Moisture and temperature effects on the dielectric spectrum of transformer pressboard

The dielectrometry spectrum of pressboard is a function of its temperature and moisture content. In transformer pressboard, it has been shown that the dielectric spectrum does not vary in shape with temperature and moisture content, but that there is a shift in amplitude and frequency. Thus it is possible to create a master curve, with appropriate temperature correction factors, containing information about the moisture contents from transformer pressboard dielectrometry measurements. Extensive measurements of dielectric properties of oil-free transformer pressboard as a function of mine moisture levels (0-5.7%) and five temperature levels (30-70°C) were conducted in order to quantify effects that will allow the prediction of moisture content of insulation by measuring its dielectric spectrum, thus allowing an on-line non-destructive moisture measurement method in pressboard. A dielectric model for biological tissue is adopted for the cellulose-structured pressboard. The master curve is found by fitting the data to the model. The paper presents measured curves that show similar trends as reported in the literature. In addition, new quantitative analysis is presented for oil-free pressboard.