Influence of vegetable oil on the thermal aging of transformer paper and its mechanism

With the development of new insulation materials, vegetable oil-the best substitute for mineral oil-has gradually been widely used in the liquid insulation of transformers. To investigate the influence of vegetable oil on the thermal aging rate of oil paper, transformer paper impregnated with mineral oil and vegetable oil underwent thermally accelerated aging at three different temperatures. Degree Polymerization (DP) of paper was measured to indicate the aging degree of transformer paper. The aging rate of paper in mineral oil and vegetable oil was compared quantitatively, and results showed that vegetable oil retarded paper\´s degradation rate and extended its useful lifetime. The reasons contributing to such phenomenon were analyzed using X-ray Photoelectron Spectroscopy (XPS) and molecular simulation software. Transformer paper in vegetable oil had larger activation energy. Due to the larger interaction force between water and natural ester molecules, water molecule was easily bonded with natural ester, weakening the hydrolysis process of cellulose. Cellulose was chemically modified by natural ester during thermal aging process, and the reactive -OH (hydroxyl) groups on the cellulose became esterified with fatty acid esters. The water molecule was firmly bounded to the ester groups on glucose produced by esterification. The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a "water barrier" to further weaken the hydrolysis process.