Conference paper

Considerations for the Design, Manufacture, and Retro – filling of Power Transformers with High Fire Point, Biodegradable Ester Fluids


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Publication Details

Author list: Torbjörn, Skytt

Publication year: 2012


Because of a number of exceptional properties over mineral oil, ester fluids are now being used in more medium and large power transformers. At the same time, there are significant differences in the dielectric & thermal properties and other material parameters, between mineral oil and ester fluids, which need to be considered in the design, manufacture, retro – filling, and service of these transformers.

This paper presents results of investigations, calculations, and measurements made to identify the special material characteristics that need to be considered when designing the core and windings of these transformers. The paper also presents the factors that must be considered in the selection, design, and determination of the voltage and current ratings of bushings and tap – changers to be used in ester fluid – filled power transformers. In addition, the paper identifies properties of ester fluids that would require different procedures in the processing of th e fluid during manufacture and impregnation of the solid insulation in the transformer.

The factors that should be considered when an old mineral oil – filled power transformer is to be retro

– filled with an ester fluid are presented. These allow the transformer to benefit from the advantages of the ester fluid but not negatively impact its performance and reliability during future years of service. During transformer service, some oil quality parameters of ester fluids; such as power factor, acidity, interfacial tension, etc do not reflect the same correlation to fluid dielectric or thermal performance as they do in mineral oil. Also, some diagnostic parameters which determine the condition of the solid insulation in a mineral oil - filled power transformers will need to be considered differently in relationship to the different properties of ester fluids.

Finally, the paper presents how the advantageous property of longer life of cellulose in ester fluids could be evaluated and benefited from, resulting in more optimized design of ester fluid – filled power transformers.


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