Dielectric spectroscopy in time and frequency domain applied to diagnostics of power transformers

Dielectric spectroscopy in the time- and frequency domain has been used to assess the quality of the insulation systems of a number of power transformers at the Ringhals nuclear power station. Three different methods were used: low frequency dielectric spectroscopy measurements (FDS), time domain polarisation/depolarisation current measurements (PDC) and return voltage polarisation spectra (RVM). The results were analysed and quantitatively modelled using a general dielectric linear response model. Significant differences in oil quality and moisture content in the board were detected. FDS and PDC gave very similar results but RVM predicted significantly higher moisture levels in some transformers. FDS seems most rugged for field work and allows use of experience from traditional power frequency measurements in the interpretation.