Microcosmic mechanism and correction of moisture effect on furfural diffusion and equilibrium in oil‐paper insulation

Abstract

Abstract Furfural content in oil is considered as an effective parameter to evaluate the ageing state of oil‐immersed transformers. In previous thermal ageing experiments at 130°C, we found that moisture increment in oil‐paper system makes more furfural distribute in oil. However, the microcosmic mechanism of moisture effect on furfural diffusion from paper to oil is still unknown, and the furfural equilibrium law at transformers' typical operating temperatures is also unclear, resulting in the unreliability of the existed furfural evaluation model. To elucidate the above problems, molecular dynamics simulation and furfural equilibrium experiments at 60°C were carried out at different moisture contents. The results show that high moisture content makes part of furfural diffuse from paper to oil indeed. Supposing moisture content increases from 1% to 2%, mass fraction of furfural in oil may increase from 32.98% to 49.83%, leading to furfural content in oil increases from 0.76 mg/L to 1.06 mg/L when the degree of polymerisation of insulation paper is about 524, which increased by 39.5%. The increase in moisture content makes cellulose form hydrogen bonds with H2O preferentially, decreasing the hydrogen bonds interaction between cellulose and furfural. Thus, partial furfural falls off cellulose and diffuses into oil, leading to an increase in furfural content in oil. To eliminate the moisture interference, a corrected furfural assessment model based on the moisture equilibrium curve has been constructed, which can accurately evaluate the ageing state of insulation paper regardless of the moisture influence.