Research and application of key technology of energy saving and capacity increasing for overhead conductors with carbon fiber composite core reinforced

Aluminum conductor composite core (ACCC) has many excellent comprehensive properties and is gradually popularized and applied in overhead transmission lines. As an energy-saving and environmentally friendly capacitance-increasing conductor, ACCC gives full play to the advantages of carbon fiber materials. This article analyzes the test results of ACCC and compares them with traditional aluminum conductor steel reinforced (ACSR) in terms of mechanical and electrical properties and proposes the development trend of ACCC.


Introduction
With the rapid growth of my country's economy, the electricity demand is rising rapidly.However, the construction of transmission projects is restricted by factors such as long construction periods, difficult land acquisition, and environmental protection.Especially in areas with tight land use, it is extremely difficult to build new transshipments for transmission network construction, and their manufacturing technology and industrial development are closely related to the country's transmission engineering construction and national economic development.On the premise of not building new lines or changing the existing network structure and ensuring the safe and reliable operation of the power grid, improving the conductor transmission capacity and easing the tight power supply through technological upgrading and transformation have become hot and difficult issues in the current transmission network construction industry.
To increase the transmission capacity of existing lines, the most economical and applicable way is to replace the conductors with capacity-increasing conductors without changing the existing conditions as much as possible.This can increase the transmission capacity of the original lines, which depends on the capacity of the capacity-increasing conductors: type and new set long-term operating temperature.For the reconstruction of lines, it is of considerable economic benefit to use the existing corridor conditions, only replace the conductors, and meet the design sag requirements to increase the transmission capacity.The ACCC emerged under these factors.As an energy-saving and environmentally friendly capacitanceincreasing conductor [1] , it has significant characteristics such as lightweight, high strength, large capacity, low line loss, low sag, high temperature, and corrosion resistance [2] .This article proposes the development trend of ACCC, analyzes its test results, and compares it with ACSR in terms of mechanical and electrical properties.

Characteristics of ACCC
Compared with traditional wires, ACCC has the advantages of high strength, low line loss, small sag, high-temperature resistance, large carrying capacity, lightweight, corrosion resistance, etc., realizing energy saving, environmental protection, and power transmission safety.Its specific characteristics are as follows.

Reinforced core with high strength
The tensile strength of general steel wire is 1240 MPa, high-strength steel wire is 1410 MPa and extrahigh-strength steel wire is 1770 MPa.The tensile strength of the carbon fiber composite core can reach 2600 MPa, which is 2.1 times, 1.8 times, and 1.5 times that of the previous steel wire [3] .ACCC and ACSR is shown in Figure 1.

Low line loss and high conductivity
Since overhead conductors with ACCC do not have the magnetic loss heat effect caused by steel wire materials and have lower operating temperatures under the same load transmission conditions, they can reduce power transmission losses by about 6% compared with ACSR [4] .In addition, the outer surface formed by the trapezoidal cross-section of overhead conductors with ACCC reinforced is much smoother than the surface of the ACSR, thereby increasing the surface roughness coefficient of the wire, which is beneficial to increasing the corona starting voltage of the wire, reducing corona losses and reducing corona noise and radio interference levels. [5]

Thermal expansion and elongation are small
The linear expansion coefficient of the carbon fiber composite core is low (1.6×10 -6 /℃), which is about 1/8 of the ordinary steel core [6] .Therefore, ACCC has significantly lower thermal expansion characteristics compared with conventional wires. [7]

Lightweight
The density of carbon fiber composite core material is small (1.9 g/cm 3 ), about 1/4 of that of ordinary steel core (7.78 g/cm 3 ) [8] .The mass per unit length of ACCC is about 60% to 80% of that of conventional conductors.

Sag characteristics of ACCC
Taking the common ACSR (JL/G1A-400/50) and ACCC (JLRX1/F2B-400/45) commonly used in overhead conductor lines as representatives (comparison of specific conductor parameters is shown in Table 1), according to the comparison with ordinary ACSR based on the principle of equal cross-sections of stranded conductor, a true type test of 200 m span is carried out under the following test conditions:      In Figure 4, the sag of the ACCC increases with the increase in temperature, and there is an obvious migration point temperature, which is around 70°C.It can also be seen from the current-sag curve in Figure 4 that as the current increases, the sag also increases, but after reaching a certain current, the sag no longer changes significantly.

Comparison of sag characteristics of ACSR / ACCC
In Figure 5, the ACSR has no temperature migration point within the test temperature range, while the ACCC has an obvious temperature migration point.ACCC has significantly lower sag characteristics than ACSR.Under the same test conditions, when the temperature rises from 25.6°C to 110°C, the sag of the ACSR increases from 3.6 m to 6.5 m, an increase of 0.8 times, while the sag of the ACCC only increases from 3.5 m to 5.3 m, an increase of 0.5 times; the change in the sag of the ACCC is 62% of that of ACSR.As the operating temperature increases, the sag of the ACSR will continue to increase.The migration point temperature of the ACCC is much lower than that of the ACSR, and above this temperature, the sag increases with the temperature.Significantly lower than ACSR, it can effectively reduce the insulation space in overhead line corridors and improve the safety and reliability of conductor operation.

Electrical properties of ACCC
Taking ASCR (JL/G1A-300/40) and ACCC (JLRX1/F2B-300/40) as representatives (see Table 2 for comparison of specific wire parameters), according to the wire parameters in Table 2, commonly used domestic calculation parameters are used to calculate the current carrying capacity at 70°C, 100°C, and 120°C, and the results are shown in Table 3 3, the current carrying capacity of ACCC of equal cross-section is equivalent to that of ACSR when the operating temperature is 70°C.However, when the operating temperature of the conductor increases to 120°C, the current carrying capacity of the ACCC is more than 1.45 times that of ACSR.As the ambient temperature increases, at 40°C, the carrying capacity of ACCC is more than 1.88 times that of ACSR.

Problems with ACCC
ACCC has obvious advantages over traditional conductors in terms of mechanical and electrical properties and has become an ideal new technological product in overhead transmission lines worldwide.At present, ACCC is used both at home and abroad.There are more than 1, 000 ACCCs in operation in the domestic network, and the number of line kilometers is about 20, 000 km. Rod-type composite cores have been used in China for many years and have accumulated rich experience.However, it has also been discovered that this reinforced core has difficult-to-overcome weaknesses, such as poor flexibility, easy to break, difficulty in construction and wiring, and difficulty hardware structures [9] .Many breakage accidents occurred during the application process of rod-type ACCC (Figure 6).Later studies attributed them to damage to the composite core caused by violent construction and fretting damage during operation.When the rod-type composite core is impacted by an external force (see Figure 7), the outer glass fiber and epoxy resin composite are shattered and peeled off from the matrix [10] .The cracks in the inner layer of carbon fiber and epoxy resin composite severely damaged the overall mechanical properties of the material, causing the rod-type composite cores to break under a load far lower than the calculated tensile breaking force, causing the conductor to fail.A stranded composite core has been developed in recent years to improve these weaknesses of the rod-type composite core (see Figure 8).Compared with the rod-type structure, it has the advantages of large bending radius, good compression resistance, and simple supporting hardware [11] .At present, overhead electrical strand conductors with reinforced stranded carbon fiber composite cores are used in Hainan, Japan, Southeast Asia, the European Union, and other places and have certain advantages in improving the safety and reliability of the power grid.
Since the overhead electrical strand conductors with stranded carbon fiber composite core reinforced is a new type of conductor that has appeared in recent years, current research on its performance at home and abroad mainly focuses on mechanical properties (high-temperature tensile strength test, tensile holding force test, and stress strain test), material properties (vitrification Transformation temperature, ultraviolet light, acid, alkali, and salt environment), etc.However, there has yet to be in-depth theoretical research on the changes in the mechanical properties of the carbon fiber composite core under different aging environments or multi-factor composite conditions, and the material and structure itself cannot explain it.The change in mechanical properties is because a single artificial accelerated aging test cannot fully simulate the conditions of carbon fiber composite cores under production, transportation, wiring, and operating conditions.If overhead electrical strand conductors with stranded carbon fiber composite core reinforced are to be used in overhead conductors in the future, basic theoretical research and testing must be strengthened, especially their online detection technology in production, online monitoring technology during wiring and operation, and re-establish user's confidence in its use [12] .a) Stranded composite core and stranded composite core conductors b) Metal-clad stranded composite core and metal-clad stranded composite core conductors

Conclusions
1) ACCC gives full play to the advantages of carbon fiber materials.They have corrosion resistance characteristics, high tensile strength, high elastic modulus, low sag, small thermal expansion coefficient, and lightweight.They are an excellent capacity-increasing conductor.
2) ACCC has good sag characteristics below and above the inflection point temperature.The sag is much smaller than ACCC at the same operating temperature.When the operating temperature exceeds the inflection point temperature, the sag has No changes.
3) ACCC of equal cross-section and ACSR can greatly increase the current carrying capacity under the same temperature conditions.In reconstructing old lines, not only can the capacity be doubled, but the weight of the conductors is also light.The existing line corridors can be used without replacing the poles and towers to increase the transmission capacity of the unit corridor.
4) A stranded composite core was developed based on the shortcomings of the rod-type carbon fiber composite core.However, in-depth basic theoretical research and online detection technology are needed for new stranded carbon fiber composite core conductors, which will help promote the application of carbon fiber composite core conductors and have important practical significance for power grid capacity expansion and reconstruction projects and new large-capacity lines.

Figure 2 .
Figure 2. Test site and test line segment structure.

3. 1 Figure 3 .
Figure 3.The temperature/current-sag curve of ACSR It can be seen from Figure3that the sag of the ACSR increases with the increase in temperature, which is a linear relationship, and there is no temperature migration point.

Figure 4 .
Figure 4.The temperature/ current -sag curve of ACCC.In Figure4, the sag of the ACCC increases with the increase in temperature, and there is an obvious migration point temperature, which is around 70°C.It can also be seen from the current-sag curve in Figure4that as the current increases, the sag also increases, but after reaching a certain current, the sag no longer changes significantly.

Figure 6 .
Figure 6.Core break of typical accidents.When the rod-type composite core is impacted by an external force (see Figure7), the outer glass fiber and epoxy resin composite are shattered and peeled off from the matrix[10] .The cracks in the inner layer of carbon fiber and epoxy resin composite severely damaged the overall mechanical properties of the material, causing the rod-type composite cores to break under a load far lower than the calculated tensile breaking force, causing the conductor to fail.

Figure 7 .
Figure 7. Rod-type composite core failure state.A stranded composite core has been developed in recent years to improve these weaknesses of the rod-type composite core (see Figure8).Compared with the rod-type structure, it has the advantages of large bending radius, good compression resistance, and simple supporting hardware[11] .At present, overhead electrical strand conductors with reinforced stranded carbon fiber composite cores are used in Hainan, Japan, Southeast Asia, the European Union, and other places and have certain advantages in improving the safety and reliability of the power grid.Since the overhead electrical strand conductors with stranded carbon fiber composite core reinforced is a new type of conductor that has appeared in recent years, current research on its performance at home and abroad mainly focuses on mechanical properties (high-temperature tensile strength test, tensile holding force test, and stress strain test), material properties (vitrification Transformation temperature, ultraviolet light, acid, alkali, and salt environment), etc.However, there has yet to be in-depth theoretical research on the changes in the mechanical properties of the carbon fiber composite core under different aging environments or multi-factor composite conditions, and the material and structure itself cannot

Figure 8 .
Figure 8. Overhead electrical strand conductors with stranded carbon fiber composite core reinforced of different structural types.

Table 3 .
Comparison of current carrying capacity test data.