Dielectric diagnosis of water content in transformer insulation systems

This paper describes dielectric measurements at transformer insulation models made to Transformerboard barriers and oil channels. It was intended to quantify the water content in barriers and to distinguish the influences of water and geometric structure. Recovery-voltage measurements (RVM) did not show a clear correlation with water content, probably because of "parasitic" influences like oil parameters, geometry and parallel resistances. Better correlations were found for insulation resistances i. e. for polarization currents. Network simulations show that common circuits - normally used for homogeneous materials - are not able to explain the measurements. Therefore circuits had to be adapted to the geometric structures of the insulation. Thereby it could be shown that recovery voltages are dominated by interfacial polarisation between oil channels and barriers. Signals are strongly dependent on oil resistivities, possibly "masking" the influence of water in the barriers.