Sequential comparative study of dielectric response and analyses of oil and paper from a power transformer undergoing repair

Methods, based on characterisation of dielectric response, for assessing moisture content in pressboard and paper are gaining ground. However, there is a need for calibrating obtained results with results from other techniques. This communication presents results from a study of a 50/10 kV - 40 MVA power transformer, which was sent to a workshop for repair, thus allowing for collecting samples of the solid insulation. Dielectric spectroscopy measurements in frequency domain were conducted at several occasions before and after the repair. Moisture estimation based on dielectric response measurements as well as on titration of oil samples yielded similar results to those obtained by direct titration of paper samples, as long as appropriate temperature values were used in all the analyses mentioned. The temperature value during measurements should be used for the analysis of the dielectric response, while in-service temperature of oil should be used for the evaluation based on Karl Fisher titration.