Experimental And Simulation Research On The Positive Stem During Dark Period In Long Air Gap Discharge

Stem is the focus of attention for studying the leader initiation and development of long air gap discharge.Grasping the evolution characteristics of stem in dark period is of great significance for understanding the physical mechanism of long air gap discharge.The inadequacies of the existing research limit the in-depth understanding of the evolution characteristics of stem in dark period,which leads to the majority of simulation models assuming that the dimention of stem did not change during the dark period.In this thesis,the stem in the positive impulses was taken as the research object,by improving the experimental platform and algorithm,the initial structure and the evolution characteristics of stem during dark period under the positive switching impulse voltage were obtained.Based on the observation results,a simulation model for anlayzing the evolution characteristics of stem during dark period was built,and the effects of different conditions on the evolution characteristics in dark period were simulated.According to the characteristics and observation requirements of stem,the quantitative schlieren observation system was improved by reasonably designing the high-speed imaging system,optical lens group and light source system.The Schlieren system has high sensitivity,with a spatial resolution of 70 μm and a temporal resolution of 5 μs.An experimental platform was built,and the synchronous measurement of Schlieren images,voltage,and current were achieved.Aiming at the ill-posed problem of inverse Abel transform,high-order Tikhonov regularization(hereinafter referred to as TV regularization)method is introduced to realize the calculation of gas density and gas temperature of stem,and the anti-noise ability of inverse Abel transform was improved.The calculation error of the thermodynamic parameters of stem is reduced to within 10%.The observation experiments of the stem under operating impulse voltage were carried out,and the thermal diameter and thermodynamic parameters of the streamer stem were calculated.According to the axial thermal diameter distribution characteristics of stem,two initial structures of r stem at the end of the electrode are defined,namely cylinder-shape and frustum-shape,and the typical parameters of two types of stem were obtained.The statistical results showed that the magnitude of the first corona current directly affected the initial structure of stem.The influences of humidity and voltage rise rate on the dark period time and initial corona charge were analyzed.The evolution characteristics of thermal diameter and axial length in dark period were acquired.It was found that initial path of newly-born channel was different between the two types of stem.Based on the observation results of stem,by considering the migration process of space charge,as well as thermodynamic processes such as heat conduction,heat convection and thermal diffusion,a model used to analyze the evolution characteristics of stem in dark period was established.The time-varying results of thermal diameter and axial length was used to verify the rationality of the model.Based on the proposed model,simulations were carried out to obtain the effects of voltage rise rate and electrode shape on the morphological evolution characteristics of stem.It was found that the migration process of space charge and the thermodynamic process have a common effect on the axial elongation of stem.And with the change of voltage rise rate and electrode shape,the contributions of the two conditions are different.The gas temperature evolution of two types of stem was simulated,the difference of the thermodynamic processes at the axis between two types of stem was analyzed.Combined with the calculation of the equivalent electric field,the reason for the difference that initial path of newly-born channel was clarified.