Breakdown failure in insulation material is one of the key problems that threaten the safe operation of high‐voltage direct current cable. In this work, the effect of boron nitride nanosheets (BNNSs) concentration, space charge and temperature on DC breakdown strength have been explored. Cross‐linked polyethylene (XLPE)/BNNS nanocomposites were prepared by the melt blending method, and the basic characteristics of nanoparticles and composite were characterised. The experimental results indicate that DC breakdown strength of nanocomposite can be effectively improved when a small amount of BN nanosheet is doped into the matrix. The breakdown strength of the sample reaches the maximum value of 407.52 kV/mm when BNNS content is 0.5 wt%, which is about 33% higher than that of pure XLPE. Further, the effect of space charge on the breakdown of nanocomposites has been studied by pre‐injecting charges. For the samples with different BNNS contents, all the breakdown strength present ascending trend when the polarity of the applied voltage is the same as that of the pre‐injected charges. Besides, it can be found that the breakdown strength of the XLPE/BNNSs composite decreases significantly at 50°C, which is due to more charge accumulation at 50°C. It reaches 2.06 × 10 ⁻⁸ C which increases by about 2.2 times than the room temperature.

The performance parameters and service life of the insulation layer of high-voltage cables are the key to ensure safe cable operation. Through extracting the mechanical parameters of high voltage cable insulation layer under different thermal aging temperatures, a normal linear regression model is established to predict the service life of high voltage cables. Meanwhile, the physicochemical characteristics such as microscopic morphology and molecular structure changes of the cable insulation layer are analyzed, the dielectric properties and breakdown field strength under AC condition are tested and the macroscopic electrical properties of the insulation layer are studied, and the insulation failure mechanism is further analyzed. The experimental results show that the elongation at break retention rate of XLPE increases slowly with the aging time, and then decreases rapidly when it reaches 50% of the initial elongation at break retention rate. The normal linear regression prediction model is established based on the Arrhenius formula, and it can be obtained the life end point of XLPE high voltage cable about 65 years at an operating temperature of 70°C. Further analysis, under the action of heat and oxygen, the crystal zone structure of cable insulation is seriously damaged, and the carbonyl index is significantly increased. Accordingly, when the aging failure point is reached, the dielectric permittivity and dielectric loss of insulation are significantly increased, and the breakdown performance is significantly decreased. The work has an important guiding significance for insulation condition assessment and life prediction of high voltage cable.

To fully understand the distribution network cable accessories’ moisture fault, the variation in insulation parameters of the cable main insulation of cross-linked polyethylene (XLPE) and the cable accessory reinforced insulation of silicone rubber (SIR) under different moisture conditions are studied experimentally. Furthermore, the effects of typical moisture defect forms and moisture positions on electric field distributions are studied by establishing the interface defect model. The experimental results show that the insulation properties of XLPE after water immersion are more seriously reduced than SIR. The resistivity of XLPE and SIR specimens decreases significantly with the increase in moisture time, decreasing by 90.3% and 48.3% after 720 h, respectively. The relative dielectric constants of the two kinds of specimens increase with different degrees after moisture, increasing by 5.16% and 7.83%, respectively, and the breakdown strength decreases by 18.3% and 13.1% after 720 h, respectively. The simulated results show that the moisture form and position have a great influence on electric field distribution. The electric field distortion caused by water trees’ defect is the most serious, followed by water film defects and water droplet defects. Besides, for different types of moisture defects, the position of the maximum distortion electric field is different.

Temperature and electric field are important factors affecting space charge accumulation in high voltage dc (HVDC) cable insulation. Charge conduction and accumulation in XLPE have been studied by correlating the polarization and depolarization processes. The experimental results indicate that the steady current of XLPE increases about 2 or 3 orders with the increasing temperature from 25 °C to 90 °C, and the stronger electric field, the less the increased amplitudes. The trap level in XLPE stressed by different electric fields and temperatures have a tiny change from 0.89 eV to 1.15 eV. Besides, there exists an inflection temperature of charge accumulation in XLPE, around 50 °C∼ 60 °C. At the room temperature, the trapped charges are difficult to release from the traps, and these homo-charges near the electrode can depress the further injection of charges. With the increase of temperature, part of trapped charges near the interface will gradually migrate towards the bulk of the material, and more charges are injected. When the temperature exceeds around 50 °C, the molecular movement is accelerated which can dramatically enhance the hopping probability of charges between the adjacent traps, and little accumulated charges are left.

In engineering applications, the material properties of the cable accessories are affected by electrical, thermal, and mechanical stress. In this work, the comprehensive properties of the insulation material and the semi-conductive material for the cable accessory are studied. Furthermore, the multiphysics field coupling simulation model is established to study the distribution and variation in electric, thermal, and stress fields. The experimental results show that the relative permittivity of silicone rubber (SiR) and cross-linked polyethylene (XLPE) gradually decreases with the increasing temperature, with values of 2.29 and 3.39 at room temperature, respectively. The thermal conductivity of the semi-conductive material is higher than that of SiR and XLPE, with values of 0.39, 0.21, and 0.30 W/( $\text{m}\cdot \text{K}$ ) at room temperature. In contrast, the semi-conductive material has a relatively low thermal expansion coefficient. The elasticity modulus of XLPE at room temperature is approximately 97.62 MPa. Moreover, the mechanical properties of the three materials are relatively good. The simulation results show that the electric field distortion caused by the temperature change is slight. The maximum electric field distortion point locates at the stress cone root, with a value of 2.22 kV/mm. The highest temperature of the cable accessory is approximately 50.31 °C, and the temperature difference between the inside and outside reaches 5.84 °C, which will cause a significant interface stress change. Besides, the internal temperature and temperature difference in the cable accessory vary widely with ambient temperatures and current loadings. The research will be guiding significance for material selection, structural design, and interfacial stress parameter selection of the cable accessories.