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Doing a good job of lightning protection is not only to learn about lightning protection products, but also to have a full understanding of thunder and lightning, and only to understand that thunder and lightning supplemented with corresponding lightning protection measures can be better to prevent lightning. This article describes: The generation of thunder and lightning, the division of lightning protection areas, the parameters of thunder and lightning, etc. Hope that everyone can have a clearer understanding of thunder and lightning!
First, how did lightning occur?
Lightning is a common phenomenon of atmospheric discharge. In the summer afternoon or evening, the hot air on the ground carries a large amount of water vapor to rise to high altitudes, forming a large-scale cumulonimbus cloud. Accumulation of a large number of positive or negative charges in different parts of the cumulonimbus cloud forms a thunder cloud. The ground is charged by near-surface thunderclouds, and it also carries charges opposite to the cloud's bottom. When the electric charge in the clouds builds up, and reaches a certain intensity, it will break through the air and open a narrow channel to force discharge.
II. Division of Lightning Protection Areas for Lightning Protection Projects
The space to be protected is divided into different lightning protection areas to specify the electromagnetic environment (different degree of damage from lightning and electromagnetic plants) in each part of the space, and the position of the equipotential bonding point at the junction of each area is specified.
Taking into account the significant changes in the electromagnetic environment at their junctions, the characteristics of the different lightning protection zones were divided.
LPZ0A: Each object in the area may be directly struck by lightning and the electromagnetic field is not attenuated;
LPZ0B: Objects in the area are unlikely to suffer direct lightning strikes and the electromagnetic field is not attenuated;
LPZ1: Each object in the area is unlikely to suffer direct lightning strikes, and the electromagnetic field may be attenuated;
LPZ2: Objects in the area are unlikely to suffer direct lightning strikes and the electromagnetic field is further attenuated
A protected area, from the perspective of electromagnetic compatibility, can be divided into several levels of protection from the outside to the inside, and the outermost layer is a level 0. It is a direct lightning strike area with the highest danger, and the more dangerous it is, the lower the degree of danger is. . The overvoltage is mainly entered along the line, and the interface of the protection zone is formed by a shielding layer formed of an external lightning protection system, reinforced concrete and a metal cover, and the electrical passage and the metal pipe pass through these interfaces. Figure 3-1 is a schematic diagram of the division of lightning protection areas.
SPD (Surge Protect Device): The abbreviation of Surge Protect Device, also known as surge protector in the company, is used to protect the interface of the device from lightning overvoltage and overcurrent damage. In this article, the SPD is called a lightning arrester.
Third, lightning protection design should pay attention to lightning parameters
Lightning discharge involves many natural factors such as weather, terrain, and geology, and has a certain degree of randomness. Therefore, parameters that characterize lightning characteristics also have certain statistical properties. In lightning protection design, we are concerned about thunderstorm day, lightning current waveform, amplitude and other parameters.
1, thunderstorm day
In order to characterize the frequency of lightning activity, the annual average thunderstorm day is used as the unit of calculation. No matter how many thunders are heard in a single day, as long as there are once, the day is recorded as a thunderstorm day. There are many times a day and it is still recorded as a thunderstorm day. The number of thunderstorm days is related to latitude. The number of thunderstorm days near the hot and humid equator is the highest, and the number of poles is the least. . After October, lightning activities in other regions except Jiangnan were almost stopped. (See also Thunderstorm Day in the lightning protection technical column of this site for details.
2, lightning current waveform
The lightning current is a non-periodic transient current that usually rises quickly to its peak value and then falls more slowly. The head time of the lightning current is the time when the lightning current rises from zero to the peak, also known as the wavefront time; the wavelength time refers to the time from zero to peak and then to half of the peak value, also known as the half-peak time. . Since the oscillation is often superimposed at the beginning and the peak of the lightning current wave, it is difficult to determine the true zero point and the time to reach the peak value. Therefore, we often use the apparent wave head time T1 and the apparent wavelength time T2 to indicate the increase of the lightning current. Time and half-peak width, generally denoted as T1 / T2. Lightning strike test waveforms specified in IEC standards and national standards are: 8/20us, 10/350us (current wave), 10/700us and 1.2/50us (voltage wave) Wait.
3, lightning wave spectrum analysis
Lightning spectrum is an important basis for studying lightning protection. From the structure of the lightning wave spectrum, the distribution of the energy of the lightning wave voltage and current in each frequency band can be obtained. Based on these data, the amplitude and energy magnitude of the lightning surge in the information system band can be estimated, and the appropriate lightning protection measures can be determined. Through the spectrum analysis of the lightning wave, it can be seen that: 1. The lightning current is mainly distributed in the low frequency part, and decreases as the frequency increases. When the wave tails are the same, the steeper and more harmonic wavefronts are more abundant. In the same case of wavefront, the longer the wave tail, the more abundant the low-frequency part; 2. The energy of lightning is mainly concentrated in the low-frequency part, and about 90% of the lightning energy is distributed in the frequency of 10kHz or less. This shows that in the information system, as long as the prevention of lightning wave infringement below 10kHz, the energy of the lightning wave can be reduced by more than 90%, which has important guiding significance for the lightning protection project.
4, the formation of lightning overvoltage
The root cause of lightning damage to information equipment is lightning electromagnetic pulses. Lightning electromagnetic pulses include two aspects, lightning currents and lightning electromagnetic fields. Lightning current is the root cause of direct lightning overvoltage, and lightning electromagnetic field is the root cause of induced lightning overvoltage.
For communication devices, the main sources of lightning overvoltages are the following:
1. Induced overvoltage: Inductive overvoltage refers to the damage caused by the transient space electromagnetic field when the lightning strikes the building or its near area. Inductive overvoltage includes both electromagnetic induction and electrostatic induction. Electrostatically induced overvoltages are generated by capacitive coupling, and electromagnetically induced overvoltages are generated by inductive coupling. For various metal loops or electronic devices in a building, the electromagnetic induction component is greater than the electrostatic induction component.
2, lightning invasion wave. Lightning intrusion waves are also called line waves. When the thunderclouds or thunderclouds are discharged to the ground, induced overvoltages (including electrostatic induction and electromagnetic induction components) are generated on the nearby metal pipelines, but for long-distance lines, the electrostatic induction overvoltage component is much larger than the electromagnetic induction. Voltage component). The induced overvoltage can also enter the room in a traveling wave manner, causing damage to the electronic device.
3, counterattack overvoltage. Lightning counterattack refers to the increase in ground potential at the grounding point of nearby equipment when a lightning strikes a building or its near area, causing a high overvoltage between the device housing and the conductive part of the equipment (called counter-attack overvoltage). The phenomenon of equipment damage.
Summary: The division of lightning protection areas is very important for lightning protection. It must be remembered that the parameters of lightning: thunderstorm days, waveforms, overvoltage formation, etc. are also of great significance for lightning protection of electronic equipment. Learn about thunder and lightning, and use scientific lightning protection for their performance. One day, humans will fight thunder and lightning!
October 08, 2024