Degradation of Electrical Performance in Transformer Oil Under Low-Temperature Moist Conditions and Its Microscopic Mechanisms

Low-temperature environments pose challenges to transformer insulation reliability, as altered solubility and physical states of moisture in oil may trigger degradation mechanisms absent at ambient temperatures, yet the physicochemical processes remain poorly understood. This study combines macroscopic measurements with molecular dynamics (MD) simulations to investigate the bulk conductivity and alternating current (AC) breakdown characteristics of moist transformer oil across a wide temperature range. Conductivity and breakdown voltage were measured under controlled cooling conditions, while MD simulations tracked water cluster evolution and interfacial behavior at the molecular level. The results reveal a non-monotonic temperature dependence: conductivity exhibits a single-peak maximum near −5 °C, whereas breakdown strength reaches a pronounced minimum around −10 °C, with an offset between the two phenomena. MD simulations demonstrate that, as temperature decreases, water molecules transition from a dispersed state to nucleated aggregates and eventually form stable large-scale clusters. The low-temperature-induced nucleation and aggregation of moisture, which are much less pronounced under normal-temperature conditions, increase the oil–water interfacial area and enhance interfacial polarization, contributing to the anomalous increase in conductivity. At lower temperatures, stable water clusters act as dielectric “soft defects”, inducing local electric field distortion and thereby reducing the breakdown strength. The results establish a link between temperature-dependent moisture aggregation and macroscopic electrical degradation, providing a microscopic explanation for the anomalous temperature-dependent insulation behavior. These findings help to optimize insulation design and develop condition-assessment strategies for transformers in frigid regions.