Ester insulating liquids for power transformers

The properties of ester insulating liquids, including both synthetic and natural esters, offer the potential for transformers with lower fire risk and greater environmental compatibility when compared with those filled with mineral oil. This potential has led to intensive studies of the electrical and thermal performance of ester liquids in recent years. A collective research effort was made to study ester liquids by a number of companies within the UK electric power industry particularly in respect of their application to large power transformers. With a liquid provider and a transformer manufacturer, extensive research has been carried out at a university into the dielectric aspects of the insulating liquids to prove the feasibility of using an ester liquid in place of mineral oil in large high voltage power transformers. This paper sets out the results of this research focusing mainly on the electrical performance of ester liquids. Experimental tests in both quasi-uniform and non-uniform electric fields including discharge initiation, propagation and breakdown were carried out under both AC and lightning impulse voltages. In addition to open oil gaps, the influence of pressboard in parallel to the oil gap was also investigated. The comparison between the performance of ester liquids and mineral oil, allows conclusions to be made regarding the application of ester liquids in power transformers.