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Design Features Of Overhead Power Lines In Distribution Networks

Pulished on Jan. 18, 2020

The appearance of high-voltage transmission lines and low-voltage distribution network lines is very different. High-voltage transmission lines mainly use truss towers, and distribution networks use a large number of concrete poles and auxiliary cable systems. Generally, lightning protection lines are not installed on distribution lines.

Distribution network lines are generally composed of wires, electric insulation fittings, crossarms and supporting iron accessories, concrete poles, cables, chassis, chucks, etc. Taking a straight single rod with a pull line as an example.

1) Lead:

In general, bare wires can be used. They can be used in forest areas and towns. Insulated wires can be used. The wires are divided into steel core and non-steel core.

2) Insulator:

Common insulators are pin insulators, suspension insulators, dish insulators, porcelain line post insulators, and post insulators.

3) Crossarms:

Angle steel products are divided into single angle steel cross arm and combined angle steel cross arm.

4) Pull line:

Commonly used GJ-50, GJ-70, GJ-100 galvanized steel stranded wire, single or double drawn wire, used with the pull disk.

5) Concrete pole:

According to the shape, it is divided into tapered rod, equal diameter rod and square rod; according to the form of reinforcement, it is divided into ordinary concrete rod, prestressed rod and partial prestressed rod; tapered rod, equal diameter rod is divided into A, B, C ... K, L, M ... and other series, used with bottom and chuck according to geological conditions.

3. Brief design steps

1) Determine the wire type

The basis of selection is the transmission capacity of the line, the cross section of the conductor is determined according to the ampacity, and the type of the conductor is determined according to the weather and terrain conditions. Special conditions such as high wind, thick icing, large pitch, and large height differences should be considered with steel core conductors. The cross-sectional area of the conductors should not be too small.

2) Determine the route of the route

Most of the distribution network lines are short lines, close to the load center, but affected by the village and the plant. These factors must be considered, and then determine the path plan with small impact, short path, small footprint, and easy implementation.

3) Use standard design rod type

In order to improve the efficiency of design and review, and facilitate the bidding, procurement, processing, construction, and operation and maintenance of materials, the provincial company promotes standardized design. Before starting the design work, you should be familiar with the design results of each module of the standard design. Select the appropriate standard design. There are several aspects to consider.

(A) Select a suitable pole height. Take the 10kV line as an example. In the case of maximum sag, the height of the conductor to ground should be greater than 6.5 meters, and the vertical distance from the road surface should be greater than 7.0 meters. 2.0 meters, the minimum vertical distance from buildings and trees is not less than 3.0 meters. Assume that the height of the head is 0.6 meters and the arc is 2.0 meters. The buried part should be at least 1/6 of the full height of the pole. The height above the ground of the cross pole should be at least 7 + 2 + 0.6 = 9.6 meters and the total height should be greater than 9.6 * 6/5 = 11.52 meters, you can choose 12 meters pole.

(B) Select the appropriate crossarm length according to the distance and arc sag. The larger the sag, the larger the distance between the wires. The "Design Specification for Overhead Power Lines of 66kV and Below" (GB50061-2010) 7.0.3 There are related regulations for the distance between lines. The standard design of the South China Network is divided into two design series according to whether the pitch is greater than 120 meters, which are the small pitch series and the large pitch series, and the corresponding design standards are applied.

4) Determine the safety factor of the wire

The code stipulates that the maximum use tension of the wire or ground wire should not be greater than 40% of the instantaneous breaking tension of the stranded wire, that is, the safety factor should be greater than 2.5. According to the sag formula, the sag f = K * L2, the sag has a square relationship with the pitch, and the small pitch has a relatively small impact on the sag. The template K value K = γ / 8σ, the sag is inversely proportional to the tension. The impact is relatively small. Considering that the distribution network lines are mostly close to residential areas and densely populated places, it is recommended to use a large safety factor for small pitch line conductors to relax the tension of the conductors and to reduce the longitudinal load of the conductors and reduce the investment in tensile towers. the goal of.

5) Design pull line

The pole is a slender structure. The structural characteristics of the pole determine its bearing capacity. A prominent feature of the distribution network is that a large number of cables are used because the concrete rod cannot bear excessive bending moments. For corner cables, terminal cables, windproof cables, etc., to achieve the purpose of balancing the longitudinal tension of the cables, the cables should be steel strands, and the safety factor should be selected according to the design code 8.2.5.

4.Insulator selection

10kV line straight poles generally use pin insulators and post insulators. Tensile poles use tension insulator strings composed of suspension insulators. 400V line straight poles and small corner poles use dish insulators, and tension rods use suspension insulators or dish insulators.

5.Bottom and chuck design

Both the chassis and the chuck have corresponding standard designs. According to the geological conditions, it is determined whether or not to choose and how large the chassis and chuck are.

Design Features Of Overhead Power Lines In Distribution Networks

Porcelain Line Post Insulators