Research On Characteristics Of The Stem In Long Air Gap Discharges Under Positive Impulse Voltage

The characteristic of stem is one of the key problems in the initiation and development of leader discharge.Due to the lack of experimental data,the research on the stem is limited,and the model of the stem can’t be verified directly.Therefore,this paper will focus on the experimental study on the characteristics of stem under positive impulses in long air gaps.Based on the characteristics of stem in long air gaps and the principle of schlieren system,the integral optical path was designed and the parameters of each module were calculated.Finally,a quantitative schlieren system was developed for 0.7～1.0 m air gaps.The temporal resolution was 5 μs,the spatial resolution was increased from 100 μm to 70μm,and the sensitivity was improved by 200%compared with the existing quantitative schlieren system.The number of stems on the surface of the H.V.electrode was counted based on the schlieren photographs.The statistical results showed that the number of stems on the surface of the H.V.electrode meets the logarithmic normal distribution approximately.With the decrease in the curvature radius of the H.V.electrode and the rising rate of the impulse,the average number of stems decreased gradually.The variation of thermal diameter of stems was calculated under different voltage waveforms.The relationship between the initial cross-sectional area and the initial corona current amplitude in 0.8 A to 7.5 A was obtained.The experimental results shown above laid the foundation for determining the initial conditions of the simulation model of stems.The method of calculating the temperature distribution of the stem was proposed by studying the background noise processing method of the shadow image,the coordinate correction method of the sample points and the Abel inverse transformation algorithm.The radial temperature distribution of stems was calculated under different voltage waveforms.The results provided direct evidence for the verification of stream-leader model.Two kinds of relaxation processes near the electrode and far away from the electrode were observed.The motion characteristics of mushroom air masses were obtained in the relaxation process which was in the vicinity of the H.V.electrode.The mechanism of this type of relaxation process was proposed,and a simulation model of the formation and movement of mushroom air masses was established at the same time.The research improved the point of views of the scholar Domens and Les Renardieres Group.The temperature distribution and thermal radius of the stems were calculated during the second type of relaxation phase which was a little far away from the H.V.electrode.The simulation model of type Ⅱ relaxation process was established,and was verified by comparing with the experimental results.