Effect of oil replacement on furfural analysis and aging assessment of power transformers

Furfural analysis is considered as a reliable method to estimate aging of cellulose insulation paper and is widely applied in assessing the condition of field transformers. However, the actual concentration of furfural in insulating oil is influenced by oil replacement, and the correction method is seldom studied. In this research, an accelerated thermal aging test of oil-paper samples was conducted at 130 °C for 35 days. Insulating oil was replaced at early, middle, and late aging stages to investigate furfural loss during the oil replacement process and its effects on assessing the aging of insulation paper. Results showed that furfural in oil obviously declined after oil replacement, which caused a severe error in paper aging evaluation. To correct furfural loss, the furfural correction factor, which is defined as the ratio of the furfural content in oil after oil replacement to the furfural content before oil replacement, was introduced into the furfural analysis method. The parameters and influencing factors of the furfural correction function were researched. The furfural correction function was verified using experimental data obtained under laboratory conditions and field data collected from 44 in-service transformers. The results showed that the furfural correction function effectively corrects furfural loss during oil replacement and improves the accuracy of estimating paper aging. Finally, the furfural correction function was extended to the operating conditions of field transformers.