Analysis of the Effect of the Degree of Mixing of Synthetic Ester with Mineral Oil as an Impregnating Liquid of NOMEX® 910 Cellulose–Aramid Insulation on the Time Characteristics of Polarization and Depolarization Currents Using the PDC Method

This article continues the authors’ research on NOMEX® 910 cellulose–aramid insulation saturated with modern electrical insulating liquids, which is increasingly used in the construction of high-power transformers The increase in technical requirements and environmental awareness influences, nowadays, shows that, during the overhaul and modernization of power transformers, petroleum-based mineral oils are increasingly being replaced by biodegradable synthetic esters (oil retrofilling). As a result of this process, the solid insulation of the windings are saturated with an oil–ester liquid mixture with a percentage composition that is difficult to predict. The purpose of the research described in this paper was to test the effect of the degree of mixing of synthetic ester with mineral oil on the diagnostic measurements of NOMEX® 910 cellulose–aramid insulation realized via the polarization PDC method. Thus, the research conducted included determining the influence of such factors as the degree of mixing of synthetic ester with mineral oil and the measurement temperature on the value of the recorded time courses of the polarization and depolarization current. The final stage of the research involved analyzing the extent to which the aforementioned factors affect parameters characterizing polarization processes in the dielectric, i.e., the dominant dielectric relaxation time constants τ1 and τ2, and the activation energy EA. The test and analysis results described in the paper will allow better interpretation of the results of diagnostic tests of transformers with solid insulation built on NOMEX® 910 paper, in which mineral oil was replaced with synthetic ester as a result of the upgrade.