In the power system, lightning protection measures for strong electrical equipment are relatively complete and experience is abundant, but lightning protection for weak electrical equipment (such as communications equipment, automation equipment, computers and network equipment, weak electrical power equipment, etc.) appears to be weak. Every year, various kinds of weak electrical equipment are damaged by lightning strikes. With the modernization of power systems and the development of informatization processes, weak systems have taken a pivotal role in the entire power system. Therefore, how to protect weak systems from damage has attracted increasing attention from all sides. Discussion.
With the continuous development of modern electronic technology, the use and networking of a large number of sophisticated electronic devices have enabled equipment installed in weak current systems to suffer from poor power quality (such as power harmonics amplification, switching electromagnetic pulses), direct lightning strikes, and induced lightning. Industrial operation instantaneous overvoltage, zero potential drift and other surge and overvoltage invasion, often by various overvoltage, overcurrent hazards. Since some electronic devices operate at voltages of only a few volts, the current carrying information is also very small and extremely sensitive to external disturbances. The lightning voltage can be as high as several million V and the instantaneous current can be as high as several 100,000 A. Therefore, it is extremely significant. Destructive.
Lightning rods can prevent direct lightning strikes, but they cannot prevent induced lightning overvoltages, overvoltages, zero-voltage floating overvoltages, and strong overvoltages generated by these overvoltages when they discharge current. It is the culprit that destroys a large number of electronic devices. The harm caused by lightning is pervasive. Especially harmful to computer network systems. According to the study, when the magnetic field strength Bm ≥ 0.07 × 10-4T, the unshielded computer will have a temporary failure or malfunction; when Bm ≥ 2.4 × 10-4T, the computer components will be permanently damaged. The transient electromagnetic field intensity around the lightning current often exceeds 2.4×10-4T.
Therefore, effectively preventing the harm caused by thunder and lightning to the weak current system equipment is an important prerequisite for ensuring the safe and stable operation of the weak current system equipment.
First, the electronic equipment anti-surge requirements (1) pressure requirements When the instantaneous voltage exceeds the insulation voltage of electronic equipment, its safety performance will be reduced or even destroyed. Therefore, the momentary overvoltage of the electronic device should be less than the insulation voltage, and the normal operating voltage should be less than the protection voltage.
(2) Over-current protection requirements The over-current capability of electronic devices is generally designed to be 1.5 to 2 times the rated current, and electronic components are selected as the standard. If the rated current is 0.22A, the maximum current capacity of the computer is about 0.45A. When the current is greater than this value, the electronic components selected by the electronic equipment will be burned out and cannot work normally, so it should be guaranteed to reach the electronic equipment. The instantaneous overcurrent is less than 1.5 to 2 times of its rated current.
(3) Requirements for dynamic response time During the design process of electronic devices, many protective devices have been used, such as fast fuses, varistors, air switches, and relay protection devices. Each type of protection device has a unique dynamic response. Time (such as the air switch, relay protection device dynamic response time is about 200ms or so), and each electronic device also has its protection response time, so the transient time flowing through the electronic device surge should be greater than the dynamic response of the electronic device Time, to prevent the protection device from responding to surges through the electronic device.
(4) Ground protection requirements When the electronic equipment is installed, it should be well grounded, otherwise the surge energy generated by lightning cannot be effectively discharged to the ground to destroy the device. The grounding line is inductively present at a momentary surge. The typical value is 1μH/m. The voltage drop on the ground line is U1=L×di/dt. For a 1.5m long ground wire L≈1.5μH, a few hundred ampere (500A) surge pulses generated by lightning at an instant (for example, 100μs), the di/dt=5×10A/s, at this time, the grounding wire Pressure drop U1=L×di/dt=1.5×10×5×10=7.5V, the device will withstand surge energy of 500A×7.5V=3750W. This energy will probably damage or destroy most electronic devices. .
Therefore, a reliable grounding protection for the electronic device can make the voltage reaching the electronic device housing smaller and play a role in security protection. However, it is not enough to provide ground protection only. Surge protection devices must also be installed. Since the invading surge energy will be discharged to the ground first through the electronic device, the inrush current flowing through the electronic device will remain basically unchanged, its energy may be great, and the electronic device may still be damaged; therefore, the grounding protection is Electronic devices can only be an auxiliary protection.
Second, system anti-surge measures
According to the IEC 61312 standard, weak current equipment should be provided with multiple lightning protection measures, which are generally three-level configurations. Since the lightning current is mainly composed of the first lightning current and the subsequent lightning current, the lightning overvoltage protection must also consider how to suppress (or divert) the first lightning current and the subsequent lightning current. While taking multi-level protection measures, energy coordination and decoupling measures between levels must also be considered. Lightning protection for weak systems can take two measures, namely external lightning protection and internal lightning protection. The external lightning protection can directly introduce most of the lightning current into the underground to leak; the internal lightning protection can block the lightning waves introduced along the power or signal lines. These two lines of defense, cooperating with each other and performing their duties, are indispensable.
1. External lightning protection and grounding External lightning protection mainly refers to the lightning protection of buildings. Generally, it is a direct lightning protection. It is the main component of lightning protection technology innovation. Its technical measures can be divided into lightning arresters (lightning rods, lightning conductors, and lightning rods) Mesh and other metal lightning receptor), deflectors, grounding bodies, and Faraday cages.
Grounding resistance should comply with relevant standards, generally 4Ω. There are special grounding requirements for certain equipment manufacturers. Separate the DC ground from the other 6 grounding types to avoid electromagnetic interference and zero ground potential rise. However, when lightning is discharged to the ground, the high voltage will counterattack the equipment through DC. Therefore, it is advisable to install a ground potential equalizer between the lightning protection zone and the DC ground in order to avoid counterattack.
2. Internal lightning protection The internal lightning protection system is mainly used to install overvoltage protection devices in buildings that are susceptible to overvoltage damage. When the device is subjected to overvoltage, the protection device can quickly act to discharge energy, thereby protecting the device from being damaged. damage. Internal lightning protection is divided into power lightning protection and signal lightning protection.
(1) Power supply lightning protection system The power supply lightning protection system is mainly to prevent the lightning wave from causing harm to the computer and related equipment through the power supply line. In order to avoid the residual voltage of the high voltage passing through the arrester after discharge to the ground, or to continue destroying the follow-up equipment after the lightning arrester is destroyed due to greater lightning current, and to prevent the secondary induction of the cable, the draft of the lightning protection project is in accordance with the draft The principle of graded protection and gradual discharge shall be adopted. One is to install a primary power surge arrester with a large discharge current at the total incoming line of the building power supply, and the second is to install a secondary or tertiary power surge arrester at the incoming line of important floors or important equipment power supplies. In order to ensure lightning strikes, the high voltage first passes through the primary power surge arrester, and then passes through the secondary power surge arrester. The distance between the primary power source lightning arrester and the secondary power source surge arrester is greater than 10 to 15 m. If the spacing between the two is not enough, a lightning protection box with a coil can be used, so that the secondary power source lightning protector can be prevented from being damaged by a lightning strike first.
(2) Signal lightning protection system Because lightning waves can induce high instantaneous impact energy on the line, network communication equipment is required to be able to withstand the transient impact of higher energy, and most devices are currently due to the high degree of integration of electronic components. To make the overvoltage and withstand current levels drop, the necessary lightning protection devices must be installed at the network communication interface to ensure the safe operation of the network communication system.
It is very important to protect the communication system against lightning strikes and select appropriate protection devices. It is necessary to fully consider the match between lightning protection products and communication systems. For information systems, it should be divided into coarse protection and fine protection. The coarse protection level is determined according to the level of the protected area, and the fine protection is determined based on the sensitivity of the electronic device.
3. Selection Principles of Surge Protection Devices (1) Selection of Maximum Discharge Current According to the geographical location of the building and the average annual thunderstorm day, the value of Ng (the average annual number of lightning strokes within an area of ​​1 km2) is calculated to determine the maximum discharge current of the lightning protection device. . Generally, 100kA or 65kA can be selected as the primary lightning protection of the system power supply; the secondary and tertiary lightning protection can be selected as 40kA, and the terminal selects the socket type arrester (such as the Einmax2 has the filtering function, eliminating 99% electromagnetic interference, radio frequency interference, and implementing the terminal The combination of lightning protection in the energy domain and lightning protection in the frequency domain.
(2) Selection of the maximum continuous pressure resistance We know that when a current of 1 mA is applied across the varistor, the measured voltage is the pressure sensitive voltage of the varistor, which is also the nominal conduction voltage of the lightning arrester. Actually, manufacturers or merchants announce the actual maximum continuous withstand voltage of surge arresters suitable for 220V or 380V power supply. This value is less than the varistor voltage of surge arresters. The design considers that the maximum continuous withstand voltage of the power surge arrester is a Critical value, exceeding this value, SPD action.
In order to ensure the reliability and continuity of power supply and to consider the actual situation in China, China's power system allows the grid to be single-phase earthed for 2 hours. If the ± 15% fluctuation of the grid is taken into account, it is possible that the single-phase continuous voltage of the grid can reach 437V if the power supply The maximum continuous withstand voltage of lightning protection products is only 385V. When the operating voltage on the power grid fluctuates in the above manner, the power surge protector will start frequently, thereby increasing the probability of accident and wasting electrical energy. Therefore, it is appropriate to choose a maximum continuous pressure of 440V, especially in rural areas.
(3) Selection of residual voltage At present, the residual voltage Ur of the lightning protection products sold in China is similar to the rated discharge current, and the difference is only 100 to 200 V. However, the line voltage drop after installation of the power surge arrester UL = L × di/dt is very large, so only consider the residual voltage Ur of the lightning arrester itself is not enough, but should consider the residual pressure U = Ur + UL of the entire system. For electronic devices such as computers, the insulation withstand voltage can be as high as 1800V or more. Through reasonable construction, it can meet equipment protection requirements. Figure 1 shows the conventional installation of a lightning arrester. For the equipment, it is not only Ur that is affected, but the system residual voltage U=Uab=Ur+U1+U2, where U1+U2=UL=L*di/dt.
Experiments have shown that when a 10mA (8/20μs) simulated lightning wave is passed through a conductor with a length of 1m and a cross-sectional area of ​​10 to 16mm, its inductance is equivalent to 1μH, and the voltage at both ends is about 1200V. Another test, the general into the indoor lightning current is about 3kA, when the induced lightning current is about 3kA, for the arrester on the market, the residual voltage Ur = 1100 ~ 1200V. Assume that there is an electronic device with an insulation voltage of 1800V. The length of the grounding wire at both ends of the arrester is L1=0.5m, and L2=1m, then U1≈0.5×1200×3/10=180V, U2≈1×1200× 3/10=360V, voltage drop between lines UL=U1+U2=540V, then the residual voltage at both ends of the device is U=Uab=540+1200=1740V<1800V (after the surge discharges through the surge arrester, the current flowing through the device is very high. Small, so the surge energy reaching the electronic device is small, less than its damage power, the device will operate safely and reliably). It can be seen from U=UL+Ur that it is necessary to reduce the line-to-line voltage drop UL (minimize the length of the ground wire or select a wire with a larger cross-section during installation). Therefore, you should not only consider the residual voltage of the arrester when selecting the arrester, but you should also consider the impact of the system residual pressure generated during installation on the electronic equipment.
(4) Selection of leakage current Under the 75% of the nominal conduction voltage, the measured current flowing through the lightning arrester is called the leakage current I0 of the lightning protection device. According to national standards, this parameter should be less than 20μA. The greater the leakage current I0, the more the power surge protector will accumulate more energy and the possibility of the power surge protector heating up, and the leakage current increases with the temperature of the varistor, so this The varistor is in a vicious cycle, which also indicates that the greater the rate of change (increasing rate) of the leakage current with time, the faster the power surge arrester will accumulate energy, thereby deteriorating the performance of the power SPD. .
Under normal circumstances, the explosion of the power surge arrester (self-explosion) occurs. In addition to the structural design flaw of the power surge protector, it is mainly due to the improper selection of varistor varistor voltage and leakage current, thus making the power surge arrester Frequent start-ups and excessive leakage currents cause damage.
(5) Selection of alarm modes There are three types of alarm modes that can be provided at present. One is remote alarm and telemetering alarm, which is suitable for unattended work situations; the other is visual alarm, and the alarm function is achieved through mechanical design. This type of alarm method should be used to check or periodically check the facilities after a thunderstorm. It is applicable to all occasions and is the most used alarm mode. There are also acousto-optic alarms. An alarm module needs to be added to this alarm mode. Many current experts recommend it. Use with caution, because when the lightning strikes, the electronic components in the sound and light alarm module may be damaged first and lose their acousto-optic alarm function. In this case, the lightning protection product is also just damaged, and people rely on sound and light alarms. It is noticed that when the second lightning strikes, the thunder and lightning will take advantage and destroy the subsequent protected equipment. Lightning protection products are safety protection products, and their structures should be as simple as possible. Therefore, visual alarms are recommended.
(6)Structural design The structural design of the power surge protector is very important. If the varistor is sealed by the resin, the heat dissipation effect will be poor and the varistor will be in a vicious cycle due to heat generation. The overall performance of the lightning device declines. At present, there are two types of lightning protection products: integrated modular design and plug-in modular design.
In the plug-in type structure, when the plug is inserted, it is inevitable that the discharge interference occurs due to the existence of the gap, especially in a place where the air humidity is relatively large, this phenomenon will be more serious, and the performance of the lightning arrester will be degraded. The monolithic modular design does not have any gaps, and at the same time, hot (charged) replacement is also possible due to the rail-mounted installation. Therefore, it is more appropriate to choose an integrated modular power supply lightning protection product.
Third, a specific embodiment of the lightning protection device lightning <br> <br> weak objects are: network equipment ancillary equipment, communications equipment, automation equipment, and various types of power supplies associated with each other transmitted signal. Figure 2 shows the lightning voltage overvoltage protection configuration diagram for our office computer room. S1 is DEHNventil280/4 type 1 and 2 composite lightning arrester, S2 is DEHNguardT385 type 2 power surge protector, and S3 is DEHNrail48FML type 3 power protection. Thunder (DC 48V), A is UGK/N (2.5G) antenna feeder lightning protection device, B is YG20-A audio isolation transformer, C is ZH type neutral transformer, and D is coaxial lightning arrester. Before laying lightning protection equipment, the relevant lightning protection points should be tested and inspected. The main ones are:
(1) Whether the lightning protection belt is continuous and reliable, and whether it is connected with the lead wire uniformly and reliably.
(2) Confirm whether it is necessary to lay lightning protection nets and install lightning rods.
(3) Check whether the grounding resistance meets the relevant standards.
The ground of the equipment room shall be grounded using the same ground floor of the same building as the lightning protection ground, working communication ground, static electricity ground, shielding ground, insulation ground, and safety protection ground. The copper bars with a cross-sectional area of ​​30×3 mm shall be uniformly distributed around the wall in the computer room. In the ring network, the grounded copper ring ring type grid is about 300-350 mm high from the ground and insulated from the wall. The grounding of all indoor equipments is connected to the ring busbar by a single point earthing method, and the ring busbar is commonly grounded to the bottom layer. Body, using 90mm multiple strands of insulated copper core wire connected through the grounding flat steel laid in the pipeline well of the building, as the grounding wire of the ring busbar; an AC distribution box is specially provided in the machine room. The power supply for the distribution box uses three-phase, four-wire power supply to strictly implement the grounding of equipment in the equipment room. The power supply and signal lightning arrester are used. The lightning arrester grounding is combined with a lightning arrester. S2 is a DEHNguard T385 power supply secondary lightning arrester. S3 is DEHNrail48FML type 3 power surge protector (DC 48V), A is UGK/N (2.5G) antenna feeder lightning protector, B is YG20-A audio isolation transformer, C is ZH type neutralization transformer, D Coaxial arrester.
Before laying lightning protection equipment, the relevant lightning protection points should be tested and inspected. The main ones are:
Whether the lightning protection zone is continuous and reliable, whether it is connected with the lead-off line evenly and reliably, whether it is necessary to lay the lightning protection network, install the lightning rod, and check whether the grounding resistance meets the relevant standards.
The ground of the equipment room shall be grounded using the same ground floor of the same building as the lightning protection ground, working communication ground, static electricity ground, shielding ground, insulation ground, and safety protection ground. The copper bars with a cross-sectional area of ​​30×3 mm shall be uniformly distributed around the wall in the computer room. In the ring network, the grounded copper ring ring type grid is about 300-350 mm high from the ground and insulated from the wall. The grounding of all indoor equipments is connected to the ring busbar by a single point earthing method, and the ring busbar is commonly grounded to the bottom layer. The body uses a 90mm multi-strand insulated copper core wire to connect the grounded flat steel that has been laid in the building's pipe well as the ground wire of the ring busbar. There is a special AC distribution box in the machine room. The distribution box is powered by a three-phase, four-wire system. It strictly implements the grounding of the equipment in the equipment room. The power supply and the signal lightning arrester are grounded. The lightning arrester is grounded in a single point, and the outer jacket of the microwave antenna is multi-pointed. For reliable grounding. After more than one year of actual operation tests, the protected equipment has not suffered any lightning damage.
Fourth, the conclusion
With the large-scale use of communication equipment, network equipment, and computer application systems, the damage caused by lightning and instantaneous overvoltages has become more and more serious. The previous protection system can no longer meet the security requirements of communications, networks, and computers. It should be changed from simple one-dimensional protection (lightning lead lightning-into-ground-passive protection) to three-dimensional protection (active and passive protection), including: direct lightning strike, inductive lightning wave immersion, lightning electromagnetic induction, ground potential protection Counterattacks and the effects of instantaneous over-voltage operation, etc. as a comprehensive consideration of the system. Modern lightning protection technology emphasizes all-round protection, comprehensive management, and layer defense. It uses a combination of shunting (discharge), voltage equalization (equal potential), shielding, earthing and protection (clamping) and other technologies to form a complete protection. system.
With the continuous development of modern electronic technology, the use and networking of a large number of sophisticated electronic devices have enabled equipment installed in weak current systems to suffer from poor power quality (such as power harmonics amplification, switching electromagnetic pulses), direct lightning strikes, and induced lightning. Industrial operation instantaneous overvoltage, zero potential drift and other surge and overvoltage invasion, often by various overvoltage, overcurrent hazards. Since some electronic devices operate at voltages of only a few volts, the current carrying information is also very small and extremely sensitive to external disturbances. The lightning voltage can be as high as several million V and the instantaneous current can be as high as several 100,000 A. Therefore, it is extremely significant. Destructive.
Lightning rods can prevent direct lightning strikes, but they cannot prevent induced lightning overvoltages, overvoltages, zero-voltage floating overvoltages, and strong overvoltages generated by these overvoltages when they discharge current. It is the culprit that destroys a large number of electronic devices. The harm caused by lightning is pervasive. Especially harmful to computer network systems. According to the study, when the magnetic field strength Bm ≥ 0.07 × 10-4T, the unshielded computer will have a temporary failure or malfunction; when Bm ≥ 2.4 × 10-4T, the computer components will be permanently damaged. The transient electromagnetic field intensity around the lightning current often exceeds 2.4×10-4T.
Therefore, effectively preventing the harm caused by thunder and lightning to the weak current system equipment is an important prerequisite for ensuring the safe and stable operation of the weak current system equipment.
First, the electronic equipment anti-surge requirements (1) pressure requirements When the instantaneous voltage exceeds the insulation voltage of electronic equipment, its safety performance will be reduced or even destroyed. Therefore, the momentary overvoltage of the electronic device should be less than the insulation voltage, and the normal operating voltage should be less than the protection voltage.
(2) Over-current protection requirements The over-current capability of electronic devices is generally designed to be 1.5 to 2 times the rated current, and electronic components are selected as the standard. If the rated current is 0.22A, the maximum current capacity of the computer is about 0.45A. When the current is greater than this value, the electronic components selected by the electronic equipment will be burned out and cannot work normally, so it should be guaranteed to reach the electronic equipment. The instantaneous overcurrent is less than 1.5 to 2 times of its rated current.
(3) Requirements for dynamic response time During the design process of electronic devices, many protective devices have been used, such as fast fuses, varistors, air switches, and relay protection devices. Each type of protection device has a unique dynamic response. Time (such as the air switch, relay protection device dynamic response time is about 200ms or so), and each electronic device also has its protection response time, so the transient time flowing through the electronic device surge should be greater than the dynamic response of the electronic device Time, to prevent the protection device from responding to surges through the electronic device.
(4) Ground protection requirements When the electronic equipment is installed, it should be well grounded, otherwise the surge energy generated by lightning cannot be effectively discharged to the ground to destroy the device. The grounding line is inductively present at a momentary surge. The typical value is 1μH/m. The voltage drop on the ground line is U1=L×di/dt. For a 1.5m long ground wire L≈1.5μH, a few hundred ampere (500A) surge pulses generated by lightning at an instant (for example, 100μs), the di/dt=5×10A/s, at this time, the grounding wire Pressure drop U1=L×di/dt=1.5×10×5×10=7.5V, the device will withstand surge energy of 500A×7.5V=3750W. This energy will probably damage or destroy most electronic devices. .
Therefore, a reliable grounding protection for the electronic device can make the voltage reaching the electronic device housing smaller and play a role in security protection. However, it is not enough to provide ground protection only. Surge protection devices must also be installed. Since the invading surge energy will be discharged to the ground first through the electronic device, the inrush current flowing through the electronic device will remain basically unchanged, its energy may be great, and the electronic device may still be damaged; therefore, the grounding protection is Electronic devices can only be an auxiliary protection.
Second, system anti-surge measures
According to the IEC 61312 standard, weak current equipment should be provided with multiple lightning protection measures, which are generally three-level configurations. Since the lightning current is mainly composed of the first lightning current and the subsequent lightning current, the lightning overvoltage protection must also consider how to suppress (or divert) the first lightning current and the subsequent lightning current. While taking multi-level protection measures, energy coordination and decoupling measures between levels must also be considered. Lightning protection for weak systems can take two measures, namely external lightning protection and internal lightning protection. The external lightning protection can directly introduce most of the lightning current into the underground to leak; the internal lightning protection can block the lightning waves introduced along the power or signal lines. These two lines of defense, cooperating with each other and performing their duties, are indispensable.
1. External lightning protection and grounding External lightning protection mainly refers to the lightning protection of buildings. Generally, it is a direct lightning protection. It is the main component of lightning protection technology innovation. Its technical measures can be divided into lightning arresters (lightning rods, lightning conductors, and lightning rods) Mesh and other metal lightning receptor), deflectors, grounding bodies, and Faraday cages.
Grounding resistance should comply with relevant standards, generally 4Ω. There are special grounding requirements for certain equipment manufacturers. Separate the DC ground from the other 6 grounding types to avoid electromagnetic interference and zero ground potential rise. However, when lightning is discharged to the ground, the high voltage will counterattack the equipment through DC. Therefore, it is advisable to install a ground potential equalizer between the lightning protection zone and the DC ground in order to avoid counterattack.
2. Internal lightning protection The internal lightning protection system is mainly used to install overvoltage protection devices in buildings that are susceptible to overvoltage damage. When the device is subjected to overvoltage, the protection device can quickly act to discharge energy, thereby protecting the device from being damaged. damage. Internal lightning protection is divided into power lightning protection and signal lightning protection.
(1) Power supply lightning protection system The power supply lightning protection system is mainly to prevent the lightning wave from causing harm to the computer and related equipment through the power supply line. In order to avoid the residual voltage of the high voltage passing through the arrester after discharge to the ground, or to continue destroying the follow-up equipment after the lightning arrester is destroyed due to greater lightning current, and to prevent the secondary induction of the cable, the draft of the lightning protection project is in accordance with the draft The principle of graded protection and gradual discharge shall be adopted. One is to install a primary power surge arrester with a large discharge current at the total incoming line of the building power supply, and the second is to install a secondary or tertiary power surge arrester at the incoming line of important floors or important equipment power supplies. In order to ensure lightning strikes, the high voltage first passes through the primary power surge arrester, and then passes through the secondary power surge arrester. The distance between the primary power source lightning arrester and the secondary power source surge arrester is greater than 10 to 15 m. If the spacing between the two is not enough, a lightning protection box with a coil can be used, so that the secondary power source lightning protector can be prevented from being damaged by a lightning strike first.
(2) Signal lightning protection system Because lightning waves can induce high instantaneous impact energy on the line, network communication equipment is required to be able to withstand the transient impact of higher energy, and most devices are currently due to the high degree of integration of electronic components. To make the overvoltage and withstand current levels drop, the necessary lightning protection devices must be installed at the network communication interface to ensure the safe operation of the network communication system.
It is very important to protect the communication system against lightning strikes and select appropriate protection devices. It is necessary to fully consider the match between lightning protection products and communication systems. For information systems, it should be divided into coarse protection and fine protection. The coarse protection level is determined according to the level of the protected area, and the fine protection is determined based on the sensitivity of the electronic device.
3. Selection Principles of Surge Protection Devices (1) Selection of Maximum Discharge Current According to the geographical location of the building and the average annual thunderstorm day, the value of Ng (the average annual number of lightning strokes within an area of ​​1 km2) is calculated to determine the maximum discharge current of the lightning protection device. . Generally, 100kA or 65kA can be selected as the primary lightning protection of the system power supply; the secondary and tertiary lightning protection can be selected as 40kA, and the terminal selects the socket type arrester (such as the Einmax2 has the filtering function, eliminating 99% electromagnetic interference, radio frequency interference, and implementing the terminal The combination of lightning protection in the energy domain and lightning protection in the frequency domain.
(2) Selection of the maximum continuous pressure resistance We know that when a current of 1 mA is applied across the varistor, the measured voltage is the pressure sensitive voltage of the varistor, which is also the nominal conduction voltage of the lightning arrester. Actually, manufacturers or merchants announce the actual maximum continuous withstand voltage of surge arresters suitable for 220V or 380V power supply. This value is less than the varistor voltage of surge arresters. The design considers that the maximum continuous withstand voltage of the power surge arrester is a Critical value, exceeding this value, SPD action.
In order to ensure the reliability and continuity of power supply and to consider the actual situation in China, China's power system allows the grid to be single-phase earthed for 2 hours. If the ± 15% fluctuation of the grid is taken into account, it is possible that the single-phase continuous voltage of the grid can reach 437V if the power supply The maximum continuous withstand voltage of lightning protection products is only 385V. When the operating voltage on the power grid fluctuates in the above manner, the power surge protector will start frequently, thereby increasing the probability of accident and wasting electrical energy. Therefore, it is appropriate to choose a maximum continuous pressure of 440V, especially in rural areas.
(3) Selection of residual voltage At present, the residual voltage Ur of the lightning protection products sold in China is similar to the rated discharge current, and the difference is only 100 to 200 V. However, the line voltage drop after installation of the power surge arrester UL = L × di/dt is very large, so only consider the residual voltage Ur of the lightning arrester itself is not enough, but should consider the residual pressure U = Ur + UL of the entire system. For electronic devices such as computers, the insulation withstand voltage can be as high as 1800V or more. Through reasonable construction, it can meet equipment protection requirements. Figure 1 shows the conventional installation of a lightning arrester. For the equipment, it is not only Ur that is affected, but the system residual voltage U=Uab=Ur+U1+U2, where U1+U2=UL=L*di/dt.
Experiments have shown that when a 10mA (8/20μs) simulated lightning wave is passed through a conductor with a length of 1m and a cross-sectional area of ​​10 to 16mm, its inductance is equivalent to 1μH, and the voltage at both ends is about 1200V. Another test, the general into the indoor lightning current is about 3kA, when the induced lightning current is about 3kA, for the arrester on the market, the residual voltage Ur = 1100 ~ 1200V. Assume that there is an electronic device with an insulation voltage of 1800V. The length of the grounding wire at both ends of the arrester is L1=0.5m, and L2=1m, then U1≈0.5×1200×3/10=180V, U2≈1×1200× 3/10=360V, voltage drop between lines UL=U1+U2=540V, then the residual voltage at both ends of the device is U=Uab=540+1200=1740V<1800V (after the surge discharges through the surge arrester, the current flowing through the device is very high. Small, so the surge energy reaching the electronic device is small, less than its damage power, the device will operate safely and reliably). It can be seen from U=UL+Ur that it is necessary to reduce the line-to-line voltage drop UL (minimize the length of the ground wire or select a wire with a larger cross-section during installation). Therefore, you should not only consider the residual voltage of the arrester when selecting the arrester, but you should also consider the impact of the system residual pressure generated during installation on the electronic equipment.
(4) Selection of leakage current Under the 75% of the nominal conduction voltage, the measured current flowing through the lightning arrester is called the leakage current I0 of the lightning protection device. According to national standards, this parameter should be less than 20μA. The greater the leakage current I0, the more the power surge protector will accumulate more energy and the possibility of the power surge protector heating up, and the leakage current increases with the temperature of the varistor, so this The varistor is in a vicious cycle, which also indicates that the greater the rate of change (increasing rate) of the leakage current with time, the faster the power surge arrester will accumulate energy, thereby deteriorating the performance of the power SPD. .
Under normal circumstances, the explosion of the power surge arrester (self-explosion) occurs. In addition to the structural design flaw of the power surge protector, it is mainly due to the improper selection of varistor varistor voltage and leakage current, thus making the power surge arrester Frequent start-ups and excessive leakage currents cause damage.
(5) Selection of alarm modes There are three types of alarm modes that can be provided at present. One is remote alarm and telemetering alarm, which is suitable for unattended work situations; the other is visual alarm, and the alarm function is achieved through mechanical design. This type of alarm method should be used to check or periodically check the facilities after a thunderstorm. It is applicable to all occasions and is the most used alarm mode. There are also acousto-optic alarms. An alarm module needs to be added to this alarm mode. Many current experts recommend it. Use with caution, because when the lightning strikes, the electronic components in the sound and light alarm module may be damaged first and lose their acousto-optic alarm function. In this case, the lightning protection product is also just damaged, and people rely on sound and light alarms. It is noticed that when the second lightning strikes, the thunder and lightning will take advantage and destroy the subsequent protected equipment. Lightning protection products are safety protection products, and their structures should be as simple as possible. Therefore, visual alarms are recommended.
(6)Structural design The structural design of the power surge protector is very important. If the varistor is sealed by the resin, the heat dissipation effect will be poor and the varistor will be in a vicious cycle due to heat generation. The overall performance of the lightning device declines. At present, there are two types of lightning protection products: integrated modular design and plug-in modular design.
In the plug-in type structure, when the plug is inserted, it is inevitable that the discharge interference occurs due to the existence of the gap, especially in a place where the air humidity is relatively large, this phenomenon will be more serious, and the performance of the lightning arrester will be degraded. The monolithic modular design does not have any gaps, and at the same time, hot (charged) replacement is also possible due to the rail-mounted installation. Therefore, it is more appropriate to choose an integrated modular power supply lightning protection product.
Third, a specific embodiment of the lightning protection device lightning <br> <br> weak objects are: network equipment ancillary equipment, communications equipment, automation equipment, and various types of power supplies associated with each other transmitted signal. Figure 2 shows the lightning voltage overvoltage protection configuration diagram for our office computer room. S1 is DEHNventil280/4 type 1 and 2 composite lightning arrester, S2 is DEHNguardT385 type 2 power surge protector, and S3 is DEHNrail48FML type 3 power protection. Thunder (DC 48V), A is UGK/N (2.5G) antenna feeder lightning protection device, B is YG20-A audio isolation transformer, C is ZH type neutral transformer, and D is coaxial lightning arrester. Before laying lightning protection equipment, the relevant lightning protection points should be tested and inspected. The main ones are:
(1) Whether the lightning protection belt is continuous and reliable, and whether it is connected with the lead wire uniformly and reliably.
(2) Confirm whether it is necessary to lay lightning protection nets and install lightning rods.
(3) Check whether the grounding resistance meets the relevant standards.
The ground of the equipment room shall be grounded using the same ground floor of the same building as the lightning protection ground, working communication ground, static electricity ground, shielding ground, insulation ground, and safety protection ground. The copper bars with a cross-sectional area of ​​30×3 mm shall be uniformly distributed around the wall in the computer room. In the ring network, the grounded copper ring ring type grid is about 300-350 mm high from the ground and insulated from the wall. The grounding of all indoor equipments is connected to the ring busbar by a single point earthing method, and the ring busbar is commonly grounded to the bottom layer. Body, using 90mm multiple strands of insulated copper core wire connected through the grounding flat steel laid in the pipeline well of the building, as the grounding wire of the ring busbar; an AC distribution box is specially provided in the machine room. The power supply for the distribution box uses three-phase, four-wire power supply to strictly implement the grounding of equipment in the equipment room. The power supply and signal lightning arrester are used. The lightning arrester grounding is combined with a lightning arrester. S2 is a DEHNguard T385 power supply secondary lightning arrester. S3 is DEHNrail48FML type 3 power surge protector (DC 48V), A is UGK/N (2.5G) antenna feeder lightning protector, B is YG20-A audio isolation transformer, C is ZH type neutralization transformer, D Coaxial arrester.
Before laying lightning protection equipment, the relevant lightning protection points should be tested and inspected. The main ones are:
Whether the lightning protection zone is continuous and reliable, whether it is connected with the lead-off line evenly and reliably, whether it is necessary to lay the lightning protection network, install the lightning rod, and check whether the grounding resistance meets the relevant standards.
The ground of the equipment room shall be grounded using the same ground floor of the same building as the lightning protection ground, working communication ground, static electricity ground, shielding ground, insulation ground, and safety protection ground. The copper bars with a cross-sectional area of ​​30×3 mm shall be uniformly distributed around the wall in the computer room. In the ring network, the grounded copper ring ring type grid is about 300-350 mm high from the ground and insulated from the wall. The grounding of all indoor equipments is connected to the ring busbar by a single point earthing method, and the ring busbar is commonly grounded to the bottom layer. The body uses a 90mm multi-strand insulated copper core wire to connect the grounded flat steel that has been laid in the building's pipe well as the ground wire of the ring busbar. There is a special AC distribution box in the machine room. The distribution box is powered by a three-phase, four-wire system. It strictly implements the grounding of the equipment in the equipment room. The power supply and the signal lightning arrester are grounded. The lightning arrester is grounded in a single point, and the outer jacket of the microwave antenna is multi-pointed. For reliable grounding. After more than one year of actual operation tests, the protected equipment has not suffered any lightning damage.
Fourth, the conclusion
With the large-scale use of communication equipment, network equipment, and computer application systems, the damage caused by lightning and instantaneous overvoltages has become more and more serious. The previous protection system can no longer meet the security requirements of communications, networks, and computers. It should be changed from simple one-dimensional protection (lightning lead lightning-into-ground-passive protection) to three-dimensional protection (active and passive protection), including: direct lightning strike, inductive lightning wave immersion, lightning electromagnetic induction, ground potential protection Counterattacks and the effects of instantaneous over-voltage operation, etc. as a comprehensive consideration of the system. Modern lightning protection technology emphasizes all-round protection, comprehensive management, and layer defense. It uses a combination of shunting (discharge), voltage equalization (equal potential), shielding, earthing and protection (clamping) and other technologies to form a complete protection. system.
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