The effect of organic additive content on the direct voltage breakdown strength of degassed transformer oil and liquid paraffin is reported for a wide range of concentration of additives known to be effective gassing inhibitors. Their effect on the breakdown strength of oils saturated with hydrogen, nitrogen and oxygen is also reported. Measurements of the gassing properties of the oils were carried out for the same extended range of additive concentrations. The observed breakdown and gassing versus concentration characteristics indicate the presence of maximum and minimum points at concentrations which are independent of the matrix liquid and of dissolved gas and gas phase. There is a remarkable degree of correlation between the breakdown and the gassing properties of the liquids tested. This and other relevant observations indicate that gas generation is an intrinsic part of the breakdown process and strongly support the bubble theory of breakdown. This forms the basis for the discussion of the results.

The breakdown voltage of degassed and gas-saturated transformer oil has been measured as a function of concentration of organic additives known to affect the breakdown strength of the oil. By using a wide range of additive concentrations, it is shown that the existence of optimum concentrations in the breakdown voltage against concentration characteristics which have hitherto been reported is in fact only one phase of a more complex relationship between breakdown voltage and additive concentration. The characteristics are markedly affected by the presence of gases dissolved in the oil.

The direct breakdown voltage of degassed transformer oil has been measured as a function of gap length using point-sphere electrodes, for both polarities of the point. Although changing the material of the sphere electrode altered the magnitude of the breakdown voltage for both point polarities, the critical gap length at which the breakdown voltage was the same for both polarities remained unchanged. This critical gap length was markedly reduced by the presence of dissolved oxygen.

The dependence of the direct electrical breakdown voltage of degassed transformer oil and of gas-saturated oil on gap length for a point-sphere electrode system has been studied for the two polarities of the point. The effects of anodization of the sphere electrode on the breakdown voltage against gap characteristics have also been examined. The critical gap length at which the breakdown voltage is the same for both polarities of the point has been found to depend on the nature of the insulating film on the sphere electrode and on the type of gas dissolved in the oil. The discussion of the results is based on the propagation of a corona discharge from the point electrode and the possibility of formation of a back streamer from the sphere electrode.

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