Bridging Characteristics of Moist Cellulosic Particles in Flowing Transformer Oil

The presence of cellulose impurities in transformer oil can compromise the insulation performance of transformers. Cellulose impurities, characterized by molecular structures containing significant amounts of hemicellulose and hydroxyl groups, tend to adsorb considerable moisture. The bridging behavior of particles in transformer oil is influenced by oil flow. Presently, the bridging characteristics of moisture-laden cellulose particles in flowing transformer oil are not well-defined. In this study, utilizing the established transformer oil circulation device and oil observation system for particles, we conducted research on the bridging characteristics of particles with varying moisture content and different electric field intensities in flowing transformer oil. The features of small bridges in flowing oil were extracted, and the influence of cellulose moisture content on the bridging in oil was elucidated from a force perspective. The results indicate that as cellulose moisture content increases, the electrical conductivity of particles rises, resulting in an increased dielectrophoretic force. Consequently, higher cellulose moisture content makes particles more likely to converge towards regions of higher electric field intensity, specifically the center of the electrodes. The center width of bridges increases, and the leakage current flowing through the electrode gap rises due to elevated cellulose moisture content.

I. Introduction

Large power transformers are crucial equipment in highvoltage transmission projects, and their safe and stable operation is of great significance to the entire transmission system. Transformers primarily utilize oil-paper composite insulation, and the performance of insulating oil directly affects the safety and reliability of the transformer[1–3]. According to CIGRE, one of the main causes of insulation faults in transformers with voltages of 400kV and above is the contamination of transformer oil by impurity particles [4]. Numerous studies indicate that due to material processing, mechanical vibrations, oil pump wear, and electrical-thermal aging, transformer oil inevitably gets contaminated by particles such as metals, fibers, and carbon, with over 90% being cellulose impurities [5–6]. Furthermore, cellulose impurities in oil, due to the porous structure of cellulose materials containing a significant amount of hemicellulose and hydroxyl groups, tend to absorb substantial amounts of moisture. Moisture-laden cellulose impurities, under the influence of an electric field, migrate, accumulate, and tend to form impurity bridges, posing a high risk of local discharge, insulation breakdown, and potentially causing severe insulation accidents [7–9].