Breakdown time lags and prebreakdown phenomena in transformer oil, effects of hydrostatic pressure

Abstract

Effects of hydrostatic pressure, degassing, voltage magnitude and polarity on time lags to breakdown and the prebreakdown phenomena in transformer oil were investigated using rectangular high voltage pulses and point-plane electrodes. The time lags measured under positive point polarity were significantly shorter than the negative point ones under the same conditions. Degassing the sample increased the time lags and the applied hydrostatic pressure caused the time lags to increase especially at the lower voltage range. At very high applied voltages, the dependence of time lags on hydrostatic pressure at the magnitudes used became insignificant especially when the point is positive. Prebreakdown disturbances in the liquid, spark-photographed by a high speed shadowgraph system were significantly suppressed by applied hydrostatic pressures above 10 atmospheres but they seemed to be unaffected by subatmospheric pressures. It was concluded that positive and negative point discharges had various differences both in the initiation and propagation phases. However, it was also found most probable that in both cases the discharges developed in pressure sensitive, low-density regions and charge multiplication was critical in the propagation mechanism.