Study on Physicochemical and Electrical Properties of Different Oil-based Natural Ester Insulating Oils during Thermal Aging

Natural ester, has the ability of protecting cellulose which could efficiently delay the aging of insulation pressboard, and is regard as the best alternative to mineral oil (MO). In this paper, an accelerated thermal aging test of oilpaper samples $120^{\circ}C$ was conducted, including mineral oil, FR3 oil, PFAE oil, cellulose insulation pressboard and thermally upgraded paper. The key parameters of insulating oils were analyzed, including liquid colour, power frequency breakdown voltage, thermal conductivity, specific heat capacity and infrared spectrum. Results show that PFAE has the highest power frequency breakdown voltage and slightest colour change during the aging process. The thermal conductivity and specific heat capacity of MO, PFAE and FR3 oil with different aging degrees all increased with the increase of temperature. Among the three insulating oils, PFAE has a relatively high specific heat capacity, which would be beneficial to reduce the transformer operating temperature. The infrared spectrum analysis shows that C-H vibration peaks appears in MO, but in PFAE and FR3 oil, the C=O stretching vibration peaks also exist which reflects the different structure of three insulating oils. This study has a certain reference value for the fault diagnosis and evaluation of natural ester transformer in the future.