Characterization of inhomogeneous field breakdown in natural ester liquid compared to mineral oil

Natural ester liquids as insulation material in power transformers offer many advantages compared to conventional mineral oils. Some have, for example, higher thermal capacity, higher flash/fire points 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 indicated a lower breakdown voltage for vegetable-based oils. Our contribution illustrates the influence of the electric field distribution on the breakdown voltage of a natural ester liquid compared to mineral oil for different degrees of inhomogeneity. The breakdown voltage tests are performed for AC and lightning impulse 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. AC breakdown voltage is determined by a stepwise rise of voltage as described in IEC 60060-1. Different types of electrodes are used in different gap distances, for representing several levels of inhomogeneity of the electric field. Envirotemp™ FR3
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natural ester liquid and Lyra X mineral oil were used in this study. The results of the measurements are compared, taking into account the degree of inhomogeneity. An electrode configuration composed of a needle of 0.06 mm tip radius, despite not representative of any real construction of any transformer, is widely applied for investigation of breakdown phenomena. The use of a needle of very small radius qualifies the D3300 test as a “scale test” as it creates high levels of voltage gradient using relatively low voltage application, allowing researchers to study partial discharge inception behavior and other breakdown characteristics. In mineral oil the partial discharges start to occur at lower voltages than in natural ester, but the difference of voltage between the inception voltage and the disruptive condition is larger, resulting in a higher breakdown voltage. Studying several different configurations from spherical electrodes to needle configuration, allows correlating the degree of inhomogeneity of the electric field with the breakdown characteristics where the equivalency of the breakdown voltage is confirmed. This paper includes also an assessment of the impacts of these differences for the dielectric design of power transformers.