First, the basic principles of lightning protection
The damage caused by thunder and other strong interference to the communication system and the resulting damage are serious and lightning protection will become necessary. Lightning consists of high-energy low-frequency components and highly permeable high-frequency components. There are two main forms, one is to directly conduct lightning damage equipment through metal pipelines or ground wires; the other is lightning lightning electromagnetic pulses of lightning passages and discharge passages to induce metal pipelines or ground wires to generate waves in various coupling manners. Damage to equipment. Most of the damage caused by this induction. For electronic information equipment, the hazard mainly comes from the lightning coupled electromagnetic pulse electromagnetic energy, through the following three channels generated by the transient surge. Metal pipelines, such as water pipes, power lines, antenna feeders, signal lines, aviation obstruction lamp leads and other surges; ground channel, ground power counterattack; space channel, electromagnetic group radiation energy.
Among them, the surge of the metal pipeline and the ground potential counterattack of the ground channel are the main causes of the damage of the electronic information system. Its most visible form of damage is damage caused on the power line. Therefore, it must be used as the focus of anti-expansion. As thunder and lightning penetrate the electronic information system, lightning protection will be a systematic project. The centerpiece of lightning protection is divergence and balance.
1. Releasing is the discharge of lightning and lightning electromagnetic pulse energy through the earth, and should comply with the principle of hierarchy, that is, as much as possible, as far as possible to excess energy into the communications system before the release into the ground; hierarchy is Lightning energy is weakened according to the level of lightning protection established. The lightning protection zone is also called electromagnetic compatibility zone. It divides the environment into several areas according to the intensity of the feeling of lightning, lightning and electromagnetic pulses according to people, objects, and information systems. In the LPZOA area, all objects in the area may be directly struck by lightning. Therefore, all the special objects may lead away all the lightning currents, and the electromagnetic field in the area does not decay. In the LPZOB area, objects in the area could not be directly struck by lightning, but the electromagnetic field in the area did not decay. In LPZ1 area, objects in this area cannot be directly struck by lightning. The current flowing to each conductor is further reduced than in LPZOB area. The electromagnetic field attenuation and effect depend on the overall shielding measures. Subsequent lightning protection zones (LPZ2 zone, etc.), if it is necessary to further reduce the guided current and electromagnetic field, shall be introduced into the subsequent lightning protection zone. The requirements for the protection zone shall be selected according to the environmental zone required by the system to be protected and the requirements of the lightning protection zone shall be continued. condition. The higher the protection zone number, the lower the expected interference energy and interference voltage. In modern lightning protection technology, the installation of lightning protection zones is of great significance. It can guide us in the implementation of technical measures such as shielding, grounding, and other power connections.
2. Equilibrium is to keep the potential difference of each part of the system not enough to cause damage, that is, the potential of all the metal conductors in the system environment and the system itself remain substantially equal in the transient phenomenon, which is essentially based on equal voltage equipotential bonding. A potential compensation system consists of a reliable grounding system, metal wires for equipotential bonding, and equipotential bonding (lightning protection). This potential compensation system can be quickly protected in the presence of transients for a very short time. An equipotential is established between all conductive components in the area where the system is located. These conductive components also include active wires. Through this complete potential compensation system, an equipotential region can be formed in a very short time. This region may have a potential difference of tens of kilovolts with respect to the distance. It is important that there is no significant potential difference between all conductive parts within the area where the system to be protected is located.
3. The lightning protection system consists of three parts. Each part has its own important role and no alternative exists. The external protection consists of a lightning arrester, a down conductor, and a grounding body. It can direct most of the lightning energy into the underground discharge. Transition protection, consisting of reasonable shielding, grounding, and routing, can reduce or block the induction introduced through each intrusion channel. Internal protection consists of voltage equalization and overvoltage protection to balance the system potential and limit the overvoltage amplitude.
Second, the role of lightning protection devices and technical parameters
Lightning arresters are also called equipotential connectors, overvoltage protectors, surge suppressors, surge absorbers, and lightning protection devices. Lightning protection devices used for power line protection are called power surge protectors. In view of the current characteristics of lightning damage, lightning protection is especially in the lightning protection and rectification. The protection scheme based on lightning protection devices is the simplest and most economical lightning protection solution. The main function of the lightning protector is to maintain the same or limit the potential at both ends of the transient phenomenon and transfer the excess energy on the active conductor.
Into the underground discharge is an important part of achieving equal voltage equipotential bonding. Some of the main technical parameters of the lightning arrester: rated operating voltage, rated operating current, special batch and parallel power supply lightning protection device. Flow capacity, the ability of the lightning arrester to transfer lightning currents, in thousands of units, is related to the wave opening type. Lightning arresters can be divided into lightning protection devices that can prevent direct lightning strikes and lightning protection devices that can prevent inductive lightning. Lightning arresters that can prevent direct lightning strikes are generally used for line protection that may be hit by direct lightning strikes, such as the protection of LPZ area and LPZ1 area. Use a 10/35μs current waveform to test and indicate its current capability. Anti-lightning surge arresters are generally used for line protection that cannot be hit by direct lightning strikes, such as the protection of the LPZOB zone at the junction with the LPX1 zone and the LPZ1 zone. The 8/20μs current waveform is used to test and represent the response time of the current capacity. The time required for the lightning arrester to control the transient phenomenon is related to the nature of the waveform. Residual pressure, surge arrester's ability to limit voltage transients, and lightning current amplitude and waveform properties.
Third, the selection of lightning protection device
For lightning protection based protection, to achieve the desired effect, attention should be paid to “arranging suitable lightning protection devices in a suitable placeâ€. The selection of lightning protection devices is very important.
1. The distribution of lightning currents between the various facilities entering the building is as follows: approximately 50 lightning currents are discharged into the ground via external lightning protection devices, and another 50 lightning currents are distributed within the metal material of the entire system. . This evaluation model is used to estimate the current capacity of the SPDs and the metal wire specifications for equipotential bonding at the junction of the LPAOA, LPZOB, and LPZ1 regions. The lightning current at this point is a 10/35μs current waveform. In the case of lightning current distribution among various metal substances: The lightning current amplitude of each part depends on the impedance and inductive reactance of each distribution channel. The distribution channel refers to metal materials that may be allocated to lightning currents, such as power lines, signal lines, Metal pipes, such as water pipes and metal structures, and other groundings are generally estimated only by their respective grounding resistance values. In the case of uncertainty, it can be assumed that the connections are equal in resistance, that is, the metal circuits distribute the current evenly.
2. When a power line is introduced overhead and the power line may be hit by a direct lightning strike, the lightning current entering the protected area within the building depends on the impedance and inductive reactance of the external route, lightning arrester discharge branch, and user side line. If the impedance at both ends is the same, the power line is allocated to half of the direct lightning current. In this case, a lightning arrester with a lightning protection function must be used.
3. The follow-up assessment model is used to assess the distribution of lightning currents at the junction of the LPZ1 zone after the protection zone. Since the insulation impedance at the user side is much larger than the impedance of the discharge branch of the lightning arrester and the external lead line, the lightning current entering the subsequent lightning protection zone will be reduced, and no special estimation is needed in numerical value. It is generally required that the lightning protection device used in the subsequent lightning protection zone has a current flow capacity of 20 kA (8/20 μs) or less, and no lightning protection device with a large flow capacity is required. The selection of surge arresters in the subsequent lightning protection zone should consider the energy distribution and voltage coordination between each level. When many factors are difficult to determine, it is a good choice to use series-parallel lightning protection devices. The serial-parallel type is a concept proposed based on the characteristics of many applications in modern lightning protection and the protection scope level distinction (as opposed to the traditional parallel-type lightning arrester). Its essence is the effective combination of multi-level discharger and filter technology with energy matching and voltage distribution. Serial-type lightning protection has the following characteristics: Wide range of applications. Not only can it be applied routinely, but it is also suitable for sites that are difficult to distinguish in protected areas. Inducing the partial pressure and delay of the decoupling device under transient overvoltage to help achieve energy coordination. Reduce the rate of rise of transient interference to achieve low residual voltage and long life and extremely fast response time.4. The selection of other parameters of the lightning protection device depends on the level of the lightning protection zone where each protection object is located. The operating voltage of the lightning protection device is based on the rated voltage of all the components installed in the pilot circuit. Serial and lightning protectors also need to pay attention to their rated current.
5. Other factors that affect the distribution of the lightning current of the electronic wire: The reduced grounding resistance of the transformer will increase the distribution current in the electronic wire. The increase in the length of the power supply cable will reduce the distribution current in the power line and allow a balanced current distribution in several wires. Short cable lengths and too low neutral impedance will cause current imbalance, causing differential mode interference. Multiple users connected to the power supply cable will reduce the effective impedance, resulting in an increase in the distribution current. In the connected state of power supply, the temporary flow of lightning mainly flows into the power line, which is why most of the lightning damage occurs at the power line.
Fourth, the installation of lightning protection device
1. The power line should implement multi-level protection. The multi-level protection is based on each lightning protection zone as a level, reducing the energy of the lightning level step by step (energy distribution), so that the voltages at each level can cooperate with each other, and eventually limit the overvoltage value to the device. Insulation strength (voltage fit).
In the following cases, multi-level protection becomes necessary: ​​one grade of lightning protector fails or one of the lightning protector fails. The residual voltage of the lightning arrester does not match the insulation strength of the device, and the length of the cable in the building is long.
2. The cable protection in almost all cases should be divided into at least two levels. The lightning protection device at the same level may also include multiple levels of protection (such as series and lightning arresters). In order to achieve effective protection, lightning protection devices can be installed at the interfaces of lightning protection zones. The lightning protection devices can be used for a single electronic device or a space equipped with multiple electronic devices. All of the protection devices are usually shielded from space. The conductors of the minefield are connected with lightning arresters at the same time through the lightning protection zone interface. In addition, the protection scope of the SPD is limited. Generally, the protection effect will be degraded when the distance between the SPD and the equipment circuit exceeds 10m. This is because of the oscillation caused by reflection on the cable between the SPD and the device to be protected. Voltage, its magnitude is proportional to line length and load impedance.
3. In multi-level protection using a power kidder, if you do not pay attention to energy distribution, you may introduce more lightning energy into the protected area. This requires that the surge protector should be selected according to the aforementioned evaluation mode. Generally, the lightning arrester has the characteristics of higher lightning current and higher residual voltage. After the energy distribution, the lightning current flowing through the unshielded lightning arrester is extremely small, which is favorable for voltage limitation. Note that it is dangerous to select only low-response voltage surge arresters for final protection without regard to voltage matching.
The key to achieving the energy distribution and voltage matching lies in the use of the intrinsic inductance of the cable between the two levels of surge arresters. The intrinsic inductance of the cable has a certain effect on the impediment of the buried current and the partial pressure, so that the lightning current is more distributed to the pre-stage discharge. It is generally required that the length of the cable between the two lightning arresters is about 15 meters, which is suitable for the situation where the protection ground wire and other cables are laid close to each other or in the same cable. The length of the branch line on the cable affects the required cable length. When the protection cable has a certain distance (>1m) from the cable to be protected, the cable length must be greater than 5m. In some cases where it is not suitable to use the cable itself as a decoupling measure, such as the two-level lightning protection zone near the interface or the cable length is short, a special decoupling device can be used. There is no distance requirement at this time.
4. Decoupling devices are important measures for energy distribution and voltage matching. The following materials can be used as decoupling devices: cables, inductors, and resistors.
Serial-type power supply lightning protector is a combination of lightning protection device that considers energy distribution and voltage matching and uses a filter as a decoupling device, and is suitable for applications in various occasions.
5. In some extreme cases, installing lightning arrestors will increase the possibility of damage to the equipment and must be eliminated; such situations occur. The lightning protector protects several lines. The lightning arrester on one of the lines fails or responds too slowly. This may cause common mode interference to be converted into differential mode interference and damage the device. This requires the implementation of multiple levels of protection and attention to the maintenance of lightning protection devices. Irrespective of the lightning protection zone, energy coordination and voltage distribution, the lightning protection device is installed casually. For example, only a lightning protection device is installed at the front of the device. Because there is no protection at the front stage, a strong lightning current will be attracted to the front of the device. The residual voltage of the surge protector exceeds the dielectric strength of the device. This requires that lightning protection devices must be installed in accordance with the principle of hierarchy.
6. In other cases, the wrong installation will not allow the device to be effectively protected. When the surge protector's connection line and lightning arrester work too long, the voltage caused by the inductive reactance on the connection line will be extremely high, and the dangerous voltage will still be added to the device. This problem is even more serious in the application of the last stage surge arrester. obvious. The solution to this problem is to use a short connection line, but also to use two or more separate connection lines to share the magnetic field strength, reduce the pressure drop, single-line thickening the connection line is no effect. If necessary, the length of the connecting wire can be reduced by changing the wiring of the protected wire so as to be close to the equipotential connection row (grounding point).
Lightning protector output line and input line, grounding line close, side by side laying. This situation has a serious impact on serial-type lightning arresters. When the output line (protected line), input line (unprotected line), and ground line of the series-type power supply surge protector are laid close to each other, a transient surge will be induced in the output line, although its strength is smaller than the original. But it can still be dangerous. The solution to this problem is to lay the input line, ground line, and output line separately or vertically, minimizing the length of parallel laying, and pulling away the distance of laying.
The grounding wire of the SPD is not connected to the protection ground of the protected equipment, that is, a separate lightning protection grounding is adopted. This will cause dangerous voltages between the protected line and the equipment protection ground during transients. The solution to this problem is to connect the grounding of the SPD to the equipment protection ground.