Determination of Moisture Content in Mass Impregnated Cable Insulation Using Low Frequency Dielectric Spectroscopy

The variations in the low frequency dielectric spectra of mass impregnated insulation depending on its moisture content have been investigated. For the calibration purposes a number of the impregnated paper samples was kept in the desiccators where different relative humidities were maintained. Thus obtained moisture content in the samples ranged from 1 % up to 6 %. The variations of the complex capacitance in a frequency range 0.001...1000 Hz were measured on these samples as well as on paper previously dried at elevated temperatures under vacuum. The moisture content of the dried samples determined was in a range 0.2-0.5 %. Moreover the study of water penetration in a 12 kV mass impregnated cable sample was carried out. Continuous measurements of the dielectric spectra revealed the dynamics of water ingress in the cable insulation.