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

As alternative liquid, both synthetic and natural ester have been widely adopted in electrical equipment due to environmental friendly and high fire point, which applied for both newly transformers and retrofilling for operated mineral oil transformers. However, based on current retrofilling process, mixture of mineral oil and natural ester is inevitable. Thus, in this study, the compatibility between natural ester mixed with mineral oil and different construction materials, such as nitrile rubber, inner surface coating, and enameled wire enamel will be studied to address potential risks for incompatibility. According to the IEC 63177 standard, the test temperature was set at 120°C for 168 hours, and several physical, chemical and dielectric properties of insulating materials were tested before and after the compatibility test. For enameled wire and inner surface coating, a polarizing microscope was used to observe microscopic changes. After the compatibility test, the color of three samples impregnated with different construction changes from 0.5 to 1. The dielectric loss factor of the mixed insulating liquid impregnated with nitrile rubber reached as high as 0.204, with a loss in mass and volume of the nitrile rubber, and a reduction in elongation at break by 12.80%. The mixed insulating liquid impregnated with inner surface coating is up to 206.37 mg/kg. And The optical microscope test showed the appearance of cracks on the surface of the enameled wire and the increase of craters on the inner surface coating. The results indicate potential risks for incompatibility between the mixed insulating liquid and the nitrile rubber, inner surface coating, and enameled wire enamel used in this study.