Lightning and thunder is a natural phenomenon. The distribution characteristics of lightning in China are: summer is more than spring and autumn, land is more than ocean, mountain is more than plain, and south is more than north. The lightning voltage is very high, and the instantaneous current intensity is very large. Therefore, although the discharge time of one lightning is only about 0.01S, the energy released is surprisingly large. When lightning is discharged, electrical equipment can be broken down, buildings can be damaged, household appliances can be destroyed, and humans and livestock can die or be injured.
Lightning strikes are divided into direct lightning strikes and inductive lightning strikes. The phenomenon that Leiyun directly discharges ground objects or humans and animals is called direct lightning strike; the phenomenon that the induced electric potential of a strong electric field formed by an aerial cable or an outdoor antenna is reflected by an aerial charged cloud is shocked by a household electric appliance or an electronic device. The method of lightning protection depends on the specific situation.
1 Building lightning protection measures Lightning rods can be installed in ordinary houses and small buildings. On the one hand, the ground-induced charge is placed in the air through the tip, and on the other hand, the received current is quickly dispersed to the ground, and lightning strikes can be avoided.
Ultra-high buildings and mountain buildings are better with lightning protection belts and lightning protection nets; modern high-rise buildings can directly use steel bars in reinforced concrete precasts as grounding devices to prevent lightning.
The industrial building is connected to the lightning protection electric grounding device within 100m, grounded once every 25m, and the grounding resistance of each impact is not more than 20Ω; the insulator foot of the civil building is grounded, and the grounding resistance should not be greater than 30Ω; except for the annual average thunderstorm day not exceeding 30 days, or the low-voltage line is not higher than the surrounding buildings, or the line grounding point is no more than 50m from the household, or the soil resistance is less than 200Ω, and the reinforced concrete rod and irons are used. In addition, the insulator feet of the overhead line of the overhead line should be grounded, and the grounding resistance should not exceed 30Ω.
2 Lightning protection measures for household appliances From the perspective of the power supply system, the power supply voltage for civil buildings is: 380/220V low-voltage system. The transmission line used is 10kV overhead line to be introduced into the distribution transformer, and then from the low-voltage side of the transformer to the low-voltage line. Inside the civil building. When the overhead line on the high voltage side of the transformer is subjected to direct lightning strike or inductive lightning, the lightning wave invades to the low voltage side through the high voltage side of the transformer, so that the user and the household appliance are damaged by lightning strike. In order to prevent lightning strikes on household appliances, the following measures can be taken:
(1) Install a FYS-0.22kV metal oxide gapless arrester between the low-voltage phase line and the neutral line, which not only can effectively prevent lightning, but also prevent the three-phase four-wire entry line from being broken. The overvoltage generated by the displacement of the point of gravity jeopardizes the safety of the person and the household appliance. At present, there are also household appliances equipped with lightning arresters on the market, such as telephones, electrical plugs, etc., that is, a metal oxide arrester with a very small volume is buried in the plug of the household appliance, so that each household appliance passes through the low voltage. The arrester has a reliable grounding.
(2) Install a set of gapless arresters before the low-voltage line enters the room, and install lightning protection sockets inside the room to form three-way protection.
(3) The first pole before the low-voltage line enters the room will support the insulator iron foot to be reliably grounded, and act as a discharge gap to reduce the lightning overvoltage amplitude in the room.
(4) When the feeder of the outdoor antenna is adjacent to the lightning rod or the lightning rod, the feeder should be worn through a metal pipeline or a shielded wire, and the metal pipe or shield should be grounded. If the feeder does not wear a metal tube and is not a shielded wire, the lightning arrestor or discharge gap should be installed on the feeder.
In addition, before the thunderstorm, try to disconnect the plug of the household appliance, do not watch the TV, do not listen to the radio, do not call, have an outdoor antenna, and then dial the antenna plug before the thunder.
3 Human body lightning protection measures During thunderstorms, non-work must be minimized to stay outdoors or in the wild. It is best to wear plastic, non-soaked raincoats outdoors or in the wild; if available, enter buildings with large metal frame or lightning protection facilities. Things; if you rely on streets that are shielded by buildings or streets that are shielded by tall trees, you should pay attention to leaving the walls and trunks more than 8m.
Suddenly encounter thunderstorms in the wild, you must keep in mind two: First, the position of the human body should be reduced as much as possible to avoid protruding; second, the two feet should be close together, it is best to choose the dry place to reduce the exposed area and the ground potential difference, because The larger the contact area between the human body and the ground, the greater the danger, so that it can be safe. Suddenly encountering thunder and lightning in the wild, you need to do a good job of "ten do not":
(1) Do not stand on the top of the mountain, on the ridge, and on the ground;
(2) Don't stand under the big tree, hide the rain by the edge of the woods or the grass.
(3) Do not walk near isolated tall buildings, chimneys, or poles;
(4) Do not wear wet clothes to hurry;
(5) Do not swim or row on open water, or leave the water or rice fields as soon as possible;
(6) Do not approach metal objects;
(7) Don't throw high-quality tools such as hoes and shovel;
(8) Do not ride cows or horses. Don't ride in the empty field;
(9) Do not use a mobile phone;
(10) Do not stand near the lightning rod.
Pay attention to the danger of lightning intruding waves indoors, and leave the lighting line, telephone line, radio line, TV antenna and various conductors connected to it to prevent secondary discharge of these lines and conductors to the human body. According to the survey data, more than 70% of indoor accidents involving secondary discharge in the human body occurred within 1m of each other, and no fatal accidents were found at a distance of 1.5m or more. It can be seen that during a thunderstorm, it is better for the human body to leave the line and conductor that may have invaded the lightning intrusion wave more than 1.5m. In addition, hiding in the room, you should also close the door to avoid the wind, to prevent the ball from entering the room and hurt people.
4 10kV distribution line lightning protection measures
(1) In order to improve the lightning protection level of the 10kV distribution line, the porcelain cross arm should be selected as much as possible in the line modified by the rural power grid, and because the lightning resistance level of the porcelain cross arm is more than three times that of the iron cross-arm type insulator. For the existing iron crossarm line, it should be replaced with a higher level of insulation.
(2) For 10kV distribution lines with ungrounded neutral points, when single-phase grounding occurs, the lines will not cause tripping. Therefore, preventing phase-to-phase short circuit is the basic principle of line lightning protection.
(3) After 10KV distribution lines are struck by lightning, they often cause insulator breakdown and wire burnout accidents. Especially for the reinforced concrete rods in the multi-mine area, the lines are most prominent, so there must be reliable electrical connections at these insulation weaknesses. And connected to the grounding down conductor. The down conductor can be welded by means of reinforced concrete rods, and the grounding resistance should be less than 30Ω.
(4) For the insulation points of individual high towers, iron crossarms, partial towers with terminal wires and terminal rods, lightning arresters shall be installed for protection.
(5) For 10kV distribution lines that cross each other and with lower voltage lines, communication lines, and closed-circuit television lines, the minimum allowable vertical distance between the upper and lower conductors shall be in accordance with the values ​​specified in the relevant regulations. . If the working distance is small, the air gap may be broken by lightning, causing the two interconnected lines to trip, and will cause non-selective action of the line relay protection, which may expand into a system accident. Therefore, it is necessary to install a matching protection gap at both ends of the line crossing the section.
(6) The branch line installed in the multi-mine area shall be equipped with a coincidence fuse or reclosing device to prevent lightning strike hazard. After the lightning strike of the line, the lightning flashing produces a steady-state power frequency arc to short-circuit the phase. When the switch trips, the current is cut off and the arc is extinguished, and the insulation can generally recover faster. After a certain time of overlap, the arc is generally not re-ignited, and the recombination power is higher, which can increase the power supply multiplier.
5 10kV power distribution equipment lightning protection measures
(1) Distribution transformers are protected by valve arresters in accordance with current regulations. The closer the valve type arrester is to the transformer installation, the better the protection effect is. It is generally required to be installed inside the high pressure drop insurance. The residual voltage of the arrester must be less than the withstand voltage of the distribution transformer to effectively protect the transformer.
(2) The choice of arrester should be consistent with the rated voltage of the line. If the rated voltage of the arrester is higher than the rated voltage of the equipment, the equipment will lose reliable protection when it is struck by lightning; the rated voltage of the arrester is lower than the rated voltage of the equipment, and the grounding trip of the line is caused by the frequent operation of the arrester under normal overvoltage.
(3) When the transformer capacity is 100kVA and above, the grounding resistance should be reduced to 4Ω or less as much as possible; when the transformer capacity is less than 100kVA, the grounding resistance can reach 10Ω and below. If the change platform does not meet the above requirements, the grounding grid should be modified to reduce the resistance, so that the potential caused by the lightning current flowing through the ground line is reduced.
(4) A protective device is also installed on the low-voltage side of the distribution transformer. The 10kV distribution transformer only installs the lightning arrester at the incoming line and cannot protect the distribution low-voltage winding. Moreover, due to the low-voltage side lightning, the lightning impulse voltage will be directly applied to the low-voltage winding through the metering device, and the high-voltage side is induced to generate high voltage according to the ratio. It is possible to break through the high voltage winding first. At the same time, the lightning impulse voltage invades the user through the low-voltage line, causing damage to the household appliance. Therefore, a low-voltage arrester (supplied with a set of FYS-type low-voltage metal oxide arresters) or a 500V communication gap protector should be installed on the low-voltage side of the distribution transformer, and the arrester, transformer casing and neutral point should be reliably grounded.
(5) Install a reactor at the incoming line of the distribution transformer. The reactor can be made by using the incoming wire and wound into a 100 mm diameter, 10 to 20 电感 inductor coil. Prevent the invasion of lightning waves and protect the transformer.
(6) The arrester installation process should be standardized. The grounding of the arrester should be good and the grounding wire connection should be reliable. Rural distribution transformers have poor installation process and thin leads: loose or open connectors cause the lightning arrester to lose its protective effect, which is more common due to lightning strikes. Therefore, the cross-sectional area of ​​the lightning protection lead, the lead connector, and the grounding body must be buried. Meet the requirements of lightning protection grounding regulations.
(7) Pre-test and overhaul on schedule. The arrester shall be regularly tested for insulation resistance and power frequency discharge voltage according to the requirements of the regulations, and replace the unqualified and defective arresters. After the FS valve type arrester is operated for a period of time, the power frequency discharge voltage decreases due to the aging of the arrester itself, and the insulation resistance decreases. When the power frequency discharge voltage is lower than 23kV and the insulation resistance is lower than 2000MΩ, it must be replaced. For grounding down the line, the grounding device should be inspected and inspected regularly. The porcelain body should be cleaned before the thunderstorm season, the joints should be tightened, and the damaged parts should be replaced immediately.
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