Discussion on lightning protection measures for co

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Discussion on lightning protection measures for coal mine substation

1 the formation and development of lightning cloud

lightning is a natural discharge phenomenon in the atmosphere, which is generally called lightning. Its shape is divided into linear, banded, flaky and spherical. According to the spatial position, it can be divided into two types: between thunderclouds and between thunderclouds and the earth. The former occurs at high altitude and does little harm to human beings, while the latter is the ground lightning that occurs between thunderclouds and the earth, especially the negative ground lightning, which does the greatest harm to human beings and equipment, and is the main source of lightning accidents in coal mine substations. Thunder and lightning are inseparable from the existence of thunder clouds. When the weather is muggy, the hot air rises to high altitude and meets cold air. The water vapor forms water droplets, which move downward under the action of gravity, collides with the rising hot air, and the water droplets separate to form fine water droplets. These water droplets gather with the wind to form negatively charged thunder clouds. Thunder clouds are the premise of lightning discharge. The development of negative polarity ground mines can be divided into the following three stages

1.1 pilot discharge

when there is a negatively charged thunderstorm in the sky, both the ground and ground objects are positively charged due to induction. The electric field near the thunderstorm increases when there is a large amount of charge in the thunderstorm. When it increases to a certain value, the air insulation is damaged and begins to dissociate, forming a pilot discharge path. The direction is gradually developing from the thunderstorm to the earth (the discharge speed is about 10km/s), When the ground object develops downward to a certain height, it may produce an upward leader, which affects the development direction of the downward leader and the orientation of the lightning strike point

1.2 main discharge

the extremely high potential of the downward leader and the induced charge of the upward leader are relatively small from the earth. When the electric field strength is large enough, the remaining air gap will be broken down, the free electrons will soon flow into the earth, and a large number of ground charges will rush to the thunderclouds, which will produce strong brightness and huge thunder. The main discharge current is extremely large. The instantaneous amplitude of most lightning currents is about 10kA, and a few can reach hundreds of kiloamps. The abrupt lightning current produces a transition process, forming lightning shock wave, which makes the magnetic field around the lightning point change greatly. Although the main discharge time is only tens of microseconds, it has a great destructive effect, causing casualties of people and animals, damage to buildings and equipment and fire

1.3 afterglow discharge

after the main discharge, the residual charge in the thunder cloud continues to flow into the earth according to the path, forming afterglow discharge. The feel and texture of the discharge electricity are even closer to that of some flagship aircraft, and the current decreases rapidly with time. It only takes a few milliseconds to complete the discharge. When there are multiple thunder cloud centers, there will be repeated discharges, but the discharge current is much smaller

2 the law of electrical and mechanical activities and the probability of lightning current amplitude in the electronic burst intensity experiment of lightning

people have summarized the law of lightning activities in long-term practice. According to the distribution law of geographical environment, thunderstorms in mountainous areas and hot and humid areas are higher than those in plains and cold and dry areas, and more in inland areas than in coastal areas. According to the geological conditions, the places prone to lightning strike are as follows: the soil resistance is small, and the clay conductivity in the surface soil is high; Metal deposits are buried underground; Tall, protruding or isolated buildings; High voltage transmission line corner, etc. Most of the lightning activities are from 1 to 9 p.m. in the daytime, and the peak month of thunderstorm is July and August

3 lightning strike types of substation and the primary task of lightning protection; The second is induced lightning near the transmission line; The third is the direct lightning strike against the lines and equipment in the substation. Lightning wave and induction lightning have large steepness, high amplitude and serious harm. Without lightning protection measures, the insulation of electrical equipment in the substation will be broken down. According to statistics, the accident rate of 110~220kv substations in China caused by lightning wave is about 0.5 times/100 substations In, the damage rate of direct distribution motor is about 1.25 times/100 units Year

the primary task of lightning protection is to prevent lightning wave and induced lightning in substations. Reasonable lightning protection measures shall be taken for direct lightning strikes. Lightning resistance level and lightning trip rate shall be used to measure the lightning protection performance of high-voltage transmission lines to ensure the safe and normal operation of coal mine substations

4 protective measures against direct lightning strike in Coal Mine Substation. Lightning arrester consists of lightning arrester, down lead and grounding device. Lightning rod, strip, wire and are used as lightning arrester. The down lead shall ensure a good connection between the lightning arrester and the earth, and the resistance of the grounding device shall not be greater than 10 Ω

when the lightning rod is higher than the protected equipment, its protection scope includes the substation plant and all outdoor equipment. The lightning rod is like an umbrella. As long as the protected equipment is placed within the scope of the umbrella cover, it can attract lightning to itself, and lead the great lightning current to the underground grounding device through the downlead, which can be dissipated to the ground as soon as possible and neutralized with different charges, and the lightning strike probability of the equipment can be protected less than 0.1%. It is necessary to prevent counterattack accidents between them. When the rolling ball method is used for calculation, the protection range of the lightning rod is reduced, and the lightning strip can be installed on the building ()

5 lightning wave protection measures for coal mine substation

5.1 lightning wave protection measures in substation

the voltage of main transformer in substation is the most important and should be protected. A valve type lightning arrester is installed on the bus of the substation and connected to the side of the main transformer. When the lightning wave invades the main transformer, it will generate total reflection to increase the voltage on them. The lightning wave voltage curve intersects with the relatively flat volt second characteristics of the valve type arrester, making the arrester act. For 110~220kv substations with normal lightning protection, the lightning current flowing through the arrester shall not be greater than 5ka. When the impulse withstand voltage of the main transformer is greater than the impulse discharge voltage of the arrester, the main transformer shall be reliably protected. The location of lightning arrester shall be selected, and its distance from the main transformer and other equipment shall be less than the maximum allowable electrical distance. If one group does not meet the requirements, the positions of any two connected parts shall be changed. Another group can be added when the requirements are met

5.2 lightning wave protection measures for transmission lines

35~110kv transmission lines without lightning lines, when the incoming line is struck by lightning, the amplitude and steepness of lightning wave will exceed the withstand voltage value of substation equipment. Installing a lightning conductor at the incoming section 1~2km away from the substation can reduce the steepness of lightning wave, limit the lightning current flowing through the valve type arrester of the substation to not more than 5ka, greatly reduce the probability of lightning wave in the incoming section, and even if it occurs, it can only be outside the incoming section. In case of frequent switching on in heavy lightning area and rainy season, a group of tubular lightning arresters shall be installed at the head end of incoming line protection

5.3 lightning wave protection measures for various transformers

(1) three phase winding transformers only need to install a lightning arrester at the outlet of a phase of the low-voltage winding

(2) a single-chip microcomputer controlled autotransformer can install a group of lightning arresters between the high-voltage side, the medium voltage side and the circuit breaker

(3) the neutral of 35kV transformer is not protected, and the neutral of 110kV transformer is connected to lightning arrester

(4) 3~10kv distribution transformer adopts valve type arrester, and a group of zinc oxide arresters shall be installed at the low-voltage side in the multi lightning area

5.4 lightning wave protection measures for direct distribution motor

install a group of magnetic blow arresters at the bus of each motor outgoing line, and then use the incoming section protection measures to limit the lightning current flowing into the magnetic blow arrester to no more than 3KA; The capacitor installed on the motor bus can also limit the lightning wave steepness, reduce the amplitude, and protect the turn to turn and neutral point insulation of the direct distribution motor

(Bai Anlin renjianhua)

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