Nanocomposite dielectric is considered the most promising insulation material for high‐voltage direct current cables. Herein, boron nitride nanoparticles (BNNPs) and boron nitride nanosheets (BNNSs) are used as nanofillers to study the effect of nanofiller concentration and morphology on the electrical properties of cross‐linked polyethylene (XLPE) composite based on experiments and simulation. The experimental results indicate that nanofiller concentration has a great influence on permittivity, while the influence of the morphology at the same concentration is small. The maximum relative permittivity occurs at 1 wt%. They are 2.35 and 2.32, respectively, for XLPE/BNNSs and XLPE/BNNP, increasing by about 11% and 9% than that of XLPE matrix. The morphology of BN significantly influence the breakdown performance, and the breakdown strength of the nanocomposite increases first and then decreases with the increase in doping concentration. The maximum breakdown strength of the composite occurs at 0.5 wt%. They are 403.8 and 349.2 kV mm−1, respectively, for XLPE/BNNSs and XLPE/BNNP, which is about 33% and 13% higher than that of XLPE matrix. Molecular simulations show that the free volume of XLPE/BNNSs is lower than that of XLPE/BNNP. In addition, the sheet structure can effectively block the carrier transport and prolong the discharge path.

Space charge accumulation in polypropylene (PP) affect the dielectric properties and breakdown strength of the material. The pre-injected charge in PP under the action of different polarity voltage is quantitatively characterized, and the effects of the pre-injected charge inside the dielectric on the dielectric properties and breakdown strength are measured and analyzed. Based on the molecular simulations, the influence mechanism of the temperature on dielectric properties and breakdown are discussed. The experimental results show that the injected charges in PP under the negative polarity voltage is significantly larger than that of the positive polarity. These charges have a great influence on the dielectric constant and breakdown performance of PP, and the effect is different for different charge polarity. The effect of negative polarity pre-voltage conditions on the dielectric constant is much greater than that of positive polarity, and the dielectric constant of PP decreases from 2.2 to 1.3, decreasing about 41% under the negative polarity pre-voltage. By contrast, the dielectric constant slightly increases under the effect of the homopolar preload. Furthermore, the breakdown strength of the dielectric after the heteropolar preload is 249 kV/mm, which is 36% lower than that of PP without pre-voltage, and it slightly increases after the positive polarity pre-voltage. As the temperature increases, the increase in free volume favors the development of electron collision ionization and electron collapse processes, leading to a decrease in breakdown voltage at high temperatures. This work has a good guiding significance for the comprehensive evaluation of energy storage parameters.

Oil-impregnated insulation paper is an important part of transformers; its performance seriously affects the life of power equipment. It is of significance to study the aging characteristics and mechanism of oil-impregnated insulation paper under thermal stress for transformer status detection and evaluation. In the work, the accelerated thermal aging was carried out at 120 °C, and DP1490, DP787, and DP311 samples were selected to represent the new, mid-aging, and late-aging status of the transformer, respectively. The space charge distribution within the specimens was measured by the pulsed electro-acoustic (PEA) method and the trap parameters were extracted based on the measurement curves. Further, the aging mechanism was studied by molecular simulation technology. A typical molecular chain defect model was constructed to study the motion of cellulose molecules under thermal stress. The experimental results show that the corresponding trap energy levels are 0.54 eV, 0.73 eV, and 0.92 eV for the new specimen, the mid-aging specimen, and the late aging specimen, respectively. The simulation results show that the trapped energy at the beginning of aging is mainly determined by the loss of H atoms. The changes in trap energy in the middle stage of aging are mainly caused by the absence of some C atoms, and the trap energy level at the end of aging is mainly caused by the breakage of chemical bonds. This study is of great significance to reveal the aging mechanism of oil-impregnated insulation paper and the modification of insulation paper.

Space charge accumulation in polypropylene will accelerate aging of the material and lead to the degradation of its insulation performance. Here, space charge distribution, current conduction characteristics, thermally stimulated depolarization current (TSDC), and surface potential decay (SPD) characteristics of polypropylene (PP) under a strong electric field were measured and analyzed, and the bulk trap and surface trap parameters were extracted. Further, the charge dynamic transport physical processes and characteristics under strong electric field were studied. Experimental results show that the charge accumulation amount in PP under the action of a negative polarity electric field is higher than that of a positive polarity electric field, by about one order of magnitude. There are two obvious charge density peaks on the PP surface, which are 2.60 and 3.66×1020·eV -1 ·m -3 V. The simulation results show that with the extension of the applied voltage time, the injected charges by the electrode gradually migrate to the bulk of the material and eventually the positive and negative charges are offset at the middle position. The local electric field caused by the accumulation of interfacial charges will weaken the original electric field, resulting in the distortion of the internal electric field.

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