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Australia, the United States and Austria: wireless communication base station lightning protection principles and engineering design (4)
3, base station lightning protection
The mobile communication BTS cabinet is mainly connected to the outside world through the following ways: the power line is connected with the power supply equipment; the transmission trunk is connected with the BSC; and the feeder cable (or jumper cable) is connected with the antenna system. Therefore, the lightning protection of the base station mainly includes four aspects: lightning protection of the power supply, lightning protection of the relay line, lightning protection of the antenna feed line, and lightning protection of the engine room.
3.1 Power supply and power line lightning protection: The BTS cabinet supports three types of power supply methods: AC 220V, DC-48V, and DC 24V. In the lightning protection mode, AC power supply lightning protection design and DC power supply lightning protection design are mainly considered.
The lightning protection design of AC power supply: The high-voltage partial power bureau has special high-voltage lightning protection devices, the 380V low-voltage line is overvoltage protected according to the national regulations in three levels, and the first-level protection is the cable core-to-ground connection between the rear end of the high-voltage transformer and the total switchboard of the building. Arrester; Level 2 protection is the addition of arresters to the cable cores of the building's main switchboard to the floor distribution box; three-level protection is important, precision equipment, and surge arresters on the front end of the UPS. The lightning protection and inductive lightning of the BTS equipment are mainly anti-radar and inductive lightning. The basic functional block diagram of the AC lightning arrester is shown in Fig. 4. Two levels of lightning protection are used. The role of the inductor is to suppress sudden changes in the current and make the varistor fully play its role. Function, under normal conditions, open and closed, AC indicator light, once subjected to direct lightning or lightning invading waves, the varistor is a low resistance (short circuit) characteristics, open open circuit, the indicator light off.
DC power supply lightning protection design: First, when the -48V DC power supply is used, the grounding point of the DC power supply of the power supply equipment is different from the grounding point of the communication equipment. When the lightning strike occurs, the instantaneous potential difference between the two points may cause the positive DC power supply to communicate. The inrush current between the equipment ground points; the second is the grounding of the power supply equipment and the grounding of the communication equipment. If it eventually converges at a certain point, the closed loop formed by the positive wire of the DC power supply and the ground wire of the two equipments will change during the lightning strike. The induced current is generated in the magnetic field; the third is that the inrush current is introduced from the AC power line and transmitted to the DC power line through a primary power supply. As shown in Figure 5, the function diagram of the DC lightning protector is a three-level protection, and the last stage makes the residual pressure clamp.
Access to the communication station (station) of the power cable, hierarchical protection must be strictly implemented in accordance with the specification, especially when renting a residential house, the AC power must be installed in the lightning protection box re-introduced equipment, the amount of flow selected according to equipment load.
3.2 BTS trunk lightning protection design: The BTS supports 75 coaxial E1, 120 twisted pair E1, and optical fiber (SDH) trunks, of which the fiber is pigtail access to the base station, and lightning protection is not considered. For other methods, lightning protection will be implemented on the DDF. As shown in Fig. 6, the DDF is iron. When using it, connect the ground bar of the equipment room and the ground wire of the DDF with a thick wire; if it is 75 coaxial cable, you can Connect the lightning arrester directly; if it is a 120 twisted pair, connect the ground wire of this trunk to the DDF chassis. The signal line is hit with the wire cutter in the corresponding position. Since each connection unit is normally open, it must be Only when the security unit is plugged in can the circuit be closed. When the current on the trunk is large enough, the security device will be self-destructed and open to protect the BTS equipment. (Actually, in addition to the E1 line arrester added to the Huawei BTS, a special lightning protection circuit is also designed in the BIE board of the base station).
3.3 BTS antenna feeder lightning protection: Base station antenna feeder lightning protection is mainly designed in the following three aspects. The first is the iron tower (frame): at the antenna platform at the top of the tower, a grounding hole should be reserved in the middle of the tower body and at the base of the tower. When the iron tower is a tower at the top of the tower, it should be on the roof lightning deflector or the main reinforcing bar of the same building. The nearest welding, solder joints do protective treatment and the number and dispersibility of the connection points must be guaranteed; when the iron tower is a landing tower, the iron tower shall be constructed with a grounding network, galvanized flat steel strips shall be buried around it, and the flat steel strips shall be used every time. One (round steel for ground piles), in addition, vertical piles can be embedded in the four corners of the iron tower, and an annular closed earth body can be set around the four corners to form a multi-point connection, that is, the area of ​​the ground net should extend to the four corners of the tower. The range, grid size shall not be greater than, and the perimeter shall be closed type. The last iron tower ground net and the machine room ground net shall be welded and communicated with each other at intervals (at least two locations shall communicate with each other). When the antenna is directly installed by using a pole without a tower, each pole should be connected to a roof lightning protection zone respectively, and the solder joints should be treated for protection against corrosion. The roof lightning protection is accompanied by severe rust damage, or it can lead off the ground and ground wires. In the absence of detailed design information, it is best to use ground resistance measurements. If necessary, separate grounding grids and lightning deflectors should be provided.
(to be continued)