Short-Time Failure Mode Considerations Associated With Power Transformer Overloading

Model structures representative of power transformer conductors and metallic components have been tested at elevated hot-spot temperatures to simulate operation at overloads. Three different types of cellulosic insulation in contact with hot-spots evolved visible gas bubbles at temperatures as low as 140°C, while hot metal in contact with oil alone produced no visible gas bubbles up to approximately 350°C. The dielectric strength of insulated conductors was observed to deteriorate moderately at temperatures in the 100°C to 150 °C range, but more rapidly at temperatures above 150°C. The experimental results have been analyzed and are discussed with relation to their potential significance to transformer loading practices.