Lightning protection and overvoltage protection for rural distribution networks
The operating experience shows that the lightning accidents of the distribution network account for 70% to 80% of all lightning accidents in the entire power system. Therefore, it is necessary to strengthen the lightning protection work of the distribution network to ensure the safety of the power supply and improve the reliability of the power supply.
1 Lightning protection of power distribution equipment On the high-voltage power distribution line, there are many distributed power distribution equipments. For example, distribution transformers must be equipped with necessary lightning protection devices to prevent lightning strikes.
1.1 Lightning protection measures for distribution transformers.
The distribution transformer is protected by a valve type arrester according to the current specifications. The specific wiring is shown in Figure 2.
Figure 2 Transformer lightning protection wiring diagram
The closer the valve type arrester is to the transformer installation, the better the protection effect is. It is generally required to be installed inside the high pressure drop fuse. The residual voltage of the arrester must be less than the withstand voltage of the distribution transformer to effectively protect the transformer.
The grounding wire should be grounded together with the metal casing of the distribution transformer and the neutral point of the low voltage side. When the transformer capacity is 100kV.A or above, the grounding resistance should be reduced to 4Ω or less as much as possible; when the transformer capacity is less than 100kV.A, the grounding resistance can reach 10Ω or less. When the three points are connected together, when the high-voltage side lightning arrester is discharged, the transformer insulation is subjected to the residual voltage of the valve type arrester, and the voltage drop on the grounding device does not act on the insulation of the transformer. Protection is advantageous in reducing the risk of insulation breakdown between the high and low voltage windings and between the high voltage windings and the transformer housing. In order to prevent the impact of the instantaneous increase of the neutral point potential on the low-voltage side of the transformer on the safety of the user, an auxiliary grounding wire (repetitive grounding) can be installed near the user.
2.2 In high-altitude areas, most of the thunder and lightning activities in the semi-hill slopes are sometimes lower than the power lines. During operation, the lightning breakdown or reverse-transformed waves on the low-voltage side are often used to cause insulation breakdown of Y, yn0 distribution transformers. In the design and installation of the distribution transformer, it is required to install a set of breakdown fuses or low-voltage arresters on the low-voltage outlet side of the distribution transformer to protect the secondary winding of the transformer and also protect the over-voltage and low-voltage windings. When the high voltage winding is reached, the high voltage winding insulation is not damaged. After the transformer in operation is sometimes struck by lightning, the dielectric breakdown of the transformer is only partial damage, but it can barely run. After a period of time, the fault expands and is forced to stop. This is the breakdown of the transformer insulation during thunderstorms, and the accidents caused by the expansion of faults in good weather are often illusory and are not considered to be damaged by lightning strikes.
2 The overvoltage protection voltage of the distribution line is subject to lightning strikes on the overhead distribution line of 3~10kV, which often causes insulator breakdown and wire burnout accidents. Especially the reinforced concrete pole iron crossarm line is the most prominent, so it must be insulated. Weak points increase the necessary lightning protection equipment. Insulation weak points such as some special high towers, iron crossarms, partial towers with terminal wires and terminal rods shall be installed with valve type arresters or tubular arresters for protection. For the intersection of two lines, the minimum allowable value of the vertical distance between the upper and lower wires shall comply with the values ​​specified in the relevant regulations. Generally, 10kV distribution lines cross each other and cross with lower voltage lines, communication lines, and closed-circuit television lines. When the wire temperature is 40 °C, the cross distance must not be less than 2m. If the upper and lower distances are small, the air gap may be penetrated by lightning. This causes two mutually intersecting lines to trip at the same time, and will cause non-selective action of line relay protection, which may expand into a system accident. The matching protection gap is shown in Figure 1.
Figure 1 Schematic diagram of the protection gap
The grounding resistance of the grounding device shown in Figure 1 should generally not exceed the value of the line grounding resistance (20Ω). If the distance between the upper and lower crossings is greater than 4m, it is unlikely that an accident will occur, and it is not necessary to install a matching protective gap.
3 Problems that should be paid attention to during construction and installation
3.1 The design and installation of rural power grids must consider lightning protection accidents of power distribution equipment.
3.2 The multi-mine area should be equipped with a low-voltage breakdown fuse or a low-pressure valve type arrester.
3.3 The rust prevention problem of the grounding device should be noted. Almost all the steel grounding down conductors have different degrees of rust, which may cause breakage, and should be coated with black paint to prevent rust.
3.4 Grounding device Grounding resistance should be measured in accordance with the standard.
3.5 Chemical resistance reduction methods or other methods should be adopted in areas with high resistivity.
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