A New Method for the Quantification of the Remaining Mineral Oil in Natural-Ester Retrofilled Transformers

Abstract

The use of natural and synthetic esters provides great improvements to the safety and environmental impact of transformers. The application of these liquids is rapidly spreading for small and medium size transformers and is becoming more common in larger units. In some cases, new transformers are installed that were designed to be filled with ester, but it is also usual to replace the mineral oil of transformers in operation by a natural or a synthetic ester in a process called retrofilling. During transformer retrofilling, the mineral oil is drained from the tank and, after some actions aimed at removing the mineral oil from the transformer active part, the tank is filled with an ester. Despite of the efforts to remove the residual mineral oil, there is always a certain amount of oil that remains adsorbed in the solid insulation that will flow into the ester once the transformer is in operation. Esters and mineral oils are compatible materials, but the flash point and other properties of esters worsen if the presence of mineral oil is high. The presence of mineral oil will also vary functional aspects as the electric field distribution in the transformer. This paper proposes a new methodology to determine the percentage of mineral oil that is present in a natural ester. The method, based on the determination of the iodine value of the oil, will be useful to assess the quality of retrofilling processes and improve the safety of that practice.

Life Expectancy of Transformer Paper Insulation Retrofilled with Natural Ester in the Laboratory

“…Although there is agreement on the fact that cellulose insulation ages more slowly in the presence of an ester, it is not so clear that this result can be extrapolated to the insulation of retrofilled transformers. Experimental evidence on this fact is mainly based on laboratory accelerated ageing tests carried out on cellulose-ester systems, but the liquid insulation of retrofilled transformers is a mixture of mineral oil (MO) and natural (NE) or synthetic ester [7][8][9], the MO being the material that remains mostly adsorbed in the paper [10,11].…”