Relaxation process of the discharge channel near the anode in long air gaps under positive impulse voltages

Abstract

The relaxation process of the discharge channel near the anode in a long air gap was observed using a Schlieren system with a temporal resolution of 5 µs and a spatial resolution of 70 µm. The dynamic characteristics of the decay process in the vicinity of the anode are obtained. The discharge channel evolves just as a growing mushroom in nature during the relaxation phase. Two physical quantities, angle θ and velocity v, are defined to describe the process in this paper. The average value of the angle and velocity under lightning impulses are 71.7° and 3.3 m s −1 respectively, while 7.7 m s −1 under switching impulses. A simplified model was established to simulate the formation of mushroom-shaped channel. The simulation and experimental results show that the formation and development of the mushroom-shaped channel are due to two factors. One is the convection of the high temperature and high pressure air near the anode produced by the first corona discharge; the other is the ionic migration. These two factors result in the phenomena that the cooling process in the vicinity of the anode is much more efficient than further into the gap, whereas the thermal conductivity of the anode may have little contribution to that.

Observations of the Channel Illuminations During Dark Periods in Long Positive Sparks

“…It is speculated that the channel illumination contributes to the corona initiated from the electrode tip. Moreover, our previous study (Zhao et al., 2017) on the evolution of mushroom structured channels around the electrode tip also shows that periodic corona discharges may occur there during the dark periods. These findings seem to challenge the traditional definition of dark periods in long positive sparks.…”

On the Velocity‐Current Relation of Positive Leader Discharges

“…The high-speed camera was operated at 340,000 frames per second (fps), or 2.93 μs per frame with an exposure time of 1.29 μs and a dead time of 1.64 μs, using the spatial resolution of 64 × 128 pixels. To synchronize all experimental data, the signal "Trig Out" produced by the oscilloscope was used to trigger the current measurement system and the high-speed camera (Zhao et al, 2017(Zhao et al, , 2018.…”

Relaxation characteristics of ion jet formed across the short air gap in the cone–sphere electrode under positive polarity DC voltage

“…At the streamer breakdown stage, the relaxation channel brightened, and then the part near the cone electrode was separated from the cone electrode and moved toward the sphere electrode at high speed. The ion jet was then dispersed in the mushroom cloudlike form near the sphere electrode [1]. Visually, sparks of the streamer began to be ejected from the HV electrode and gradually developed to reach the low-voltage electrode.…”

“…When the discharge current disappears, the discharge channel will present for a given time, which is also called a relaxation process [1,2]. If a new discharge occurs later, it goes along the previous discharge channel in the gap when the time interval is small enough [3].…”

“…One is the convection of the hightemperature and high-pressure air produced by the discharge and the other is the ionic migration. Les [7] and Zhao et al [1] have studied the relaxation process of the leader channel by the Schlieren techniques and thought that the relaxation process is mainly due to the convection of the high-temperature and highpressure air in the vicinity of the electrode in long air gap.…”