The influence of impurities on electrohydrodynamic motion in insulating oils

The authors present results of an investigation of the influence of inherent and added impurities in transformer oils on electrohydrodynamic (EHD) instabilities. All the test samples were subjected to a voltage step applied to parallel plane electrodes. Two of the test liquids were sampled from in-service transformers. The magnitude of change in the current in transformer oils after filtration (0.45 μm) was three to five times larger than the change caused by the addition of conducting particles. The transformer oil aged in service for longer duration showed higher EHD current and flow which were associated with higher particulate concentration and conductivity. The measurement of AC breakdown strength was less reflective of the extent of oil degradation as compared to transient current response. The space charge density variation that causes EHD instability in insulating oils depends on both suspended and dissolved impurities