At present, UPS power supplies on the market can be divided into three categories according to their working methods: online, (2) online interactive, and (3) backup. Among them, the characteristics of the online UPS are independent rectification, inverter, charging, bypass and maintenance bypass systems. During the working process, the user has been in the inverter working state, and all the power supply is "regenerated". High-quality pure sine wave power supply is relatively expensive. The backup type is characterized by high conversion efficiency. When the mains power supply is normal, the inverter does not work. The load is obtained from the mains after simple voltage regulation. Only when the mains supply is abnormal, the inverter starts. The power supply to the load is relatively cheap, and it is generally not used in important applications. Online interactive products are intermediate products of UPS, which not only have the advantages of high backup conversion rate and high reliability, but also have the advantages of high online power supply quality and short switching time, and the price is moderate. In recent years, the degree of intelligence of UPS power supplies has improved rapidly. Most UPS power supplies have microprocessors, fault self-test functions, and standard communication ports.
1. Basic working principle of UPS power supply
The main working process of UPS power supply is filter rectification and inverter, and also includes many auxiliary units, such as: charger and battery, microprocessor, communication interface, etc. Because the UPS power supply is installed between the equipment and the mains, it can filter out the electromagnetic interference in the power grid. Therefore, it creates an illusion that the UPS power supply can block the intrusion of all electromagnetic pulses including lightning, in fact it's not true.
2. The harm of lightning to UPS power supply
The hazards of lightning to microelectronic devices have long been familiar to engineers and technicians. For microelectronic equipment, the most harmful is lightning electromagnetic pulse, which is permeable and implies killing. According to our statistics on related accidents, more than 70% of lightning strike accidents are intruded from the power line, and the UPS power supply cannot block the intrusion of lightning current. There are three reasons.
(1) From the discussion in 2, it can be seen that the mains input port of the UPS power supply is a filtering unit, which generally includes a MEI filter and an RFI filter. According to the spectral characteristics of the lightning current, more than 90% of its energy is concentrated below 1MHz. The DC component accounts for more than 60%. When lightning strikes, the UPS is at the forefront of the power line, and it is the first to be attacked.
(2) Many UPSs now have a lightning protection function. The principle is to add a MOV lightning protection module at the input of the UPS. Some imported brand-name UPS and several domestic famous UPS manufacturers are inside their UPS, according to the international IEC801-5 The standard is equipped with a lightning protection module to suppress the strong surge of absorbing lightning voltage and current at the input end of the power supply line. Its impulse current is 20KA, the impulse voltage is 6kV, and the waveform is 8/20. However, statistics show that the overvoltage amplitude of direct lightning strikes on general low-voltage overhead lines is as high as 100kV, and that of telecommunication lines is as high as 40 ~ 60kV. Induction lightning overvoltage amplitude is up to 20kV on unshielded overhead lines and up to 10kV on unshielded underground cables. If there is no complete lightning protection system designed according to the specifications, even such a UPS cannot protect electrical equipment from Lightning strikes.
(3) UPS power supply, especially intelligent UPS power supply, itself contains a large number of integrated circuits. And more and more UPSs are equipped with intelligent management systems, and signal lines have also become channels for lightning electromagnetic pulses to invade. Because of this, cases of lightning damage to UPS power supplies are not uncommon, especially in lightning strike areas where there are many thunderstorm days.
For example, a UPS power supply installed in a unit in Hainan has been operating normally for half a year since installation, but after encountering a lightning strike, the UPS frequently appears after a period of time after startup and inexplicable automatic conversion from the inverter power supply Failure to supply power to the AC bypass power supply.
Judging from the types of lightning disaster losses, more than 90% involve computer networks and communication systems, and basically have UPS power supplies. Therefore, we must pay enough attention to the lightning protection of the UPS power supply and its monitoring system.
3. Lightning protection of UPS power supply
Lightning protection for UPS power systems and communication ports should be based on the relevant regulations of the state and the specific circumstances of the application environment, and work out practical solutions according to local conditions to establish an effective, scientific, and economic lightning protection system. According to the characteristics of the UPS system, its lightning protection should focus on the following points:
3.1 It is necessary to improve the external lightning protection facilities and do a good job of the grounding of the computer room. According to the "Electronic Computer Room Design Code", AC, DC working ground, protective ground, and lightning protection grounding should share a set of grounding devices, and the grounding resistance should be determined according to the minimum value. If you must set up separate grounding, you must install an equipotential common ground coupling between the two grounds. Regardless of the grounding system used, equipotential bonding is very important. UPS protects often large-scale data systems, which are more sensitive to lightning strikes. Even small potential strikes often cause unnecessary losses.
3.2 Multi-level lightning protection measures shall be taken. The "Code for Design of Lightning Protection of Buildings" and IEC61312-1 have a clear concept of lightning protection zone, which divides the areas that need lightning protection into:
LPZOA (OA area), all objects in this area may be directly struck by lightning, and the electromagnetic field generated by lightning in this area can propagate freely without attenuation.
LPZOB (OB zone), all objects in this zone will not be exposed to direct lightning strikes within the protection range of the lightning receptor, but the lightning electromagnetic field in this zone has no shielding device, and the electromagnetic field generated by lightning can also propagate freely without attenuation .
LPZ1 (Zone 1), because the objects in the zone will not be directly struck by lightning, the current flowing through each conductor is smaller than that in the LPZOB zone, the lightning electromagnetic field in this zone will have different attenuation according to the different shielding measures .
LPZ2 (Zone 2), when it is necessary to further reduce the lightning and electromagnetic fields, follow-up lightning protection zone should be introduced, and the requirements of the subsequent lightning protection zone should be selected according to the environment required to protect the system.
The central content of lightning protection is discharge and balance. The discharge discharges as much lightning current as possible and as far as possible to the ground, but rejects it from the communication system. The so-called multi-level protection is to weaken the lightning current in layers according to the principle of electromagnetic compatibility. Install the appropriate specifications of lightning arresters in the power line access household distribution cabinet, floor distribution cabinet and equipment room access household distribution box. For UPS with signal or communication interface, in order to prevent the introduction of lightning waves from the signal or communication line, the corresponding signal lightning arrester must be installed at the signal or communication line interface.
3.3 Selection and installation of lightning arresters
The lightning arrester product market is relatively rich at present. You should try to choose a reliable and reliable lightning arrester. The grounding wire of the arrester should be no less than 6mm2. Connect with the straightest and shortest lead wire. Ground to reduce the induced voltage on the lead.
The special lightning protection box for UPS power supply and UPS power supply must be grounded. The grounding resistance should not be greater than 4 ohms. The lightning protection device and UPS power supply should be connected to the same potential. The grounding system is best to use high-quality grounding modules, which can ensure the reliability and corrosion resistance of the grounding resistance, and avoid the ground network transformation every 1-2 years, saving costs for the user.
4 Conclusion
With the increase in the degree of intelligence of UPS power supply, UPS power supply is often not only a complete machine product that can continue to supply power to the load after a power grid outage, but also a local highly reliable, complete and highly intelligent power supply center. It plays an important role in ensuring the data security and smoothness of the information network. It is the purpose of this article to analyze the importance and necessity of lightning protection for UPS power supplies. I hope to draw everyone's attention to this issue. Lightning protection for UPS power system is a very professional work, it is best to carry out under the guidance of professionals. Pay attention to systematic considerations. Lightning, shielding, grounding, equipotentiality, regional lightning protection and other factors are comprehensively considered. Making a grounding system is the foundation and key to the lightning protection system, especially in some new campuses. It is an important factor that cannot be ignored.
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