Behavior of ester dielectric fluids near the pour point

The pour points of dielectric fluids based on natural esters are not reliable indicators of their fluidity at cold temperatures. A 25-day isothermal test was used to determine the time to solidification of several natural ester fluids and a synthetic ester at -15/spl deg/C. The results are compared to ASTM D97 pour point measurements. Natural ester dielectric fluid properties measured before and after a 120 hour freeze/thaw cycle showed no changes occurred due to solidification. Finally, the performance of both a solidified natural ester and a high fire point high molecular weight hydrocarbon was compared to that of conventional mineral oil by energizing at full rated load transformers held at an ambient temperature of -30/spl deg/C.