Investigation on charge transport processes in liquid dielectrics on the basis of high-purity paraffins

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In insulation systems of high voltage direct current transmission the electric field distribution is determined by the electrical conductivity of the insulation materials. Liquid dielectrics (transformer oil on mineral or natural basis) play a key role in insulation systems, whereas at the moment there is no model that is able to describe the charge transport and hence the electrical conductivity of liquid dielectrics correctly. The reason is, that several physical and chemical processes, which may be superimposed and interact with each other, contribute to the conductivity of liquid dielectrics and technical insulation liquids are a mixture of different chemical components.In this project a new and fundamental approach is chosen to investigate and differentiate the charge transport processes in detail. For the first time, paraffins, that are the main component of conventional transformer oil, will be used for the investigations. Due to the defined chemical structure of the paraffins, charge transport processes can directly be linked to the physical and chemical properties of the dielectric liquid. Moreover, for the research project paraffins can be chosen that solidify at room temperature so that states of polarization can be frozen and be investigated in detail. This also enables to produce high purity deionized paraffins that are best suited for the intended investigations. In the research project, the relevant charge generation processes, like the field emission (Foweler-Nordheim-Injection) and electrochemical dissociation at the electrode shall be analyzed. Furthermore bulk processes like the chemical sample composition as well as the field dependent dissociation are investigated in detail. The research work contains beside the time-, temperature- and field strength-dependent measurement of electrical conductivity also the determination of quantitative charge carrier densities and field strength within the dielectric by the so called pulsed electroacoustic (PEA) method, as well as a laser optic measurement system based on the Kerr-effect. These methods are used to verify the physical and chemical models which should be created. The goal of the research is to contribute to a better knowledge of the different conduction processes of liquid dielectrics and to understand the relevant processes in a quantitative manner. To reach this goal extensive knowledge and laboratory equipment in the field of high voltage testing and diagnostics as well as chemistry is needed. Consequently an interdisciplinary cooperation of the Institute for Power Engineering and High Voltage Technology (FHWS) with the chair for Chemical Technology of Material Synthesis (University of Würzburg) is foreseen, to bring together the different competences.