A study on the effect of moisture on paper insulation impregnated with mineral oil and ester oil using Frequency domain Dielectric Spectroscopy (FDS)

Condition monitoring of power transformers has been a continuous process and has seen many improvement over the years. Although several diagnostic tests such as dielectric loss angle, IR/PI, DGA, furan analysis are available, interpretation of data still appears to be a challenging task due to multiple failure mechanisms. Interpretation of data requires much care and expertise. In recent times dielectric response methods have been used for detection and estimation of moisture content and ageing of pressboard/paper insulation system of power transformers. Moisture in the transformer affects the dielectric strength and the rate of insulation ageing. In some cases, there is also threat of bubble evolution above a certain temperature when the load is suddenly increased. Earlier, the water content of the cellulose of a transformer in service is determined indirectly by measuring the moisture content in the oil sample. Moisture distributes unequally between the oil and pressboard, the greater part residing within the solid insulation. As the water concentration in the oil is highly temperature dependent, the measurement of moisture in oil is not a reliable indicator of dryness of the cellulose. In addition to the conventional measurement of power frequency loss angle, recent attention has focused on measuring various dielectric response parameters, which characterize some known polarization phenomena. Frequency Domain dielectric Spectroscopy (FDS) is a promising method to estimate the moisture content in oil-paper insulation system of the transformer. Research work is going on to use vegetable ester oil as replacement to mineral oil. However, information regarding the effect of moisture on paper-oil insulation system using ester oil is limited. In this paper a comparative study has been made to understand the effect of moisture in mineral oil and ester oil using Frequency Domain dielectric Spectroscopy (FDS). Mineral oil and vegetable ester oil impregnated paper insulation samples are prepared in the laboratory with different moisture content at controlled conditions. The dielectric behavior of these composite with different moisture has been studied. It is found that dielectric behavior of ester oil impregnated sample is better compared to mineral oil impregnated paper sample. Experimental details and findings are discussed in this paper.