The interface between nanoparticles and the polymer matrix, which dominates the electrical properties of nanocomposites, can effectively improve the DC breakdown and suppress space charge accumulation in nanocomposites. To research the interface characteristics, XLPE/SiC nanocomposites with concentrations of 1 wt%, 3 wt% and 5 wt% were prepared. The DC breakdown, dielectric properties and space charge behavior were examined using pulsed electro-acoustic (PEA) equipment and a dielectric analyzer. The test results show that the nanocomposites with concentrations of 1 wt% and 3 wt% have higher DC breakdown field strength than neat XLPE. In contrast, there is a lower DC breakdown strength at a concentration of 5 wt%, possibly due to the agglomeration of nanoparticles. Nanoparticle doping increases the real and imaginary permittivities over those of neat XLPE. Furthermore, with increasing concentration, a larger increase in the permittivity amplitude was observed. Based on the space charge behavior, all nanocomposites could suppress space charge accumulation, but the nanocomposite with a concentration of 1 wt% exhibited the best effect. Meanwhile, heterocharge accumulation near electrodes was observed in neat XLPE and the nanocomposite with a concentration of 5 wt%. In contrast, homocharge accumulation near electrodes was observed in the nanocomposite with a concentration of 3 wt%. This phenomenon may be due to different amounts of shallow traps in nanocomposites with different concentrations, which might lead to differing electron or hole mobility.

This paper investigates the morphology, thermal, and electrical properties of LDPE (low-density polyethylene)-based nanocomposites after thermal aging. The FTIR (Fourier transform infrared spectroscopy) spectra results show that thermo-oxidative reactions occur in neat LDPE and LDPE/SiO2 nanocomposites when the aging time is 35 days and in LDPE/MgO nanocomposites when the aging time is 77 days. Specifically, LDPE/MgO nanocomposites delay the appearance of thermo-oxidative reactions, showing anti-thermal aging ability. Furthermore, nanocomposites present lower onset degradation temperature than neat LDPE, showing better thermal stabilization. With regard to the electrical properties, nanocomposites maintain the ability to suppress space charge accumulation after thermal aging. Additionally, in comparison with SiO2 nanocomposites and neat LDPE, the permittivity of LDPE/MgO nanocomposites changes slightly after thermal aging. It is concluded that LDPE/MgO nanocomposites have better insulation properties than neat LDPE after thermal aging, which may be caused by the interface introduced by the nanoparticles.

This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130˝C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the samples were extracted periodically for the measurement of the moisture content, the degree of polymerization (DP) and frequency domain dielectric spectroscopy (FDS). Next, the measurement results of samples were compared to the related research results of transformer oil-paper insulation, offering a theoretical basis of the parameter analysis. The obtained results show that the moisture fluctuation amplitude can reflect the different initial moisture contents of insulating paper and the mass ratio of oil and paper has little impact on the moisture content fluctuation pattern in oil-paper but has a great impact on moisture fluctuation amplitude; reduction of DP presents an accelerating trend with the increase of initial moisture content, and the aging rate of test samples is higher under low moisture content but lower under high moisture content compared to the insulation paper in transformers. Two obvious "deceleration zones" appeared in the dielectric spectrum with the decrease of frequency, and not only does the integral value of dielectric dissipation factor (tan δ) reflect the aging degree, but it reflects the moisture content in solid insulation. These types of research in this paper can be applied to evaluate the condition of humidified insulation and the aging state of solid insulation for condenser bushings.

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This paper focuses on the space charge characteristics in TiO 2 /cross-linked polyethylene (XLPE) nanocomposites; the unmodified and modified by dimethyloctylsilane (MDOS) TiO 2 nanoparticles were added to XLPE matrix with different mass concentrations (1 wt%, 3 wt%, and 5 wt%). The scanning electron microscope (SEM) showed that the MDOS coupling agent could improve the compatibility between TiO 2 nanoparticles and XLPE matrix to some extent and reduce the agglomeration of TiO 2 nanoparticles compared with unmodified TiO 2 nanoparticles; the volume resistivity testing indicated that the volume resistivity of TiO 2 /XLPE nanocomposites was higher than Pure-XLPE and increased with the increase of filling concentrations. According to the pulsed electroacoustic (PEA) measurements, it was concluded that the space charge accumulation was suppressed by filling TiO 2 nanoparticles and the distribution of electric field in samples was improved greatly. In addition, it was found that the injection of homocharge was more obvious in MDOS-TiO 2 /XLPE than that in UN-TiO 2 /XLPE and the homocharge injection decreased with the increase of filling concentration.

The ability of antithermal aging of LDPE/TiO 2 nanocomposites was investigated through SEM, FTIR, DSC, and dielectric properties in this paper. The results of SEM images showed that the thermal aging had a significant influence on the structure of Pure-LDPE and LDPE/TiO 2 samples. The measurement of FTIR showed that the content of hydroxyl and carboxyl increased with thermal aging, but the time of emerging aging characteristic peaks for the LDPE/TiO 2 samples was delayed. The DSC measurement indicated that filling TiO 2 nanoparticles changed the crystallization behavior of LDPE, played a role of heterogeneous nucleation during the process of recrystallization, and improved the crystallinity of LDPE/TiO 2 . Similarly, the aged LDPE/TiO 2 samples had lower permittivity and dissipation factor compared to the aged Pure-LDPE samples. All the results had indicated the LDPE/TiO 2 samples had the significant ability of antithermal aging, especially the LDPE/TiO 2 -0.5 samples with good dispersion of nanoparticles. A new model was proposed to illustrate the antithermal aging behaviors of LDPE/TiO 2 samples, which shows that the TiO 2 nanoparticles play a role of "crosslinking points" between LDPE molecular chains, increasing the density of crystal structure and reducing oxygen diffusion into materials to break molecular structure.

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