Principles and Engineering Design of Lightning Protection for Wireless Communication Base Stations (2) 2.1 Structure and Basic Requirements of Lightning Protection Grounding System: Lightning protection grounding system consists of earth, grounding electrode, grounding lead-in, grounding busbar, and grounding wiring. The overall composition. Among them: Earth has conductivity and infinite capacity, and it is a good reference ground for public grounds; grounding electrodes are metal strips that are in electrical contact with the earth and are used to diffuse current into the ground; grounding leads are in grounded electrodes and indoor The grounding busbars connect the copper busbars; the ground busbars are the bus copper busbars used for the collection grounding wiring; the grounding wiring is the wires connecting the equipment to the grounding busbars, as shown in Figure 3.
The requirements of the lightning protection grounding system are mainly reflected in the following two aspects: (1) Requirements for grounding resistance: Grounding resistance mainly includes soil resistance, contact resistance between soil and ground electrode, resistance of ground electrode itself, resistance of ground lead-off wire, etc. After several kinds of resistance are very small, generally negligible, so the grounding resistance mainly refers to the soil resistance. Reducing the grounding resistance is the key to realizing the lightning current discharge. The formula for calculating the total voltage drop of a lightning current through a single down conductor is
Among them, the voltage drop, unit; lightning current, unit:; grounding device resistance, unit; for the unit length of the inductor, about 1.5; for the length of the downline, unit; for the lightning current steepness, unit. From the formula, it can be known that in the lightning protection grounding device, the smaller the resistance of the grounding resistance, the smaller the instantaneous internal impact voltage drop, and the less dangerous the facility is during lightning. The grounding resistance requirements of different facilities are slightly different. The mobile communication base station base is ≤ 4Ω, the antenna line metal shield is ≤ 4Ω, the signal lightning arrester is ≤ 10Ω, the power lightning arrester is ≤ 4Ω, the safety protection ground is ≤ 4Ω, and the communication equipment room is ≤ 1Ω. The system design must be properly planned and meet the specification parameters. 2 Requirements for joint grounding: In the relevant lightning protection grounding design specifications of IEC (International Electrotechnical Commission) and ITU-T (International Telecommunication Union), there is no longer a separate grounding, but a public ground network is established for lightning protection, ie, power ground, Workplaces, protective sites, etc. are electrically integrated on public grounds to establish a zero-potential reference level platform. In the mobile communication base station, the lightning protection grounding is ground leakage current protection for lightning protection; the work ground is DC power grounding; and the protection grounding is the indoor equipment chassis grounding.
2.2 Preliminary survey, data collection, trial grounding resistance: Collect the topography, geological structure, soil properties, aquifer conditions, and local thunderstorms at the location of the communication site. The key points are: 1 Soil resistivity around the grounding electrode, 2 Resistance Rate seasonal correction factors, specific parameters can be found in the relevant information such as power engineering manuals, and the soil resistivity in different seasons can be obtained. When fixed, the grounding resistance is only related to the shape and embedment of the grounding electrode. The commonly used grounding resistance of the tube (or plate) vertical electrode is the resistivity, the electrode length, the electrode depth, and the tube electrode. Diameter, generally. When using multiple electrodes and connecting ground, ground resistance can be further reduced. If there are multiple grounding systems coexisting, then the grounding electrodes between different grounding systems should be at least 25m apart to prevent mutual interference.
2.3 Design grounding scheme and optimize grounding design: The choice of grounding electrode can be flexible and diverse. Local conditions, such as horizontal buried trench electrodes, vertical rod electrodes vertically driven into the soil, and multiple grounding electrodes in parallel to form a multi-pole grounding body are common solutions. In addition to this, reducing soil resistivity is also an important aspect of optimizing grounding design. Common methods, such as soil changing method, change the soil in the range of 1~4m around the grounding body to high-quality soil with low resistivity; - Alternately lay high-quality soils (or charcoal) and salt 6-8 layers around the grounding body, and pour them with water, and apply a long-lasting resistance-reducing agent to fill the soil around the electrodes. Can effectively reduce the resistance.
(to be continued)
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