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| Negative Temperature Characteristics of Soil Resistivity and Influence of Frozen Soil on Impulse Response of Grounding Electrode |
| Yuan Tao1, Ren Jianxing1, Sima Wenxia1, Chang Feitong1, Cai Yongxiang2, Xiao Xiaobing2 |
1. State Key Laboratory of Power Transmission Equipment Technology Chongqing University Chongqing 400044 China;
2. Electric Power Research Institute Guizhou Power Grid Co. Ltd Guizhou 550000 China |
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Abstract Due to the unique geographical location and climate environment in Tibet, the coexistence of frozen soil and lightning current exists in the low temperature area, and the green energy transmission channel is difficult to avoid the low temperature area. However, as a complex system of water, ice, air and soil particles containing ice at negative temperature, the variation of soil resistivity with temperature under different water and salt contents and the impulse characteristics of grounding devices in frozen soil under lightning current are still unclear.
Therefore, in view of the above problems, on the one hand, the change of resistivity with temperature of 16 soil samples with different water content and salt content was measured by the quadrupole method, and the reasons for the change of soil resistivity were analyzed by combining the soil conductivity mechanism and the calculation formula of unfrozen water content; on the other hand, based on the similarity principle of the scale experiment, taking the surface soil freezing thickness of the seasonal frozen soil sample and the surface soil melting thickness of the permafrost sample as variables, the influence law of the vertical grounding electrode transient potential in the corresponding frozen soil sample was explored. Furthermore, the X-ray imaging device was used to observe the discharge image of the impulse current in the frozen soil sample and the numerical simulation results of electric field intensity and current density in the frozen soil were combined to explain the experimental law.
The measurement results of soil resistivity at different temperatures show that when the temperature of saline soil is higher than the secondary freezing temperature (Ts), its resistivity increases slowly with the decrease of temperature; when the temperature is lower than Ts, the resistivity has a sudden rise. The results of impulse current dispersion experiment of grounding electrode in permafrost show that when the temperature of permafrost layer is higher than Ts, the permafrost still has good current dispersion performance; the melting thickness of surface soil increases, and the potential of grounding electrode decreases slowly; when the melting thickness exceeds its end, the potential decreases greatly; however, when the temperature of the frozen soil layer is lower than Ts, the frozen soil with great resistivity will force the current to disperse in the surface thawing soil, and the grounding electrode potential is very small. The impulse current dispersion test results of the grounding electrode in seasonal frozen soil show that the grounding electrode potential will rise slowly with the increase of the freezing thickness of the surface soil; when the freezing thickness exceeds its end, the potential rise becomes larger.
The following conclusions can be drawn from the analysis of the results: (1) When the temperature of saline frozen soil decreases to Ts, a large amount of non-conductive ice and water salt will be generated, and the water content will decrease sharply, resulting in a sudden increase in soil resistivity. (2) The discharge image observation results and numerical simulation results of impulse current in frozen soil show that the current dispersion performance of frozen soil is inferior to that of thawed soil, and the soil ionization degree around the grounding extreme is the most serious, while the soil ionization is beneficial to the end current dispersion, therefore, when the thickness of the surface soil is greater than the length of the vertical grounding electrode, the change range of the transient potential of the grounding electrode will be greater. It can be seen from the above that by reducing the Ts of the soil, the grounding performance of the grounding device can be avoided to fail under extremely cold conditions.
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Received: 29 November 2022
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2024, Vol. 39 
