Laboratory study on negative spark inception direction and breakdown characteristics in rod-rod air gaps

To better understand the inception direction and breakdown characteristics of negative sparks in rod-rod air gaps, experiments have been carried out with negative 250/2500 mu s voltage impulses in 0.4m-1.2m air gaps. Three types of air gaps were used including rod-double rods air gaps and two kinds of rod-rod air gaps. Initial angle of spark channel defined as the angle between tangential directions of the initial spark channel to vertical was found to follow Normal distribution in rod-rod air gaps. The average initial angle of negative sparks in rod-rod air gaps is 14 degrees-62 degrees which is larger than that of positive sparks in 0.5m rod-plane air gaps and close to those in 7m-8m rod-plane air gaps. The inception direction and breakdown characteristics of 0.4m-1.2m rod-rod air gaps are influenced by gap distance, grounded rod tip shape and applied voltage amplitude. The negative 50% breakdown voltage increases linearly with gap distance, which is also influenced by gap configuration. As gap distance was elongated, longer upward streamer occurred at conical grounded rod tip resulting in larger attraction and higher striking probability of grounded rod with conical tip; with higher voltage amplitude, the initial angle and striking selectivity of grounded rods become more random.