Power supply lightning protection solution for 3G mobile base stations

As the country continues to promote the 3G industry, 3G construction is constantly underway. 3G base stations have the problem of RRU (Radio Frequency Remote Unit) towers, and the lightning strike environment is relatively harsh, which poses a severe test to the lightning protection design of the system. Considering that the cables on the towers of 3G are optical fibers and power cables, and generally optical fibers without reinforcement ribs are used, there is no problem of lightning strikes, so the lightning protection of the power supply plays an important role in solving the lightning protection of 3G base stations.

Because power supply lightning protection belongs to system engineering, it must be considered as a whole. Generally, it includes the following four aspects: lightning protection of AC power cables, ground connection between base station ground network and equipment in the station, lightning protection of combined power supply system in the station, lightning protection of RRU power line and power port, etc. Only by comprehensive protection in these four aspects can the ideal lightning protection effect be achieved.

This article discusses four aspects of 3G base station power supply lightning protection and provides a complete 3G mobile base station power supply lightning protection solution.

2 3G mobile base station power supply lightning protection solution

Lightning protection for AC power cables

⑴ Protection of AC power cables entering the station. For base stations with conditions, both the high-voltage side and low-voltage side cables of the transformer should be installed underground. According to the postal standard "YD 5098-2005 Communication Bureau (Station) Lightning Protection and Grounding Engineering Design Specifications" (hereinafter referred to as "postal standard"), "When using a dedicated transformer, the buried length of the high-voltage power cable should not be less than 200M, and when the low-voltage cable enters the machine room, its buried length should not be less than 15m (when the high-voltage power cable has been buried, the low-voltage side cable is generally not required). Low-voltage buried cables should use power cables with metal armor layers or be buried in steel pipes to introduce them into the machine room. The cable metal armor layer should be connected to the transformer ground network and the machine room ground network at both ends." However, for the high-voltage side cable, the investment and construction difficulty of underground installation are relatively large, which is difficult for general base stations. According to the same standard requirements above, lightning arresters should be installed along the overhead lines at this time, and high-voltage lightning arresters should be installed on the high-voltage side of the transformer .

(2) Install a Class B lightning protection box at the entrance of the AC low-voltage power cable into the machine room. Pay special attention to the "Kevin" wiring method when installing the Class B lightning protection box to reduce the residual pressure on the lead wire and give full play to the role of the Class B lightning protection box. For base stations where the AC low-voltage power cable is buried, since the attenuation effect of the AC low-voltage power cable on the lightning current is very obvious after being buried, the Class B lightning protection box can use an ordinary varistor lightning protection module with a waveform of 8/20μs. However, for low-voltage power cables that are not buried, the lightning current may be relatively large, and it is recommended to use an active ignition gap lightning protection module with a high current capacity of a 10/350μS waveform.

Ground connection between base station ground network and equipment inside the station

The ground network of the base station should be designed in accordance with Chapter 7 of the "Postal Standard" "Lightning Protection and Grounding of Small Wireless Base Stations", and the grounding resistance should also meet the standard of less than 10Ω.

Good grounding grid design and low grounding resistance play an important role in lightning protection of base stations, but this is far from enough. The success of a lightning protection grounding system is largely determined by the ground connection (installation) relationship between the equipment in the station.

⑴ Common unreasonable ground connection relationships of equipment in stations

Figure 1 Common unreasonable ground connection relationship of station equipment

This is the most common ground connection relationship for equipment in the equipment room. Its disadvantages are very obvious: the ground lead of the Class B lightning protection box at the entrance of the equipment room is too long and cannot play its due role; in addition, the ground wire of the switching power supply is too long, and the residual voltage on the ground wire will be superimposed on the power port of the back-end equipment. For this kind of base station, no matter how good the ground network design is or how small the grounding resistance is, it cannot play a good lightning protection role.

⑵ 'Postal Label' recommends two equipotential ground connection schemes: ring equipotential connection and star equipotential connection.

Figure 2 Ring equipotential ground connection scheme for equipment in the station

According to the 'Postal Standard', "When using a ring equipotential connection, a ring grounding collection line should be set up along the wiring rack and walls in the machine room. The ring grounding collection line should be connected to the ground grid at multiple points nearby, and the equipment in the station should be grounded nearby by the ring collection line."

Figure 3 Star-shaped equipotential ground connection scheme for equipment in the station

According to the 'Postal Standard', "When using star-shaped equipotential connection, the base station's main grounding bus should be located near the distribution box and the first-level power supply SPD. The grounding busbars of the switching power supply and other equipment are all connected to the main grounding bus. If the equipment rack is far away from the main bus, a two-level bus can be used."

It is recommended to use the equipotential ground connection scheme specified in the "Postal Standard" for the ground wire connection of the base station ground network and the equipment within the station, so as to truly give full play to the role of lightning arresters at all levels and achieve good lightning protection effects.

Lightning protection for combined power supply system in the station

For the lightning protection of the combined power supply system in the station, the lightning protection circuit is relatively simple and mature. According to the "Postal Code", a "3+1" type AC C-class lightning arrester with a current capacity of 40kA (8/20μS waveform) is used on the AC side of the combined power supply system, and a "1+1" type DC lightning arrester with a current capacity of 15kA (8/20μS waveform) is used on the DC side of the combined power supply.

Figure 4 Class C lightning arrester for combined power supply in the station

Figure 5 DC side lightning arrester of combined power supply in the station

RRU power cable and power port lightning protection

The RRU power line is led from the machine room at the bottom of the tower to the top of the tower. The space span is large, and the potential difference between the top and bottom of the tower is also large. Its lightning protection is very important and relatively difficult. The following lightning protection points should be paid attention to:

⑴ Use shielded cables, and both ends of the shielding layer must be reliably grounded. The upper end of the shielding layer is connected to the shell of the RRU (for systems with an external lightning protection box for the RRU, it is connected to the shell of the RRU external lightning protection box). The lower end of the shielding layer is connected to the outdoor ground row at the feeder window, and should not be introduced indoors to prevent the RRU power line shielding layer from introducing the tower lightning current into the room;

⑵ A differential mode inductor should be connected in series to the lightning protection circuit of the RRU power port to suppress the lightning current from flowing to the RRU back-end circuit.

Figure 6 RRU power cable and power port lightning protection

Note: ① If the combined power supply is not equipped with a suitable DC lightning arrester, a primary DC lightning arrester should be installed at the point where the RRU power cable exits the equipment room;

Because the specific parameters of RRU lightning protection devices vary from manufacturer to manufacturer, they should be selected according to the specific location of use.

Conclusion

Once 3G base stations are widely deployed in rural and remote mountainous areas, they will be subjected to the test of a very harsh lightning environment. In addition to the above measures for the power supply part, the 3G base station should also fully consider the lightning protection of the antenna feeder line, the lightning protection design of the RRU internal circuit, the lightning protection of the GPS, etc., in order to ensure the stable and reliable operation of the 3G base station.