DC Electric Field Effect on Conductivity and Interface Charge of Oil-Paper Insulation

Understanding the effect of electric field strength on conductivity and interface charge is very important for the converter transformer design, operation, and maintenance. This article studies the dc electric field effect sensitivity on conductivity and interface charge of "liquid-solid" oil-paper insulation. First, polarization and depolarization current (PDC) and conductivity of mineral oil, oil-immersed pressboard, and "liquid-solid" oil-paper insulation is measured. Second, interface charge characteristics are calculated. Finally, the influence mechanism of dc electric field and moisture on interface charge density and conductivity is analyzed. Results show that PDC curve of mineral oil increases with electric field, while mineral oil conductivity decreases and then increases with dc electric field strength. PDC curve and conductivity of oil-immersed pressboard and "liquid-solid" oil-paper insulation increases with moisture content and dc electric field. Nevertheless, "liquid-solid" oil-paper insulation with low moisture content increases rapidly and then tends to be stable, while that with high moisture content keeps increasing exponentially. Interface charge density of "liquid-solid" oil-paper insulation increases with dc electric field strength and decreases with moisture content. Compared with interface charge density calculated by Maxwell-Wagner equation (M-W equation), error percentage in low moisture content is high, indicating that interface charge density effect on converter transformer "liquid-solid" oil-paper insulation should be taken into consideration in structure design and operation maintenance.