With the improvement of living standard, people put forward higher requirements for power quality, especially for reliability.
Increase the power capacity of unit corridor area transmission and reduce the area of line corridor to save line investment;
Applying new technology to improve the reliability of equipment to adapt to the new changes in the development of power system is a new task faced by power planning and design.
Path and bar type
A limited urban corridor rarely uses a single loop, while a double or multiple loop requires equal half-corridor widths on both sides of the line center.
Therefore, the overhead lines should be set up along roads, canals and green belts, and the semi-corridor on the side of roads and green belts can be freely used, which is also easy to agree with the urban planning department.
In some cases, the single-sided three-phase vertical arrangement of the rod type, although only set up a circuit, but compared with the cable line, the benefit is very considerable.
Narrow path corridor, prompted by the emergence of steel pipe rod, it can technically meet the requirements of transmission lines, such as the price comparable with Angle tower, its application will be more extensive.
The steel pipe rod is not only beautiful in shape, quick in installation, and covers an area of less area, which meets the high requirements of modern urban environment for overhead lines, but also ADAPTS to the characteristics of relatively flat urban terrain, small width of corridor and inconvenient line construction.
Since the 90 s, the domestic steel tube pole factory, material from centrifugal concrete thin-wall steel tube pole to pure steel tube pole, look from the cone to the polygon, the connection from welding, flange connection to plug type, galvanized anti-corrosion spray from the hot dip galvanized, its manufacturing technology is also increasingly to the reliable quality, beautiful appearance, convenient in the direction of development.
Recently, the Technical Provisions for the Design of Steel Pipe Rod of Transmission Line (Draft for Comments), edited by Northeast Electric Power Design Institute, has been delivered to the discussion as a standard of the electric power industry.
In the future, steel pipe rod will be the main type of urban overhead transmission line.
SDJ3-79 Technical Specification for Design of Overhead Transmission Lines stipulates that the minimum horizontal distance between the outer edge of poles and towers and the edge of roadbed is 5-6m in areas with limited paths, but it is actually difficult to implement because the center line is often arranged in the green belt of the road, and even the distance between the edge of the sidewalk and the edge of the fast lane is less than 5m.
Therefore, the steel pipe guard pier (pipe diameter 0.1m, height 1m) can be buried in the Angle steel tower leg or the side of the steel pipe rod near the fast lane to reduce the damage caused by the automobile collision.
Safety factor of insulated conductor
According to Technical Specification for Design of Overhead Insulated Distribution Lines, the designed wind speed is 25m/s, the design safety factor of the conductor should not be less than 3, and the gap between medium and low voltage lines in urban network is generally not more than 50m.
Considering the large cross section of conductor used in urban network, the safety factor of conductor is very important to the economic cost of the project, the safety of operation and maintenance and the breakage accident caused by stress concentration.
Lightning protection of insulated wire
In the early 1960s, insulated wires began to be used in France, Japan, Australia and other countries, which has more than 40 years of experience.
According to relevant data, in the early 1990s, Japan replaced all the overhead bare stranded wires with overhead insulated wires, and the number of lightning breakage accidents on the lines increased.
When lightning flashover occurs between two or three phases, the power frequency current tends to concentrate on the breakdown point of the insulation layer, thus making the insulated conductor fuse before the circuit breaker tripping.
The overhead insulation line is vulnerable to damage when vibration and lightning strike occur. Most of the damage occurs at the fixed place of the insulation line on the pole, which is 200-500mm away from the pole.
City network insulation line lightning protection, generally use the installation of lightning arrester method, according to domestic data, that in heavy minefields, should be installed every 360m each phase of the lightning arrester.
And Japan's famous lightning protection expert hengshan Shigeru believes that lightning arresters should be installed at intervals of 200m each phase.
The selection of tension-resistant metal fittings has a certain influence on the occurrence of wire breakage.
Insulated wire resistance has two kinds of peeling wire clip and non-peeling wire clip.
Jiujiang power supply bureau is mainly produced by Hongdu wedge self-locking peeling wire clip.
The first reason to use this type of wire clip is to consider lightning protection and stress concentration caused by wire breakage.
No stripping wire clip, tensile outlet for hard plastic wedge type splint, splint at the surface of the insulation wire, because of the grip force caused by the insulation layer plastic creep damage, coupled with its structure has a parallel sharp Angle will produce induced overvoltage, causing lightning break line.
When wires near the ground or tree was struck by lightning retaliate voltage will be produced on the wire, not peeling clamp and wires are insulated, insulated wires under high residual pressure and break wire, peeling clamp under the residual pressure of resistance by gold wire clip with your bear, installed on the straight needle bottle with long interval of lightning arrester or install flashover arrester is the same principle.
That's the second reason.
The third reason is that we have been tested on the peeling tension clamp and the non-peeling tension clamp respectively. The insulation wire of 240mm2 without steel core gradually increases the tension, and the peeling tension clamp always has no cramp phenomenon, until the tension exceeds 3.5t, the wire is destroyed;
When the tension clamp is not peeled, the insulation is separated from the aluminum core wire and the cramp occurs when the tension reaches 1.7T.
This shows that the peeling tension clamp for tension, in the case of over-traction force, always maintain a tight state, than the non-peeling tension clamp is much better.
In operation, due to the difference in the actual force when the wire is tightened, when the non-peeling wire clamp is used, the insulation layer creep is easy to break the wire accident.
4 conclusion
The contradiction between overhead line and corridor is the inevitable of urban development. In the design of urban overhead line, the range and the complex and changeable site conditions to be considered are also different from the previous concept of overhead line.
Under the condition of satisfying the reliability requirements, planners and designers should try their best to control the action range of overhead lines and fully limit the width of corridors so as to tap the potential of corridors.