Frequency Response Analysis for Accounting the Changes in Permittivity of Natural Ester Oil with Mineral Oil

Recently, ester based insulating oils including both natural ester and synthetic ester are being considered as potential alternatives to petroleum based mineral oil due to their better environmental friendly, higher fire safety, non-toxic and continuous overloading benefits. Ester oil has higher permittivity than mineral oil due to chemical compositions. Hence, the higher permittivity of ester oil affects electrical stress distribution in insulation system of transformer and it can also affects frequency response analysis of transformer during sweep frequency response analysis (SFRA). In this paper, to evaluate the differences in permittivity of natural ester and mineral oil, SFRA are performed for a given same winding geometry and insulation arrangement. The mineral oil transformer frequency responses are considered as a reference response and its results are compared with natural ester oil transformer. The frequency domain responses are evaluated using graphical analysis and commonly used numerical indicators of correlation coefficient, standard deviation, absolute sum of logarithmic error, mean square error and Max-min ratio.