Investigations on the Polarity Dependent Charge Carrier Injection in Dielectric Liquids at High DC Voltage Stress

A reliable operation of HVDC converter transformers requires an optimized dielectric design of the oil-paper insulation system. Therefore, the dielectric behaviour of the oil-paper-insulation at high DC voltage stress needs to be investigated. In previous investigations, a charge carrier-based approach is utilised to explain occurring effects like the nonlinear, field strength dependent electric conductivity of mineral oil and to model the dielectric behaviour of mineral oil at high DC voltage stress. This model considers different species of charge carriers, their drift and their effect on the electric field distribution. These physical processes can be modelled using the Poisson-Nernst-Planck equation system. Both species of oil-intrinsic charge carriers and charge carriers injected from electrodes play a significant role in the nonlinear conductivity of mineral oil and affect the dielectric stress of oil-paper-insulated arrangements. Furthermore, a polarity effect in the electric conductivity of mineral oil was observed, which is a result of the different injection rates of electrons and holes from bare metal electrodes. The aim of this study is to examine the nonlinear conductivity of mineral insulating oil and to apply the results on the charge carrier-based approach in order to investigate the injection of charge carriers and their influence on the dielectric stress. Current measurements are carried out at an inhomogeneous electrode arrangement at DC voltage stress at both polarites in order to investigate the electric conductivity of an aged and unaged naphthenic and a paraffinic mineral oil. Furthermore, the pure paraffin n-Dodecane was used to investigate the influence of the different components of mineral oil on its dielectric behaviour. The results show differences in the polarity dependent injection rates of the investigated liquids and between the aged and the unaged mineral oil. The different components of the used oils as well as ageing products affect the injection of charge carriers and thus the measured conductivity. Therefore, it is assumed that the different ingredients of mineral oils affect the generation of charge carriers and thus the dielectric stress of an oil-paper-insulated arrangement at DC voltage stress.