This study explores the influence of hydrostatic pressure on the discharge along the oil-paper interface under AC voltage, especially for the normal operating condition and breakdown. In this paper, an experimental platform was set up to record the partial discharge (PD) parameters of the test sample under different hydrostatic pressures, while the applied AC voltage was increased to final flashover voltage step by step. Experimental results showed that higher hydrostatic pressure had different effects on PD under different voltages. Higher pressure decreased the PD energy and increased the flashover voltage. Furthermore, under higher hydrostatic pressure, discharge traces (white mark) were found on the surface of the samples after intense discharging on the oil-paper interfaces, indicating that the hydrostatic pressure can affect the gas generation and dissipation process underneath the surface of the pressboards. Finally, the mechanism of how hydrostatic pressure influences the PD, flashover voltage, and white mark was interpreted based on the bubble theory. The results derived in this paper can be helpful for an optimal design and reasonable operation of oil-paper insulation systems, especially for power transformers.

The pressboard surface is the electric weak link of the oil-paper insulation in transformers, and long-term partial discharge (PD) erosion is the dominant cause of degradation in pressboard. To explore the development processes of surface tracking under the effect of tip curvature, the typical needle-plate model was selected to initiate an electric field with a high tangential component on pressboard surface under needle tip curvature of 4~42 μm. With the help of a high-speed camera and a PD detecting system, the development processes of surface tracking and PD were recorded under a sustained AC voltage. A profound difference between surface tracking under different curvatures was discussed. Pressboard surfaces after tests were observed under a scanning electron microscope (SEM), and the damage degree of cellulose fibers was dependent on the tip curvature.

Experimental investigations were raised to research the tracking process on mineral-oil-impregnated pressboard surface under ac voltages. This article addressed the evolution of discharge characteristics and severe stages of pressboard surface tracking by correlating partial discharge patterns from MPD600 (commercial partial discharge measurement) and images from high-speed camera. A needle-plane model was used to initiate divergent electric field and studied propagation of discharges along the solid-liquid interface. This paper compared the discharge characteristics of needle-plane model with or without pressboard under step-up sustained ac stress. It is found that the introduction of impregnated pressboard promotes partial discharges by increasing energy dissipated on or inside of the oil-pressboard interface, which is very lacking in previous references. The discharge parameters tendency and the accompanying various stages of surface tracking were investigated. A phenomenon that the stable spark discharge from needle tip extended to 30mm was called electric branch in this paper. An artificially manufactured defective pressboard was made to confirm the conjecture that the carbonized area could consume a lot of energy along the white mark. The microscopic appearance of the pressboard surface before and after the experiment was observed under SEM (Scanning Electron Microscope) and found that impregnated pressboard was severely damaged under constant ac voltage.