Under DC voltage stress dielectric liquids reveal a non-linear electrical conductivity, which cannot be sufficiently modelled with conventional RC equivalent circuit diagrams. Rather, a comprehensive chemical-physical understanding of charge generation and transport processes is required to describe the electrical conductivity of liquid dielectrics. To investigate the basics of electrical conduction phenomena, chemically pure liquids, i.e. paraffins, are used, because transformer oil consist of a mixture of several different chemical species and their influence on the electrical conductivity is difficult to describe. By adding step-by-step known impurities to a pure paraffinic model oil, the influence of these impurities on conduction and space charge formation, hence field distribution can be investigated. This contribution shows the influence of nonanoic acid on the electrical conductivity and field distribution of pure n-dodecane. By adding nonanoic acid to the paraffinic model oil, the electrical conductivity increases and is getting closer to that of transformer oil. Furthermore, a connection between the nonanoic acid and a field distortion measured by the electro-optic Kerr-effect could be found. It is shown, that the influence of a specific impurity can be assigned to the dielectric behaviour of liquids.
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