Prebreakdown phenomena in transformer oil subjected to nonuniform fields

These investigations were conducted to record Schlieren photographs of the prebreakdown phenomena in transformer oil subjected to nonuniform fields at room temperature and atmospheric pressure. Various states of contamination were also studied. Positive and negative voltage polarities were used, and an estimate of the disturbance temperature was made by measuring changes in refractive index. Breakdown was initiated by disturbance channels propagating from the point electrodes; the positive-point disturbance being more filamentary and propagating much faster. The propagation rate increased when contaminated by solid particles, the effect being enhanced by prestressing. No significant change was detected with dissolved air or oxygen. However, when either gas was added to oil containing solid particles, the propagation rate tended to decrease after prestressing. The disturbances consisted of a liquid at a higher temperature than the surrounding bulk. It was suggested that the disturbances contained significant net space charge, which distorted the applied field and which was propagated by electron-collision processes. The effect of solid particles was explained by local field enhancement and the effect of prestressing by the motion of particles into or away from regions of strong field.