Correlating UV Visible Spectral Response and Thermal Ageing of Blended Transformer Oil

Ester based vegetable oils (VOs) are potential replacement of Mineral oil (MO) filled power transformers in near future. Retro filling of power transformers filled with MO by VO is real possibility which may result into a mixture of MO and VO or simply blended oils (BOs). The effect of ageing on electrical and physiochemical properties of BOs in operation will be different than that of individual MO and VO. Fresh MO and VO samples are blended in three different proportions, 3:1, 1:1 and 1:3 and are thermally aged in open beaker for 168 hours at 150°C. Important ageing parameters namely specific resistance (SR), dielectric dissipation factor (DDF), relative permittivity, total acid number (TAN) and interfacial tension (IFT) are monitored before and after thermal ageing. Extent of ageing in BOs is compared with individual MO and VO. The changes in these parameters are different than individual MO and VO. The ageing characteristics are varying with varying MO/VO proportions. The UV-visible spectroscopy is recorded before and after ageing and areas under the curve (AUC) of absorbance between 360-600 nm wavelength range are obtained. The correlation of AUC with electrical and physiochemical parameters are discussed.