Low-Temperature Negative Lightning Impulse Breakdown in Ester and Mineral Oils: Viscosity–Voltage Correlation and Streamer Branching

To assess the insulation withstand characteristics of insulating fluids in cold environments, this study examines the negative lightning impulse characteristics of 45# mineral oil and Midel 7131 synthetic ester within a temperature range of 10 °C to −30 °C. Experimental results show that the breakdown voltage of both oils increases as the temperature decreases, with the synthetic ester exhibiting greater sensitivity to low-temperatures. A significant linear correlation is observed between the breakdown voltage and the logarithm of kinematic viscosity. A specialized observation system is developed to capture the dynamic streamer evolution in insulating oils at low-temperatures. Streamer images at different temperatures reveal that low-temperatures significantly suppress the initial streamer development and reduce the overall propagation speed. To further analyze streamer morphology, a novel image processing method combining CLAHE (Contrast-Limited Adaptive Histogram Equalization) enhancement and dynamic threshold extraction is proposed, yielding pseudo-color images that retain both grayscale and structural information. Quantitative analysis of streamer grayscale distribution at 0 °C and −30 °C indicates that low-temperatures increase the branching density and overall brightness of the streamer channel. These findings provide valuable insights for the selection of insulating oils and the design of high-voltage equipment in cold and high-altitude regions.