Electromigration Mechanism Of Ions In Soil And Effects Of Pore Structure Factors

With the development of power grid towards high voltage,high intelligence and high reliability,the electromagnetic sensitivity of the equipment becomes stronger,and the frequency of the electromagnetic interference source becomes higher,and the current passing through the grounding grid and the frequency component of the current show a diversified trend.These problems have brought great challenges to the performance of substation grounding system.It is urgent to consider the electrical characteristics of soil from the perspective of multi frequency.Ions electromigration are the main factor determining the electrical characteristics of soil,and pores are main channels for the migration of water,ions and air in soil.Therefore,it is necessary to study the spectral characteristics of relevant electrical parameters caused by the electromigration of soil characteristic ions in pores.The geological structure of grounding grids in China is becoming more and more complex.The special geological conditions such as high soil resistivity,frozen soil and saline-alkali soil pose challenges to its design,operation and maintenance,this makes it important to study the ions electromigration in complex soil structure.This paper focuses on the following aspects:Based on the capillary condensation effect of soil pores and the formation of the same electric double layer on the surface of soil pores,quartz glass and silica gel when they come into contact with electrolyte solution,the equivalent models of soil pores are established.The experimental circuit and experimental scheme are also designed,and the effects of pore diameter,solution concentration and ion type on the spectrum of electrical parameters are studied.In the COMSOL software,the current module is used to add an electric field,the general form of the partial differential equation module is used to calculate the pressure change in the fluid,and the dilute matter transfer module is used to simulate the changes of ion concentration under the influence of the electric field,soil pore models with different radius of curvature and different tortuosity are established,and carried out relevant research.It is found that the changes of the number of ions and the ratio of pore wall area to pore volume are the two main factors affecting the characteristics of the current and impedance angular spectra.It is verified that ions are electromigrated in the form of hydration in aqueous solution,and the number of water molecules that ions can stably adsorb is positively related to the charge density.It is proved that the charge amount,size and relative mass of hydrated ions are the main factors affecting their electromigration in aqueous solution.The experimental results show that all pore currents are capacitive under AC electric field,and the existence of capacitive current is confirmed by calculating the migration rate of ions.After decomposing the currents,it is found that the capacitive currents are generated in the low frequency band and high frequency band,and then it is known that the ions migration in the pores can produce capacitive effect,resulting in the existence of capacitive current.Based on the proposed equivalent circuit model and the spectral characteristics of the capacitance,the formation mechanism of the capacitive effect is analyzed in combination with the hydrate migration form of ions.The low frequency capacitance effect of soil pores is mainly caused by the electric double layer formed near the electrode,while he high frequency capacitance effect of soil pores is mainly caused by the noncoincidence of positive and negative ion groups during ions electromigration.By studying the influence of curvature radius,it is found that the change of soil pore curvature radius can cause the non-uniformity of electric field intensity and internal pore water pressure in the pores,and then affect the ions electromigration.By studying the effect of tortuosity on ion electromigration,it is found that the change of soil pores structure can also affect ions electromigration.In this paper,the electrical properties of soil are studied from the point of view of ions electromigration.The results can be used to analyze the ionization process of soil and the transient performance of grounding device in soil.It is of great significance to the optimization of soil frequency characteristic testing method,the design of transmission line grounding device and the safe operation of power system.