Experimental Study And Mechanism Analysis Of The Frequency Characteristics Of Electrical Parameters Of Soil Medium

With the development of modern science,the electrical properties of soil are attracting more and more attention.And it is not only the field of lightning protection and grounding in traditional electrotechnics,but also the environment,geotechnical engineering,soil and energy.As a typical porous medium,the conductivity of the soil is the decisive factor of the grounding system parameters,and is also the basis of grounding design.The progress of soil electrical research can also promote the development of related disciplines.The impacting ground resistance is closely related to the impact characteristics of the grounding electrode.Studying the impact characteristics of soil is the key to study the phenomenon of soil spark discharge around the grounding pole of transmission line tower under the impact of lightning current,and is also the foundation of lightning protection for power transmission and transformation engineering.First of all,in this paper,we build a soil impact test platform and study the impact characteristics of soil with different matrix types through the test method.And then we analyze the impact waveform of voltage and current.The experimental results show that: due to the influence of spark effect and inductance effect,the impulse grounding resistance of all kinds of soil samples will undergo three stages: steep descending area,jump area and flat area before and after being breakdown.In the steep descending area,soil resistance value is very high and the inductive part accounted for a very small that can be neglected.The soil at this time is in a state of resistivity.In the flat area,soil resistance value is small and the proportion of the inductive part becomes larger that can’t be neglected.The soil at this time is in a state of inductivity.And the higher the water content,the greater the proportion of the inductive part of the soil impact grounding impedance.When the soil is broken,the higher the water content is,the smaller the jump area of the impact resistance is.However,the greater the size and porosity of soil,the greater the jump zone of the ground resistance.Secondly,in order to further improve the frequency characteristics of the inductivity and the resistance changes of soil impedance,we build the experimental circuit for the frequency characteristics of soil electrical conductivity to measure the variation of impedance angle of yellow soil,latosolic red soil and red soil with frequency and the influence of water content on the frequency characteristics of soil impedance angle.It is found that when the current frequency is low,the impedance angle of the soil sample is less than 0,which presents a capacitive characteristic.With the increase of frequency,the soil impedance angle increases rapidly.However,when the frequency increases above 1000 Hz,the change of the soil impedance angle becomes slow.Then when the water content is high,the soil impedance angle is less than 0.With the decrease of water content,the impedance angle increases to more than 0,and with the further increase of the frequency,the impedance angle decreases to 0.Based on the analysis of the phenomenon,the migration of ions in soil under electric field is the main cause of capacitive characteristics of soil impedance,and the inductance of ion channels formed by pore water is the main reason of perceptual characteristics of soil impedance.The two factors interact to form the frequency characteristics of soil impedance angle.Finally,permeability and permittivity are also closely related to soil impedance angle.We also set up a test circuit to measure soil permeability to measure the rule of the relative permeability of soil represented by fine sand with the change of frequency and water content.The study found that the soil is a diamagnetic material like water.Due to the impact of diamagnetic molecules additional magnetic moment and eddy current effect,the higher the concentration of the ions in the soil medium and the greater the alternating frequency of the external magnetic field,the lower the relative permeability of the magnetic medium.Therefore,when grounding calculation and related analysis are carried out,it is concluded that the approximate calculation of magnetic permeability measured by traditional power frequency signal will produce about 13.3% relative error.Limited by laboratory conditions,the study on the related mechanism of soil permittivity needs to be further developed.