This article analyzes the shortcomings of plug-and-socket power surge arresters that are prone to fire. And propose improvements. Keywords: Lightning protection device, smoke, fire, heat release, lightning-proof fuse.
The current problem
At present, lightning protection devices (referred to as SPDs) are installed in communication power equipment. Most of these surge protectors are plug and socket type. Because of this lightning arrester, it has a compact structure, small size, convenient installation and maintenance, and low price, so it has been widely used.
However, this type of lightning protection device also has the disadvantage that it will emit smoke and catch fire. Power surge protectors always operate under the condition of power frequency voltage load. Among them, the key protective element, metal oxide varistor (MOV), is always ineffective. The failure mode is often a short circuit mode. However, once the MOV fails short circuit, the power supply will supply a strong short circuit current. If the necessary protection measures are not taken during the design and manufacture of the lightning arrester, accidents with short circuit power supply will cause smoke and fire accidents.
In order to prevent fire, users often use a circuit breaker in series with the MOV. When there is a strong short-circuit current, the circuit breaker disconnects the power supply to the lightning arrester. There is also a serious drawback to doing so. Because the residual voltage introduced by the circuit breaker is even higher than the residual voltage of the arrester itself. This will greatly reduce the protection effect of lightning protection devices.
This article gives some ideas on how to reduce the smoke and fire problems of this type of plug-in lightning arrester.
Plug and socket type lightning arrester design features
One expert grouped plug-and-socket power surge arresters into lightning protection devices of European design patterns. European OBO. The lightning protection devices of companies such as DEHN Cite lphoenix are plug and socket type power supply lightning protection devices. It designed the lightning protection mode as a plug, and it was equipped with a large-sized MOV with a pulse current of 40 kA or 65 kA. In order to prevent the overheating of the MOV, it causes fire. A thermal detachment mechanism consisting of a tension spring, a lever and a low melting point alloy is designed. In addition, the working status indication of the lightning protection module is provided in the form of a mechanical action. The remote alarm contact of the operating status indication is provided through the engagement of the plug and the socket.
However, lightning protection devices of this design do not use lightning protection fuses. Therefore, when a large power frequency short-circuit current occurs, the fault current cannot be disconnected. The so-called lightning-resistant fuses, also known as the impact of current fuses. The melt material is a special alloy. It not only has a relatively low power-frequency fuse current, but also has a large enough 8/20US pulse current tolerance. Melt materials in ordinary fuses do not have this property. The ability of ordinary fuses to withstand pulse currents is rather poor. In order to achieve a fuse with a pulse current capacity equivalent to MOV, the cross-sectional area of ​​the fuse had to be thickened. As a result, the frequency of the rapid blow current is greatly increased. It is considered that the effect of blocking the power frequency current is not achieved at all.
In order to prevent the power frequency short circuit current from being too large. A circuit breaker must be installed on the branch of the arrester to prevent fire.
Investigation and test
We collected a number of power surge arresters that failed in the lightning protection site as anatomical analysis and used impact current test benches to perform simulated impact tests on a variety of lightning protection products. We got some preliminary perceptual knowledge and rational knowledge.
The lightning arrester is exposed to a large lightning current, which causes a small percentage of sparking due to breakdown and short-circuiting. However, in most cases, the lightning arrester of the power supply fails to catch fire during days when no lightning strike has occurred.
For the actual withstand lightning current greater than the maximum impulse current tolerance of the surge protector, to prevent ignition, one approach is to increase the surge current tolerance of the surge protector, or to add a level of protection, that is, to install a pulse at the preceding stage. The lightning current device with large current tolerance; another method is to use a lightning-proof fuse in series with the lightning arrester.
For the days when lightning does not occur, the lightning protection module smokes and catches fire. We analyze: One reason is that when the lightning protection device is subjected to multiple current surges, the lightning protection device does not completely fail. The leakage current increases a lot and causes the MOV chip to generate heat, the leakage current continues to increase, a vicious cycle occurs over time, and the short circuit completely fails on a certain day. Another reason is that the user and the designer excessively pursue the low residual voltage, resulting in the use of a MOV whose threshold voltage (VlmA) is too low, so that the MOV operates under conditions in which the voltage stress (charging rate) is too high. In another case, the power surge protector is installed in a grid with extremely unstable grid voltage. When the power supply voltage fluctuates high, the MOV is subjected to excessive voltage stress. This situation is more common. For example: The power supply of an airport is the power supply of three power grids, in addition there are several sets of diesel generators standby power supply. When powered by a diesel generator, the voltage of the grid is not rated at 220Vrms, but may be above 300Vrms for short periods of time, which is unbearable for MOVs of semi-conductor nature. For those rural small hydropower stations, the regional grid voltage is also unstable. Power surge protectors are exposed to harsh temporary overvoltage stress in this environment.
(to be continued)
The current problem
At present, lightning protection devices (referred to as SPDs) are installed in communication power equipment. Most of these surge protectors are plug and socket type. Because of this lightning arrester, it has a compact structure, small size, convenient installation and maintenance, and low price, so it has been widely used.
However, this type of lightning protection device also has the disadvantage that it will emit smoke and catch fire. Power surge protectors always operate under the condition of power frequency voltage load. Among them, the key protective element, metal oxide varistor (MOV), is always ineffective. The failure mode is often a short circuit mode. However, once the MOV fails short circuit, the power supply will supply a strong short circuit current. If the necessary protection measures are not taken during the design and manufacture of the lightning arrester, accidents with short circuit power supply will cause smoke and fire accidents.
In order to prevent fire, users often use a circuit breaker in series with the MOV. When there is a strong short-circuit current, the circuit breaker disconnects the power supply to the lightning arrester. There is also a serious drawback to doing so. Because the residual voltage introduced by the circuit breaker is even higher than the residual voltage of the arrester itself. This will greatly reduce the protection effect of lightning protection devices.
This article gives some ideas on how to reduce the smoke and fire problems of this type of plug-in lightning arrester.
Plug and socket type lightning arrester design features
One expert grouped plug-and-socket power surge arresters into lightning protection devices of European design patterns. European OBO. The lightning protection devices of companies such as DEHN Cite lphoenix are plug and socket type power supply lightning protection devices. It designed the lightning protection mode as a plug, and it was equipped with a large-sized MOV with a pulse current of 40 kA or 65 kA. In order to prevent the overheating of the MOV, it causes fire. A thermal detachment mechanism consisting of a tension spring, a lever and a low melting point alloy is designed. In addition, the working status indication of the lightning protection module is provided in the form of a mechanical action. The remote alarm contact of the operating status indication is provided through the engagement of the plug and the socket.
However, lightning protection devices of this design do not use lightning protection fuses. Therefore, when a large power frequency short-circuit current occurs, the fault current cannot be disconnected. The so-called lightning-resistant fuses, also known as the impact of current fuses. The melt material is a special alloy. It not only has a relatively low power-frequency fuse current, but also has a large enough 8/20US pulse current tolerance. Melt materials in ordinary fuses do not have this property. The ability of ordinary fuses to withstand pulse currents is rather poor. In order to achieve a fuse with a pulse current capacity equivalent to MOV, the cross-sectional area of ​​the fuse had to be thickened. As a result, the frequency of the rapid blow current is greatly increased. It is considered that the effect of blocking the power frequency current is not achieved at all.
In order to prevent the power frequency short circuit current from being too large. A circuit breaker must be installed on the branch of the arrester to prevent fire.
Investigation and test
We collected a number of power surge arresters that failed in the lightning protection site as anatomical analysis and used impact current test benches to perform simulated impact tests on a variety of lightning protection products. We got some preliminary perceptual knowledge and rational knowledge.
The lightning arrester is exposed to a large lightning current, which causes a small percentage of sparking due to breakdown and short-circuiting. However, in most cases, the lightning arrester of the power supply fails to catch fire during days when no lightning strike has occurred.
For the actual withstand lightning current greater than the maximum impulse current tolerance of the surge protector, to prevent ignition, one approach is to increase the surge current tolerance of the surge protector, or to add a level of protection, that is, to install a pulse at the preceding stage. The lightning current device with large current tolerance; another method is to use a lightning-proof fuse in series with the lightning arrester.
For the days when lightning does not occur, the lightning protection module smokes and catches fire. We analyze: One reason is that when the lightning protection device is subjected to multiple current surges, the lightning protection device does not completely fail. The leakage current increases a lot and causes the MOV chip to generate heat, the leakage current continues to increase, a vicious cycle occurs over time, and the short circuit completely fails on a certain day. Another reason is that the user and the designer excessively pursue the low residual voltage, resulting in the use of a MOV whose threshold voltage (VlmA) is too low, so that the MOV operates under conditions in which the voltage stress (charging rate) is too high. In another case, the power surge protector is installed in a grid with extremely unstable grid voltage. When the power supply voltage fluctuates high, the MOV is subjected to excessive voltage stress. This situation is more common. For example: The power supply of an airport is the power supply of three power grids, in addition there are several sets of diesel generators standby power supply. When powered by a diesel generator, the voltage of the grid is not rated at 220Vrms, but may be above 300Vrms for short periods of time, which is unbearable for MOVs of semi-conductor nature. For those rural small hydropower stations, the regional grid voltage is also unstable. Power surge protectors are exposed to harsh temporary overvoltage stress in this environment.
(to be continued)
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