Research On Grounding Resistance Reduction Strategy Of Transmission Line Tower Near Water Source

Transmission line towers near water source areas are mostly located near ponds,lakes or areas with abundant underground water sources,and the resistivity of water source is much lower than that of surrounding soil.The soil resistivity near the water source is smaller,while the soil resistivity far away from the water source is higher.Therefore,the soil structure of water source area can be regarded as horizontal or vertical layered soil structure.The traditional transmission line tower grounding method mostly uses the grounding device composed of natural grounding body(concrete pile foundation)and artificial grounding body(horizontal or vertical grounding body)laid in the soil.Concrete pile foundation has the characteristics of large size,deep embedding and large number of steel bars.When the transmission line tower is struck by natural lightning,the lightning will flow into the ground through the concrete pile foundation of the tower.Because the resistivity of concrete can reach thousands of Ohm or even megaohm,it is difficult for the current flowing through the reinforcement in the pile foundation of the tower to disperse to the surrounding soil.In view of the complexity of soil structure near the water source area,this paper proposes the grounding method of laying "spiral" graphite electrode on the pile foundation of transmission line tower to improve the current dispersion efficiency of reinforcement in concrete pile foundation near the water source area and reduce the grounding resistance of transmission line tower near the water source area.The research contents of this paper are as follows:(1)Based on the comprehensive analysis of the research results of grounding resistance reduction of transmission line tower at home and abroad,the simplified and refined models of cast-in-place pile foundation and cast-in-place pile foundation of transmission line tower near water source area are built by combining electromagnetic field theory with finite element method according to the actual tower grounding project.The calculation deviation of grounding resistance is less than 6%,which verifies the rationality of the simplified model.(2)The calculation model of "single spiral" graphite electrode applied on single concrete pile foundation of transmission line tower near water source area is built,and the influence of groundwater layer position,wrapping area,wrapping position and wrapping spacing on the grounding dispersion characteristics of tower pile foundation is analyzed.The calculation results show that the ground resistance of tower pile foundation is reduced by the groundwater layer,and the closer the grounding device is to the water layer,the smaller the ground resistance is;The results show that the wrapping area of graphite electrode has a great influence on the grounding resistance and shunt coefficient of tower,and the whole wrapping method has the best resistance reduction effect;The results show that the resistance reduction efficiency of the bottom position is the highest under the conditions of upper,middle and lower winding positions;Considering the resistance reduction efficiency of grounding device and the cost of graphite electrode material,the grounding and dispersion characteristics of cast-in-place piles are optimized when the wrapping spacing of cast-in-place piles is 0.6m and the wrapping spacing of cast-in-place piles is 0.4m.(3)The calculation model of "double spiral" graphite electrode applied on the concrete single pile foundation of transmission line tower near water source area is built,and the grounding characteristics are compared with that of "single spiral" grounding device.The effects of the distance between the river and the tower bottom,the wrapping area,the wrapping distance and the wrapping position on the grounding dispersion characteristics of the tower pile foundation are compared and analyzed.According to the simulation results,the river water source will affect the dispersion characteristics of the nearby tower pile foundation,and the closer the river is to the tower pile foundation,the smaller the grounding resistance is;When the grounding characteristics of "double helix" grounding device under the influence of the above factors are analyzed separately,its variation law is consistent with that of "single helix" grounding device;When the soil resistivity is 3200 Ω· The maximum difference between the shunt coefficient of graphite in "single helix" grounding device and "double helix" grounding device is only 5%,but the material required for "double helix" grounding device is twice that of "single helix" grounding device.(4)The calculation model of the whole tower pile foundation of the transmission line near the water source area is built,and four kinds of external application schemes are put forward,including split single screw,split double screw,"two two" external application and integrated external application.The results showed that the four external application methods all played a certain role in reducing resistance,and the effect was significant.On the whole,there is little difference in the effect of resistance reduction between the two kinds of external application.However,from the perspective of construction and cost,the latter two need to carry out large-scale excavation projects,which need to consume large land and human resources;In the split external application mode,the split single helix can meet the resistance reduction needs of most grounding projects.(5)Considering the current dispersion characteristics of the "horizontal extension +single spiral" grounding device,the resistance reduction strategy of the grounding device is proposed under the multi-dimensional condition of the transmission line tower near the water source area.The oblique horizontal epitaxial grounding device is compared with the split single helix grounding device and the horizontal epitaxial + single helix grounding device.The simulation results show that the grounding characteristics of the oblique horizontal epitaxial grounding device are basically the same as that of the split single helix grounding device when the length of the ray conductor is 2m,However,compared with the spiral external grounding device,the inclined horizontal grounding device consumes75.88m2 more land resources.The external spiral electrode resistance reduction scheme is to use the vertical space of the foundation pit to lay grounding device to reduce resistance,and only use the land for the construction of pile foundation,without consuming other land resources.At the same time,in the "horizontal epitaxial + single spiral" grounding device,the shunt coefficient of the external vertical graphite grounding body is always greater than that of the horizontal epitaxial grounding body,which indicates that the vertical graphite electrode has good dispersion effect.