Application of analysis of lightning protection measures for 10 kV distribution lines
10 kV distribution lines are very complex. Once a lightning incident occurs, it is difficult to maintain and manage the power grid, which often results in significant economic losses. Lightning is a common natural phenomenon, and people cannot predict the exact time of its occurrence, thus causing great harm to 10 kV distribution lines. A large number of studies have shown that lightning accidents are mainly caused by lines being affected by induced lightning overvoltage. Therefore, it is urgent to take lightning protection measures for 10 kV distribution lines to ensure the safe operation of the power grid.
1. Factors affecting 10 kV distribution lines
1. Induction lightning overvoltage
Induced lightning overvoltage refers to the lightning overvoltage generated by lightning around overhead distribution lines. Although the line is not directly hit by lightning, a lot of charges with the opposite nature of thunderclouds are induced on the wires. Facts have proved that the number of lightning accidents caused by induced lightning overvoltage accounts for 90% of the total faults of distribution lines, and has become the main culprit of 10 kV distribution line faults. Literature research shows that if the conductors of a 10 kV distribution line are ten meters away from the ground and the location of the lightning strike is fifty meters away from the conductors, a lightning current of 100 kA will be generated. If no lightning protection measures are taken on the line, the maximum lightning overvoltage can reach 500 kV. If the insulation effect of the 10 kV distribution line is not good, it will be easily broken down by lightning overvoltage or even broken.
2. Insulation level
Failure rates due to explosions of insulators in distribution lines are also high. Whether the insulator can operate normally determines the safety of the distribution line, because the insulator determines the insulation level of the 10 kV distribution line. Therefore, insulator maintenance must be done well. When 10 kV distribution lines operate for a long time, if the insulators are not regularly tested, repaired or replaced, once the line ages, the insulation level of the entire distribution line will be greatly reduced, increasing the probability of lightning accidents.
3. Lightning protection installation
①Installation of transformer. Once the lightning overvoltage reaches more than ten times the rated voltage, it will break down the insulation protective layer around the neutral point of the transformer. Currently, most of the lightning arresters of domestic transformers are installed on the high-voltage side, and the lightning protection on the low-voltage side is very weak. Transformer lightning protection can be installed in front of the main fuse or in front of the distribution line. During installation, the low-voltage end of the arrester must be grounded. Since the neutral line behind the current-type protector cannot be grounded repeatedly, otherwise once the protector loses its working ability, the entire line cannot be protected against lightning. Therefore, the grounding wire of the low-voltage arrester must be installed at the first end of the neutral line of the transformer.
②Pole-mounted switch and knife gate. If pole-mounted switches and knife switches are installed in 10 kV distribution lines, the safety and reliability of the lines can be greatly improved. But in fact, many lines do not have lightning protection for pole switches and knife gates.
③Lightning protection for switch cabinets and other units. The 10 kV power distribution line contains many units, each of which is critical. Lightning arresters can be installed on every unit or can be installed selectively. It is obvious that although the installation cost of the former is high, the safety performance is also very high.
2. Analysis of lightning protection measures for 10 kV distribution lines
1. Reduce the probability of explosion and flashover of insulators and improve the insulation level of distribution lines.
If the voltage changes too much, it will have an adverse impact on the operation of the distribution lines. In order to improve the lightning protection effect of 10 kV distribution lines, U50% discharge voltage insulators should be used. Since the distance between loops on the same pole is very small, once it is broken down by lightning overvoltage, it is easy for the loop to be grounded, which greatly affects the power supply reliability of the 10 kV distribution line. Therefore, all wires must be insulated, and insulation must be placed between insulators and wires to improve the reliability of distribution lines.
2. Selectively put into operation automatic reclosing
Once a lightning strike fault occurs on a 10 kV distribution line, it is difficult to completely repair it. In order to avoid further expansion of lightning strike faults, automatic reclosing should be installed at certain locations in the line. If the distribution lines are all cables, automatic reclosing does not need to be installed in this case. If the distribution lines are all overhead, it is recommended to use automatic reclosing to improve the safety performance of the lines. If it is a mixed line of cables and overhead insulated conductors, and the cables account for more than 40% of the entire line, you do not need to consider installing automatic reclosing at this time. If it is a mixed line of cables and overhead bare wires, and the length of the cable reaches more than 50% of the entire line, automatic reclosing does not need to be considered.
3. Install special lightning arresters
Lightning arrester is an important lightning protection device in 10 kV distribution lines, which can provide good protection for the entire line. There are many types of lightning arresters, the common ones are gapless arresters and zinc oxide arresters. Under the combined action of power frequency voltage, freewheeling and lightning overvoltage, gapless arresters are prone to aging, which makes the lightning protection function ineffective and greatly affects the power supply reliability of distribution lines. Zinc oxide arresters do not require maintenance and can be specially installed to protect weak links in distribution lines. If arresters are also installed on pole switches and knife gates, the distribution lines can be fully protected. Therefore, it is best to install zinc oxide arresters in 10 kV distribution lines.
4. Install parallel gap insulators
When an insulator flashover occurs, do not let the arc come into contact with the surface of the insulator. If the gaps cannot withstand operating overvoltage, distribution line faults will be amplified. If a parallel gap is installed in the line, the parallel gap can protect the insulator string. In addition, the operation and maintenance of the parallel gap are extremely convenient and can be directly observed with the naked eye.
5. Reduce the grounding resistance of equipment in 10 kV distribution lines
Relevant documents stipulate the grounding resistance of power distribution equipment as follows. If the capacity of the distribution voltage device is not greater than 100kVA, the grounding resistance of the equipment should be no more than 10 ohms; if the capacity of the distribution voltage device exceeds 100kVA, the grounding resistance of the equipment should be no more than 4 ohms. When installing lightning arresters at weak links in distribution lines or at pole-mounted switches and knife gates, the grounding resistance should be controlled below 4 ohms. There are generally two methods to reduce the resistance of power distribution equipment: First, use a horizontal grounding body. The disadvantage of this method is that it is easily corroded and has a short service life. The second is to add resistance reducing agent. Adding high-efficiency bentonite that can both reduce resistance and prevent corrosion next to the horizontal grounding body has a very obvious effect and can greatly reduce the resistance of the pole. The method used to reduce resistance should be chosen based on the actual situation.
6. Overvoltage protection
Currently, there are many kinds of lightning protection technologies in our country. Although many technologies have obvious advantages, their shortcomings are also obvious. As a protection technology for overhead distribution lines, overvoltage protection can meet the needs of the development of my country's power grid. Distribution line overvoltage protection can protect distribution lines from lightning overvoltage, flash voltage, power frequency increase and other factors, thus greatly improving the operational safety of the line.
Yichang Hengyuan Technology Co., Ltd. specializes in the production of European-style cable joints, plug-and-pull heads, cable branch boxes, American-style elbow joints, high-voltage vacuum circuit breakers, high-voltage load switches, arresters, zinc oxide arresters, tank arresters, over-voltage protectors, counters, monitors, heat-shrinkable cable accessories, Manufacturer of cold shrink cable accessories, T-heads, wall bushings, European cable accessories, American cable joints, transformer neutral point protection devices, fuses, isolating switches, PT cable heads, elbow cable joints, 10kV outdoor terminals, switch cabinets, cable intermediate joints, cable accessories and other products
