Frequency-domain spectroscopy characteristics of oil-paper insulated bushings with localized moisture

Moisture assessment in oil-paper insulated transformer bushings is closely related to the service reliability of large power transformers. Based on the nondestructive frequency-domain dielectric response method, this paper establishes equivalent simulation and experimental models of localized moisture in oil-paper insulated bushings. COMSOL finite element analysis was used to compare the dielectric response under dry and localized moisture conditions. The simulation results show that as moisture distribution becomes more uniform, the low-frequency tanδ-f curve increases and an interfacial polarization loss peak with an almost fixed characteristic frequency appears in the mid-frequency range. The localized-moisture bushing model further shows that multiple polarization loss peaks appear in the low- and mid-frequency ranges, with the low-frequency peak mainly affected by the degree of moisture-distribution nonuniformity. Experiments confirm a clear mid-frequency interfacial polarization loss peak. These results suggest that tracking dielectric loss peaks may provide useful information for observing the evolution of localized moisture in the end-screen region of oil-paper insulated bushings.