Effect of Frequency, Temperature and Thermal Aging on Breakdown Characteristics of Oil-paper Insulation of 220 kV Converter Transformers

The breakdown characteristics of the oil-paper insulation in 220 kV converter transformers under varying thermal aging durations, temperatures, and voltage frequencies are investigated. A bivariate quadratic nonlinear regression model is developed to describe the deterioration of breakdown strength as a function of temperature and voltage frequency across different aging times. The results reveal the synergistic effects of thermal aging, temperature, and voltage frequency on the breakdown performance. It is found that, regardless of the aging time, the breakdown strength decreases as temperature or voltage frequency increases. Compared to unaged samples, aged samples show a greater susceptibility to the deteriorating effects of temperature and frequency, with breakdown strength exhibiting a stronger negative linear correlation with these variables. Additionally, temperature exerts a more significant influence on breakdown strength than frequency in aged samples. As thermal aging progresses, the impact of temperature and frequency on breakdown strength becomes more pronounced, with the strongest effect occurring after 10 days of aging. The proposed deterioration model can also predict the breakdown strength of oil-paper insulation under various temperatures and voltage frequencies for different operational lifetimes, providing a valuable reference for evaluating the condition of converter transformer insulation under complex operating conditions.