Evaluation of transformer oil by Frequency domain technique

In recent years measurements of different Dielectric parameters in frequency domain is being employed for assessing the condition of the paper-oil insulation systems in Transformers. Understanding the variations in the patterns of the dielectric responses as a function of frequency of each of the dielectric media is important for judicious analysis & interpretation of the data for the realistic assessment of the condition of the composite insulation system of the transformer. The paper details some of the findings of the laboratory investigations of frequency domain Dielectric Spectroscopy measurements carried out on new mineral oil samples as well as oil samples drawn from Power Transformers in service. Interpretation has been emphasised on characterizing the variations in the dielectric response as a function of frequency with change in conductivity of the oil. Temperature dependence and moisture content in the insulation system have also been evaluated for better understanding of the variations in the dielectric spectral patterns and hence for reliable assessment of the condition of each of the dielectric media of the composite insulation system of Transformer, with specific reference to mineral oil.