A three-wavelength flexible sensor for monitoring the moisture content of transformer pressboard

A three-wavelength interdigitated flexible sensor has been developed for measuring the heterogeneous properties of transformer pressboard. The imposed spatial wavelengths are 1 mm, 2.5 mm, and 5 mm, and the temporal frequency ranges from 0.005 Hz to 10000 Hz. Because the penetration depth of the electric field is approximately 1/5 of the wavelength, the sensor can be used to observe spatial as well as temporal changes in the dielectric loss of transformer pressboard. Experimental results indicate that the measured response is sensitive to changes in the moisture content of the pressboard. In addition, the sensor can be used to monitor the absorption of moisture into pressboard as a function of time. This measurement provides an estimate of the diffusion coefficient of moisture in oil-free transformer pressboard at room temperature of 3.5*10/sup -11/ m/sup 2//s.<>