Zhou Xuan

Investigation of long air gap discharges is important to explain insulation problems in UHVDC system. Leader discharge is one of the main phases of long air gap breakdown. In this paper, a long optical path (approximately 4 m) Mach-Zehnder interferometer was set up to determine thermal parameters of leader discharges in atmospheric air quantitatively. The IEC standard positive switching impulse voltage was applied to a 0.93 m p-plane gap. Filamentary column of gas density reduction can be observed in fringe images. The diameter of the column including inner leader channel and outer hot gas is 1.5-3.6 mm with an average expansion velocity of 6.7 m/s and the typical value of average gas temperature is 600-1000 K. The thermal diameter of the leader itself should be less (~1/5-1/3 of the whole column diameter) and the average temperature should be higher.