Furfural content dissolved in transformer oil is affected by the furfural diffusion in oil-paper insulation. Previous studies focused on the effects of temperature and moisture on furfural diffusion, while the effect of formic acid on furfural is ignored. Given this, the effect of formic acid on the diffusion properties of furfural is studied in this work through molecular dynamics simulations and macroscopic experiments. First, oil-cellulose-furfural molecular models with different formic acid mass fractions are built, molecular dynamics simulations are performed separately. Also, the furfural diffusion coefficients for different molecular models are determined. Then, insulating paper with different formic acid mass fractions is prepared, and the furfural diffusion coefficients are calculated by combining the measured data and Fick’s second law. Finally, the effect of intermolecular interaction energy on the furfural diffusion in the oil-paper insulation is analyzed. The results revealed that formic acid increased the van der Waals force between furfural and insulating oil, which enhanced the adsorption of furfural and promoted furfural diffusion into the oil. This work explains the effect of formic acid on furfural diffusion, and provides a theoretical basis for furfural analysis.

A self-healing coating with UV-shielding property was prepared in this paper. The self-healing property was based on the inclusion between a host (β-CD-TiO₂) and a guest HEMA-Ad). After inclusion of the host and guest, the host–guest complex (HEMA-Ad/β-CD-TiO₂) was polymerized with other reactive monomers (HEMA and BA) to obtain the final coating. The coating had good hydrophobicity (water contact angle >90°, moisture absorption rate 90%), and could be firmly bonded to a soft substrate. In addition, the coating had good self-healing property, which means that cracks in the material can recover many times after being damaged and that the UV-shielding ability can be fully restored with the self-healing process.

This paper investigated the variation law of methanol concentration in transformer oil caused by operational defects and hotspot temperature from 500 kV field transformers. The correlations between various operational defects and methanol in oil are explained in detail for the first time. It is observed that three types of operational defects such as bushing heating, fan failure and breather failure can lead to the increase of methanol concentration in oil. Furthermore, the associations of annual load factor and hotspot temperature with methanol are investigated. Besides, various hotspot temperatures of nine transformers are calculated by the temperature rise guide. The results show that the methanol concentration in oil at the bottom of the winding can indicate the hotspot temperature of the winding. To verify this view, the correlation between methanol concentration and hotspot temperature is also proved by the implementation of accelerated non-uniform thermal aging experiments. In addition, the methanol concentration is found to be unevenly distributed along the axial height of the winding. This work not only provides a novel direction for the maintenance of transformer components, but also gives a further support for the distribution mechanism of methanol in insulating oil.

Breakdown strength is an important electrical property of insulating paper. Oil-impregnated paper of various aging states was prepared. Its breakdown voltage was then measured at pulsating direct current (DC) voltage with various ripple factors, and alternating current (AC) and DC voltage for comparison, respectively. The AC breakdown voltage is the smallest, and pulsating DC (r = 1/5) breakdown voltage is the greatest before the paper reaches its end of life. A dielectric model was adopted to investigate the difference in magnitude of breakdown voltage at different voltage waveforms. Meanwhile, it was found that breakdown voltage fluctuated and even increased occasionally during the thermal aging process, and a somewhat opposite changing tendency versus aging time was observed for breakdown voltage at DC voltage and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3), compared with AC voltage. The degree of polymerization (DP) and moisture content of the paper were measured, and the characteristics of the pores and cracks of the paper were obtained to investigate the possible influencing factors of breakdown voltage at different aging states. The results showed that the moisture content, oil absorption ability associated with pores and cracks of paper, and the damage to paper structure all contributed to the variation of the breakdown voltage versus aging time, while the importance of their influence differed as the aging state of paper varied.

In order to explore a non-destructive method for measuring the polymerization degree (DP) of paper insulation in transformer, a new method that based on the optical properties of free fiber particles in transformer oil was studied. The chromatic dispersion images of fibers with different aging degree were obtained by polarizing microscope, and the eigenvalues (r, b, and Mahalanobis distance) of the images were extracted by the RGB (red, blue, and green) tricolor analysis method. Then, the correlation between the three eigenvalues and DP of paper insulation were simulated respectively. The results showed that the color of images changed from blue-purple to orange-yellow gradually with the increase of aging degree. For the three eigenvalues, the relationship between Mahalanobis distance and DP had the best goodness of fit (R² = 0.98), higher than that of r (0.94) and b (0.94). The mean square error of the relationship between Mahalanobis distance and DP (52.17) was also significantly lower than that of r and b (97.58, 98.05). Therefore, the DP of unknown paper insulation could be calculated by the simulated relationship of Mahalanobis distance and DP.