Comparative study on inhomogeneous field breakdown in natural ester liquid and mineral oil

Using natural ester liquids as insulation material in power transformers offers many advantages compared to conventional mineral oils. There are, for example, higher flash and fire point, greater water solubility, and better environmental sustainability. Despite several studies confirmed the equivalent dielectric capacity for the typical construction elements of transformers, some studies representing extremely inhomogeneous field distributions have indicated a lower breakdown voltage for vegetable-based oils. This investigation illustrates the influence of the electric field distribution on the breakdown voltage of vegetable-based oils compared to mineral oils. The breakdown voltage tests are performed using different experimental setups and different types of excitation voltage. The breakdown voltage of a standard 1.2 / 50 μs lightning impulse is measured based on the breakdown test defined in ASTM D3300. Different types of electrodes are used in different gap distances, for representing several levels of inhomogeneity of the electric field. The less inhomogeneous field distribution is achieved using two sphere electrodes at low sparking distance. For the extremely inhomogeneous field distribution, one sphere electrode is substituted by a needle electrode having a defined tip radius. Oils used are Envirotemp™ FR3™ fluid as natural ester liquid and Lyra X as mineral oil. Both oils are dried and degassed to a relative moisture content of <; 15 % saturation before starting tests. The result of the breakdown voltage obtained from a slightly inhomogeneous electric field distribution show a comparable dielectric strength for both oils. However, in extremely inhomogeneous electric fields, a decrease of breakdown voltage of natural ester liquid compared to mineral oil occurs. This is due to differences in the chemical structure and discharge mechanisms in natural ester in comparison to mineral oil. The main aim of study is to determine the limit of inhomogeneity for keeping the equivalency of dielectric behavior.