Breakdown Characteristics of Natural Ester‐Mineral Oil Blended Insulating Oils Under Cryogenic Conditions: Influence of Mixing Ratios and Moisture Migration Mechanisms

ABSTRACT This study centers on a 45# mineral oil and FR3 natural ester transformer oil blend. Under extremely cold conditions, it explores how varying moisture content impacts the breakdown characteristics of this mixture and examines the internal factors related to breakdown voltage. Moisture absorption experiments, breakdown tests, and computational simulations of moisture migration trajectories within oil matrices were conducted, with data analysis for temperatures below 0°C using the Weibull distribution. Results indicate that at a fixed oil mixture ratio, reducing moisture content greatly enhances the insulation oil's insulation performance. Around –30°C, increasing the natural ester proportion in the mix effectively raises the breakdown voltage. Under low—temperature conditions (0°C and below), when moisture content is low, increasing the natural ester share most markedly improves the insulation ability. However, as moisture content rises, this enhancement effect diminishes. Through simulation, the movement of moisture droplets in the transformer oil was analysed. It was found that they migrate to high‐electric‐field‐strength areas and gather, stretch, and connect the two poles. Further research shows that the movement speed of free water in mineral oil is about two orders of magnitude faster than in natural ester, and this speed difference grows with increasing voltage.