Relationship between Morphology Feature of Contact Surfaces and Impulse Breakdown Voltage in Vacuum Gap under a Non-uniform Electric Field

Morphology feature of contact surfaces plays an important role in the impulse breakdown voltage of vacuum gaps. However, surface roughness R_a can't describe the morphology feature of contact surfaces effectively. Fractal geometry is an effective way to describe the irregular phenomenon of the feature of the contact surfaces in nature. The objective of this paper is to research the relationship between the morphology feature of contact surfaces and the lighting impulse breakdown voltage in vacuum gap under a non-uniform electric field with rod-plane contacts. A surface roughness R_a and a fractal dimension D were used to feature the contact surfaces morphology. In experiments, the material of the rod-plane contacts was Cu. The lighting impulse insulation characteristics of the vacuum gaps were obtained by using up-down method. The morphology features of the contact surfaces were observed by a 3D laser microscopy. The structure function method was used to estimate the fractal dimension D of the contact surfaces. The results indicate that the initial morphology feature of the contact surfaces had little effect on the impulse breakdown voltage of the vacuum gap. For one certain material, it was found that the tip surface of the cathode rod contact has the same fractal dimension D. The 50% impulse breakdown voltage U₅₀ decreased with the increase of the rod tip surface roughness Ra after conditioning. Moreover, this relationship satisfied an approximate power law function. The research can help to understand and improve the insulation of the vacuum interrupters.