Barrier effect on the prebreakdown and breakdown phenomena in long oil gaps

The influence of barriers on the dielectric strength of oil gaps is analyzed according to the electrode geometry, the dimensions and the position of the barrier, and the polarity of the voltage. It shown that the effectiveness of an insulating barrier, namely the ratio of the breakdown voltage of oil gaps in presence of barrier to that one without barrier, in a divergent field, is the higher when the barrier is placed near the sharp electrode (at 0 to 25% of the electrode gap), and when the polarity is positive. The breakdown voltage of a given gap in presence of barrier is roughly equal to that one corresponding to a gap without barrier the length of which is equal to the sum of the distance from the sharp electrode to the edge of the barrier and from the edge of the barrier to the electrode plane, except when the barrier is very close to the electrodes. By using a simple model, we estimate the optimal position of the barrier and its effectiveness. The experimental results are also discussed vs the capacitance and electrical charge corresponding to each configuration.