Experimental Study on the Insulation Permittivity of Transformers Retrofilled with Natural Esters

Environmental or safety aspects might lead utilities to replace the mineral oil that serves as liquid insulation of transformers with a natural or synthetic ester. Although transformer's retrofilling is becoming common for certain applications, the replacement of the dielectric fluid by a liquid with different properties has its own challenges and some aspects of the process have not been studied in depth yet. One of the factors that might vary when the mineral oil is replaced with an ester is its impact on the electric field distribution within the transformer. The relative permittivities of esters and mineral oil differ and, in the case of retrofilled transformers, where the insulation system becomes a mixture of both fluids, the values of this parameter are uncertain. In this paper, an experimental study is carried out to investigate how the retrofilling process affects the permittivities of the transformer insulation system. The evolution of this parameter along time has also been studied, finding that, although there is certainly some migration of mineral oil towards the ester, the process is slow and it mostly affects the surface of the solid insulation.