Nanocomposites have been developed as the third-generation insulating materials. This article introduces the crosslinked polyethylene (XLPE) nanocomposite dielectric doped with a small amount of nano-hexagonal boron nitride ( ${h}$ -BN), which exhibits excellent insulation properties under high dc voltage. The test results show that XLPE/ ${h}$ -BN nanocomposites possess significant improvements in dc breakdown strength and 0.1 wt% nano ${h}$ -BN doping can actually increase the dc breakdown strength by 39%. The space charge properties of the XLPE/ ${h}$ -BN obtained by the pulsed electro-acoustic (PEA) measurements and the direct current integrated charge [DCIC- ${Q}({t})$ ] measurements show that the positive homocharge injection under dc voltage is obviously suppressed, and the electrical conductivity under relatively high electric field decreases. The thermally stimulated current results show that the nano ${h}$ -BN doping introduces only deep charge traps (above 1 eV) to the XLPE matrix. The trap energy is deeper than that of the original traps, and this is the main factor leading to the improvement of the dc insulation properties. Besides, the doping of ${h}$ -BN does not introduce impurities or change the crystallinity of XLPE. This article presents the novel XLPE/ ${h}$ -BN nanocomposites with excellent dc insulation properties and indicates the large potential of the nano ${h}$ -BN-doped insulation materials.

Thermal ageing of cross-linked polyethylene (XLPE) cable insulation is an important issue threatening the safe operation of power cables. In this paper, thermal ageing of XLPE was carried out at 160 °C in air for 240 h. The influence of oxygen diffusion on thermal ageing of XLPE was investigated by Ultraviolet–visible spectrophotometer (UV–Vis), tensile testing, and Fourier transformed infrared spectroscopy (FTIR). It was observed that the degradation degree not only depended on ageing time but also on sample positions. The thermally aged samples were more oxidized in the surface region, presented a darker color, more carbon atoms appeared in the conjugate cluster, had smaller elongation at break and tensile strength, and a larger carbonyl index. As ageing time increased, the non-uniform oxidation of the XLPE samples became more prominent. The degree of non-uniform oxidation caused by oxygen diffusion was quantitatively studied by first order oxidation kinetic. The calculated results demonstrated that carbonyl index measured by FTIR was more sensitive to non-uniform oxidation with a shape parameter in the range of 1–2. The result shown in this paper is helpful for interpreting and predicting the non-uniform ageing behavior of high voltage XLPE cables.

Epoxy resin-based nanocomposites have been widely researched for being potential insulating materials in high voltage power equipment. In this paper, nano-TiO₂ particles were chosen and surface-modified by a silane coupling agent containing an epoxy group. The effect of functionalized nano-TiO₂ doping on the physical properties of epoxy resin was studied. The results of differential scanning calorimetry show that T_g increased significantly and can be increased by up to 35 °C. Therefore, it is believed that the suppression of molecular motion by the addition of nanofillers works effectively in the case of this functionalized nano-TiO₂ and a strong interaction between the epoxy resin and the nano-TiO₂ was formed after surface modification. Consequently, dynamic mechanical properties, thermal conductivity, electrical conductivity, and trap characteristics of epoxy resin are all adjusted after introducing functionalized nano-TiO₂. All of these physical properties were analyzed from the perspective of suppression of molecular motion, and it is of significance to establish the theory of a nanocomposite dielectric. Besides, the results show that the epoxy/TiO₂ nanocomposite is expected to be applied in the insulation system of electrical equipment.

Micro/nano- BN co-doped epoxy composites were prepared and their thermal conductivity, breakdown strength at power frequency and voltage endurance time under high frequency bipolar square wave voltage were investigated. The thermal conductivity and breakdown performance were enhanced simultaneously in the composite with a loading concentration of 20 wt% BN at a micro/nano proportion of 95/5. The breakdown strength of 132 kV/mm at power frequency, the thermal conductivity of 0.81 W·m⁻¹·K⁻¹ and voltage endurance time of 166 s were obtained in the composites, which were approximately 28%, 286% and 349% higher than that of pristine epoxy resin. It is proposed that thermal conductive pathways are mainly constructed by micro-BN, leading to improved thermal conductivity and voltage endurance time. A model was introduced to illustrate the enhancement of the breakdown strength. The epoxy composites with high thermal conductivity and excellent breakdown performance could be feasible for insulating materials in high-frequency devices.

To further study the thermal‐oxidative ageing mechanism of crosslinked polyethylene (XLPE), the terahertz frequency domain dielectric parameters and relaxation phenomena of thermal‐oxidative aged XLPE are analysed using terahertz time domain spectroscopy. It is found that the real part of the terahertz dielectric constant increases with the increase of thermal‐oxidative ageing duration, and the imaginary part shows an overall upward trend. Relaxation of the complex dielectric constant of the aged XLPE samples is obvious. Through the second‐order Cole–Cole equation fitting, it is found that there are two relaxation processes around 0.15 and 1.5 THz, and the relaxation strength gradually increases with the ageing time. To explain this phenomena, microstructure changes and crystallisation behaviours are characterised using Fourier transform infrared spectrum and differential scanning calorimetry, respectively. It is suggested that the two relaxations may be caused by elastic dipole polarisation originating from the angular deflection of polar groups and stretching rotation of hydrogen bond. In addition, by comparing the relaxation strength in different temperatures, the source of the relaxation process is further confirmed.

Insulation fails quickly under high-frequency AC high voltage, especially bipolar square-wave voltage with a high dV/dt. It is of great significance to study the failure mechanism of epoxy casting insulation under such kind of voltage. In this paper, pin-plane epoxy casting insulation samples with air gaps were prepared, and the relation between the electrical trees under the high frequency bipolar square-wave voltage and the air gap conditions and voltage frequencies (1~20 kHz) were studied. Results indicated that, with the presence of air gaps, the electrical trees were bush-type and had a relatively slow growth rate, which was different from the fast-growing branch-type trees in the samples without air gap. The electrical tree characteristics related with the size of air gap and voltage frequency were also studied. The electrical tree grew faster under higher voltage frequency or with a smaller air gap. Results proved that discharge introduced a lot of defects for the surface layer of the epoxy resin samples and hence induced the possibility of multi-directional expansion of electrical trees. In addition, the resulting heat accumulation and unique charge transport synergistically affected the electrical tree characteristics under the high frequency bipolar square-wave voltage.