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Summary of lightning protection technology for CATV system


Lei is a kind of discharge phenomenon in the atmosphere, which often causes serious damage to cable TV equipment. In CATV system, lightning protection design is a very important task, and in actual engineering, lightning protection has not caused enough attention of technicians. In the event of a lightning strike, systems without good lightning protection measures will be severely damaged or even embarrassed. For the large system with long trunk line, lightning protection design is an important task. This paper expounds the lightning mechanism from the mechanism of lightning strike and the distribution law of lightning, in order to have a general understanding of lightning strikes, and then explain the lightning protection measures and CATV equipment. Anti-lightning performance.

There are two main types of lightning strikes: “straight strike lightning” and “inductive lightning”. The lightning strike rate is only about 10% of the lightning strike rate. The damage range is generally small. The lightning rod, the lightning protection line and the lightning protection net can be used to prevent it. The "inductive lightning" with much greater damage accounts for nearly 90% of the lightning strike rate, and the damage range is wide. The electronic equipment of the CATV system is damaged by lightning strikes, mainly caused by inductive lightning. The direct lightning strike is caused by the discharge between the charged cloud layer and the earth. In the process of forming the thundercloud, some clouds accumulate a positive charge and the cloud is close to a certain degree, and a sudden discharge occurs. A dazzling flash of light appeared. When the thundercloud is very low and there is no thundercloud with heterogeneous charge around it, it will induce the opposite charge on the ground or the building, forming a charged cloud layer to discharge to the ground or the building; the discharge current can reach tens or even hundreds of kiloamperes. The discharge time is 50-100μ S. This kind of discharge is a direct lightning strike. The direct lightning strike is very harmful to the safety of buildings and people and animals. After being installed on the lightning rod, the CATV system's electronic equipment may be damaged by lightning even if it is within its protection range. Most of them are burned fuses, power transformers, rectifier components, three-terminal regulators, and serious damage may also occur. Components such as integrated circuits, which means that lightning strikes are not introduced from the antenna, but are introduced from the power line. It can be seen that although the lightning rod protects the building, it cannot protect the CATV electronic equipment placed therein, which is caused by the induced lightning.

      Caused by the induction of lightning electrostatic induction and the electromagnetic induction caused by lightning current, when the charged clouds (Thundercloud) are close to the transmission line, they will induce opposite charges on them, and these opposite charges are bound by the Thundercloud charge. When Thundercloud discharges a nearby target or lightning receptor (the lightning rod is the earliest and most commonly used lightning receptor), its charge is quickly neutralized, and the charge trapped on the transmission line is free charge, forming a local induction high potential. This induction high potential can also reach 100KV when it is on the low-voltage overhead line; it can reach 40-60KV on the telecommunication line. And it can be passed along the line to the electronic device, causing damage. After the lightning strike, the huge lightning current generates an alternating magnetic field in the surrounding space, and the electromagnetic induction causes the nearby equipment to induce a high voltage, thereby causing damage to the equipment.

       The distribution law of lightning in China is: usually hot and humid areas have more lightning than cold and dry areas, and the frequency of lightning increases with the decrease of geographical latitude, that is, the closer to the frequency of the equatorial, the more South China >Southwest>Yangtze River Basin>North China>Northeast>Northwest, in addition, the frequency of lightning varies with the region, namely: mountain> plain> desert, land> lake and sea, the peak of lightning every year is in July and August, activities Most of the time is between 14-22 hours. The presence or absence of lightning in an area is determined by meteorological conditions, but the specific lightning point is greatly affected by local conditions. 1 The influence of the terrain is usually more than the northwest slope of the mountain on the east slope and the south slope of the mountain. The probability of falling lightning in the mountain flat is greater than that of the canyon. The probability of lightning on the lakeside is small, but the water on the lake is like a mountain. There are more chances of thundering on the slopes; the valleys of the estuaries or downwinds are prone to thunder. 2 The influence of ground objects, the towering buildings in the isolated buildings and buildings in the open space are vulnerable to lightning strikes, the roofs and towers of metal structures are vulnerable to lightning strikes, the trees next to the houses, the receiving antennas, the mountainous power lines, etc. Lightning strikes, because these ground objects are very conducive to the establishment of a good discharge channel between the thundercloud and the earth. 3 geological influence, the relative value of soil and resistivity is small, and the place where the charge is relatively accumulated is easy to fall. When there is a large soil resistance, the local soil resistivity is easy to drop; the soil resistivity is small, the slope is easy to fall. Areas with conductive deposits in the soil are prone to thunder; areas with high groundwater or mineral springs are prone to thunder.

The most favorable measure for lightning protection of the system is that the system has good grounding. Good grounding can not only discharge the voltage generated by the inductive lightning early, but also discharge the ground voltage generated by the leakage of the equipment to protect the equipment and the human body. For the purpose of safety, the specific system lightning protection also starts from the grounding. At the same time, in the place where it is needed to avoid lightning and lightning arresters, but the lightning resistance performance of the selected equipment is not neglected. When selecting equipment, the anti-lightning performance of the equipment is also an important part. At present, China produces and uses arresters in large quantities, and various valve arresters with electrical silicon carbide valve discs as basic components; zinc oxide surge arresters with zinc oxide valve discs as basic components, zinc oxide surge arresters have action compared with valve arresters It has the advantages of rapid, large flow capacity, low residual pressure, no freewheeling, simple structure, high reliability, long life and easy maintenance. The following focuses on the zinc oxide surge arrester, and the zinc oxide pressure sensitive arrester is effective for preventing the intrusion of lightning and lightning waves. Its working principle is the nonlinearity of its volt-ampere characteristics. At normal voltage, the arrester is in a high-impedance state, with only a small leakage current (on the order of μA), and the power loss is small. When an overvoltage occurs in the line, the arrester is in a low-resistance state (time effect is on the order of 10-9) overvoltage. In the form of discharge current flowing into the earth through the arrester, the overvoltage is suppressed. When the line voltage returns to normal after the surge voltage is over, the arrester is in a high-resistance insulation state. Therefore, the arrester must have a good grounding device to cooperate with it.

The current of the lightning overvoltage is a pulse that rises quickly to the peak and then falls slowly. The time value is on the order of 10-6S. The national standard is 8/20μS, that is, the time rises to the peak with 8μS, then With a time of 20μS, the response time is reduced to half-peak, and the response time of the zinc oxide arrester is 10-9, so the response time is available, followed by the amplitude problem. The power supply of the cable TV passes through the multi-level lightning protection of the power system, so the lightning The probability of pulse current greater than 5 KA will not be 15% larger. As long as the arrester with a flow capacity of 5 KA is selected, the residual voltage is also able to withstand the equipment, and the required grounding will achieve the lightning protection effect.

1. Wireless lightning protection grounding. The receiving antenna and the vertical rod of the cable TV are generally installed at the top of the building. All the receiving antennas, including the satellite receiving wireless grounding, should be welded together. The lightning rod should be installed on the vertical rod (frame) of the antenna. The height of the lightning rod should be installed. Should be able to meet the protection of the antenna facilities, when installing a separate lightning protection clock, because the protection range of a single lightning rod is tent-like, the boundary line is hyperbolic, so the length of lightning protection higher than the top of the antenna should be greater than the maximum size of the antenna, lightning rod The minimum horizontal spacing between the antenna and the antenna should be greater than 3M. When the building has a lightning protection grounding system, the grounding of the lightning rod and the antenna vertical rod should be connected with the lightning protection grounding system of the building; the building has no special lightning protection grounding. When using, special grounding device should be set up. From the lightning receptor to the grounding device, two down conductors are used. The grounding resistance should be less than 4 ohms from the different directions and the shortest distance along the building. The grounding system is also the grounding wire of the original building. The grounding resistance should be less than 4 ohms. In addition to the antenna should have a good lightning protection grounding, the following measures should be taken. Shi: 1 wireless output should be installed with dedicated CATV security device; 2 antenna output cable should be grounded according to grounding requirements; 3 use antenna amplifier or channel amplifier with gas discharge tube and fast reaction protection diode.

2. Lightning protection grounding of the front-end equipment. If a lightning strike occurs near the front end, high voltage is induced in the metal case and the outer casing of the equipment room, which jeopardizes equipment and personal safety. Leakage of the power supply of the front-end equipment will also endanger the safety of personnel. Therefore, all equipment in the equipment room, shields of input and output cables, metal pipes, etc. need to be grounded, and cannot be grounded with the grounding of the top-layer antenna. The grounding of the lightning rod of the house and the grounding of the power frequency AC power supply system should be connected at the total grounding. The grounding device of the electrical equipment and the buried metal pipe in the system shall be connected with the lightning protection grounding device. When the connection is not connected, the distance between the two should be more than 3 meters. The surface of the grounding busbar in the equipment room should be complete without obvious hammer marks and residual flux residue. The copper busbar should be smooth and burr-free. The aged layer of the insulated wire should not have aging cracking. Some front-end devices, such as modulators, receivers, etc., do not have over-voltage protection, but only over-current protection. In case of lightning strikes, the power supply will burn out and the insurance will continue. For this case, a lightning arrester should be installed at the main power supply. To better protect the front-end equipment.

3. Lightning protection grounding of trunk and distribution systems. 1 Underground cables laid in the open area. When the annual thunderstorm days in the area is greater than 20 days and the soil resistivity is greater than 100 ohms, the shield or metal sheath of the cable should be grounded once every 2KM. 2 The shield of the overhead cable and the metal sheath and steel hanging wire are grounded once every 250M. The overhead cable metal sheath at the cable distribution box, the shielding layer and the steel strand should be grounded with the cable. . The underground cable buried in the open area, its shielding layer and sheath should be grounded once every 2KM to prevent the influence of induction electricity. 3 When the cable enters the building, the outer conductive shielding layer of the cable should be grounded near the building. When the overhead cable is directly introduced, a lightning arrester should be added at the entrance and the outer conductor of the cable should be connected to the grounding device of the electrical equipment. When the cable is directly buried, the metal sheath of the cable should be connected to the grounding device at the entrance end. 4 Do not lay cables directly between the roofs of the two buildings. The cables can be lowered along the wall into the lightning protection zone, and the operation of the vehicle should not be hindered. The suspension wires should be grounded. 5 The input and output of the equipment in the system should have a gas discharge protection tube. The power supply end of the 220V power supply amplifier should have an overvoltage protection device. At present, the amplifiers on the market are mixed, and in order to reduce the cost, even the overvoltage prevention measures are omitted. Input and output Ruiyuan gas over-voltage discharge tube, 220V power supply amplifier only has over-current protection, and no over-voltage protection. When using trunk equipment, anti-overvoltage protection should be considered as an important precondition. Both the coaxial cable shielding net in the 5CATV system and the galvanized iron wire for the overhead supporting cable have good grounding.

4. The coaxial cable shielding net and the galvanized iron wire for the overhead support cable in the CATV system have good grounding, and the chance of inductive lightning is small. The lightning is the easiest to enter the electronic equipment from the power line, and the power supply line is grounded to the porcelain iron foot. It can play a certain role in protecting electrical equipment and personal safety. However, since electronic equipment such as CATV can withstand much higher voltage than electric equipment, it must be installed in addition to low-voltage arresters on the incoming line. Light-shielded wires, cables, etc. are embedded in the metal pipe to allow lightning waves to pass into the ground. The power cord can be wound around several turns before entering the electronic device to form a small inductance, which has no effect on the 50HZ current, and has a certain effect on blocking the lightning wave from entering the device.

When the system is grounded, be sure to minimize the grounding resistance. The grounding resistance is large and the lightning protection effect is poor. It is best to reduce the grounding resistance as much as possible and control it below 8 ohms.

The lightning protection of the cable TV system is a comprehensive technical project. Any loophole in the link will affect the lightning protection effect of the entire system. In the lightning protection design of the system, we must do a scientific and rigorous attitude and do a good job in system protection design.

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