Measurements of ion mobility in transformer oil: Evaluation in terms of ion drift

The aim of this paper is to develop a theoretical foundation of the reversed polarity method for measuring the mobility of ions in transformer oil. The results of the measurements of transient currents in oil, showing very distinct transient current peaks appearing after polarity reversal, are evaluated by computer simulations using an ion drift model. It is found from the analysis that the peaks in the measured currents occurred at instants much shorter than the so-called transit time that is the time for an ion to cross the oil gap between electrodes. A relation between the transit time and the current peak position is found that can be used to extract the ion mobility from data obtained with an experimental set up in which the transit time is shorter than the dielectric relaxation time of the liquid. On the other hand, for a setup providing the dielectric relaxation time shorter than the transit time, the current peak position strongly depends on the former and no simple correlation between the current peak position and transit times can be established.