Investigation on DC conductivity of alternative insulating oils as an application for HVDC converter transformers

The application of alternative insulating oils in HVDC converter transformers is new and is still at the beginning of the realization. Alternative insulating liquids such as Midel7131 and FR3 promise the use of environmentally friendly and resource-saving insulation systems. In order to be able to use these ester-based insulating liquids in HVDC equipment, it is necessary to characterize the DC conductivity among others. For this purpose, the investigations deal with the statistical evaluation of the leakage currents at different polarities. The temperature dependence of the insulating liquids is also considered in detail and conductivity minima are shown. In addition, deviations between end values are displayed after different times. The modelling and simulation of the results shows a possibility of predicting the newly gained insights.