Correlation of Dielectric Parameters and Insulation Properties of Power Transformers

Abstract

Oil-paper insulated power transformers exist since more than 125 years and since nearly a century, electrical measurements are in use for the assessment of their insulation condition. For a long time, high voltage DC resistance measurements and high voltage dielectric loss measurements at power frequency have been the commonly used methods. Frequency variable high voltage dielectric measurements, Recovery voltage measurements (RVM) and polarization-depolarization-current measurements (PDC) were developed towards the end of the last century in order to gain more information about the insulation condition. Modern technology enables more advanced analysis and with the use of broadband dielectric measurements, also known as dielectric frequency response (DFR), it is possible to determine the absolute moisture content in the cellulose insulation as well as the oil conductivity. Nonetheless, the old measurement methods are still widely used for condition assessment of power transformers but often without a profound knowledge of the exact meaning of the measured values. This paper uses DFR measurements on more than 100 power transformers and tries to correlate the different measurement values and the condition of the insulation, represented by the moisture content in the cellulose and the oil conductivity. It shows that most measurement values obtained by classical measurements are dominated by the influence of the oil conductivity and that the moisture content only has minor influence.