1. Introduction
The grounding of transmission line towers is crucial to the lightning protection of transmission lines, especially the lightning resistance level of transmission lines. However, due to the high soil resistivity, complex terrain, inconvenient transportation, and difficult construction of transmission lines in mountainous areas, the grounding resistance of towers is generally high. For example, an in-depth investigation of lightning accidents on transmission lines also found that the line sections where lightning accidents often occur generally have several towers with continuously high grounding resistance, or there are large spans and large spans. This is because in these areas, once the tower is struck by lightning, the adjacent towers cannot be effectively diverted, and most of the lightning current flows through the struck tower. Due to the high grounding resistance, more tower top potential is caused. Once the potential difference between the two ends of the insulator string is greater than 50% of the impulse discharge voltage in the insulator, the insulator breaks down, that is, “counterattack”.
2. Analysis of the reasons for the high grounding resistance of transmission line towers
An investigation of the reasons for the high grounding resistance of transmission line towers found that there are many reasons for the high grounding resistance of towers, including objective reasons and problems in operation and maintenance. In summary, there are the following reasons:
2.1 Complex geology and terrain, especially in mountainous areas, the main reason is the high soil resistivity.
According to our investigation, the soil resistivity in northern mountainous areas is generally 1300Ωm-3000Ωm, and the soil resistivity in southern mountainous areas is even as high as 5000-10000Ωm. In addition, some mountainous areas have thin soil layers or no soil at all, which are basically all rocks, and transportation is inconvenient. Grounding construction is difficult. In addition, the soil in the north is dry, and the earth conducts electricity basically by ions. Various inorganic salts can only be dissociated into conductive metal ions in the presence of water. Therefore, the conductivity of dry soil is very poor, which is the main reason for the high grounding resistance of towers in mountainous areas or northern arid areas.
2.2 Reasons in design and construction
Due to the complex terrain, uneven soil, and large changes in soil resistance in mountainous areas, careful field exploration is required when designing the grounding of towers, and careful design is carried out in combination with actual conditions. However, when investigating the actual project, it was found that there were some problems in the design. Some of them did not conduct soil resistivity measurement on site, did not conduct terrain, topography and geological exploration on site, and made a design that met the site conditions based on the actual situation, but took an average resistivity for a fairly large range. Or they applied typical design drawings and did not calculate the grounding resistance, resulting in the design not being consistent with the actual site. During construction, since the grounding project is a hidden project, there are also inadequate engineering and technical supervision, and it cannot be strictly constructed according to the drawings. For example, the length of the grounding body, the burial depth, welding and backfilling soil are not in compliance with the requirements. As a result, there are a large number of tower grounding resistances that exceed the standard after the line construction. If the test is not strictly carried out during the project acceptance, these hidden dangers will not be eliminated until the line is put into operation.
2.3 Reasons for operation and maintenance
Some poles and towers are qualified when they are first built, but as the operation time goes by, the grounding resistance of the poles and towers will become larger and larger. This is mainly due to the following reasons:
1) Corrosion of the grounding body, especially in acidic soil in mountainous areas or weathered soil, is most prone to electrochemical corrosion and oxygen absorption corrosion. The most prone to corrosion is the connection between the grounding lead and the horizontal grounding body, which is caused by electrochemical corrosion caused by different corrosion potential differences. Sometimes the pole tower “loses ground” due to corrosion and fracture. Another reason is that the grounding body is not buried deep enough, or backfilled with gravel or sand, and the soil has a high oxygen content, which makes the grounding body prone to oxygen absorption corrosion. Due to corrosion, the contact resistance between the grounding body and the surrounding soil increases, and even the grounding body breaks at the welding joint, resulting in an increase in the grounding resistance of the pole tower or loss of grounding.
2) On the slope of the hill, due to the erosion of rainwater, the grounding body is exposed and loses contact with the earth.
3) Chemical resistance reducing agents or resistance reducing agents with unstable performance are used during construction. As time goes by, the resistance reducing components of the resistance reducing agents are lost or fail, which increases the grounding resistance.
4) External force damage, the grounding lead or grounding body of the tower is stolen or damaged by external force.
3. Discussion on resistance reduction measures
Resistance reduction transformation for towers with excessive grounding resistance is an important measure to improve the lightning resistance level of the line and ensure the safe operation of the line. However, for transmission lines, since resistance reduction is mainly for the need of lightning protection, there are clear requirements for resistance reduction measures, that is, the main purpose is to reduce the impact grounding resistance of the tower. Therefore, the following issues should be considered for tower resistance reduction measures.
3.1 About horizontal grounding bodies.
Since it is to reduce the impact grounding resistance, it cannot be extended to the horizontal grounding body in such a large range as in power plants and substations to reduce the power frequency grounding resistance. Instead, there are certain requirements for the length of the grounding body. This is mainly due to the consideration that if the horizontal grounding body is too long, it will be ineffective in reducing the impact grounding resistance due to the influence of inductance. For horizontal grounding bodies, the horizontal rays should be mainly radiated outward from the surroundings of the tower according to the terrain and topography of the site. The terrain and geology of the site should be fully utilized, such as rock cracks, and resistance reduction agents should be used for resistance reduction treatment. To prevent rain erosion, the horizontal grounding bodies should be arranged along the contour lines as much as possible, and combined with waterproof walls for protection. The buried depth of the horizontal grounding body should be as low as possible below 0.8m. In the north, it should be buried below the frozen soil layer. For example, the tower resistance reduction transformation of the aforementioned 110KV Xinli Line, 110KV Pingbao Line and Liaoning Benxi 60KV Xihu Line all use resistance reduction agents to reduce the ground pull resistance to within the qualified range in combination with the favorable terrain of the site.
3.2 About vertical grounding bodies.
Vertical grounding bodies are a common measure for grounding line towers, but the lines located in mountainous areas are not easy to construct vertical grounding electrodes due to the large number of stones, especially the towers located in rocky areas. At this time, vertical grounding electrodes can be used in combination with rock cracks. When there are metal mines or geological structures with low resistivity underground, the shaft grounding resistance reduction method can be used. However, if there is no geological structure with low resistivity underground, it is not economical to use the shaft method to reduce resistance. Besides, lightning current is a high-frequency current with strong skin tendency. Lightning current generally flows along the surface soil, but not in the deep soil. Therefore, the use of deep well grounding electrodes in general geological structures is not very effective in reducing the impact grounding resistance. Therefore, the grounding body for tower grounding should be mainly horizontal grounding body, supplemented by vertical grounding body. The length of the vertical grounding body should be 1.5-2m, and it is generally set at the top of the horizontal grounding body, or in the middle of the horizontal grounding body, where it is easy to drive in.
3.3 About the use of resistance reducing agent.
A large number of engineering practices have proved that the use of resistance reducing agent is very effective in reducing the grounding resistance of towers. Because tower grounding is a small and medium-sized grounding device, the resistance reducing effect of the resistance reducing agent can be fully exerted. However, some problems have also occurred in actual projects, mainly:
(1) The stability of the resistance reducing agent. Some resistance reducing agents, especially some chemical resistance reducing agents, have a good resistance reducing effect in a short period of time, but their performance is unstable. With the penetration and diffusion of the resistance reducing agent, especially with the loss of rainwater, the resistance reducing effect is easy to fail;
(2) The corrosion of the resistance reducing agent. Some resistance reducing agents are very corrosive and can cause great corrosion to the steel grounding body. For example, a certain type of resistance reducing agent used in a power plant in Guangdong Province corroded and rotted the grounding body in less than 5 years, and also caused great corrosion to the adjacent fire water pipe system underground;
(3) The resistance reducing effect problem. The resistance reducing effect of the resistance reducing agent is mainly determined by the resistivity, water retention, penetration and diffusion of the resistance reducing agent itself. Therefore, in the selection of resistance reducing agents, it is necessary to pay attention to the selection of resistance reducing agents with good resistance reducing performance, low corrosion to steel grounding bodies, stable performance, long life, good water retention, and not easy to be lost with soil and water.
Regardless of the type of resistance reducing agent, the resistance reducing effect is reflected through certain design and construction. The dosage of the resistance reducing agent should be combined with the design to carefully calculate the grounding resistance. For example, the XS128 long-term physical resistance reducing agent gives the relationship between the resistance reducing coefficient and the dosage. The design should be calculated according to the specific engineering requirements and soil resistivity. The resistance reducing agent and the horizontal makeup ground body should be buried below the frozen soil layer, and the burial depth should preferably be less than 0.6m. The backfill soil should be backfilled with fine soil and compacted in layers. Sand and gravel should not be backfilled. Because most resistance reducing agents have a higher corrosion potential than soil, all grounding bodies should be evenly wrapped in the middle of the resistance reducing agent. Disconnection or exposure of the grounding body is not allowed, because this will cause different corrosion potential differences and cause electrochemical corrosion, which has been confirmed by a large number of engineering practices. For example, the Nanyang Electric Power Bureau of Henan Province found that the steel grounding body evenly wrapped in the resistance reducing agent in the line tower grounding was basically free of corrosion, while there was a disconnection or the grounding body was exposed due to uneven application of the resistance reducing agent, and serious corrosion occurred. In Xinyang, Henan, there is a tower grounding project. Because the burial depth is not enough, it is backfilled with crushed stone, which results in the failure of the resistance reducing agent and the corrosion of the grounding body.
3.4 About the construction of the project.
Because the grounding project is a concealed project, every link in the project must be carefully supervised throughout the entire process. For the newly built tower, it is best to lay the grounding body and resistance reducing agent at the bottom of the pit during the construction of the tower foundation and the wire pulling foundation, so that twice the result with half the effort can be achieved. For the transformation and resistance reduction project, the actual design should be made in combination with the favorable geology and terrain of the site, and the horizontal grounding and vertical grounding bodies should be constructed as required. Pay special attention to the burial depth of the horizontal grounding body, and the welding should be qualified. The backfill soil should be backfilled with fine soil and compacted in layers. The connectors of the grounding down conductor should be treated with anti-corrosion. The grounding down conductor should be treated with asphalt varnish and anti-corrosion paint until the connection with the horizontal grounding body.
3.5 Regarding operation and maintenance, the grounding device of the tower should be regularly maintained and inspected, such as regular anti-corrosion treatment of the grounding lead, regular testing of the tower grounding resistance and loop resistance, regular inspection of the grounding body for erosion and external force damage, etc.
4. Conclusion
The grounding resistance reduction treatment of the transmission line tower is mainly for lightning protection, so it should be mainly based on reducing the impact grounding resistance. Then all resistance reduction measures should be carried out around this purpose. It is not advisable to use extra-long extension grounding and deep well grounding. However, it can be combined with the on-site terrain to use radial grounding, deep buried grounding body and appropriate resistance reduction agent to reduce resistance. Specific technical and economic analysis should be made for specific projects, practical designs should be made, and careful construction should be carried out, and operation and maintenance should be strengthened to achieve ideal lightning protection effects.
