A Comparative Study of the Chemical Stability of Esters for Use in Large Power Transformers

There is uncertainty as to whether ester oils will be suitable replacements for mineral oil used in large free breathing power transformers, such as those found on the transmission system of the UK. This is partly due to concerns regarding the chemical stability of esters and how stability will affect the dielectric properties of the oil. This paper presents research investigating how ester dielectric properties change due to the effects of ageing compared to mineral oil and seeks to advise Utilities which parameters to monitor in service. This investigation considered the dielectric properties of AC and impulse dielectric strength, dielectric dissipation factor and acidity. It is recommended to monitor trends over working life rather than simply comparing values, and to use at least two parameters including the dielectric dissipation factor and acidity.