Surface Backward Discharge Patterns Under Full and Chopped Negative Impulses in C4F7N/CO2 Mixture

Backward discharge, or reverse discharge, has been observed in experimental investigations under impulse discharge. We found the different residual charge accumulation characteristics under full and chopped lightning impulses in previous work, which is suspected to be caused by different surface backward discharge processes. However, the specific processes and mechanisms are not yet clear. In this study, the surface backward discharge patterns under full and chopped negative standard lightning impulses in C4F7N/CO2 mixtures are investigated. The residual charge distribution of the surface discharges under full and chopped impulses is studied, and their difference is discussed. The backward discharge under full impulse appears as streamer discharge with a lower intensity, and more charges are accumulated on insulator surface after discharge. The backward discharge under chopped impulse occurs through activating the discharge channels of forward discharge at the chopping moment. It exhibits a strong intensity and a longer discharge distance. Only a small amount of charge accumulates on insulator surface after backward channel activation discharge. The effect of chopping moment on backward discharge patterns is also studied. The formation conditions of the two types of backward discharges are determined, and the formation process of backward channel activation discharge is established. Due to the different patterns of surface backward discharge, surface charge accumulation is more prominent under full lightning impulse compared to chopped impulse. This will have an impact on the subsequent development of the surface discharge.