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The converter transformer is an important equipment in the bulk power and long-distance power transmission projects, and the oil-pressboard insulation at the rectifying valve side, which mainly deals with combined ac–dc voltage, requires more study of discharge and its influence to ensure equipment safety. In this research, an oil-pressboard insulated experimental platform with needle and plate electrodes, which is similar to the sharp defects of components, such as the corner ring and the gasket of converter transformer, is established, and the photographs of partial discharge (PD) are used to analyze the influence of PD under the above experimental conditions; then, the generation and evolution of various PDs and their by-products can be researched in detail, which can influence the electrical aging process of oil-pressboard insulation. Results indicate that the main by-products brought by PD are bubbles and paper fibers, which can affect the proportion and development of subsequent PDs. These results can provide experimental support to research the internal mechanism of the electrical aging phenomenon of converter transformer, especially at its rectifying valve side.

Power transformer may generate bubbles in the transformer oil due to various factors, such as internal insulation winding vibration and pressure change. Bubbles distort the spatial electric field, which makes the discharge breakdown along the gas channel and accelerates the discharge breakdown of insulating medium. Therefore, in order to prevent the arc discharge induced by microbubbles inside the transformer from increasing the risk of transformer combustion damage, it is necessary to conduct in-depth research on the dynamic law of suspended bubbles before insulation breakdown from the source by combining experiments and simulations. In this article, the bubble dynamic model of a needle plate electrode under the multicoupling physical field of transformer is established in terms of the migration and deformation of suspended bubbles in liquid phase. By comparing the migration trajectories of single suspended bubbles obtained from experiments and simulations, the reliability of the model is verified. The variation of velocity component of voltage amplitude and the change of shape stretching of single suspended bubble in power frequency period are simulated, and the fluctuation law of pressure formed by electrostriction force inside and outside the bubble during migration is analyzed. The capacity absorption process of multibubbles in the strong field region near the needle electrode was simulated. The changes of the surrounding liquid pressure and the spatial electric field distribution during the formation of the gas channel were analyzed, and the internal relationship between the gas channel and the formation of the discharge breakdown was revealed.

To study the change characteristics of space charge in oil-paper insulation under a slightly nonuniform electric field and a uniform electric field, the existing space charge measuring device was improved, and sphere- and column-plate electrode models were prepared. Oil-paper insulation samples at different aging stages were obtained via electrothermal aging. The real-time waveforms of the space charge distribution in the samples at different aging stages were measured in the process of polarization with a direct-current (dc) voltage and depolarization with a dc voltage, and the effects of different electric-field types on the charge distribution and motion characteristics of oil-paper insulation samples at different aging stages were examined. The results indicated that the existence of a slightly nonuniform electric field accelerated the change in the charge-density peak at the interface and increased the fluctuation range of the charge-density curve in the process of applying a dc voltage. With the aggravation of electrothermal aging, the amount of charge injected into the sample increased, and the slightly nonuniform electric field intensified this process, affecting the accumulation and dissipation of space charge in the sample. Electrothermal aging increased the trap distribution density and energy-level depth. However, for an oil-paper insulation sample at the same aging stage, compared with the uniform electric field, the slightly nonuniform electric field had no obvious effect on the trap energy-level depth, but it increased the amount of trapped charge and changed the trap density distribution in the sample. The results of this study provide a theoretical basis for examining the influence of slightly nonuniform electric fields on the space charge distribution characteristics of oil-paper insulation subjected to ac/dc combined electrothermal aging.