Mechanisms and testing of moisture content measurement of transformer pressboard

In this paper, the method to measure moisture content of oil-free pressboard with single frequency experimental method is studied and the paper focuses on the implementation of experimental studies based on dielectrometry. Accurately monitoring and controlling of moisture can be implemented by the measurement of the permittivity of pressboard. A capacitive sensor is used to monitor the dielectric response of the pressboard. The effects of frequency and temperature to the signal of capacitive sensor are analyzed. The models of effects of frequency and temperature on signal are developed with studying of the dielectric polarization and dielectric loss of pressboard. A universal expression relating permittivity and moisture content of oil-free pressboard is developed to calculate the moisture concentration in the pressboard by fitting the moisture data obtained from the drying experiments to the dielectric properties of pressboard. Real-time on-line measurement of the moisture diffusion process is achieved in this study and the moisture profile in transformer pressboard is non-destructively estimated.