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The gas discharge tube has a strong ability to withstand large energy shocks, but in the specific use, since the residual voltage of the gas discharge tube during discharge is extremely low, which is similar to the short circuit state, it cannot be used alone in the power supply arrester, and the gas discharge tube is used. The flow resistance is related to the pipe diameter. The larger the pipe diameter, the better the flow resistance. The quality problem of the gas discharge tube is mainly characterized by chronic air leakage, reliability problem of long-term use (that is, the DC breakdown voltage value is shifted after suffering multiple lightning impulses), and the photosensitivity effect and dispersion are large. Although the domestic gas discharge tubes have been greatly improved in recent years, the quality is gradually improving, but the overall quality problems still exist, especially the reliability problems and chronic air leakage problems. Therefore, the choice of imported brand-name gas discharge tube in the power supply arrester should be the first choice, and the diameter of the gas discharge tube is better than Ф8mm.
The choice of capacitors and thermal fuses in the power supply arrester is also important. The power supply arrester has been working in the power grid for a long time. There are many cases in which the power supply arrester is damaged due to the quality problem of the capacitor. Therefore, the voltage withstand selection of the capacitor is very important, especially the impulse withstand high voltage. In contrast, foreign products are better than domestic products, Hitachi, OKAYA's capacitor quality is good. The function of the thermal fuse in the lightning arrester is that when the lightning current exceeds the maximum capacity of the lightning arrester, the fuse can be disconnected due to the overcurrent, and the fuse can be disconnected due to the temperature rise of the zinc oxide varistor due to overcutting. It acts as both overcurrent and temperature protection. Due to the normal operating conditions of the power supply arrester, the current is very small, only in the case of lightning shock or pulse voltage shock, under the transient conditions, it is different from the conventional hot-melt fuse, so the power supply arrester Hot melt fuses should have unique properties, ie, blow characteristics under transient conditions.
Advanced design
The design of the arrester has good components, and the advanced design is the necessary condition to ensure the quality of the lightning arrester. According to the analysis and comparison of domestic and foreign products, the following aspects should be fully considered when designing the power supply arrester. Reasonable positioning of the lightning arrester of the power supply lightning arrester, that is, the rated surge current value and the maximum surge current value of the power surge arrester. Nowadays, some manufacturers of power supply arresters on the market, for commercial purposes such as advertising and product competition, are free to increase the level of lightning current surge, which is a very irresponsible attitude towards users. Lightning strikes are extremely destructive to modern electronic equipment. The magnitude of lightning current in a certain area is difficult to determine with a digital quantity due to many uncertain factors such as geographical environment, meteorological conditions and power supply wiring of electronic equipment. Therefore, manufacturers should have a large margin for the design of power supply arresters. . The general inrush current should be designed to double the maximum surge current of the power supply arrester, and the maximum inrush current value should be double the rated surge current of the power supply arrester. This design margin is for the user. Responsible attitude. In the specific circuit designed by the manufacturer, a redundant circuit structure with multiple surge current absorption should be adopted, that is, when a certain surge current absorption circuit is damaged due to a certain component, the whole circuit of the power supply arrester is automatically removed, and the whole circuit is not affected. The power supply arrester works normally. Due to the above design margin, even if one or even two absorption circuits exit the overall circuit, the lightning protection capability of the entire power supply arrester is not affected. This redundant design will greatly improve the reliability of the power supply arrester and is the preferred protection device for lightning protection of power lines in multiple minefields.
Production process and quality management system
A reasonable and scientific production process is a guarantee for ensuring the quality of the power supply arrester. In the production process of the power supply arrester, the manufacturer should pay attention to the following aspects. Damp heat has always been an important cause of varistor failure. The phenomenon is that under the influence of long-term humid environment, the varistor has a significant increase in leakage current and a significant decrease in varistor voltage. For the entire power supply arrester, due to the influence of the humid environment, once the transient overvoltage or lightning current surge occurs in the power grid, it is likely to cause partial short circuit and damage. Due to the thunderstorm season
The section is often a hot and humid meteorological environment, so the anti-humidity process of the power supply arrester is very important. Usually manufacturers use epoxy resin potting production process. Some manufacturers can vacuum evacuate during epoxy potting, and the effect is better. Therefore, in the selection of the power supply arrester, in addition to viewing the choice of components of the manufacturer, design and production process, quality management is also very important. This includes components procurement, storage, inspection, assembly, aging, residual pressure and leakage current testing system, safety system and other aspects.
In summary, the selection of high-quality power supply arresters can not only stay in the advertising of manufacturers, but also to the manufacturers to look at the above aspects, especially the selection of key components, design solutions, production processes are understood Focus. In addition, the local meteorological conditions, the number of annual thunderstorm days and the damage caused by thunderstorms should also be considered in combination with the protection level of the power supply arrester.
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Design principle
In view of the emergence of various types of lightning arresters on the market, the quality is uneven, and some are even unheard of (such as: lightning arresters without grounding, until now, can not understand how it works), so By introducing the working principle and composition of the arrester, it is helpful to identify the true and false, good and bad customers.
The lightning arrester components are available in both soft and hard from the perspective of response characteristics. The discharge element belonging to the hard response characteristic has a spark gap (angular spark gap and coaxial discharge spark gap based on xenon arc technology) and a gas discharge tube, and the discharge element belonging to the soft response characteristic has a metal oxide varistor and transient suppression. diode. The difference between these components is the discharge capacity, response characteristics and residual voltage. The arresters use their different advantages and disadvantages, and they are combined to form various arresters and protect circuits.
Spark gap (Arc chopping)
1. Discharge gap: The principle is two electrodes such as horns. The distance is very short and separated by insulating material. When the electric field strength between the two electrodes reaches the breakdown strength, a current path is formed between the electrodes. When the lightning wave arrives, it first breaks down at the gap, so that the air in the gap is ionized, forming a short circuit, and the lightning current flows into the earth through the gap. At this time, the voltage across the gap is very low, thereby achieving the purpose of protecting the line. When the electric field strength is lower than the breakdown gap, the discharge gap type arrester returns to the insulated state. Commonly used in lightning protection of high voltage lines. In low voltage systems, it is often used for pre-protection of power supplies.
The advantages and disadvantages of spark gap type arrester products are the material of the electrode, the gap distance and the insulating material.
Advantages: Strong discharge capacity, large flow rate, 10/350μs pulse waveform can divert 50KA pulse current, for 8/20μs pulse current, can be greater than 100KA, high insulation resistance and small parasitic capacitance, leakage current small. It does not have any detrimental effect on equipment that is working properly.
Disadvantages: high residual voltage (2.5~3.5KV), long reaction time (≦100ns), low operating voltage accuracy, continuous power flow, so a fuse should be connected in series in the protection circuit, so that the power frequency freewheeling is quickly Cut off.
Note: Since the two discharge tubes are respectively installed on the two wires of one circuit, they sometimes discharge at different times, so that a potential difference occurs between the two wires. In order to make the discharge tube on the two wires close to a uniform time discharge, reduce two A three-stage discharge tube was developed for the potential difference between the lines. It can be seen as consisting of two secondary discharge tubes combined. The first stage in the middle of the three-stage discharge tube serves as a common ground line, and the other two stages are respectively connected to the two wires of the loop.
2, Gas discharge tube (GDT): is a ceramic or glass package, the tube is filled with a certain pressure of inert gas (such as argon), switch-type protection components, there are two electrodes and three electrodes structure. When the electric field strength reaches the breakdown inert gas intensity, a gap discharge is caused, thereby limiting the voltage between the poles. 8/20μs pulse current can guide 10KA. The discharge voltage is unstable. When the voltage is greater than 12V and the current voltage is 100mA, the subsequent current will be generated. Usually used in measurement, control, regulation technology circuits and electronic data processing transmission circuits.
Metal oxide varistor
Metal oxide varistor (MOV)
A metal oxide semiconductor non-linear resistor containing zinc oxide as a main component, when the voltage applied across the resistor is less than the varistor voltage, the varistor is in a high-resistance state, and if it is connected in parallel on the circuit, the valve is in an open state; When the voltage applied across the varistor is greater than the varistor voltage, the varistor will break down, exhibiting a low resistance value, even close to a short circuit condition. The breakdown state of the varistor is recoverable, and when the voltage higher than the varistor voltage is cancelled, it returns to a high resistance state. When the power line is struck by lightning, the high voltage of the lightning wave causes the varistor to break down, and the lightning current flows into the earth through the varistor, so that the voltage on the power line is clamped within a safe range.
Zinc oxide varistor arresters are now circulating on the market. China was only mass-produced in the late 1980s. It is considered to be the latest and most advanced technology, and will be featured in detail. At present, lightning arresters for transmission lines in China use zinc oxide arresters.
Advantages: wide switching voltage range: 6V~1.5KV, fast response speed (25ns), low residual voltage (can achieve safe working voltage of terminal equipment), large flow rate (2KA/cm2), no freewheeling, long life.
Disadvantages: It is easy to aging. After several actions, the leakage current will increase, which will cause the varistor to overheat and eventually lead to aging failure.
The capacitance is large and in many cases is not used in high frequency, ultra high frequency systems. This capacitor in turn forms a low pass with the wire capacitance. This low pass can cause severe attenuation of the signal. However, at frequencies below 30 kHz, this attenuation is negligible.
Transient suppression diode
Transient voltage suppressor (TVS):
1, two-pole discharge tube: There are two forms: one is Zener type (for one-way avalanche breakdown), and the other is bidirectional silicon varistor. Performance is similar to switching diodes. Under the specified reverse voltage, when the voltage at both ends is greater than the threshold voltage, the working impedance can be immediately reduced to a very low level to allow a large current to pass, and the voltage at both ends is clamped at a very low level, thereby effectively protecting Precision components in the end electronics avoid damage. The bidirectional TVS can absorb instantaneous large pulsating power in both positive and negative directions and clamp the voltage at a predetermined level. Suitable for AC circuits.
Advantages: The action time is extremely fast, reaching picoseconds. It has low voltage limit and low breakdown voltage and is used in various electronic fields.
Disadvantages: The current load is small, the capacitance is quite high, generally below 20pF, and the current ceramic discharge tube can achieve 3~5pF.
The surge protection system required for electronic information systems generally consists of two or three levels. It uses gas discharge tubes, varistors and suppression diodes, and utilizes the characteristics of various surge suppressors to achieve reliable protection. The gas discharge tube is generally placed at the input end of the line as a first-order surge protection device, and is subjected to a large surge current, and is a bleed-type device. The secondary protection device uses a varistor that limits the surge voltage to a low level in a very short time (ns). For highly sensitive electronic circuits, a suppression diode can be used as a three-level protection. Limit the surge voltage to the insulation level of the end electronics in a shorter period of time. As shown in the figure, when a surge such as lightning comes, the suppression diode is first turned on, and the instantaneous overvoltage is accurately controlled to a certain level. If the surge current is large, the varistor starts and discharges a certain surge current. At this time, the voltage across the varistor will increase until the pre-stage gas discharge tube is pushed to discharge the large current to the ground. When the distance between the three devices in the line is long, the conduction sequence starts from the gas discharge tube and is turned on in turn.
The operation of the arrester starts from the fastest reaction time, the end of the equipment, and then starts to the front end step by step.
Equipment that protects the back end with a gas discharge tube alone has the following problems: the conduction time is too long, and the residual voltage is too large, which may exceed the withstand voltage level of the back-end equipment. After discharge, a power frequency freewheeling will occur. To avoid the above problem, another circuit (Figure 3) is used. In order to solve the problem of generating power frequency freewheeling, and also avoiding the varistor due to excessive leakage current and self-explosion or aging, we connect a varistor in series with the gas discharge tube, so as to avoid the generation of power frequency freewheeling, It can prevent the varistor from self-explosion and aging due to leakage current. However, a new problem arises, so that the operating time of the arrester is the sum of the on-time of the gas discharge tube and the on-time of the varistor. Assuming that the on-time of the gas discharge tube is 100 ns and the on-time of the varistor is 25 ns, their total reaction time is 125 ns. In order to reduce the reaction time, a varistor is incorporated in the circuit, so that the total The reaction time is 25ns.
When the overvoltage occurs, the suppression diode acts as the fastest component first. The circuit is designed to convert the discharge current to the pre-discharge path as the suppression diode may be destroyed. Discharge on the road.
Us+△u≥Ug
Us: suppress the voltage on the diode
â–³u: voltage on the decoupling induction coil
Ug: operating voltage of the gas discharge tube
If the discharge current is less than this value, the gas-filled discharge tube does not operate. The use of such a circuit not only makes it possible to take advantage of the rapid action of the arrester under conditions of low protection level, but also achieves a high discharge capacitance. This eliminates the disadvantage of suppressing the diode overload and the primary fuse frequently cutting off the circuit in the event of a power flow.
Higher frequency lines can also use ohmic resistors as decoupling components for use with low capacitance bridged lines.
2, three-pole discharge tube: two two-pole discharge tube on the two wires, there will be a potential difference, so there is a three-pole discharge tube, more than one pole for common ground, can reduce the time difference (0.15 ~ 0.2μs ), and the resulting lateral lightning voltage amplitude.
Common power supply arresters on the market generally use varistor for primary, secondary and tertiary power supplies. In this combination, when the distance is greater than 5 meters, the conduction time is gradually turned back from the first stage.
If the first stage uses a gas discharge tube and the second and third stages use a varistor, the first stage and the second stage must satisfy a distance greater than ten meters, and the second stage and the third stage satisfy a distance greater than 5 meters. This will ensure that the previous level moves first. Otherwise, the first stage may not operate, and the second and third stage arresters do not have such a large flow rate, so that the arrester cannot effectively protect the equipment. This must be noticed in engineering design.
The function of the lightning arrester is to protect various electrical equipment in the power system from lightning electrical overvoltage, operating overvoltage, power frequency transient overvoltage and damage. The types of lightning arresters mainly include protection gaps, valve type lightning arresters and zinc oxide lightning arresters. The protection gap is mainly used to limit atmospheric overvoltage, and is generally used for protection of power distribution systems, lines, and substation incoming segments. Valve type lightning arrester and zinc oxide lightning protection device are used for protection of substations and power plants. The system is mainly used to limit atmospheric overvoltage in 500KV and below, and will also be used to limit internal overvoltage or internal in ultra high voltage system. Backup protection for overvoltage.
Basic composition and use of lightning arresters
The zinc oxide varistor is a voltage-limiting protection device. When there is no pulse voltage, it exhibits a high-resistance state. Once the pulse voltage is responded, the voltage is immediately limited to a certain value, and the impedance is abruptly changed to a low-resistance state. Compared with the gas discharge tube, its biggest advantage is that when it absorbs the pulse voltage, the residual voltage is higher than the working voltage, it will not cause a short circuit of the power supply, and no freewheeling will occur. The zinc oxide varistor has a faster response time than the gas discharge tube. The breakdown voltage of the gas discharge tube is very sensitive to the rising rate of the pulse voltage. The faster the voltage rise rate, the higher the ignition voltage and the faster the response time. It is possible to correctly select the two types of components, such as varistor and gas discharge tube, and use their respective advantages to combine the power supply arresters, and the overall performance is relatively good. Zinc oxide varistor is required in the power supply arrester and has excellent energy withstand characteristics. The energy withstand characteristic is mainly described by three indicators: rated lightning impulse current, maximum lightning impulse current and energy tolerance. These characteristics are sensitive to zinc oxide. The surface area of ​​the resistor is related to the heat dissipation conditions of the component. The same type of varistor, due to different manufacturers' manufacturing processes and raw material formulations, their energy tolerance will vary greatly.