Measurement of ion mobility in transformer oils for HVDC applications

Conduction process in dielectric liquids usually occurs due to ion migration and is therefore characterized by concentration and mobility of the ionic charge carriers. For this reason accurately determining these parameters and especially ion mobility in insulating oils for HVDC applications needs to be done precisely. In this report various methods for determining ion mobility are described, including single polarity, reversal polarity methods in time domain as well as frequency domain based measurement of dielectric response. The advantages and disadvantages of each of them are presented and illustrated with the results of measurements on various oils using two different test cells. The dependence of ion mobility on measuring voltage level and temperature are analyzed and compared.