Space charge behavior evolution with thermal aging of oil-paper insulation

Space charge is a threat to insulation materials and oil-paper insulation gradually degrades in service. The degradation of cellulose affects space charge formation, accumulation and dissipation, and the presence of space charge affects the performance of dielectric. In this paper, the effect of thermal aging on space charge behaviors and other properties including permittivity, conductivity, and tensile strength was investigated. Oil-paper samples were aged at a hot spot temperature (180°C). During thermal aging, space charge distribution was measured at a regular interval at room temperature. And the pulse electro-acoustic (PEA) method was used for space charge measurement. The results showed that the thermal aging at hot spot temperature caused increase of conductivity and tensile strength degradation. The permittivity increased at the beginning but then it decreased, while dissipation factor showed an upward trend. Besides these, trap distribution and carriers characteristics were also affected. The formations of space charge of unaged and aged oil paper were consequently different under low electric field. Due to the generation of traps by the degradation of cellulose, more charge, especially positive charge was trapped, but the increased traps mostly were shallow traps. Based on the Schottky model, a correlation between space charge injection and the permittivity was drawn. This, together with the appearance of positive bulk charge could be used for oil paper aging status diagnosis.