The Mysterious Motion of Long AC Arcs Understood

Michael A. Greenwood

The motion characteristics of long AC arcs in atmospheric air have been explained by researchers who say that the findings could result in improved safety and stability for power systems.

A typical pattern of a long AC arc burning in atmospheric air. Reprinted with permission of Applied Physics Letters.

The research team, led by Jinliang He of the State Key Lab of Power System at Tsinghua University in Beijing, noted that, whereas the motion of short arcs, DC arcs and arcs in closed space have received much attention, relatively little was known about the very distinct motion characteristics of long AC arcs in atmospheric air. They also noted that the long AC arc is a frequent phenomenon in power systems that can potentially damage equipment, oftentimes causing serious faults in a power system.

In the team’s experimental system, an arc horn with two steel electrodes was added parallel to the insulator. The distance between the upper and lower electrodes ranged from 0.9 to 2.1 m, and the arc current was as high as 31.5 kA. A fuse wire connected to the electrodes triggered the arc. By capturing images of the long AC arc with a Photron Fastcam video camera, the researchers determined the arc’s motion and characteristics.

They found that anode and cathode arc roots have different motion characteristics because of different formation mechanisms. During a half cycle of the arc current, the anode arc root moved toward a magnetic force and occasionally exhibited a jumping motion. The cathode arc root was slow enough to be considered stationary and essentially had no jump. The investigators noted that an arc column is governed by magnetic force and also is quickened by the lower arc root.

Applied Physics Letters, Jan. 29, 2007, Vol. 90, 051501.