The effect of the electrode gap on the breakdown process in liquid dielectrics

It is well known that the breakdown voltage of an insulating liquid increases nonlinearly with increasing gap between the electrodes. Under nonuniform field DC conditions, two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm or larger, it is found that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. The implications of these observations are discussed in some detail