Non-Uniform Field Flashover Characteristics in SF6 Gas under Very Fast Transient Overvoltages

SF6 gas has an excellent dielectric strength for the uniform electric field, however, flashover voltages are reduced for non-uniforrn electric field. To clarify the flashover mechanism, this paper deals with the effects of applied voltage waveforms and gas pressure in the presence of strong inhomogeneous field. For discharge detection, high-speed streak photographs and current measurements were taken. Experimental results proved that voltage-time characteristics have a minimum value in the time range between several micro seconds and 10 micro seconds. Flashover voltages increase for long wave front duration by corona stabilization effects, however, under fast-rising impulse and fast-oscillating impulse voltages leader type flashover appears immediately after the voltage application. Under fast-oscillating impulse voltages, flashover voltages are nearly equal to those of nonoscillating impulse voltages in the time range of several micro seconds, however, bipolar currents flow before flashover occurrs. The corresponding high-speed streak photograph reveals that luminous events and eliminations appear alternately. This indicates that the back discharges occur in the injected space charges. It also clarifies that discharge propagations occur when positive leader currents flow, and they do not occur when negative leader currents flow. These results show that the flashover mechanism for oscillating impulse voltages is a little different from that for non-oscillating impulse voltages, however, flashover voltage is determined by positive leader velocity.