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To investigate the effect of aging on surface discharge of the pressboard on the shielding electrode at the end of bushing, a thermal aging preparation methods of different aging degrees of pressboard samples, surface defect model, and the associated hemisphere are designed and the step‐up method is used for subsequent experimental work. Finally, the partial discharges (PDs) under different degrees of aging are compared and combined with Fourier Transform Infrared spectra and electric field simulation. Results show that the pre‐discharge occurs mainly in the oil gap between the hemispherical electrode edge and the pressboard. The aging of the pressboard exerts no obvious effect on the discharge characteristics in the initial stage of PD; with the development of the discharge, the development of aging of pressboard leads to a higher electric field intensity acting over a greater area, the more intense the discharge, the faster the relative discharge increases and the larger its amplitude. This discharge will cause decomposition of the oil in the pores of the aging pressboard thus generating gas bubbles which will move as discharge develops. When the bubbles reach the surface of the pressboard, the local electric field intensity will increase and the discharge will become more intense. [ABSTRACT FROM AUTHOR]

The water concentration in transformer paper insulation must be monitored because, if it becomes too high, the paper insulation will age rapidly [1]-[4] and the transformer may fail on overload [5]-[9]. A failure can occur on overload if the temperature of the paper rises sufficiently to cause evolution of water bubbles. If these bubbles cause dielectric failure of the oil gap, the transformer may fail catastrophically. Paper insulation becomes brittle as it ages, and eventually becomes too brittle to act as an effective insulator. Oxygen increases the aging rate because the acids created by chemical reactions occurring in the oil, paper, and oxygen react further with water [10]-[12]. A study of the thermal aging of Kraft paper found that wet oxygenated paper degrades 40 times faster than the same paper heated in a dry oxygen-free environment [4]. The paper within a transformer is likely to degrade fastest at the top of the winding, where it is hottest [13]. Since the water concentration in the paper significantly influences its aging, it is important that the water concentration be measured accurately at the top of the winding. [ABSTRACT FROM PUBLISHER]

The article presents a study which investigated the mechanism of foaming with a combination of high-speed photography, scanning electron microscopy, and infrared thermography using foamed paperboard of different compositions prepared both in the laboratory and on a commercial machine. The study addresses the fundamental mechanism of foaming and identify the characteristics of the board that lead to a product with evenly distributed bubbles of a uniform size. The study attempts to understand the fundamental mechanism of foaming on paper-polymer composites.