Analysis of the field ageing of natural ester transformer dielectrics: Ten years of data

Since 2003 power transformers have been manufactured using natural esters as the insulating dielectric. Before this, these natural ester based dielectrics had been used in factory and distribution type transformers successfully since the mid-1990s. Currently, international standards which would recommend in-service limits for the properties of natural ester are still under development. One reason is that there is insufficient data on which to base these limits. For this study, data from seventeen power transformers containing a natural ester dielectric was gathered, compared, and contrasted with standard mineral oil to evaluate the changes in the fluid expected for normal operation over the first ten years of a power transformer's life. The oldest transformer was ten years old, nearly the same age as the earliest units which use this dielectric. It is hoped that this information will help the industry evaluate the fluid data for their power transformers by understanding what is considered to reflect that of normal operation.