Compatibility Test of Coatings with Natural Ester at Elevated Temperature for Transformers

Natural esters are widely used as insulating liquids due to their higher fire safety and environmental safety. The method generally used to evaluate the compatibility of construction materials with mineral insulating oil is according to the standard ASTM D3455. As natural esters can be applied to high-temperature insulation systems for transformers according to IEC 60076-14, the test temperature 100 °C indicated in ASTM D3455 will no longer meet the requirements of the specific application of natural esters. In this paper, three types of coatings used in soybean natural ester transformers have been tested. The testing temperature met the insulating liquid/kraft paper system. The testing duration was increased to multiples of 168 h (7 days), and the highest was 1008 h. Not only the insulating liquids were analyzed before and after the heating process, but also coatings were analyzed which were not required in ASTM D3455. The changing of liquid composition was also observed and analyzed. Throughout the experiment, except colour, dielectric dissipation factor, breakdown voltage, water content and interfacial tension of insulating liquids, hardness of coatings and breakdown voltage of enamelled wire were also important properties to evaluate the compatibility of construction materials with natural esters. This study will support the new proposal of the “IEC 63177 Test method for compatibility of construction materials with electrical insulating liquids” standard.