Y. Sheiretov et M. Zahn, DIELECTROMETRY MEASUREMENTS OF MOISTURE DYNAMICS IN OIL-IMPREGNATED PRESSBOARD, IEEE transactions on dielectrics and electrical insulation, 2(3), 1995, pp. 329-351
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.