Study On The Characteristics Of Soil Discharge And Recovery And Transient Model Under Successive Impulse Currents

Grounding system acts as an important safety protection apparatus to ensure the stable operation of power systems,reliable operation of equipment and personal safety,which is one of the vital components of power systems.The transient characteristics of lightning impulses on grounding devices directly affect the accuracy of research results such as lightning protection calculation of transmission lines,lightning current divergence performance of grounding systems and lightning striking characteristics of upper equipment in grounding systems.Soil media as the main dispersive medium,its impulse discharge characteristic is also one of the important research content in the field of grounding.However,because it is difficult to observe the impulse discharge process in soil and to measure the key parameters accurately,the mechanism of soil impulse discharge is not fully understood at present.The accuracy of the soil discharge parameters and the calculation results of transient analysis model need to be improved.In addition,previous studies on soil discharge characteristics use single-impulse currents to simulate actual lightning strikes,which ignore the fact that a lightning discharge contains several subsequent strokes,and cannot fully characterize the actual dispersive process of lightning currents in soil.Based on the above research background,this paper carries out a study on the characteristics of soil discharge and recovery and transient model under successive impulse currents,which reveals the development process of soil impulse discharge and the effects of several key factors on it.The main research contents and results are as follows:(1)An experimental platform was established for studying the soil discharge characteristics under successive impulse currents.The time interval between successive impulses can be continuously adjusted from 0 to 500 ms,and the parameters of impulse current waveform can be independently controlled.The soil discharge characteristics under successive impulse currents was experimentally obtained.The research results show that in the actual average time interval of lightning multiple impulses,the subsequent impulse voltage of soil is obviously lower than the first impulse voltage in the same current peaks,and the maximum attenuation of subsequent impulse voltage can be above 80%.The attenuation of the subsequent impulse voltage has recoverable characteristic,and the recovery time varies from ten to tens of milliseconds.It was found that the recovery rate of subsequent impulse voltage increases with the increasing soil water content,but the soil salt content had no significant effect on the recovery rate of impulse voltage.(2)Based on X-ray imaging technology,the morphology and structure of discharge channels were carried out in soil under successive impulse currents.It was found that under successive impulse currents,the discharge channels in soil have various morphologies,such as single channels and multiply-branched channels.And classified them according to morphological characteristics.Excavate the gray value information of discharge channel images,a method of extracting discharge channel volume based on gray-level difference of single-view image is proposed.Also the changing rules of structure characteristics of discharge channels in soil with different time intervals between successive impulse currents are analyzed.The results indicate that as time interval increases,the generation probability of multiply-branched channels increase.Based on the analysis of the characteristics of soil discharge and morphology of discharge channel under successive impulse currents,the development process of successive impulse discharges in soil are analyzed.(3)The calculation methods of soil discharge characteristic parameters under successive impulse currents were studied.The change rules of residual resistivity and critical breakdown field strength of soil medium were obtained during subsequent impulse discharges.The results indicate that when the soil breakdown in second time caused by subsequent impulse current,the residual resistivity and critical breakdown field strength in the discharge area decrease significantly compared with that during the first breakdown process.And the value of attenuation coefficient is approximately equal to that of the recovery coefficient.Within the scope of the test in this paper,the lowest residual resistivity of soil when it is breakdown in second times can be 0.00014%??₀,and lowest secondary critical breakdown field strength of soil is approximately 50k V/m.(4)A dynamic growth model was proposed for discharge channel,comprehensively considering the effects of successive impulse currents and the structure characteristics of discharge channels.An improved four-zone structure model of impulse discharge in soil was proposed,and a finite element analysis model for dynamic growth of discharge channel considering the interaction between spatial structure of arc channel and time-varying electric field is proposed as well.Which improves the fit between the results of transient analysis and the actual situation,and improves the comprehensiveness of analysis of impulse discharge area.Besides,the accuracy of the proposed model is verified by comparing the simulation results with the experimental results.The evolution of transient potential,current density and the growth process of discharge channels are obtained by analyzing the typical soil discharge process under the successive impulse currents.