Lightning protection grounding of the hottest tran

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Lightning protection grounding of transformer and column switch

distribution transformer area; Pole mounted switch; Grounding device; System grounding mode at present, the power supply company adopts the method of connecting the lightning arrester at the high-voltage side to the lightning protection grounding of the distribution transformer area, and then connecting the grounding down lead of the lightning arrester with the distribution transformer shell and low-voltage neutral point, sharing a grounding device. It is required that the grounding resistance of the grounding device of the distribution transformer of 100kVA and above is less than 4 Ω, and the grounding resistance of the distribution transformer of 100kVA and below is less than 10 Ω, The manual grounding device is required to be made into a ring. These Provisions are the conclusions of relevant standards. However, each provision in the standard has a specific scope of application, and many specific provisions are not reflected in the site regulations of the power supply company. Therefore, it is necessary to explain these provisions and make some specific provisions easier to implement

1 lightning protection wiring of distribution transformer

1.1 the three-point common grounding in terms of grounding resistance means that lightning protection grounding (high-voltage lightning arrester), protective grounding (shell) and working grounding (low-voltage neutral point) share a grounding device, and its grounding resistance shall meet the minimum value of the three. Lightning protection grounding is generally less than 10 Ω, but there shall be a vertical grounding electrode to facilitate dissipation of current. Low voltage working grounding shall be generally less than 4 Ω. Therefore, the grounding resistance mainly depends on the protective grounding when the high-voltage side breaks down to the ground. Generally, the distribution transformer supplies power to the vicious competitors among class B construction enterprises in terms of the output and price of low-end products. The standard stipulates that only when the protective grounding grounding resistance R ≤ 5 metal heating block 0/i, the lightning protection and protective grounding at the high-voltage side can share a grounding device with the working grounding at the low-voltage side. Conversely, if three points are grounded together, when R ≤ 50/i, where I is the single-phase grounding current of the high-voltage system. For ungrounded system, I is the capacitive current of the system; for arc suppression coil grounded system, I is the residual current at the fault point. Although some systems are equipped with arc suppression coils, they often run abnormally and exit from operation. At present, many 10kV system ICs are about 40a, so r should be taken as 1 Ω in large high-voltage systems. If it is greater than 4 Ω according to the above calculation results, it shall not be greater than 4 Ω according to the low-voltage working grounding requirements. In the formula R ≤ 50/i, 50 is the safety voltage of the low system, that is, when the high voltage side is grounded to the shell, the voltage drop of the grounding current through the grounding device shall not exceed 50V. However, the capacitive current in 10kV system varies greatly, some are less than 10a, and some are as high as hundreds of amps or hundreds of amps. Therefore, when the three points of distribution transformer are grounded together, the grounding resistance of grounding device should be determined according to the situation of the high-voltage system, and 4 Ω or 10 Ω should not be generally specified. Since the grounding resistance is related to the single-phase grounding current of the system and has nothing to do with the capacity of distribution transformer, the statement of field regulations is unreasonable. Some data suggest that when the low-voltage working grounding is set separately, the working grounding resistance at the low-voltage side of the distribution transformer below 100kVA can be relaxed to 10 Ω, because the transformer is small and the internal resistance is large, which limits the grounding current and the rise of the ground potential

1.2 there will be other grounding methods in the construction. The three common grounding methods in the construction are common grounding. The analysis is as follows. The high-voltage side arrester is used to protect the insulation between the high-voltage coil and the shell of the transformer. According to the connection method in Figure 2, in addition to the residual voltage of the arrester, the voltage between the high-voltage coil and the shell also increases the voltage drop on the inductance and resistance of the grounding down lead. This voltage drop can not be ignored under the impact of lightning current, which greatly reduces its protection effect. The connection method in Figure 1 will also cause a problem, that is, the low-voltage coil and neutral line all bear the voltage drop on the grounding device, especially when there is repeated grounding at the neutral point, the grounding resistance is less than that of the distribution transformer, and it is close to the distribution transformer, the discharge impulse current of the lightning arrester at the high-voltage side will flow more to the repeated grounding, and sometimes the down lead of the repeated grounding will be burnt out (the repeated grounding wire is generally thin). Therefore, the connection method shown in Figure 3 is reasonable, the lightning protection for high-voltage coil is reasonable, and the impact on low-voltage neutral line is also small, because part of lightning current has flowed into the ground through the grounding device

1.3 as for the design of grounding device, according to the standard, the grounding device in the distribution transformer substation area should be laid as a closed ring with vertical grounding electrode. This is because the contact voltage in the ring is relatively low, and the step voltage of pedestrians walking along the ring grounding body is also small. Most distribution transformers in urban areas are installed at the roadside. Because people often walk around, they must be laid as a ring for the sake of pedestrian safety. The size of the ring is generally 5m in diameter, which is necessary to give play to the current dissipation effect of the horizontal grounding electrode and the vertical grounding electrode, reduce mutual shielding and reduce the grounding resistance. However, if some installation sites are too narrow, they can be oval, and the short wheelbase shall not be less than 3m. Two vertical grounding electrodes should be laid near the two ends of the short shaft. The high-voltage arrester, shell grounding and neutral point grounding should be respectively led near the vertical grounding electrode to facilitate current dissipation. If the soil resistivity is high and the test grounding resistance is unqualified after making a ring, a large ring shall be made outside the ring. The distance between the two rings is 4 ~ 5m, and the buried depth is deeper than the first ring. At least two parts are connected until the requirements are met

1.4 as for the connection mode of grounding down lead, according to the ministerial standard, except that the connecting terminal of the equipment can be connected by bolts, the down lead and grounding device shall be welded. However, for the convenience of installation, there is usually a disconnection card at 1.8 ~ 2.0m under the pole, which can also be connected by bolts. Flat steel is generally used for down lead, but steel strand is also used. The connection between steel strand and flat steel shall be made of terminal block, preferably connected with double bolts to facilitate good contact. At present, the actual situation is that the grounding terminal of the high-voltage arrester is connected with steel strands respectively, and the three steel strands are connected together, and all of them are twisted. The grounding wire of the distribution transformer shell is also twisted with the grounding wire of the arrester, and then connected with the lead-in wire of the grounding device with bolts. Some of them have not pressed the wiring nose. These connections do not meet the standard requirements, and there are too many connectors, resulting in poor contact. It is recommended to use 30 × The flat steel of 4 is connected into a whole, which is led down from the middle to connect with the grounding flat steel of the shell. Welding is adopted, and it is not suitable to set a disconnection card in the middle. However, the direct grounding and grounding device is a part that can store energy for welding. The low-voltage neutral point is directly led to the grounding device by flat steel and welded with it. The flat steel should be 30 × 40mm2。

1.5 for construction of grounding device, the underground horizontal grounding electrode of grounding device shall be 40 × 4 flat steel, L40 for vertical grounding electrode × 4. If the buried depth is greater than 60cm, clean original soil shall be used and compacted during filling. If possible, the area of the ring horizontal grounding electrode shall be appropriately increased, or a ring shall be made outside the ring, and the two parts shall be connected to reduce the grounding resistance to 1 Ω as far as possible. Underground connections shall be welded and meet the requirements. The lap length of flat steel shall be twice the width of flat steel, and shall be welded on three or four sides. When welding on three sides, try to have two short sides and one long side to facilitate the passage of current. The welding length of round steel is 6 times the diameter of round steel where the lead screw is the most severely worn part. Both sides shall be welded without virtual welding. Anti corrosion measures shall be taken at the welding position

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