The dynamic expansion of positive leaders observed using Mach-Zehnder interferometry in a 1-m air gap

Abstract

The leader plays an important role in long-air-gap discharges. In this paper, Mach-Zehnder interferometry and a high-speed video camera were used to observe the dynamic expansion process of positive leaders near the anode in a 1 m air gap. The leader diameters under lightning and switching impulse are obtained through the analysis of interference fringes. The influences of the applied voltage, including the amplitude and the front time, as well as the electrode sizes on leader expansion are obtained and analysed. For a 0.5-cm-diameter cone electrode, when the applied voltage amplitudes are 330-419 kV, the diameters of the leaders are 1.5-2.5 mm at time scales of less than 195 {\mu}s, and the diameters increase as the voltage rises. The diameters of the leaders are larger and the expansion rates are higher for shorter front times. The average expansion rates are 72.30-9.54, 28.09-5.05, 14.38-3.02 and 5.73-1.44 m/s for front times of 1.2, 40, 100 and 250 {\mu}s for a 0.5-cm-diameter cone electrode. A larger electrode size leads to a wider diameter. A numerical model was employed to analyse the expansion of the leaders, and the calculated results are in good agreement with the experimental data. Based on the model, the mechanism underlying the leader expansion is discussed in detail.