Analysis of the Influence of the Degree of Mixing of Insulating Oil with Synthetic Ester on the Dielectric Loss Factor of Impregnated Electrotechnical Papers in a Wide Frequency Spectrum

The article presents the results of laboratory tests of cellulose insulation samples TERTRANS® N 125 and cellulose-aramid NOMEX®910 impregnated with a mixture of synthetic ester MIDEL®7131 and mineral oil NYTRO® 10X, in the form of dispersion characteristics of capacitance, dielectric loss factor and complex permittivity in the range of measuring voltage frequency from 10 mHz to 5 MHz. The aim of the tests was to analyze the effect of the degree of mixing of dielectric liquids on the above characteristics. It was shown that this effect is significant for the operation of power transformers, in which a decision was made to replace mineral oil with a modern, biodegradable dielectric liquid, which is synthetic ester MIDEL®7131. The final result of the research was the determination of frequency windows in which the analysis of changes in the composition of dielectric liquids is most visible. The presented test results may be helpful in future extended diagnostics of power transformer insulation, which will be subjected to the dielectric liquid replacement procedure.