Starting from the importance and special significance of rural power grid reconstruction, this paper focuses on the selection and installation of distribution transformers in rural power grid construction.
Keywords: network reconstruction and construction; 10kV distribution transformer; Selection and installation
With the rapid development of China's economy, power grid construction has also reached a new level. As an important part of the power grid, the number of transformers is also increasing rapidly. The installation of transformer is a complicated and important process, and the installation quality directly affects the safe and stable operation of transformer. Therefore, how to choose distribution transformers reasonably and install them correctly is also a key problem to be solved in the design and construction of rural power grid reconstruction. According to the practice of participating in rural power grid transformation and referring to the relevant electric power technical regulations, I put forward the following opinions on the installation of transformers for reference.
I. Location of 10kV distribution transformer station area
Rural distribution transformer substation should be constructed and reconstructed according to the principle of "small capacity, dense point and short radius". The transformer should be installed near the load center or important load as far as possible, and at the same time, it should avoid places where there are many vehicles and pedestrians and it is convenient to replace and maintain equipment. The best location refers to the location that can minimize the line loss, investment and consumption of low-voltage lines. Before site selection, the existing and future loads in 10 should be fully investigated and predicted, so that the installation position of distribution transformer is located in the load center. So that the investment of the low-voltage power supply line is minimal, the voltage drop is minimal, and the loss of the low-voltage power supply line is small. This is closely related to the economic benefits of the power supply unit itself and reducing the burden on farmers. The regional power supply radius of the reformed low-voltage substation is generally less than 300m, which not only reduces the line loss, but also improves the voltage quality.
In a word, the selection of installation location of distribution transformer is related to ensuring low voltage quality, reducing line loss, safe operation, reducing project investment, facilitating construction and not affecting the city appearance. We should proceed from reality and consider comprehensively.
Second, 10kV distribution transformer model selection
Before the network reform, SJ series transformers with high loss were mostly used for power supply, and the loss ratio was significant. In recent years, there are three new types of energy-saving transformers newly developed in China: S8 and S9, S 1 1.
S9 series distribution transformers reduce losses by increasing the number of effective materials, mainly by increasing the cross-sectional area of the iron core to reduce the magnetic flux density. Both high and low voltage windings adopt copper wire, which increases the cross-sectional area of the wire to reduce the winding current density, thus reducing the no-load loss and load loss.
Compared with S7 series transformers, S9 has an average reduction of 10% in no-load loss and 25% in load loss. S 1 1 series transformers are developed on the basis of S9 series by improving the structural design and selecting ultra-thin silicon steel sheets to further reduce the no-load loss. At present, the no-load loss of S 1 1 series transformers is 30% lower than that of S9 series, but the investment is relatively high. Therefore, from the point of view of cost performance, S9 low-loss transformer should be generally selected for new or modified transformers. The original high-loss distribution transformers have all been eliminated, and S7 series distribution transformers have also been replaced.
III. Capacity Selection of 10kV Distribution Transformer
In the past, when selecting distribution transformers, due to the lack of scientific analysis and calculation, the phenomenon of "big horse-drawn car" generally existed. There is no scientific basis for selecting transformer capacity only according to the number of users, and the phenomenon of "big horse-drawn car" appears when the capacity is too large, which not only increases one-time investment, but also increases no-load loss. If the selected capacity is too small, the transformer will be overloaded, the overload loss will increase, and finally the transformer will be burned. Therefore, when selecting the distribution transformer capacity, it should be selected according to the actual load and the power development plan of 5 ~ 10 years, which is generally 45% ~ 70% of the transformer capacity. In addition, considering that rural areas have their own electricity consumption characteristics, they are affected by seasonality, timeliness and large fluctuation of electricity load. Conditional villages can use mother-child transformers or capacity-regulating transformers to supply power to meet the needs of different seasons and different times.
Four. 10kV distribution transformer installation
10KV distribution network is equipped with a transformer on the pole. Generally, the maximum capacity is controlled at or below 400KVA, the center distance between the two poles is 2.5m, the inclination of the transformer on the pole is not more than 20mm, and the distribution transformer bracket is fixed on the two poles with two [12×3000] channel steels. The bracket is not less than 3m from the ground, and the horizontal inclination of the bracket should not be greater than/kloc-of the bracket length. The feet and workbench of the transformer are tightened with 4 screws, and the high and low voltage columns of the transformer should be equipped with dust covers, and warning signs should be hung on the transformer. In addition, the installation of iron pieces should be galvanized, and transformers above 100KVA should be equipped with disconnectors.
Verb (abbreviation for verb) Install a drop fuse.
The high and low voltage sides of the distribution transformer shall be equipped with fuses. The vertical distance between the bottom of the high-voltage fuse and the ground is not less than 4.5m, and the horizontal distance of each fuse is not less than 0.5m. In order to facilitate the operation and the fuse tube can fall smoothly after the fuse is blown, the axis of the drop fuse should form an angle of 15% ~ 30% with the vertical. The vertical distance between the bottom of the low-voltage fuse and the ground is not less than 3.5m, and the horizontal distance of each fuse is not less than 0.2m.
The fuse of the drop fuse switch should be selected according to the principle of "short circuit in distribution transformer or high and low voltage outlet pipe can be blown quickly", and the blowing time must be less than or equal to 0.1s. When the capacity of distribution transformer is below 100kVA, the rated current of high-voltage side fuse is 2 ~ 3 times of the rated current of transformer capacity; When the capacity is above 100kVh, the rated current of high-voltage fuse should be 0.5 ~ 2 times of the rated current of 65438+ transformer capacity. The fuse on the low voltage side of the transformer is selected according to the rated current of the low voltage side.
Installation of low-voltage JP cabinet with intransitive verb
Because the low-voltage JP cabinet integrates power distribution, metering, protection (overload, short circuit, leakage and lightning protection) and capacitor reactive power compensation, it provides a guarantee for safe electricity use. Therefore, since the rural power grid transformation, a large number of JP cabinets have been used in the IOKV distribution station area, and their selection and installation requirements are as follows:
(1) The capacity of JP cabinet must match the capacity of transformer.
(2) The JP cabinet installed on the bracket of the angle steel (2L70*7*3000) at the lower part of the pole frame transformer must be installed firmly, and the horizontal inclination is less than1100 of the bracket length.
(3) The lead wires are well connected and have waterproof bends.
(4) Insulators shall be in good appearance, clean and without leakage.
(5) The opening and closing actions are correct, reliable and clear.
(six) low voltage cable inlet and outlet line installation is reliable. And it can prevent small animals from entering and leaving, causing short circuit in the cabinet.
(seven) low voltage insulation lead installation is reliable.
(8) The door of 8)JP cabinet must be tightly closed to prevent rainwater from entering the cabinet, causing electrical short circuit or insulation breakdown and leakage.
Seven, lightning arrester. device
Operation experience has proved that most external hazards affecting the safe operation of distribution transformers come from lightning strikes. Therefore, transformers should be equipped with lightning protection devices. The metal oxide arrester with seamless synthetic insulating sheath is selected to replace the original valve porcelain sleeve arrester, which has the characteristics of strong power frequency withstand voltage, good sealing performance and stable protection characteristics.
The high-voltage arrester should be installed between the high-voltage fuse and the transformer, as close as possible to the transformer, but it must keep a distance of more than 0.5m from the end cover of the transformer. This not only reduces the influence of the down conductor inductance on the distribution transformer when lightning strikes, but also avoids the power failure of the whole line during the maintenance and repair of the lightning arrester, and also prevents the lightning arrester from exploding and damaging the transformer porcelain bushing. In addition, in order to prevent the invasion of low-voltage inverse transformation wave and low-voltage side lightning wave, low-voltage arrester should be installed in the low-voltage side distribution box to protect the distribution transformer and its total metering device. Lightning arresters shall be connected by multi-strand copper-core plastic wires with a cross section of not less than 25mm2. In order to prevent the voltage drop of lightning current on the grounding resistance and the residual voltage of the lightning arrester from superimposed on the transformer insulation, the grounding terminal of the lightning arrester, the transformer shell and the neutral point on the low-voltage side should be connected together by a plurality of copper-core plastic wires with a cross section of not less than 25mm2, and then connected with the lead of the grounding device.
Eight, grounding device
At present, in the rural power grid transformation, there are many small-capacity transformers in rural areas, and 10kV distribution transformers are often struck by lightning in thunderstorm season. If the grounding resistance is too large, it can't reach the specified value in the code, and the lightning current can't leak into the earth quickly, resulting in the high residual voltage of the arrester itself, or a high voltage drop on the grounding resistance, resulting in a transformer burning accident. Therefore, the grounding resistance of grounding device must conform to the value specified in the regulations. 100kV distribution transformer: the capacity is 100kv. A and below, the grounding resistance should not be greater than10q; ; The capacity is above 100kVh, and its grounding resistance should not be greater than 4Q. Grounding resistance test should be carried out after grounding device construction, and backfilling can be carried out only after it is qualified. At the same time, transformer shell must be well grounded, and the shell grounding should be fastened with bolts, and direct welding maintenance is not allowed.
The underground part of grounding device consists of horizontal grounding body and vertical grounding body. Generally, the horizontal grounding body adopts four 40mm×4mm long 5m flat steel, and the vertical grounding body adopts five 50mm×50mm×5mm long 2.5m angle steel, which are welded with the horizontal grounding body every 5 m. ..
The horizontal grounding body is buried in the soil to a depth of 0.6~0.8m, and the vertical grounding body is driven into the ground on the basis of the horizontal grounding body. 40mm×4mm flat steel shall be used for grounding lead. In order to facilitate the detection and safe use of electricity, the lead connection point of the transformer installed on the column should be located at the channel steel position below the transformer.
Nine, transformer area drop line
Multi-strand insulated conductors should be used for the down lead of newly-built and reconstructed distribution transformers, and the cross section should be selected according to the rated capacity of the transformer. However, the copper core and aluminum core of the high-voltage side down conductor shall not be less than 65438+25mm2, and single conductor and unqualified conductor shall not be used. At the same time, the safe distance between the landing line and the surrounding buildings should also be considered.
The distance between high-voltage down lead and hoop, palm iron, pole and transformer shell shall not be less than 200mm, the distance between high-voltage down lead and lead shall not be less than 300mm, and the distance between low-voltage down lead and other objects shall not be less than 150mm.
In recent years, with the implementation of rural power grid reconstruction project, the distribution network structure in our city is becoming more and more reasonable, and the distribution network facilities have been greatly improved, making the power grid realize reasonable structure, safe and reliable power supply and economical operation.
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