The Influence of Pressure on the Discharge along Oil-Paper Interface under AC Stress

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.

Mixing-demixing phase diagram for simple liquids in nonuniform electric fields

We deduce the mixing-demixing phase diagram for binary liquid mixtures in an electric field for various electrode geometries and arbitrary constitutive relation for the dielectric constant. By focusing on the behavior of the liquid-liquid interface, we produce simple analytic expressions for the dependence of the interface location on experimental parameters. We also show that the phase diagram contains regions where liquid separation cannot occur under any applied field. The analytic expression for the boundary "electrostatic binodal" line reveals that the regions' size and shape depend strongly on the dielectric relation between the liquids. Moreover, we predict the existence of an "electrostatic spinodal" line that identifies conditions where the liquids are in a metastable state. We finally construct the phase diagram for closed systems by mapping solutions onto those of an open system via an effective liquid composition. For closed systems at a fixed temperature and mixture composition, liquid separation occurs in a finite "window" of surface potential (or charge density). Higher potentials or charge densities counterintuitively destroy the interface, leading to liquid mixing. These results give valuable guides for experiments by providing easily testable predictions for how liquids behave in nonuniform electric fields.

Experimental investigation of transient pressure waves produced in dielectric liquids

The experimental results reported in this paper mainly concern the two first transient pressure waves emitted after a fast (approximately ns) and localized (approximately micron) injection of electrical energy Wi in insulating liquids. The influence of various parameters (hydrostatic pressure P infinity, injected energy Wi, etc.) on the relative amplitude of these two transient pressure waves are presented and discussed. The analysis shows clearly a difference of behavior, in particular their dependence as a function of hydrostatic pressure, between these two waves. Basically, the physical processes leading to their emission are quite different. The first pressure transient is a consequence of the plasma relaxation, whereas the second one is due to the bubble collapse. According to these results, the chronology of the events is more improved, as well as the theoretical analysis.