Abstract: This paper describes the characteristics and experience of urban high voltage overhead transmission line design, and puts forward the development trend of urban power grid.
Key words: urban overhead transmission line; power corridor; insulated conductor
With the improvement of living standards, people put forward higher requirements for power quality, especially for reliability. It is a new task for power planning and design to improve the power transmission capacity per unit corridor area, reduce the floor area of line corridor, and save line investment; to apply new technology to improve the reliability of equipment to adapt to the new changes of power system development.
1 path and rod type
The limited corridors in the city rarely use single loop, while double loop or multi loop need equal half corridor width on both sides of the line center. Therefore, the overhead lines should be erected along roads, canals and greenbelts. The half corridor on the side of roads and greenbelts can be used freely, and it is easy to agree with the urban planning department. In some cases, single side three-phase vertical arrangement of the pole type, although only one circuit of the line is erected, but compared with the cable line, the benefit is very considerable.
Narrow path corridor promotes the emergence of steel pipe pole, which can meet the requirements of transmission line in technology. If the price is comparable with angle steel tower, its application field will be more extensive. The steel pipe pole not only has the advantages of beautiful appearance, fast installation and less floor area, which meets the high requirements of modern urban environment for overhead lines, but also adapts to the characteristics of flat urban terrain, small corridor width and inconvenient line construction. Since the 1990s, the domestic steel pipe rod manufacturers have been emerging. The material is from thin-walled centrifugal concrete steel pipe rod to pure steel pipe rod, the appearance is from conical to polygonal, the connection method is from welding, flange connection to plug-in, and the anti-corrosion is from spray galvanizing to hot galvanizing. The manufacturing process is also developing towards the direction of reliable quality, beautiful and convenient. Recently, the "technical regulations for design of steel pipe poles for power transmission lines (Draft for comments)" edited by Northeast Electric Power Design Institute has been submitted for discussion as a power industry standard. In the future, steel pipe pole will become the main pole type of urban overhead transmission line.
"Sdj3-79 technical specification for design of overhead transmission line" stipulates that the minimum horizontal distance from the outer edge of the tower to the edge of the subgrade is 5-6m in the restricted area, but it is difficult to implement in practice, because the center line of the line is often arranged in the road green belt, even the edge of the sidewalk, and the distance from the edge of the expressway is less than 5m. Therefore, steel pipe piers (0.1M in diameter and 1m in height) can be embedded on the side of angle steel tower legs or steel pipe poles near the fast lane to reduce the damage caused by vehicle collision.
2. Safety factor of insulated conductor
According to the technical specification for design of overhead insulated distribution lines, the design wind speed is 25m / s, the design safety factor of conductor should not be less than 3, and the span of medium and low voltage lines in urban network should not exceed 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 disconnection accident caused by stress concentration.
3. Lightning protection of insulated conductor
In the early 1960s, insulated conductors were used in France, Japan, Australia and other countries, which has more than 40 years of experience. According to relevant information, in the early 1990s, Japan has replaced all overhead bare stranded wires with overhead insulated wires, resulting in an increase in lightning line breaking accidents. When lightning flashover occurs between two or three phases, the power frequency current tends to concentrate on the breakdown point of the insulation layer, so that the insulated conductor will fuse before the circuit breaker trips. Overhead insulated lines are easy to be damaged in case of vibration and lightning strike. Most of the damaged parts are located at the fixed place of insulated lines on the pole, 200-500mm away from the pole. According to the domestic data, it is considered that the arrester should be installed every 360m in each phase in heavy lightning area.
Yokoyama Mao, a famous lightning protection expert in Japan, thinks that arresters should be installed in each phase with an interval of 200m. The selection of strain fittings has a certain influence on the occurrence of disconnection accident. There are two kinds of tension metal for insulated conductor: stripping clamp and non stripping clamp. Jiujiang Power Supply Bureau mainly adopts wedge self-locking stripping clamp produced by Hongdu. The first reason for using this kind of clamp is to consider the disconnection caused by lightning protection and stress concentration. There is no peeling clamp. The tension outlet is made of two hard plastic wedge-shaped splints. The surface of insulated wire at the splint is damaged by plastic creep of insulating layer due to grip force. In addition, its structure has parallel sharp angle, which will produce induced overvoltage and cause lightning break. When the ground or trees near the conductor are struck by lightning, the counterattack voltage will strike back on the conductor. The wire without peeling is insulated from the conductor. The insulated conductor will bear high residual voltage and break the conductor. The residual voltage borne by the stripping clamp is borne by the strain clamp itself. It is the same principle to install the lightning arrester with interval or long flashover arrester on the straight wire rod bottle. This is the second reason. The third reason is that we have tested the stripped tension clamp and non stripped tension clamp respectively. When the tensile force of 240mm2 insulated conductor without steel core is gradually increased, there is no cramp in the stripped tension clamp until the tensile force exceeds 3.5t, and the conductor is damaged; when the tensile force of non stripped tension clamp reaches 1.7t, the insulating sheath is separated from the aluminum core, and the cramp occurs. This shows that it is much better to keep the tension clamp in a tight state when the tension clamp is peeled under excessive traction force than the tension clamp without peeling. In operation, due to the different actual force when the wire is tightened, when the wire clamp is not peeled, the insulation layer will creep, and the wire breaking accident is easy to occur.
Conclusion
The contradiction between overhead lines and corridors is inevitable for urban development. In the design of urban overhead lines, the complexity of the scope and site conditions to be considered is also different from the previous concept of overhead lines. Under the condition of meeting the reliability requirements, planners and designers should try every means to control the scope of action of overhead lines and fully limit the corridor width, so as to tap the potential of the corridor.