Barrier effect on the time lag to breakdown of transformer oil gaps under lightning impulse voltage

This paper deals with the variation of the time lag to breakdown upon the physical and geometrical parameters of a barrier inserted in a divergent point-to-plane electrode system under lightning impulse voltages. It is shown that in presence of an insulating barrier, the time lag to breakdown is significantly increased. This increase mainly depends on the barrier position in the gap, its radius and the amplitude and polarity of the applied voltage. The highest time lag to breakdown value is observed when the barrier is placed near the sharp electrode whatever the other parameters (barrier radius, applied voltage,...). On the other hand, the variation of the time lag to breakdown seems to be inversely proportional to the amplitude of the applied voltage. It increases when the minimum leakage length of the discharges (i.e. the sum of the distance between the point and the barrier edge, the barrier thickness and the distance between the barrier edge and the plane electrode) increases.