Natural ester and synthetic ester fluids, applications and maintenance

These days the use of ester-based transformer fluids as an alternative to mineral oil is on the increase. The environmental and fire benefits of these fluids are being realised by more and more users, which has led to esters being used in a wide range of transformers, even up to the 400kV transmission level. These larger power transformers are closely monitored to ensure their correct operation; this includes fluid testing, such as dissolved gas analysis. There are some fundamental chemical differences between mineral oil and ester-based fluids and it is important for asset operators to understand how to monitor and maintain these liquids. This paper aims to give an overview of the key chemical differences between mineral oil and esters and advice on how these differences affect the correct maintenance of ester fluids, as well as providing a review of the ongoing research into the area of dissolved gas analysis.

I. Chemistry of Fluids

It is useful for non-chemists working within the electrical industry to know a little about chemistry in order to understand the differences between mineral oil and esters, which in turn have an influence on maintenance and DGA. To start with there is a short hand notation used by chemists to describe the molecular structure of substances. Fig. 1 illustrates how this shorthand can be expanded into a longer form, showing the carbon and hydrogen elements and their bonds. Fig. 1.

Chemist's shorthand