The service history of ester-based fluids used in railway transformers

Polyol ester fluids have been used as a substitute for PCBs in dielectric applications. A project in which samples of ester fluid from in-service railroad traction transformers were obtained and tested is described. Analysis of the samples´ electrical, physical, and chemical characteristics confirm that polyol ester fluid is exceptionally stable in service. The functional properties studied showed little or no change in their values, even in samples with more than ten years of service aging. The test values are well within the recommended values set by ASTM and IEEE for continued use of both new and used conventional transformer oil. In most cases, the test values for service-aged fluid exceeded the recommended acceptance values for conventional transformer oil