1 Causes and negative effects of lightning
Lightning is discharged in the atmosphere. Generally speaking, most of the cloud weather that occurs in lightning is cumulonimbus, especially thundercloud. For Lei Yuyun's theory of electricity generation, the benevolent sees the wise and sees wisdom. Nowadays, the international consensus on the temperature difference, the induction of electricity, the ice crystal and the large water droplets, the three doctrines, which are different from different angles. The reason for the formation of the negative charge structure is analyzed, thus laying a solid foundation for human in-depth research.
Lightning includes ground flash, cloud flash and special ball flash. Among them, the ground flash has the most destructive power. The time of one flash is only a few tens of microseconds. It refers to the lightning that releases the energy stored in thunderclouds to the ground. Point, the potential difference is tens of thousands of volts, and the lightning current is about tens to hundreds of thousands of amps. The thermal effect of the lightning current can melt the metal with a relatively small flash point or evaporate the internal moisture of the non-metal in an instant to cause an explosion. In addition, the lightning current can also form adverse effects such as impact breaking and infrasound waves, which can generate local high potential on the ground structure after discharge, and the steepness and maximum peak of the lightning current can generate huge electromagnetic field to form Inductive current, large inductive current and high potential will damage its internal equipment and buildings within a certain period of time, and may also damage the equipment, causing the metal to discharge sparks, and even worse, it will cause a fire. Industrial plants, especially chemical plants, can even cause explosions, causing great damage and personal injury.
2 Classification and method of grounding protection
The simplest method of lightning protection is grounding. The earth is an extremely low-resistance, extremely large-capacity object. It has the ability to absorb infinite charges and maintains its potential after absorbing a large amount of charge. Therefore, it is most suitable as a reference potential body in electrical systems. It is the purpose of lightning protection grounding protection to discharge abnormal current through the conductor to the earth to protect people and equipment from electric shock. Grounding is generally classified into two types: protective grounding and functional grounding.
2.1 Protective grounding
(1) Anti-shock grounding In order to prevent damage to the external insulation of the electrical equipment or accidental leakage current, the exposed conductive part that is not normally charged is charged and the electric shock is caused. The exposed conductive part of the equipment is grounded, which is called electric shock grounding. This kind of grounding can also limit the high voltage caused by the inrush current or low voltage line and equipment due to high voltage intrusion; when the electrical equipment fails, it also helps the overcurrent protection device to operate and cut off the power supply. This kind of grounding is also a narrow "protective grounding".
(2) Lightning strike grounding The lightning current is conducted to the ground to prevent lightning current from causing personal injury to the ground or damage to the equipment.
(3) Anti-static grounding The static charge is directed to the ground to prevent grounding due to the accumulation of static electricity on the human body and equipment. The instantaneous maximum value of the static induction current can be very large, and it is possible to destroy the line device. Anti-static grounding in chemical plants is mostly used to prevent static electricity from protecting the grounding of inflammable and explosive oil, gas storage tanks, containers, reaction devices and pipelines. It is also used to prevent electrostatic sensitive electronic components from being damaged. Assume.
(4) Anti-corrosion grounding Underground metal bodies are buried as sacrificial anodes or cathodes to prevent electrical corrosion of cables and metal pipes.
2.2 Functional grounding
(1) Working grounding In order to ensure the normal operation of the power system and prevent system oscillation. To ensure the reliability of relay protection, grounding is carried out in the appropriate place of the AC/DC power system. The AC system is generally a neutral point, and the DC system is generally the midpoint.
(2) Logical grounding In order to ensure a stable reference potential, a suitable metal device in an electronic device is used as a "logic ground", and a metal substrate is generally used as a logic ground.
(3) Shield grounding Introduce electrical interference sources into the earth, suppress the influence of external electromagnetic interference on electronic equipment, and reduce the interference generated by electronic equipment itself to other equipment.
2.3 signal grounding
The signal ground is grounded to ensure that the signal has a stable reference potential, such as the grounding of electrical parameters such as leakage current, impedance measurement bridge, and corona discharge loss.
3 grounding resistance
The sum of the grounding resistance of the grounding electrode or the natural grounding electrode and the grounding wire resistance is called the grounding resistance of the grounding device. The value of the grounding resistance is equal to the ratio of the grounding device's voltage to ground and the current flowing into the ground through the grounding electrode. The resistance obtained by flowing the AC current into the ground through the grounding pole is called the power frequency grounding resistance; the grounding resistance obtained by the inrush current flowing into the ground through the grounding pole is called the grounding electric current. The grounding pole refers to various metal conductor members, metal well pipes, reinforced concrete building foundations, metal pipes and equipment buried in the ground and directly in contact with the earth.
Numerous facts prove that the magnitude of the grounding resistance directly affects the effect of the grounding system. Where soil resistivity is high, the first step in building an industrial project is to reduce soil resistivity. In order to obtain a lower grounding resistance, the method of changing the soil moisture is usually used to solve the problem of high grounding resistance. The humidity must have acid-base corrosion and bacterial free material corrosion; in order to increase the conductivity of the soil, conductive is also added to the soil. Medium, most of the conductive medium is chemically corrosive; there are some potential differences that lead to electrical corrosion, acid and alkali corrosion caused by the integration of factory products into the soil. This creates a contradiction between low ground resistance and corrosion. We should find the balance point in the design, choose the appropriate method to reduce the grounding resistance, choose the appropriate grounding material corrosion resistance, and make the grounding system run stably for a long time.
The methods for reducing the grounding resistance include: resistance reducing agent and grounding module, soil replacement, increasing the area of ​​the grounding grid, deep well method, blasting method, and increasing the grounding pole. The use of the resistance reducing agent and the grounding module and the method of changing the soil have a relatively large construction cost and difficulty, and the effect of reducing the resistance is not obvious, and it is easy to follow the soil and water loss after a certain time, and the effect of continuous resistance reduction cannot be achieved.
Increasing the area of ​​the grounding grid is one of the most common methods, but the effect of increasing the area to reduce the resistance is limited. When the area is doubled, the grounding resistance is reduced by 29.3%. Some plants are limited by the area, and it is difficult to achieve using this method. Low grounding resistance. The deep well method and the blasting method are often used in geologically harsh conditions, which are technically difficult, have high risk, and have high construction costs.
The grounding electrode is often used in a variety of environments due to its small footprint, low construction difficulty, and long-lasting stability. However, the contact range between the grounding electrode and the soil is limited, and the leakage effect is limited. Each grounding pole must be kept at a certain distance to prevent the current from colliding. Therefore, the grounding pole cannot be increased indefinitely within the plant area. Therefore, we must choose the appropriate grounding electrode to work with the grounding grid to achieve a satisfactory grounding effect. Because copper has good conductivity and high degree of oxidation resistance, copper is commonly used as a grounding material in common corrosive environments. Considering cost factors, copper-clad steel grounding bodies are more suitable for large-area use.
Lead is an inactive metal that is extremely resistant to corrosion, lead-coated copper, which has copper conductivity and lead. It is the best choice for grounding at present. It is suitable for substation, transmission line, and core part of power generation. It is especially suitable for desulfurization equipment. Chemical equipment used in areas with strong acid corrosion.
The material properties of zinc also have cathodic protection. Zinc-clad steel and other materials with thick galvanized layer are suitable for use in alkaline corrosion areas.
4 lightning protection grounding system design
The lightning protection of industrial plants shall be based on the different nature of their use, the possibility and consequences of lightning accidents, and the GB50057-2010 "Lightning Protection Design Code for Buildings" shall strictly classify their lightning protection categories, and then adopt different types according to their lightning protection categories. Lightning protection measures. Industrial plants, especially those with equipment on the roof, lightning protection nets should consider lightning protection measures according to different materials of their equipment. The objects such as the discharge pipes, ducts, chimneys, etc. of the roof should be highlighted according to whether they are metal objects and their emissions. It is a different protective measure for explosive hazardous gases, vapours or dusts.
Buildings should be equipped with internal lightning protection measures. This is indispensable in the design of industrial plants. Because industrial plants have more electrical equipment than civil buildings, their electrical environment is more complicated than civil buildings. Designed as follows: metal body of building, metal installation, system inside building, metal pipeline entering and exiting building should be connected with lightning protection device for lightning protection; external lightning protection device and building metal body, metal device, building The distance between the internal systems should still meet the requirements of the separation distance.
In the design of chemical industry plants, integrated grounding systems are generally used. A grounding system is used in combination with lightning protection grounding and anti-static grounding. The grounding resistance is taken as the minimum value of each system. Incorporated into the entire plant grounding network. Includes: lightning rod, down conductor and grounding body.
Incorporating various grounding into a grounding system can make the entire plant a smooth, large grounding grid. It can effectively avoid the potential difference between the grounding points of various systems during lightning strikes, causing lightning strikes. Otherwise, in the same plant area, the grounding system is separated, and the lightning protection counterattack measures should be designed according to the “Code for lightning protection design of buildings†to increase the complexity of the grounding system. There are a large number of steel bars in the foundation of the building, according to the “Lightning Protection Design of Buildingsâ€. "Specifications" require welding to be made into a basic grounding body. The contact area between the foundation grounding body and the soil is good. The use of the foundation and the steel bars in the column is used to make equipotential bonding plates, and the devices inside the building that need to be grounded are all connected to form. A large good equipotential body. When the lightning strikes, it can protect the internal personnel equipment from lightning strikes. Then use the artificial grounding device to connect the basic grounding body with the whole plant grounding net, so that the grounding net can be enlarged. The grounding area overcomes the limitations of the grounding body of the single structure.
In the case where the electrical grounding device is shared with the lightning protection grounding device, a surge protector of Class I test is installed at the total distribution box introduced by the low voltage power supply line, which is often omitted or designed by the designer in the specific engineering design. The specifications of the selected surge protector do not meet the specifications.
5 Conclusion
In summary, a large lightning protection grounding system must meet the requirements of the national standard for lightning protection grounding design, and should be combined with the actual situation, try to use the natural grounding body or the existing grounding body and the artificial grounding body. And choose a reasonable lightning protection grounding material, so that the system can effectively protect personnel equipment from injury and achieve long-term effective operation. Under the action of a strong electric field, electrons in the air are prone to electronic avalanches. In the avalanche, excited atoms will appear, and then high-energy photons will be emitted. Once the ionization energy of the gas molecules is less than the photon energy, the gas molecules will generate a lot of electrons and positive ions under the action of photons. Such new electrons will have a new source of electronic avalanche, and the process of photoionization and electron avalanche will be repeated. The speed of streamering will exceed the speed of electronic avalanche. In fact, the process of streamering is the basic process of lightning discharge.
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