Aging Behaviors of Ester-Insulated Oil–Paper Composite Systems under Combined Electrical and Thermal Stresses – Part II: Aging Characteristics and Microscopic Mechanisms of Oil-Impregnated Paper

To investigate the aging characteristics and differences of insulating paper in various ester-based insulating oils, this study conducted accelerated electro-thermal aging tests on soybean-based natural ester and palm fatty acid ester (PFAE) insulating oil-paper composite insulation systems. Molecular dynamics simulations were employed to reveal the underlying mechanisms of the aging characteristics of the insulating paper. The results showed that during the early stages of aging, cellulose with a higher degree of polymerization formed more hydrogen bonds with H2O and H3O+, leading to stronger binding energies, which accelerated the degradation of cellulose. After aging, the number of hydrogen bonds and binding energies between cellulose and H2O/H3O+decreased, slowing the aging rate. As a result, the aging process of the insulating paper exhibited a characteristic pattern of rapid aging initially, followed by a slower rate. Crystalline cellulose, which is less prone to forming hydrogen bonds with H2O and H3O+, is less susceptible to hydrolytic aging, leading to a limit on the degree of polymerization at the end of the aging process. Compared to soybean-based natural ester, PFAE insulating oil showed stronger net interaction energies with H2O and H3O+, which reduced the number of hydrogen bonds between cellulose and H2O/H3O+, thus delaying the hydrolysis of cellulose. Therefore, the aging degree of insulating paper in PFAE insulating oil is lower, and the final residual degree of polymerization in the insulating paper of PFAE insulating oil is approximately 140 higher than that in soybean-based natural ester. This paper provides theoretical insights into the aging characteristics of insulating paper in ester-based insulating oils under electro-thermal combined effects from both experimental and molecular scales.