The present study aims at the understanding of charge injection from metal electrodes into mineral oil under DC stress. The work focuses on the field dependence of the charge injection. In a previous article the electrical field in a blank oil gap with parallel electrodes is measured with a so called Kerr electro-optic technique. The data show that the electrical field is close to symmetric for low fields (~ 0.1 kV/mm) but strongly asymmetric at higher fields (~ 1 kV/mm). In the present study an ion drift model is used to analyze the data. We show that an ion drift model including a field dependent injection term can reproduce the measured results fairly well. Furthermore, the comparison of measured and simulated results shows that the charge injection is stronger from an electrode with positive polarity compared to one of negative polarity.During the last years it has been a push for Ultra High Voltage Direct Current (UHVDC) transmission systems, due to the rising demands on transmitting electrical power over long distances. A key component in such systems is the mineral oil insulated HVDC converter transformer. The increased voltage levels are calling for a deeper understanding of high field phenomena under dc stress in mineral oil based insulation systems.
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