Compatibility of transformer construction materials with natural ester-based insulating fluids

The main objective of this study was to evaluate the effect of transformer construction materials, such as kraft insulation paper, varnish, core steel and gasket materials (rubber), on the physicochemical properties of natural ester-based insulating oils (NEIs) and mineral insulating oil (MIO) under heating. We evaluated two NEIs, one formulated with soya oil (Envirotemp FR3) and other with corn oil (Biovolt A). The test specimens were placed in a 1-L jar with 800 mL of insulating oil and then the jar was placed in an oven at 100 °C for 716 h. The insulating fluids were analyzed before and after the heating tests by physicochemical analyses, such as kinematic viscosity, acid number (or neutralization number), dissipation factor and interfacial tension for MIO. Properties of the aged reference and the test specimen oils for a standard testing method for compatibility of construction materials with NEIs are proposed. The only parameter that showed variation after the NEI compatibility testing with different materials was the dissipation factor at 90 °C. The results show that besides compatibility and aging tests, mechanical tests for the gasket or seal materials should also be performed to conclude whether a material is suitable for construction of transformers insulated with NEIs.