Study on the Compatibility Between Synthetic Ester Mixed with Mineral Oil and Construction Materials for Liquid Filled Transformer

Retrofilling mineral oil transformers with synthetic esters has been widely adopted as a promising solution for better fire resistance and ecological friendliness. However, compatibility risk exists between insulating oil and construction materials after the retrofilling process, as the current process inevitably results in residual mineral oil mixing with synthetic esters. Therefore, it's necessary to study the compatibility between the mixed oil and construction materials. In this paper, compatibility tests were conducted for gasket, inner surface coating, and enamelled wire with SE/MO (synthetic ester and mineral oil) mixed insulating oil. The proportions of mineral oil of mixed oils were set at 0% and 3.5%. According to the IEC 63177 standard, the test temperature was set at 120°C for 168 hours. The physicochemical and dielectric properties of the insulating fluids, and mechanical properties and polarizing microscope analysis of construction materials were measured before and after 168h heating process. The results show that, under the test conditions and samples we used, there exists compatibility risk between SE/MO mixed oils with three typical construction materials: nitrile rubber, epoxy-based inner surface coating, and enameled wire with a base coat of polyvinyl formal and a top coat of epoxy resin, and low mineral oil concentration has no effects on the compatibility of the SE/MO insulating oil with construction materials.