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.

中文翻译：

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油纸绝缘材料中水分对糠醛扩散和平衡影响的微观机制及修正

油中糠醛含量被认为是评价油浸式变压器老化状态的有效参数。在之前130℃的热老化实验中，我们发现油纸体系中水分的增加使得更多的糠醛分布在油中。然而，水分影响糠醛从纸到油扩散的微观机制仍不清楚，变压器典型工作温度下的糠醛平衡规律也不清楚，导致现有的糠醛评价模型不可靠。为了阐明上述问题，在不同含水量下进行了分子动力学模拟和60℃下糠醛平衡实验。结果表明，高水分含量确实使部分糠醛从纸中扩散到油中。假设水分含量从1%增加到2%，油中糠醛质量分数由32.98%增加至49.83%，导致绝缘纸聚合度约为524时，油中糠醛含量由0.76mg/L增加至1.06mg/L，增加了39.5%。水分含量的增加使纤维素与H形成氢键₂ O优先，减少纤维素和糠醛之间的氢键相互作用。因此，部分糠醛从纤维素上脱落并扩散到油中，导致油中糠醛含量增加。为了消除水分干扰，构建了基于水分平衡曲线的糠醛修正评估模型，无论水分影响如何，都能准确评估绝缘纸的老化状态。