Retrofilling aging transformers with natural ester based dielectric coolant for safety and life extension

The number of mineral oil filled distribution and substation transformers at cement and mineral extraction plants that are approaching the end of their normal life expectancy is increasing. A proposed method of extending their remaining life is possible by retrofilling with a natural ester dielectric coolant. Additional benefits include reduced fire and environmental risks if the unit fails or when it is decommissioned. This paper reports on tests of miscibility and compatibility of mixtures of the two types of dielectric coolants. The paper also discusses field experience with retrofilling mineral oil-filled distribution and power transformers with a natural ester. Finally, results of an accelerated paper aging retrofill study are presented. The data collected to date suggests that the aging rate of the paper is significantly improved. Other benefits indicated by data include an increase in the fire point by approximately 200°C, a significant reduction in the moisture content of the insulation paper, the probable elimination of bubble formation due to sudden overload, and the elimination of oil sludge formation. The increase in the dielectric coolant fire point to over 300°C essentially eliminates the risk of oil pool fires by arcing in the event of internal fault arcing.