DIELECTROMETRY MEASUREMENTS OF MOISTURE DYNAMICS IN OIL-IMPREGNATED PRESSBOARD

The complex dielectric spectrum of pressboard is a function of its moi sture content and temperature. In oil-impregnated pressboard of medium and low humidity the dielectric spectrum's shape and amplitude do not change with variations in temperature and moisture content, but only shift in frequency. Thus it is possible to create a 'universal' curve, with appropriate temperature correction factors, which can be used to extract information about the moisture dynamics of solid transformer insulation from dielectrometry measurements. To create the universal c urve, measurements are taken with the material placed in a parallel-pl ate structure whose complex impedance is measured. To measure spatial distributions of dielectric properties, imposed omega-k (frequency-wav enumber) dielectrometry is used, where a set of interdigitated electro des on one surface of the material cause the electric field to have a limited depth of penetration into the dielectric, proportional to the electrode spacing. Measurements using the parallel-plate methodology e stablish a mapping of the dielectric spectrum of EHV-Weidmann HIVAL pr essboard impregnated with Shell Diala A(TM) transformer oil, as a func tion of temperature and water content. This mapping is then used to de termine spatial moisture profiles in pressboard in other experiments w hich make use of a three-wavelength interdigitated sensor.