DC Breakdown Characteristics of Transformer Oil With Different Cellulose Impurity Concentrations

Cellulose impurities in transformer oil pose a significant threat to the reliability of converter transformers, with their concentration increasing during long-term operation, further compromising insulation performance. Therefore, it is crucial to investigate how the concentration of cellulose impurities affects the dc breakdown characteristics of transformer oil. For this purpose, the study constructed an experimental platform to explore the breakdown characteristics of transformer oil containing cellulose impurities under dc voltage. The results indicate that the breakdown voltage of transformer oil initially decreases, then increases, and eventually stabilizes with increasing cellulose impurity concentration. The maximum decrease and increase in breakdown voltage of transformer oil are 39.0% and 25.2%, respectively. Subsequently, observations were made using an optical diagnostic system, revealing that at low concentrations (≤0.005%), cellulose bridges exhibited periodic collapse, while at high concentrations (>0.005%), the bridges remained intact and bubbles appeared due to partial discharge (PD). Furthermore, physical and simulation models indicate that at low concentrations, breakdown is primarily associated with electric field distortion caused by cellulose impurities, while at high concentrations, thermal effects from leakage current and PD dominate. Therefore, the cellulose retards bubble growth (CR-BG) model is proposed to describe how cellulose impurity concentrations inhibit bubble bridging and affect the breakdown voltage of transformer oil at high concentrations. These findings establish a quantitative relationship between cellulose concentration and the electrical strength of transformer oil under the dc electric field, providing valuable engineering guidance for monitoring and maintenance work at the site.