Frequency–Temperature Characteristics of the Cellulose—Insulating Oil–Water Nanodroplet Nanocomposite Components for Diagnostic Evaluation of Power Transformer Insulation

We determined the reference characteristics of the loss tangent and the real component of the complex permittivity of the cellulose-insulating oil–water nanodroplet nanocomposite with a moisture content of 5.17% by weight in pressboard. Such a high moisture content was selected because a value close to 5% by weight is critical, and reaching it may lead to catastrophic transformer failure as well as contamination of the natural environment with poorly biodegradable mineral oil and products of its incomplete combustion. Based on the measurement results, the values of the loss tangent and the real and imaginary components of the complex permittivity of the power transformer insulation system, consisting of moistened pressboard and insulating oil, were determined according to CIGRE. These values were obtained for both factory-new and moistened mineral oil. It was found that oil moisture content has a significant impact on the tanδ characteristics of strongly moistened liquid–solid insulation in the lowest frequency range. In the intermediate frequency range, this effect gradually decreases and then practically disappears. In the frequency range above 50 Hz, the tanδ values depend on the moisture content in cellulose and on the geometrical parameters of the insulation components in the CIGRE system, and do not depend on the oil moisture content. The influence of oil moisture on the estimation of cellulose moisture content becomes noticeable starting from a water content of 2% in pressboard. This should be taken into account in insulation condition analysis and in moisture level estimation in order to detect a critical state threatening catastrophic failure of a power transformer. It was also determined that the real component of the complex permittivity depends only weakly on oil moisture, and only in the low-temperature and low-frequency ranges. In contrast, the imaginary component of the complex permittivity depends on oil moisture practically in the same way as the loss tangent of the power transformer insulation system.