The Breakdown Voltage of Moistened Mineral Oil During Its Cooling

In high voltage oil-filled equipment, mineral insulating oil has two main functions—to remove heat from heated parts and to insulate conductors. Regardless of the temperature, transformer oil must always provide reliable insulation of high-voltage equipment elements. The primary indicator characterizing the electrical insulating properties of mineral oil is its electrical strength. This work presents the results of the study, which reflect the change in the value of the breakdown voltage of the oil during its cooling from +60 °C to −20 °C. Modeling shows that when the cell is cooled, together with its contents, conditions are realized on the electrodes for the predominant formation of water in the form of drops or ice particles. Such an experiment and simulation shows the processes occurring in the insulation of a power transformer during its forced commissioning and decommissioning at negative temperatures. Experimental data on the nature of changes in the breakdown voltage of a liquid dielectric depending on the cooling method and the degree of oil oxidation are presented. Ice crystals form in the oil as it cools, the shape of which is different. It was found that in the process of oil cooling in the interelectrode gap, an “ice cloud” or “ice bridge” is formed. During thawing, emulsion water is formed from ice inclusions, which are concentrated on the electrodes of the measuring cell. It was experimentally determined that under high voltage action in small gaps of the measuring cell, water droplets are drawn out, forming a “water bridge” between the electrodes.