Visualization of Positive Leader Channel Expansion in a 3m rod-plane gap by Schlieren Method

Long air gap discharge plays an important role in the breakdown of external insulation and lightning physics, and the leader propagation is the main process of long air gap breakdown. The leader channel is thermal plasma with high conductivity. The thermal radius of leader channel is an important macroscopic parameter to represent the expansion of a continuously propagating leader. In this paper, the schlieren diagnostics was employed to record the leader channel expansion in a 3m rod-plane air gap under positive switching impulses. The change of thermal radius with time was derived from the recorded Schlieren photos under the breakdown and the withstand scenario, respectively. The observed thermal radius changes are compared with prediction results by using the Gallimberti model. Finally, the impact of thermal conduction and convection loss on the leader expansion is discussed.