Research On The Strategy Of Reducing The Impulse Grounding Impedance Of Transmission Line Tower

With the rapid development of national economy,the demand for electricity is gradually increasing,and the scale of power grid is expanding.Especially in recent years,the development of power transmission and transformation projects puts forward higher requirements for the safety and reliability of power system.Power system operation experience shows that most transmission line accidents are caused by lightning trip of transmission lines or towers.Reducing impulse grounding impedance of transmission line towers is an effective measure to improve lightning withstand level and reduce lightning trip rate.The traditional metal grounding material is easy to corrosion,and the service life is limited.The common resistance reduction measures need large area excavation and large construction area.Nonmetallic flexible graphite grounding material can effectively solve the corrosion problem of buried metal grounding material.The non-metallic flexible graphite grounding material is used to replace the traditional metal grounding material,and the resistance reduction strategy of tower impulse grounding impedance under limited construction area is proposed,which has become one of the problems to be solved to improve the stability and long-term effect of lightning resistance of transmission lines in China.In this paper,the ionized breakdown effect of soil under impact current is considered.By analyzing the impact dispersion characteristics of tower grounding device,the impedance reduction strategy of tower grounding device based on surface electrode and slot electrode is proposed.The main research contents include :(1)The physical process of impact dispersion of grounding device is theoretically analyzed,and the finite element model of grounding electrode impact characteristics is established considering the soil ionization effect.The influence of soil ionization effect on grounding impedance,electric field strength and peak potential of typical tower grounding electrodes impact is analyzed.The simulation results show that when the soil resistivity is1000Ω · m,the soil ionization effect reduces the impulse grounding impedance of single horizontal grounding electrode by 18.95 % and the impulse grounding impedance of box ray grounding electrode by 13.75 %.(2)The impact dispersion model of needle-punched grounding electrode considering ionization effect is established to analyze the influence of needle-punched conductor arrangement angle,arrangement spacing and arrangement number on the impact grounding characteristics of grounding electrode.The simulation results show that when the soil resistivity is 1000 Ω·m,the resistance reduction effect is the best when the arrangement angle of the needle-punched conductor is 90°,and the resistance reduction rate is 8.79 %.When the total length of the needle-punched conductor unchanged and the number of needlepunched conductors increases from 1 to 6,the resistance reduction rate decreases from 8.79 %to 2.37 %.When the spacing of needle-punched conductors increases from 0 m to 0.4 m,the resistance reduction rate increases from 8.95 % to 14.19 %.(3)Considering the soil ionization effect,the impact dispersion model of the grounding electrode filled with the surface electrode is established.The impact dispersion characteristics of the single grounding electrode,the square frame grounding electrode and the square frame ray grounding electrode under the surface electrode filling are analyzed.The influence of the number of surface electrodes,the filling length,the connection position and the filling spacing on the resistance reduction effect and the spark breakdown volume in the soil is studied.The simulation results show that when the soil resistivity is 1000 Ω·m,the resistance reduction rate of the planar electrode is 28.74 %,The influence of the number of segmented planar electrodes and the connection position on the impulse grounding impedance of a single horizontal grounding electrode is less than 0.1 %,The resistance reduction effect is enhanced by increasing the filling spacing and filling length of the planar electrode,and the soil breakdown volume is the largest when the planar electrode adopts the2 / 3 filling type.(4)Considering the soil ionization effect,the impact dispersion model of grounding electrode filled with slot electrode is established.The drag reduction effect of surface electrode and slot electrode on tower grounding electrodes is compared,and the influence of slot electrode filling length,filling spacing and filling height is analyzed.The simulation results show that under the same conditions,the drag reduction effect of the grooved electrode is stronger than that of the planar electrode.The drag reduction effect can be enhanced by increasing the filling length,filling spacing and filling height of grooved electrodes.When the distance between trench electrode and single horizontal grounding electrode is 0.8 m,the resistance reduction rate can reach 50.40 % under the condition of soil resistivity of 1000Ω · m.The research conclusions of this paper can provide reference for the resistance reduction design and construction of transmission line tower grounding device,and have high engineering application value.