During the 12th Five-Year Plan period, the State Grid Corporation of China increased its investment and built a "three horizontal and three vertical" high-voltage backbone grid connecting large energy bases and major load centers, as well as long-distance tributary transmission projects, initially constructing a core world-class robust and intelligent power grid. This requires power lines or cables to have long spans and to be able to perform online monitoring during power transmission, i.e., to play an early warning role. In view of this situation, this article discusses the cables used in long-distance tributary transmission projects.
I. Selection of Reinforcing Core
Since this is a long-distance, large-span overhead cable, our first consideration is that the cable must have high tensile strength. In ordinary conductors, we would use steel strand as the reinforcing core. However, in recent years, the various drawbacks of using steel strand as the reinforcing core have become increasingly apparent, such as heavy weight and poor corrosion resistance. After research and testing, we finally chose carbon fiber rods as the reinforcing core. These carbon fiber rods have the following advantages:
1. High strength. For the same cross-section, the strength of a carbon fiber core rod is twice that of a steel core, while its mass is only one-third that of a steel core;
2. Corrosion resistance. The carbon fiber core rod, as an insulating material, will not react with the aluminum conductor, thus avoiding the electrochemical corrosion problem between the aluminum wire and the core rod when energized.
3. Low cost. Due to its high strength, the laying span can be increased, the number of towers can be reduced, the laying efficiency can be improved, and the total cost of towers, accessories, etc. will be reduced.
However, carbon fiber mandrels also have their drawbacks, the most significant being their relatively low radial compressive strength. When twisted with ordinary round aluminum wire, the carbon fiber mandrel is easily subjected to radial "cracks" during compression, ultimately affecting its strength. Therefore, we adopted the following structure in the design, as shown in Figure 1:
This structure can reduce the radial pressure on the carbon fiber reinforcing core and increase the fill factor and improve the current carrying capacity while maintaining the same outer diameter.
II. Optical Unit
Intelligent overhead insulated cables primarily rely on optical fibers in the optical unit to transmit signals and sense changes in insulation temperature, so the optical fibers must be in good condition. We use a stainless steel tube type optical unit, mainly because this structure can be filled with grease to protect the optical fiber from corrosion, provides a larger excess length for the fiber, and offers radial compressive strength protection. Since no potential difference is generated when the stainless steel tube and aluminum wire surfaces are in contact, corrosion is not a concern. However, when the conductor is copper wire, a copper layer must be coated on the outside of the stainless steel tube to eliminate the potential difference, as shown in the figures below. Figure 2 shows the case with an aluminum conductor, and Figure 3 shows the case with a copper conductor.
III. Selection of Insulation
For overhead insulated cables, insulation plays a very important role. Commonly used insulation materials such as polyvinyl chloride and polyethylene cannot meet the harsh conditions of long-term exposure to sunlight, acids and alkalis. Therefore, this smart cable should use irradiated cross-linked polyolefin. This insulation material has excellent properties such as good heat resistance, insolubility, non-melting, non-flammability, high mechanical strength, heat deformation resistance, and good crack resistance.
IV. Finished Product Structure and Analysis
In this design, we will temporarily use aluminum or aluminum alloy as the conductor, as shown in Figure 4 below.
The reason for adopting this structure is:
1) Since the stainless steel tube is circular, the outer aluminum conductor must also be circular, and it cannot be tightly compressed during stranding;
2) Because the optical fiber in the optical unit needs to perform temperature measurement and insulation monitoring, the optical unit must be in contact with the insulation layer on the outer layer to constantly monitor the temperature of the insulation;
3) The inner layer uses profiled wires to protect the carbon fiber core rod, while also increasing the current carrying capacity of the conductor.
With the increasing level of industrial automation, the use of overhead cables is also growing, leading to a higher probability of cable fires. Currently, traditional cables are commonly used for power transmission. This article introduces the combined application of intelligent overhead cables and backend software. These cables can promptly sense changes in insulation temperature and conductor tension, issuing alarms to protect the lines. Furthermore, the intelligent overhead cable's optoelectronic composite structure avoids secondary construction, saving costs. I believe that with the deepening of industrial automation, the demand for this type of cable will increase significantly.
