Lightning protection equipment
Lightning protection equipment is through modern electricity and other technology to prevent the equipment being struck by lightning. Lightning protection equipment can be divided into power lightning protection, power protection socket, antenna feeder protection, signal lightning protection, lightning protection testing tools, measurement, and control system lightning protection, earth pole protection.
According to the theory of sub-area lightning protection and multi-level protection according to IEC (international electrotechnical committee) standard, b-level lightning protection belongs to the first-level lightning protection device, which can be applied to the main distribution cabinet in the building; Class C belongs to the second level lightning protection device, which is used in the sub-circuit distribution cabinet of the building; Class D is a third-class lightning arrester, which is applied to the front end of important equipment for fine protection.
Overview/Lightning protection equipment
Information age today, the computer network and communication equipment is more and more sophisticated, its work environment is becoming more and more demanding, and thunder and lightning and instantaneous overvoltage of large electrical equipment will be more and more frequently by power supply, antenna, a radio signal to send and receive equipment lines into indoor electrical equipment and network equipment, equipment or components damage, casualties, transfer or store the data of interference or lost, or even make electronic equipment to produce misoperation or pause, temporary paralysis, system data transmission interrupt, LAN and wan. Its harm is striking, indirect loss is more than direct economic loss in general. Lightning protection equipment is through modern electricity and other technology to prevent the equipment being struck by lightning.
Change/Lightning protection equipment
When people know that thunder is an electrical phenomenon, their worship and fear of thunder gradually disappear, and they begin to observe this mysterious natural phenomenon from a scientific perspective, in the hope of using or controlling the lightning activity for the benefit of mankind. Franklin took the lead in technology more than 200 years ago launched a challenge to the thunder, he invented the lightning rod is likely to be the first of the lightning protection products, in fact, when Franklin invented the lightning rod is that the tip of the metal rods function can be integrated in the thundercloud charge-discharge, reduce the thunder electric field between cloud and the earth to the level of the breakdown of air, to avoid the occurrence of lightning, so the lightning rod must requirements are pointed. But later research showed that the lightning rod is unable to avoid the occurrence of lightning, lightning rod, it can prevent lightning because a towering changed the atmospheric electric field, makes a range of thunderclouds is always to the lightning discharge, that is to say, the lightning rod is easier than other objects around it to answer the flash of lightning, lightning rod protection being struck by lightning and other objects, it is the lightning protection principle of lightning rod. Further studies have shown that the lightning contact effect of the lightning rod is almost related to its height, but not related to its appearance, which means that the lightning rod is not necessarily pointed. Now in the field of lightning protection technology, this kind of lightning protection device is called lightning receptor.
Development/Lightning protection equipment
The widespread use of electricity has promoted the development of lightning protection products. When high-voltage transmission networks provide power and lighting for thousands of households, lightning also greatly jeopardizes high-voltage transmission and transformation equipment. The high-voltage line is erected high, the distance is long, the terrain is complex, and it is easy to be hit by lightning. The protection scope of the lightning rod is not enough to protect thousands of kilometers of transmission lines. Therefore, the lightning protection line has emerged as a new type of lightning receptor for protecting high voltage lines. After the high-voltage line is protected, the power and distribution equipment connected to the high-voltage line is still damaged by over-voltage. It is found that this is due to the “induction lightning”. (Inductive lightning is induced by direct lightning strikes in the nearby metal conductors. Inductive lightning can invade the conductor through two different sensing methods. First, electrostatic induction: when the charge in the thundercloud accumulates, the nearby conductor will also induce On the opposite charge, when the lightning strikes, the charge in the thundercloud is quickly released, and the static electricity in the conductor that is bound by the thundercloud electric field will also flow along the conductor to find the release channel, which will form electricity in the circuit pulse. The second is electromagnetic induction: when the thundercloud discharges, the rapidly changing lightning current generates a strong transient electromagnetic field around it, which produces a high induced electromotive force in the conductor nearby. Studies have shown that the surge caused by electrostatic induction is several times greater than the surge caused by electromagnetic induction. Thunderbolt induces a surge on the high-voltage line and propagates along the wire to the hair and power distribution equipment connected to it. When the withstand voltage of these devices is low, it will be damaged by the induced lightning. To suppress the surge in the wire, people A line arrester was invented.
Early line arresters were open-air gaps. The breakdown voltage of air is very high, about 500kV/m, and when it is broken down by high voltage, it has only a few volts of low voltage. Using this characteristic of air, an early line arrester was designed. One end of one wire was connected to the power line, one end of the other wire was grounded, and the other end of the two wires was separated by a certain distance to form two air gaps. The electrode and the gap distance determine the breakdown voltage of the arrester. The breakdown voltage should be slightly higher than the working voltage of the power line. When the circuit works normally, the air gap is equivalent to an open circuit and will not affect the normal operation of the line. When the overvoltage is invaded, the air gap is broken, the overvoltage is clamped to a very low level, and the overcurrent is also discharged into the ground through the air gap, thereby realizing the protection of the lightning arrester. There are too many shortcomings in the open gap. For example, the breakdown voltage is greatly affected by the environment; the air discharge will oxidize the electrode; after the air arc is formed, it takes several AC cycles to extinguish the arc, which may cause a lightning arrester failure or a line failure. Gas discharge tubes, tube arresters, and magnetic blow arresters developed in the future have largely overcome these problems, but they are still based on the principle of gas discharge. The inherent disadvantages of gas discharge arresters are high impact breakdown voltage; long discharge delay (microsecond level); steep residual voltage waveform (dV/dt is large). These shortcomings determine that gas-discharge arresters are not very resistant to sensitive electrical equipment.
The development of semiconductor technology provides us with new lightning protection materials, such as Zener diodes. Its volt-ampere characteristics are in line with the lightning protection requirements of the line, but its ability to pass lightning current is weak so that ordinary regulator tubes cannot be directly used. lightning arrester. Early semiconductor The arrester is a valve arrester made of silicon carbide material, which has similar volt-ampere characteristics to the Zener tube, but has a strong ability to pass lightning current. However, metal oxide semiconductor varistor (MOV) has been discovered very quickly, and its volt-ampere characteristics are better, and it has many advantages such as fast response time and large current capacity. Therefore, MOV line arresters are currently widely used.
With the development of communication, many lightning arresters for communication lines have been produced. Due to the constraints of communication line transmission parameters, such arresters should consider the factors affecting transmission parameters such as capacitance and inductance. However, its lightning protection principle is basically the same as MOV.
Type/Lightning protection equipment
Lightning protection equipment can be roughly divided into types: power supply lightning protection device, power protection socket, and antenna feeder line protectors, signal lightning arresters, lightning protection test tools, lightning protection devices for measuring and control systems, and ground protectors.
The power supply lightning arrester is divided into three levels: B, C, and D. According to the IEC (International Electrotechnical Commission) standard for the theory of zone lightning protection and multi-level protection, Class B lightning protection belongs to the first-level lightning protection device and can be applied to the main power distribution cabinet in the building; The lightning device is applied to the branch distribution cabinet of the building; the D-class is a third-level lightning protection device, which is applied to the front end of important equipment to finely protect the equipment.
The communication line signal lightning arrester is divided into B, C and F levels according to the requirements of IEC 61644. Base protection basic protection level (rough protection level), C level (Combination protection) comprehensive protection level, Class F (Medium & fine protection) medium & fine protection level.
Measurement & Control devices/Lightning protection equipment
Measurement and control devices have a wide range of applications, such as production plants, building management, heating systems, warning device, etc. Overvoltages caused by lightning or other causes not only cause damage to the control system, but also cause damage to expensive converters and sensors. Failure of the control system often results in product loss and impact on production. Measurement and control units are typically more sensitive than power system reactions to surge overvoltages. When selecting and installing a lightning arrester in a measurement and control system, the following factors must be considered:
1, the maximum operating voltage of the system
2, the maximum working current
3, the maximum data transmission frequency
4, whether to allow the resistance value to increase
5, Whether the wire is imported from the outside of the building, and whether the building has an external lightning protection device.
Low voltage power arrester/Lightning protection equipment
The analysis of the former post and telecommunications department shows that 80% of the lightning strike accidents of the communication station are caused by the intrusion of the lightning wave into the power line. Therefore, the low voltage alternating current arresters develop very rapidly, while the major lightning arresters with MOV materials occupy a dominant position in the market. There are many manufacturers of MOV arresters, and the differences of their products are mainly shown in:
The flow capacity is the maximum lightning current (8/20μs) that the arrester can withstand. The Ministry of Information Industry Standard “Technical Regulations for Lightning Protection of Communication Engineering Power System” stipulates the flow capacity of the lightning arrester for power supply. The first-level arrester is greater than 20KA. However, the current surge capacity of the arrester on the market is getting bigger and bigger. The large current-carrying arrester is not easily damaged by lightning strikes. The number of times that the small lightning current is tolerated is increased, and the residual voltage is also slightly reduced. The redundant parallel technology is adopted. The arrester also improves the protection of the ability. However, the damage of the arrester is not always caused by lightning strikes.
At present, it has been proposed that a 10/350 μs current wave should be used for detecting a lightning arrester. The reason is that the IEC1024 and IEC1312 standards use a 10/350 μs wave when describing a lightning wave. This statement is not comprehensive, because 8/20μs current wave is still used in the matching calculation of the arrester in IEC1312, and 8/20μs wave is also used in IEC1643 “SPD” – Principle of Selection” It is used as the main current waveform for detecting the arrester (SPD). Therefore, it cannot be said that the flow capacity of the arrester with the 8/20 μs wave is outdated, and it cannot be said that the flow capacity of the arrester with the 8/20 μs wave is not in compliance with international standards.
Protect the circuit
The failure of the MOV arrester is short-circuited and open-circuited. A powerful lightning current may damage the arrester and form an open-circuit fault. At this time, the shape of the arrester module is often destroyed. The arrester may also decrease the operating voltage due to aging of the material for a long time. When the operating voltage drops below the working voltage of the line, the arrester increases the alternating current, and the arrester generates heat, which will eventually destroy the nonlinear characteristics of the MOV device, resulting in partial short circuit of the arrester. burn. A similar situation may occur due to an increase in operating voltage caused by a power line failure.
The open circuit fault of the arrester does not affect the power supply. It is necessary to check the operating voltage to find out, so the arrester needs to be checked regularly.
The short-circuit fault of the arrester affects the power supply. When the heat is severe, the wire will be burned. The alarm circuit needs to be protected to ensure the safety of the power supply. In the past, the fuse was connected in series on the arrester module, but the fuse must ensure the lightning current and the short-circuit current to be blown. It is difficult to implement technically. In particular, the arrester module is mostly short-circuited. The current flowing during the short circuit is not large, but the continuous current is enough to cause the lightning arrester mainly used for discharging the pulse current to be severely heated. The temperature disconnecting device that appeared later solved this problem better. The partial short circuit of the arrester was detected by setting the disconnection temperature of the device. Once the arrester heating device was automatically disconnected, the light, electric and acoustic alarm signals were given.
The Ministry of Information Industry Standard “Technical Regulations for Lightning Protection of Communication Engineering Power System” (YD5078-98) has made specific requirements for the residual voltage of lightning arresters at all levels. It should be said that the standard requirements are easily achieved. The residual voltage of the MOV arrester is Its operating voltage is 2.5-3.5 times. The residual voltage difference of the direct-parallel single-stage arrester is not large. The measure to reduce the residual voltage is to reduce the operating voltage and increase the current capacity of the arrester, but the operating voltage is too low, and the arrester damage caused by the unstable power supply will increase. Some foreign products entered the Chinese market at an early stage, the operating voltage was very low, and later greatly increased the operating voltage.
The residual voltage can be reduced by a two-stage arrester.
When the lightning wave invades, the arrester 1 discharges, and the residual voltage generated is V1; the current flowing through the arrester 1 is I1;
The residual voltage of the arrester 2 is V2, and the current flowing is I2. This is: V2=V1-I2Z
It is obvious that the residual voltage of the arrester 2 is lower than the residual voltage of the arrester 1.
There are manufacturers to provide two-level lightning arrester for single-phase power supply lightning protection, because the power of single-phase power supply is generally below 5KW, the line current is not large, and the impedance inductance is easy to wind. There are also manufacturers that provide three-phase two-stage arresters. Because the power of the three-phase power supply may be large, the arrester is bulky and expensive.
In the standard, it is required to install a lightning arrester in multiple stages on the power line. In fact, the effect of reducing the residual voltage can be achieved, but the self-inductance of the wire is utilized to make the isolation impedance inductance between the arresters at all levels.
The residual voltage of the arrester is only the technical indicator of the arrester. The overvoltage applied to the equipment is also based on the residual voltage. The additional voltage generated by the two conductors of the lightning arrester connected to the power line and the ground wire is added. Therefore, the correct installation is performed. Lightning arresters are also an important measure to reduce the overvoltage of equipment.
Other/Lightning protection equipment
The arrester can also provide lightning strike counters, monitoring interfaces and different installation methods according to user needs.
Communication line arrester
The technical requirements of the lightning arrester for communication lines are high, because in addition to meeting the requirements of lightning protection technology, it is necessary to ensure that the transmission indicators meet the requirements. In addition, the equipment connected to the communication line has a low withstand voltage, and the residual voltage of the lightning protection device is strict. Therefore, it is difficult to select the lightning protection device. The ideal communication line lightning protection device should have small capacitance, low residual voltage, large current flow and fast response. Obviously, the devices in the table are not ideal. The discharge tube can be used for almost all communication frequencies, but its lightning protection capability is weak. MOV capacitors are large and only suitable for audio transmission. The ability of TVS to withstand lightning current is weak. Protective effects. Different lightning protection devices have different residual voltage waveforms under the impact of current waves. According to the characteristics of the residual voltage waveform, the arrester can be divided into a switch type and a voltage limit type, or the two types can be combined to make the strength and avoid the short.
The solution is to use two different devices to form a two-stage arrester. The schematic diagram is the same as the two-stage arrester of the power supply. Only the first stage uses a discharge tube, the intermediate isolation resistor uses a resistor or PTC, and the second stage uses a TVS, so that the length of each device can be exerted. Such a lightning arrester can be up to a few tens of MHZ.
Higher-frequency arresters mainly use discharge tubes, such as mobile feeders and paging antenna feeders, otherwise it is difficult to meet the transmission requirements. There are also products that use the principle of a high-pass filter. Since the energy spectrum of a lightning wave is concentrated between several kilohertz and several hundred kilohertz, the frequency of the antenna is very low, and the filter is easy to manufacture.
The simplest circuit is to connect a small core inductor in parallel with the high-frequency core wire to form a high-pass filter arrester. For the point frequency communication antenna, a quarter-wavelength short-circuit line can also be used to form a band-pass filter, and the lightning protection effect is better, but both methods will short-circuit the DC transmitted on the antenna feeder line, and the application range is limited.
Grounding is the basis of lightning protection. The grounding method specified by the standard is to use horizontal or vertical ground poles with metal profiles. In areas with strong corrosion, galvanization and the cross-sectional area of metal profiles can be used to resist corrosion. Non-metallic materials can also be used. The conductor acts as a ground pole, such as a graphite ground electrode and a Portland cement ground electrode. A more reasonable method is to use the basic reinforcement of modern architecture as the ground pole. Due to the limitations of lightning protection in the past, the importance of reducing the grounding resistance is emphasized. Some manufacturers have introduced various grounding products, claiming to reduce the ground resistance. Such as resistance reducer, polymer ground electrode, non-metal ground electrode and so on.
In fact, in terms of lightning protection, the understanding of grounding resistance has changed, the requirements for the layout of the grounding grid are high, and the resistance requirements are relaxed. In GB50057–94, only the grounding network forms of various buildings are emphasized. There is no resistance requirement, because in the lightning protection theory of the equipotential principle, the ground network is only a total potential reference point, not an absolute zero potential point. The shape of the ground grid is required for equipotential needs, and the resistance value is not logical. Of course, there is nothing wrong with obtaining a low grounding resistance when conditions permit. In addition, power supply and communication have requirements for grounding resistance, which is beyond the scope of lightning protection technology.
The grounding resistance is mainly related to the soil resistivity and the contact resistance between the ground and the soil. It is also related to the shape and the number of the ground when forming the ground. The resistance reducer and various grounding electrodes are nothing to improve the contact resistance or contact between the ground and the soil. area. However, the soil resistivity plays a decisive role, and the others are relatively easy to change. If the soil resistivity is too high, only the engineering method of changing soil or improving the soil can be effective, and other methods are difficult to work.
Lightning protection is an old topic, but it is still evolving. It should be said that there is no product to try out. There are still many things to be explored in lightning protection technology. At present, the mechanism of lightning power generation is still unclear. The quantitative research on lightning induction is also very weak. Therefore, lightning protection products are also developing. Some new products claimed by lightning protection products, It needs to be tested in practice with a scientific attitude and developed in theory. Since lightning itself is a small probability event, it requires a lot of long-term statistical analysis to get beneficial results, which requires the cooperation of all parties to achieve.