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The performance of oil-impregnated paper significantly affects the safety and reliability of transformers. Previous studies on water reducing the breakdown strength of oil-impregnated paper have been mainly due to increasing charge carriers and conductivity. However, the influence of water on the cellulose crystal structure of oil-impregnated paper is still not clear; in particular, its role on breakdown strength remains ambiguous. In this study, the AC breakdown strength and crystal structure of vegetable and mineral oil-impregnated paper with different moisture contents were investigated. The results show that with increasing moisture content, the cellulose crystallinity and crystal size increases linearly, while the breakdown strength decreases. The breakdown strength of vegetable oil-impregnated paper is higher than that of mineral oil-impregnated paper and is less affected by moisture. Herein, a model is proposed on the effects of water on the crystal structure of oil-impregnated paper and the correlation of breakdown strength with crystal structure is reported. [ABSTRACT FROM AUTHOR]

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual furfural content in oil is influenced by temperature because furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different furfural contents were prepared through an accelerated thermal aging test. A furfural equilibrium experiment was conducted at different temperatures to construct a furfural partitioning diagram. The mass fraction of furfural in oil and paper was calculated. Results showed that the mass fraction of furfural in oil increases with temperature rising and aging deepening, which means that furfural tends to migrate from paper to oil at high temperatures and deep aging conditions. The change in furfural partitioning between oil and paper at different temperatures causes a deviation in paper aging assessment. To correct the temperature effect on furfural partitioning, a furfural correction factor was developed to normalize furfural contents in oil at different temperatures to a reference temperature. The furfural correction factor was verified by experimental data and then introduced into the relation between degree of polymerization of paper and furfural content of oil. Verification results showed that the corrected equation can enhance the accuracy of paper aging estimation at different temperatures. The findings of this work indicated temperature effect on furfural partitioning should be considered when determining the furfural thresholds and evaluating the aging condition of power transformers. [ABSTRACT FROM PUBLISHER]