Discussion on measures to reduce grounding resista

Talking about the measures to reduce the grounding resistance of grounding devices

Abstract: according to the accidents caused by grounding problems and electric accidents caused by lightning strikes, this paper discusses the reduction of the grounding resistance of grounding devices, and puts forward constructive suggestions

key words: grounding resistance of grounding device

Preface:

with the continuous economic development of Shehong County, the demand for power is increasing year by year, and all kinds of electrical equipment are increasing sharply. Every year, only a lot of distribution transformers are newly added, relocated and installed by our company. In the actual project, it is found that the installation position of many distribution transformers or other towers that need grounding devices, some of which have high grounding resistance values, such as mountains, slopes, beaches and other places, need special treatment to meet the requirements of the regulations. In the West agricultural improvement project implemented by our company last year, such as the high-voltage and low-voltage power distribution project in five villages of Taixing Township, many sand quarries for sand and stone mining have been added along the Fujiang River in recent years, with high-voltage and low-voltage lines with different long and short distances, or distribution transformers have been installed, and some have taken special measures to reduce the grounding resistance, so as to meet the requirements of safe operation. In the thunderstorm season, the distribution transformer is often struck by lightning. Because the grounding resistance is too large, it cannot reach the specified value, and the lightning current cannot be quickly discharged into the earth, causing the residual voltage of the arrester itself to be too high, or a high voltage drop on the grounding resistance 7, causing the transformer burning accident. Therefore, the grounding resistance of the grounding device must conform to the value specified in the specification. For 10kV distribution transformer: if the capacity is 100kVA or below, the grounding resistance should not be greater than 10, if the capacity is 100kVA or above, the grounding resistance should not be greater than 4. After the construction of the grounding device, the grounding resistance test should be carried out, and the backfill can be carried out only after it is qualified. At the same time, the transformer shell must be well grounded, and the shell grounding shall be tightened with bolts. It can't be welded directly for maintenance

I. whether the grounding device meets the requirements of the regulations, and the main index is the grounding resistance

the main function of grounding is to prevent people from being shocked, equipment and lines from being damaged, fire, lightning, electrostatic damage and ensure the normal operation of the power system. In recent years, many equipment damage accidents caused by the failure of grounding to meet the requirements have occurred continuously in many regions of China. At the same time, lightning stroke is one of the main natural disasters leading to electrical accidents, and the electrical accidents caused by lightning stroke account for more than 50% of the total accidents. Therefore, a good grounding device should also be an important technical measure for lightning protection. The grounding resistance is actually the sum of two parts of resistance, one part is the resistance of the metal object of the grounding body, and the other part is the resistance of the whole earth, also known as the stray resistance. Because the resistance of the metal grounding body is very small, the grounding resistance mainly depends on the magnitude of the stray resistance. The dissipation resistance is mainly determined by the structure of the grounding device and the soil resistivity. The lower the soil resistivity, the lower the dissipation resistance. The soil resistivity in some areas is large, resulting in the grounding resistance value exceeding the requirements of the specification

II. Selection of grounding materials

the grounding material is generally made of structural steel, and the material must be checked during selection. The material should not have serious corrosion, thickness or uneven thickness. Vertically installed grounding body is usually made of angle steel or steel pipe. Although the grounding body made of angle steel is a little worse than steel pipe in terms of flow dispersion effect, the construction is less difficult and the cost is low, so angle steel is generally used for on-site installation. The ideal galvanized angle steel required in the specification is 50mm, 50mm and 5mm. However, from the perspective of corrosion prevention and increasing service life, 63mm, 63mm and 6mm galvanized angle steel is gradually used. Practice has also proved that its corrosion prevention effect is good. Hot dip galvanized materials are recommended for galvanized angle steel or flat steel

seasonal factors should be considered for grounding devices. Because the soil resistivity changes with seasons, the grounding resistance required by the specification is actually the maximum allowable value of grounding resistance from the perspective of R & D strength. In order to meet this requirement, the grounding resistance of grounding is required to reach:

r=rmax/where:

rmax. Therefore, for testing the tensile performance of flexible packaging materials, a tension machine with a large stroke needs to be equipped -- the maximum value of grounding resistance, which is what we call the grounding resistance value of 10 and 4

-- it is a seasonal factor, which is taken according to the region and the nature of the project, and the common value is 1.45. Therefore, what we call the grounding resistance is actually r=6.9 (rmax=10), r=2.75 (rmax=4)

in this way, the grounding is in line with the specification requirements, and it also meets the design requirements when the soil resistivity is the highest (usually in winter)

third, the technical measures to reduce the grounding device have the following aspects for the reference of engineers and technicians:

(1) replace the soil. This method uses soil with low resistivity (such as clay, black soil and sandy clay) to replace the soil with high resistivity, and the replacement range is within 0.5m around the grounding body. This method can be used in rocky areas, such as mountains, slopes, etc., but this method of soil borrowing and replacement costs a lot of manpower and man hours

(2) artificial treatment of soil. Add chemicals to the soil around the grounding body, such as salt, charcoal, furnace ash, nitrogen fertilizer slag, calcium carbide slag, lime, etc., to improve the conductivity of the soil around the grounding body. Although the engineering cost of this method is low and the effect is obvious, the artificial treatment of soil will reduce the thermal stability of grounding, accelerate the corrosion of grounding body, and reduce the service life of grounding body. Therefore, it is usually recommended only under the conditions that are absolutely unavoidable

(3) deep buried grounding electrode. When the method of increasing the size of the grounding level cannot be used to reduce the dissipation resistance, if the resistivity of the surrounding soil is uneven and the resistivity of the soil or water deep underground is low, the deep buried grounding electrode can be used to reduce the grounding resistance value. This method is most effective for sandy soil. According to relevant data, the soil resistance coefficient at 3M depth is 100%, 75% at 4m depth, 60% at 5m depth, 50% at 6.5m depth and 20% at 9m depth. This method does not consider the resistance coefficient increased by soil freezing and drying, but it is difficult to construct, with large earthwork and high cost, which is even more difficult in rock areas

(4) multiple external lead-in grounding devices. For example, there are rivers, lakes, wells, springs, reservoirs, trees and other places with low soil resistivity with good conductivity and non freezing near the grounding device, or underwater grounding is laid to reduce the grounding resistance. The external grounding device should avoid the sidewalk to prevent step-by-step voltage shock, but the influence of the resistance of the main line connecting the grounding electrode must be considered in the design and installation. Therefore, the length of the external grounding electrode should not exceed 100m

(5) use grounding resistance reducing agent. After laying resistance reducing agent around the grounding electrode, it can increase the overall size of the grounding electrode and reduce the contact resistance

there is an active agent on the surface of the resistance reducing agent, with fine particle size. After absorbing water, it is applied between the grounding body and the soil, which can make the metal and the soil closely contact, form a large enough current flow surface, and effectively reduce the grounding resistance; On the other hand, it can penetrate into the surrounding soil, reduce the resistivity of the surrounding soil, and form a low resistivity area with gentle changes around the grounding body. The resistance reducing agent is a strong electrolyte and water with good conductivity. These strong electrolytes and water are surrounded by colloids, and the spaces of colloids are filled with partially hydrolyzed colloids, so that it will not be lost with groundwater and rain, so it can maintain good conductivity for a long time. This is a relatively new and actively popularized method currently adopted. At present, the resistance reducing agent of Chengdu Franklin Minsheng is widely used in China. This resistance reducing agent is a good conductor, with remarkable resistance reduction effect, stable performance, long service life and non corrosiveness

(6) adopt extended horizontal grounding body. In the practical application of the project, the analysis results show that when the length of the horizontal grounding body increases, the influence of inductance increases, so that the impact coefficient increases. When the length of the grounding body reaches a certain length, and then increase its length, the impact grounding resistance will not decrease

IV. conclusion

a good grounding device is an important guarantee for the safe and stable operation of electricity. It is necessary to correctly master the construction method of reducing the grounding resistance. During the stretching process, the materials should check and periodically operate and maintain the items in the design planning and demonstration stage, the selection of grounding materials and forms, the verification of thermal stability and mechanical strength of conductor section, the quality management of construction process, and the handover and acceptance of the project. Only in this way can we ensure the good quality of grounding devices for a long time and fundamentally prevent electrical accidents