A Comparative Test and Consequent Improvements on Dielectric Response Methods

Abstract: This paper starts with a systematic benchmark test of the three commercialised dielectric diagnostic methods RVM (recovery voltage method), PDC (polarisation and depolarisation currents) and FDS (frequency domain spectroscopy), performed on a large transformer model. The water content as analysed by the specific software was compared under three parame-ters: insulation geometry, insulation temperature and oil conductivity. It was found, that for the RV method the interpretation scheme based on the central time constant is too simplistic. The other two methods PDC and FDS compensate much better for the mentioned parameters but nevertheless analyse a drier cellulose with increas-ing temperature and a wetter cellulose for increasing oil conductivity. Actually the moisture in cellulose re-mained constant at 1 % during all the tests. Starting from these weaknesses a more sophisticated software was developed. The new software called “Dirana ” bases on a new data pool, measured at new and aged press-board with various moisture contents, temperatures and oil impregnation. The analysis algorithm works in fre-quency domain and features weighting of low frequency data and an extended XY-model for analysing oil-paper-insulations. Results obtained from this software show especially a much better compensation for varying oil conductivities and insulation geometries. The new soft-ware was successfully utilized for a on-site measure-ment in comparison to other measurement methods.