Effects of combined electric field configuration and moisture under cryogenic conditions on breakdown characteristics of transformer oil blends

• Breakdown voltage of blended oil shows U-shaped dependence on temperature. • Mixed oil breakdown voltage's sensitivity to electric field types decreases with increasing water content. • The breakdown voltage of mixed oil is higher than that of mineral oil. • Water bridges take 4.5 times longer to form in mixed oil than mineral oil. With increasing performance requirements for transformer insulating oils in power systems, particularly in extreme cold regions, the application of oil blends as both insulating and cooling media has garnered significant attention. This study investigates the breakdown characteristics of oil blends under low-temperature conditions, focusing on their application in cold regions. A blended oil sample was prepared by mixing mineral oil (A common naphthenic mineral insulating oil) and FR3 vegetable oil (A typical natural ester insulating oil) at an 8:2 mass ratio. The breakdown voltage variations were systematically studied under controlled moisture content and temperature conditions, considering different electric field configurations. The results reveal that the breakdown voltage of the oil blend exhibits a U-shaped trend, reaching its minimum between –10°C and 0°C. Above 0°C, with the moisture content increasing from 40 mg/L to 175 mg/L, the influence of temperature on the breakdown voltage diminishes significantly, and the amplitude of its variation decreases by approximately 50%. The breakdown voltage is simultaneously influenced by both moisture content and electric field configuration. Under a uniform electric field, the effect of moisture content is particularly pronounced, with the breakdown voltages for Samples A, B, and C being 46 kV, 38 kV, and 32 kV, respectively. In contrast, under a non-uniform electric field, the corresponding breakdown voltages are 32 kV, 29 kV, and 28 kV, representing a maximum reduction in the difference of up to 71.4%. Weibull statistical analysis indicates that increased moisture content reduces the influence of electric field configuration on breakdown voltage, with a more significant reduction observed in non-uniform fields. Furthermore, the impact of temperature on breakdown voltage becomes more pronounced with higher moisture content. Simulation studies elucidated the motion trajectories of water droplets in the oil and their deformation processes. It was found that the aggregation rate of water droplets in mineral oil is approximately 68 times faster than that in vegetable oil, while the time required to form a conductive "water bridge" in blended oil is 4.5 times longer than that in mineral oil. These findings validate the reliability of the experimental results. This research provides critical reference data for the application of oil blends in extremely cold regions, offering insights for optimizing their formulation and performance to enhance the safety and stability of power systems.