Discussion on safety lightning protection of power

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In recent years, with the rapid development of electronic technology, automatic control system is more and more widely used in all aspects of power production. While benefiting from the great convenience of microelectronics technology, power workers are also troubled by the huge loss once it is damaged. In fact, when the automatic control system is added to the power system, the awareness of safety and lightning protection of the automatic control system is relatively weak. Once there is lightning wave intrusion, the equipment damage is generally huge, and some even paralyze the whole system, causing irreparable losses

1 causes of lightning stroke

lightning stroke is a natural phenomenon, which can release huge energy and has extremely strong destructive ability. For a long time, the researchers who are committed to power production and power equipment research have taken certain preventive measures against the harm of lightning through the research and exploration of the destructive nature of lightning, which has effectively reduced the lightning damage

when the lightning discharge path does not pass through the lightning protection device, a strong transient electromagnetic field is generated in the discharge process, and a strong electromagnetic pulse is induced in the nearby conductor, which is called induced lightning. Induction lightning can invade conductors through two different induction methods. One is that when the charge accumulates in the thunder cloud, the nearby conductor will induce opposite charges. When the lightning discharge, the charge in the thunder cloud will be released rapidly, and the static charge in the conductor will flow along the conductor after losing the restraint of the thunder cloud electric field, looking for the release channel, which will form electrostatic induction in the circuit. Secondly, when the thunder cloud discharges, the rapidly changing thunder current will produce a strong transient electromagnetic field around it, A high induced electromotive force will be generated in the nearby conductor, forming electromagnetic induction in the circuit, and the inductive lightning will spread along the conductor, damaging the equipment in the circuit or devices in the equipment. In the information system, there are many system interfaces and long lines, which provide a good environment for the generation, coupling and propagation of induction lightning. With the development of science and technology, the integration of information system equipment is getting higher and higher. The anti overvoltage axial force can be applied, and the weight force is getting worse and worse. It is extremely vulnerable to the attack of induction lightning, and the damage is often the core components of the system with high integration, so we can't take it lightly. Induction lightning can be discharged from the cloud, It can also come from lightning strike to the ground. The information system is connected with the outside world with various long-distance cables, which can generate inductive lightning in a larger range and transmit it to the information system along the cable. Therefore, anti induction lightning is an experimental automation system in which the substation, which is widely used in power system, especially microelectronic technology, can realize two situations of simply supported beam and cantilever beam. Therefore, lightning protection of information system is the key to ensure the safety of power system

2. Lightning protection technology of high-voltage power devices in power system

2.1 the original high-voltage lightning protection technology

power devices in the early stage of their development and use are mostly transmitted through bare conductor overhead lines, which are generally 6 ~ 18m away from the ground, and are damaged by the insulation breakdown of lines or equipment caused by the overvoltage of lightning abrasion reducing and wear-resisting parts, seals, insulating parts and medical device parts generated by lightning intrusion waves. At that time, people artificially made insulation weak points, namely gap devices, on the road or equipment. The breakdown voltage of the gap was lower than the lightning impulse insulation level of the line or equipment. Under the normal operating voltage, the gap was in the state of isolation and insulation. When lightning occurred, the strong overvoltage made the gap breakdown, thus producing grounding protection and protecting the insulation of the line or equipment

2.2 gap protection technology: gap protection is that the two poles of the line are generally composed of angle rods. One pole is fixed on the insulator to connect the live wire, and the other pole is grounded. After gap breakdown, the arc rises and lengthens between the angle rods. When the arc current becomes small, it can self extinguish. The disadvantage of gap protection technology is that when the arc current is more than dozens of AMPS, it cannot self extinguish. When lightning overvoltage, single-phase The two-phase or three-phase gap may break through the grounding, causing grounding fault, two-phase or three-phase short circuit fault, so that the line power circuit breaker protection action opens

2.3 tube type arrester technology: tube type arrester technology uses a gap device with air-jet arc extinguishing function. This device has internal and external gaps. The external gap is similar to the protective gap. The two poles are fixed on the insulator, and the internal gap is placed in the arrester tube. When the internal and external gaps of lightning overvoltage break down, the lightning current and power frequency short-circuit current are grounded through the inner wall of the tube, and the material on the tube wall is heated and vaporized, The gas with large pressure is ejected out of the pipe through the inner gap, and the arc extinguishing is forced through the gap. Tube type arrester technology also has many disadvantages: the selection of this device is limited by the installation location, and then by the maximum and minimum short-circuit current of the line. When the maximum short-circuit current is greater than the upper limit of the arrester, the arrester will explode; Quhongle, head of the wall reconstruction Office of Shaanxi Provincial Department of construction, said that when the current is less than the lower limit of the lightning arrester, the arc cannot be extinguished, and the lightning arrester may be burned out. In addition, after the tube arrester acts for many times, the inner diameter of the tube will gradually increase, and the arc extinguishing capacity will decline or even disappear

3. The application of new lightning protection technology

gap protection technology and tube type lightning arrester technology both rely on gap breakdown, grounding discharge and voltage reduction to play a protective role. The above two lightning protection technologies often cause grounding fault or phase to phase short circuit fault, which cannot achieve scientific and reasonable protection function. At present, they are only used for lightning protection of transmission lines in the power system. At the same time, they are used in conjunction with automatic reclosing devices in order to minimize line outage accidents. A more scientific and reasonable lightning protection measure is the valve type lightning arrester technology, which is the most common lightning protection technology for electrical equipment in high-voltage lightning protection. Its principle is to automatically open the gate to discharge and reduce the voltage under overvoltage, and close the gate and cut off the current when the operating voltage is restored. This protective effect is realized by the current and voltage limiting effect of the resistance element in the arrester. Under overvoltage, the resistance element can limit the lightning current within 5ka, and the residual voltage is limited below the lightning impulse insulation level of the equipment; Some resistance elements still have freewheeling under the operating voltage, and long-term freewheeling will damage the tubular arrester, so it is generally necessary to add a series gap to isolate the operating voltage, and rely on the gap to extinguish the arc and cut off the freewheeling. The outstanding advantage of the valve type arrester is that the resistance element of the arrester can avoid the direct grounding or phase to phase short circuit fault of the power system, and its protective effect will not affect the normal and safe operation of the power system

3.1 silicon carbide arrester technology: the structure of silicon carbide arrester is to compress and seal the gap and several SiC valve sheets in the arrester porcelain bushing. The protection function is to use the nonlinear characteristics of SiC valve sheets, and the resistance becomes very small under overvoltage, which can discharge a large amount of lightning current to limit the residual voltage. After the lightning voltage passes, the resistance automatically increases, limiting the continuous current within tens of AMPS, so that the gap can extinguish arc and cut off current. Silicon carbide arrester technology is the main lightning protection appliance in the current lightning protection technology

3.2 zinc oxide arrester: zinc oxide arrester is abbreviated as MOA. Compared with traditional silicon carbide arrester, MOA has the characteristics of good protection characteristics, large flow capacity, strong pollution resistance, simple structure, high reliability, and can provide the best protection for power transmission and transformation equipment

silicon carbide arrester technology has outstanding advantages in lightning protection performance and is widely used by high-voltage equipment in power system, but it also has some disadvantages: first, it only has the maximum amplitude voltage limit protection function of lightning, but has no lightning steep wave protection function, and the lightning protection function is incomplete; Second, there is no continuous lightning impulse protection capability; Third, the stability of action characteristics is poor and may suffer from transient overvoltage; Fourth, the action load is heavy and the service life is short. These potential shortcomings have exposed that there are hidden dangers affecting power safety in the use of silicon carbide arrester, and its product technology is relatively backward. According to the shell material, zinc oxide arresters are divided into three categories: porcelain bushing type, tank type and composite jacket type; According to the six categories of power distribution, power station, line, parallel compensation capacitor, transformer and motor neutral point, generator and motor protection, zinc oxide arrester technology inherits the silicon carbide arrester technology, and is recognized as the most advanced lightning protection appliance in the world in terms of design idea and product function improvement. The structure of zinc oxide arrester is to compress and seal several ZnO valve plates in the arrester porcelain bushing. ZnO valve slice has very excellent nonlinear characteristics. Under high voltage, the resistance is very small, which can discharge the amplified lightning current. The residual voltage is very low. Under electric operation voltage, the resistance is very large, and the leakage current is only 50 ~ 150 μ A. The small current can be regarded as no power frequency freewheeling, which is the reason why the gapless zinc oxide arrester is made. Its outstanding advantage is that it has the same limited voltage effect on Lightning steep wave and lightning amplitude, and the lightning protection function is complete

gapless zinc oxide arrester is the first one produced and used in China. After long-term operation practice, it is found that it has shortcomings such as high damage explosion rate and short service life. The reason is that its fatal weakness is its poor transient overvoltage withstand capacity. However, the series gap zinc oxide arrester still has the advantages of the protection performance of the gapless zinc oxide arrester, and has the characteristics of strong transient overvoltage withstand capacity. It is an ideal product to develop strengths and avoid weaknesses. Combined with the national conditions, the series gap zinc oxide arrester in 3 ~ 35kV system is the most advanced lightning protection appliance in the contemporary era

4 lightning protection technology for weak current devices in power system

4.1 the formation and invasion ways of lightning stroke

4.1.1 the formation of lightning stroke mainly has two forms: direct lightning stroke and induction lightning stroke

direct lightning stroke refers to the direct action of lightning on objects, generating electric energy effect, thermal effect and mechanical force, which cause harm to objects

inductive lightning strike refers to the electrostatic effect and electromagnetic induction generated on the nearby conductor during lightning discharge, and the resulting discharge effect causes sparks between metal parts, which is called inductive lightning strike

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