Heat Transfer Coefficient of Mineral Oil/Natural Ester Mixture Resulting from Retrofilling in Power Transformers

Mineral oil is being replaced by esters in power equipment. This is due to the environmental and fire protection properties of esters. One such example is retrofilling in power transformers, which involves changing oil into esters. A small amount of oil, up to 15%, remains after such a process. As a consequence, an ester/mineral oil mixture is formed. The cooling efficiency of transformers is determined by the heat transfer coefficient α of insulating liquids. This coefficient for oil is higher compared to esters. The following question arises: what is the value of coefficient α of the mixture after the retrofilling? Using a laboratory system, coefficient α of the mixture was measured, the oil content of which was equal to 0%, 5%, 10%, 15% and 100% (pure oil). Measurements were made at the top and bottom of the system for various surface heat loads. It was determined that as the oil content in the mixture increases, coefficient α disproportionately increases. Coefficient α had higher values in the upper part of the system than in the lower part. With the increase in heat load, factor α of the mixture increased, and the influence of the mineral oil content on the values of coefficient α decreased.