Electrical conductivity measurement and determination of ion mobility in insulating oil

The insulation system of power transformers is a key topic which has to be taken into account in order to achieve an optimized design and safe longterm operation. The design of feasible insulation for high voltage direct current transformers requires the understanding of field distributions in insulating materials in both AC and DC operation because the transient stresses are dependent on both conductivities and permittivities and geometry. The steady state DC field stresses are determined by conductivity of insulating material which is normally a combination of oil and paper. Therefore knowledge about the conductivity of the oil is of great importance for design and safe operation in DC case. Moreover the space and surface charges will occur in field space and also will influence the electric field. If such charges are present in a noteworthy amount, models depending on material properties only will probably be an approximation. The conductivity of insulating material is dependent on several parameters, e.g. time, temperature and electrical field strength and the geometry of the arrangement. In this contribution, a new measuring cell with plate type electrodes is presented for analysis of oil conductivity. The measurement accuracy of the set-up is investigated both analytically and experimentally. Consequently, measurements are performed for investigating the time behavior of conductivity. The results show that the determination of ion mobility using polarity reversal test represents to a suitable approach to investigate the transit times more precisely.