Low Temperature Property Improvement on Green and Low-Carbon Natural Ester Insulating Oil

In order to improve the low-temperature operational reliability of natural ester insulating oil-immersed transformers, the low-temperature property improvement on natural ester is investigated. The results show that poly alpha-olefin, hexyl naphthalene, and polymethacrylate have good effects of pour point depression, but poly alpha-olefin and hexyl naphthalene cause properties deterioration of natural ester such as acid value and dielectric dissipation factor because of their own characteristics. When the adding proportion of polymethacrylate is 1%, the pour point of natural ester is reduced to −20.5 °C, which can maintain good physicochemical and electrical insulating properties. Polyacrylate has no depression effect and reduces the insulating property. During the crystallizing fractionation (CF) process, when the stirring speed of low-temperature crystallization (II) is 3 r/min, and the temperature difference between cooling medium and natural ester is 3 °C, the pour point is reduced to −20.3 °C. However, due to the reduction of high melting point glycerides content, the oxidation resistance of natural ester decreases. The effective combination of CF process and polymethacrylate as a pour point depressant makes the pour point reduce to −27.3 °C, which also ensures good physicochemical and electrical properties. Also, the synergistic antioxidation between phenolic antioxidants and polymethacrylate enhances the antioxygenic property, which effectively expands the application conditions and ranges of natural ester.