Study On AC Stray Current Distribution Law In Grounding Grid Of Mine Power Supply System

This dissertation is a sub topic of Shanxi Youth Science and Technology Research Fund Project,"The Research of Distribution Law and Protection Method of Stray Current in Mine Power Supply System"(No.201701D221240),The subject is put forward in view of the increasingly prominent risk of AC stray current in coal mines and frequent occurrence of coal mine accidents in our country,the lack of relevant theoretical research on the formation mechanism and distribution law of AC stray current in coal mines at home and abroad.With the popularization of fully mechanized mining technology,the increase of load capacity and voltage level and power supply distance,the safety problem of AC stray current in mines has attracted widespread attention.The AC stray current in mine is a potential threat to normal production and health of workers in mine.Therefore,the distribution law and influencing factors of AC stray current in mine grounding grid are studied to provide theoretical and technical support for effective prevention and control of AC stray current,which has very important theoretical and practical significance.Taking the actual AC power supply system of mine as the research object,and combining the structure of underground grounding grid,the AC stray current model of mine was established on the basis of analyzing the formation mechanism of AC stray current and its possible flow through path in mine.Through simulation analysis,the distribution law of AC stray current in the grounding grid under the relevant factors were summarized,and the dispersing characteristics of AC stray current at the local grounding electrode under the different influencing factors were studied,and corresponding optimization measures were put forward to guide production practice.The specific research contents are as follows.By consulting relevant literature,learning and summarizing the theoretical knowledge and research methods of stray current,the main sources and formation mechanism of AC stray current in mine are ensured.On the basis of comprehensively mastering AC power supply system and protective grounding grid and cable structure of mine,the AC stray current model of mine is built by using the software platform of MATLAB/Simulink.The distribution law of AC stray current in grounding grid is studied under the conditions of asymmetric insulation of low voltage cable,asymmetric insulation of high voltage cable,asymmetric insulation of both high voltage cable and low voltage cable,resistance of metal mesh and grounding resistance.The research shows that as the degree of cable insulation asymmetry(unbalance factor K)increases,the AC stray current flowing through the shield layer or ground core,the metal mesh and the ground electrode also increases.The AC stray current mainly distributes in the shielding layer or ground core of the cable and the corresponding metal mesh structure of the cable.In the process of AC stray current return to power supply,due to the asymmetric insulation of the whole cable,the AC stray current flowing through shielding layer or ground core and metal mesh increases continuously from the end of the cable to the power source.The larger the resistance of the metal mesh,the smaller the AC stray current flowing through the metal mesh,and the larger the AC stray current flowing through the shield layer or ground core and grounding electrode.Because the grounding resistance is much larger than the resistance of shielding layer or ground core and metal mesh,the change of grounding resistance has little effect on AC stray current distribution in shield layer or ground core and metal mesh.The research results lay a solid foundation for further management of AC stray current.On the basis of knowing the distribution law of AC stray current in each local grounding electrode,in order to further analyze the dispersing field of AC stray current in rock stratum through local grounding electrode,a three-dimensional finite element model of vertical steel pipe grounding electrode is established based on the rock structure around the vertical steel pipe grounding electrode and the actual structure of the grounding electrode.Theinfluence of shotcrete structure,rock parameters,grounding electrode structure and grounding electrode buried depth on the ground electrode dispersion field are studied.The research findings that the dispersing field of AC stray current are influenced by rock parameters,grounding electrode structure and grounding electrode buried depth.End effect exists in the vertical steel pipe grounding electrode.The lower the resistivity,the higher the current density.The distribution of electric field intensity is uniform except at the grounding electrode terminal position.When AC stray current is constant,grounding electrode potential decreases with the rock resistivity decreases and the increase of grounding electrode length and radius.When the rock structure is fixed,changing the buried depth of grounding electrode to reduce grounding electrode potential.By studying the dispersing fieid of the ground electrode under the relevant factors,it has a certain guiding significance to reduce the harm of AC stray current.