charge accumulation than the neat LDPE. It is indicated that the graphene addition with a filler content of 0.005 wt% introduces large quantities of deep carrier traps that reduce charge carrier mobility and result in the homocharge accumulation near the electrodes. The deep trap modulated charge carrier transport attributes to reduce the dc conductivity, suppress the injection of space charges into polymer bulks and enhance the breakdown strength, which is of great significance in improving electrical properties of polymer dielectrics.

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.

Electric field distortion caused by the DC conductivity variation and space charge accumulation under temperature gradient in the cross-linked polyethylene (XLPE) insulation is the most serious problem in HVDC cable application. In this paper, three different voltage stabilizers, Bis(4-fluorophenyl)-methanon, Bis(4-hydroxyphenyl) ketone, and Bis[4-(dimethylamino)phenyl] diketone, denoted as A, B, and C, respectively, were mechanically dispersed into the XLPE material with an addition of 0.5 wt%, aiming at improving the DC conductivity characteristics and space charge behaviors of the XLPE insulation at different temperature. The DC conductivity under different temperature and the electric field was tested by a three-electrode system. Meanwhile, space charge properties at 30 °C and 90 °C were investigated using PEA method, and the trap level distribution was estimated by the surface potential decay (SPD) process. The experimental results indicate that all the three voltage stabilizers have improved the DC conductivity characteristics of the XLPE, especially the voltage stabilizers B and C. In addition, the XLPEs modified by the voltage-stabilizers B and C with higher deep trap level demonstrate an excellent ability on space charge suppression at different temperatures. It is concluded that owing to the excellent voltage stabilizing effect, the voltage stabilizers B and C show much potential in HVDC cable application.