Dielectric properties of transformer oils for HVDC applications

The knowledge of the behavior of electric conductivity in mineral oils for possible applications in HVDC converter transformers is of paramount importance for proper design of their insulation system. This study presents the results of measurements of dielectric properties of various transformer oils by means of frequency response technique as well as time domain measurements, including measurements of ion mobility using the reversal polarity method. Influences imposed by varying the measurement voltage level and temperature are investigated. Some important parameters of the investigated oils, e.g. their conductivities, ion mobilities, ionic concentrations and effective ionic radiuses are compared and discussed. The results show that despite of similarities in various physical parameters of insulating mineral oils available on the market, the dielectric behavior and especially ionic conduction vary greatly between different oil types. Changes of these properties with temperature are characterized by different activation energies.