Effect of Additives on Durability of Insulating Paper in Natural Ester Oil

The life of insulating paper is key to transformer reliability and service life. For transformers using natural ester oil, the effect of a surface protection layer is examined to suppress the degradation of insulating paper that occurs when moisture is continuously taken up. From first-principles calculations, carboxy and amide groups are extracted as functional groups with a high adsorption energy to the insulating paper surface. Adsorption experiments using additives with those functional groups are performed, and Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements suggest that stearic acid has a strong ability to form a surface protective layer. Accelerated degradation tests using stearic acid as an additive show that the residual tensile strength after 25 h at 170 °C is significantly higher than that of without additive, verifying the idea that forming a surface protective layer extends the life of insulating paper.