A brief review of the impregnation process with dielectric fluids of cellulosic materials used in electric power transformers

In the manufacturing of power transformers, the impregnation of the solid electric insulation systems (cellulosic materials) with a dielectric liquid is a key issue for increasing the breakdown voltage of the insulation, and this prevents the apparition of partial discharges that deteriorate the insulation system. After introducing the problem, this article presents the theory of impregnation and later carries out a bibliographical review. Traditionally, mineral oils have been used as the dielectric liquid in electrical transformers, but for environmental (low biodegradability) and safety (low ignition temperature) reasons, since the mid-1980s, their substitution with other ester-type fluids has been studied. However, these liquids have some drawbacks, including their higher viscosity (especially at low temperatures). This property, among other aspects, makes the impregnation of cellulosic materials, which is part of the transformer manufacturing process, difficult, and therefore this tends to lengthen the manufacturing times of these machines.This work was supported in part by the Spanish Ministry of Science and Innovation by the National Research Project Asset Management of Biodegradable-Fluid-Based Transformers under Grant PID2019-107126RB-C22/AEI/10.13039/501100011033, in part by the Universities and Research Council of the Government of Cantabria by the Grant “Biodegradable Fluids in Electrical Power Transformers: Solid Dielectric Impregnation and Thermal Modeling with Thermal Hydraulic Network Models (THNM)” under Grant VP32, 2019-2, and in part by the University of Cantabria through the Industrial Doctoral Program 2016, Scholarship DI13