Investigation of the electrical breakdown of transformer oil in Skowronski´s cup-sphere spark gap

The cup-sphere spark gap designed by Skowronski is used to check the effects of impurities and dispersed water on the breakdown of transformer oil. Several different filters, with porosities varying from 0.05 to 2¿m, are used for filtering oil samples. Filtering through a sintered-glass filter with a porosity of 2¿m does not affect the breakdown of the oil, but some improvement in oil properties is observed when sintered-glass filters with porosities of 1.5 and 0.8¿m are used. Filtering through plastic filters with porosities of 0.4, 0.1 and 0.05¿m results in considerable increase in breakdown stress for all gap settings, accompanied by significant changes in the breakdown-voltage/gap-setting characteristics. The values recorded are much greater than those obtained using Kok´s formula for the size of remaining particles in the oil. The effect of dispersed water is also studied, using the same cell and a 1 mm gap setting. The presence of water greatly affects the breakdown stress, which drops from 406kV/cm for dry oil to 63kV/cm for oil containing 0.06% of water. It is also found that the effect of particles in dry oil is less pronounced than that in oil containing dispersed water. The results presented are in good agreement with those published by Skowronski, who used the same cell in his work.