The contribution of oil-flow electrification to the leakage current of multi-point gaps under direct and alternating voltages

This paper demonstrates the effect of oil-flow electrification (OFE) on the leakage current in oil-cooled electric power apparatus using a closed cycle model. The current in multi-point gaps is measured as a function of oil-flow velocity and temperature, type and polarity of the applied voltage, number of points and their interspacing, and type of the solid insulating material used in such composite insulation system. The results reveal that OFE phenomenon exists for both energized and unenergized cases. It causes a polarity reversal for the current under positive direct voltage, while it augments that current for negative voltage case. The measured current increases exponentially with temperature and/or voltage. Higher number of points or interspacing between points leads to higher current under both direct (both polarities) and alternating voltages for both stationary and flowing conditions. Also, the experimental results are interpreted and discussed in the light of electric field computation using a package based on the charge simulation method. The electric field profiles, corona inception voltage and number of streamers are invoked to explain the trend of the experimental results.