Lightning protection and overvoltage protection of rural distribution network
Operating experience shows that lightning accidents in the distribution network account for about 70% to 80% of all lightning accidents in the entire power system. Therefore, the lightning protection work of the distribution network must be strengthened to ensure the safety of power supply and improve the reliability of power supply.
1 Lightning protection of power distribution equipment On high-voltage power distribution lines, there are many scattered power distribution equipment, such as distribution transformers, to prevent lightning accidents, necessary lightning protection devices must be installed.
1.1 Lightning protection measures for distribution transformers.
Distribution transformers are protected by valve-type arresters in accordance with current regulations. The specific wiring diagram is shown in Figure 2.
Figure 2 Wiring diagram of Transformer lightning protection
The valve-type arrester is required to be installed closer to the transformer, and the protection effect is better. Generally, it is required to be installed inside the high-voltage drop fuse. The residual voltage of the arrester must be less than the withstand voltage of the distribution transformer in order to effectively protect the transformer.
The grounding wire should be connected with the metal shell of the distribution transformer and the neutral point of the low-voltage side together and grounded together. When the transformer capacity is 100kV.A and above, the grounding resistance should be as low as possible to below 4Î©; when the transformer capacity is less than 100kV.A, the grounding resistance should be 10Î© and below. When the three points are connected together and grounded, when the high-voltage side lightning arrester discharges, what the transformer insulation bears is 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. The protection is very advantageous and can reduce the risk of insulation breakdown between the high-voltage and low-voltage windings and between the high-voltage windings and the transformer housing. In order to prevent the instant rise of the neutral point potential on the low-voltage side of the transformer from affecting user safety, an auxiliary grounding wire (repeated grounding) can be installed near the user.
2.2 In high-altitude areas, most of the lightning activity is on the half-hill slope, sometimes lower than the power line. During operation, due to the impact of lightning on the low-voltage side or the inverse transform wave, it is easy to cause the insulation breakdown of the Y, yn0 distribution transformer, so rural mountainous areas During the design and installation of distribution transformers, it is required to install a set of breakdown fuse or low-voltage arrester on the low-voltage outlet side of the distribution transformer to protect the secondary winding of the transformer, and also to protect the transmission of overvoltage to the low-voltage winding When the high-voltage winding is reached, the insulation of the high-voltage winding will not be damaged. After a transformer in operation is sometimes struck by lightning, the insulation breakdown between the transformer turns is only a partial damage, but it can barely run. After a period of time, the fault expands and the operation is forced to stop. This is the insulation breakdown of the transformer during a thunderstorm, and the accidents caused by the expansion of the fault in good weather are often easy to give people the illusion that it is not due to lightning damage.
2 Overvoltage protection of distribution lines After lightning strikes on overhead distribution lines with a voltage of 3ï½ž10kV, they often cause insulator breakdown and wire breakage accidents. Especially the lines with reinforced concrete poles and iron crossarms are the most prominent, so they must be insulated Add necessary lightning protection equipment for weak points. Individual extremely high poles, iron crossarms, parts of poles with cables, terminal poles and other weak points of insulation should be protected by valve-type arresters or tube-type arresters. When two lines cross, the minimum allowable value of the vertical distance between the upper and lower conductors shall comply with the value specified in the relevant regulations. Generally, 10kV power distribution lines cross each other and lower voltage lines, communication lines, and closed-circuit television lines. When the wire temperature is 40â„ƒ, the crossing distance shall not be less than 2m; if the upper and lower distances are small, the air gap may be broken down by lightning , Cause two mutually crossing lines to have fault trips at the same time, and will cause non-selective actions of line relay protection, which may expand into system accidents. The coordinated protection gap is shown in Figure 1.
Figure 1 Schematic diagram of protection gap
The grounding resistance of the grounding device shown in Figure 1 should generally not exceed the value of the line grounding resistance (20Î©). If the up-and-down distance at the crossover is greater than 4m, accidents are unlikely to occur, and there is no need to install a matching protective gap.
3 Problems that should be paid attention to during construction and installation
3.1 The planned design and installation of rural power grids must consider the lightning protection of power distribution equipment.
3.2 Low-voltage breakdown fuse or low-pressure valve-type arrester should be installed in mine areas.
3.3 The anti-rust problem of the grounding device should be paid attention to. Almost all the steel bar ground down conductors have different degrees of rust, which may cause breakage. They should be painted with black paint to prevent rust.
3.4 The measurement of the grounding resistance of the grounding device shall comply with the standard.
3.5 In areas with high resistivity, chemical resistance reduction methods or other methods should be adopted.
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