A Study on Critical Mixing Ratio Estimation of Transformer Main Tank Oil in Bushings and Field Operation Algorithm Based on Accelerated Aging

As the service years of power transformers increase, ensuring the insulation reliability of bushings has become critical for power system stability. Although the influx of main tank oil during maintenance can degrade bushing insulation, standardized permissible mixing limits remain undefined. In this study, internal residual oil volumes during sampling were quantified through dismantling and structural modeling of three decommissioned bushings. Based on these findings, the impact of oil mixture on bushing integrity was evaluated using accelerated aging tests. Experimental results demonstrated that while dielectric characteristics remained stable, the pour point and kinematic viscosity variation rate functioned as the primary limiting factors for operational reliability. Analysis of each parameter yielded conservative critical mixing ratios: 30% for pour-point-dominant oils, and 20% for oils governed by viscosity variation standards. Furthermore, this study confirmed that the viscosity variation rate exhibited a high correlation with a linear regression model, enabling precise property prediction based on the mixing ratio. Finally, a maintenance algorithm was proposed to determine replenishment limits and sampling availability in the field by integrating critical values with residual oil data. This study is expected to provide data-driven guidelines for systematic bushing oil mixture management, thereby enhancing the long-term reliability of transformer assets.