In recent years, China’s wind power industry has been developing rapidly, and the total installed capacity and annual new installed capacity of wind power are in the forefront of the world. However, due to the harsh environmental conditions of wind farms, the cable terminals, cable bodies, etc. of the power collection lines in the site are prone to failure, so it is necessary to constantly use new materials and processes with advanced technology and reliable performance to improve the operation quality of the power collection lines for the safe and stable operation of wind farms.
The power cable is an important part of the collection line of the wind farm. Generally, there are two types of cable conductors for the wind farm: aluminum core and copper core. Aluminum core cable and copper core cable have their own shortcomings. The disadvantage of aluminum core cable is poor flexibility, and it is easy to break after repeated bending; Poor stability, easy to be corroded and oxidized; High resistivity, high energy consumption and large loss compared with copper cores of the same section; Poor ductility, inconvenient installation, etc. The disadvantage of copper core cable is its high price; Heavy weight; High construction and transportation costs.
Based on the above situation, domestic cable manufacturers have started to produce aluminum alloy cables. At present, there are more than 10 domestic enterprises producing aluminum alloy cables. In fact, as early as 1968, the American Southern Cable Company invented aluminum alloy cable. The United States, Canada and other countries have had more than 40 years of application history. Therefore, it is necessary to analyze the use of aluminum alloy cable products in order to improve the operation quality of the collection line of the wind farm and reduce the project cost.
Analysis of Characteristics of Aluminum Alloy Cable
The main components of the conductor of aluminum alloy cable are: copper, iron, silicon, magnesium, manganese, titanium, chromium, zinc, rare earth, etc. The role of copper increases the resistance stability of the conductor at high temperatures; The role of iron is to increase the creep resistance of conductor; The function of magnesium is to improve the tensile strength of conductor; The function of rare earth is to improve the corrosion resistance of conductor. 5083 aluminum alloy plate is also widely used in wind power generation and can be used as the pedal of wind tower
1、 Electrical performance of aluminum alloy cable
（1） Conductivity: The conductivity of aluminum alloy is between aluminum and copper, inferior to copper and slightly superior to aluminum.
（2） Cable compression characteristics: aluminum alloy conductor adopts layered compression stranding technology, and the compression coefficient of conductor can reach 0.93, while the compression coefficient of copper conductor is generally only 0.80. The maximum limit compaction can make up for the shortage of aluminum alloy conductor in volume conductivity, so that the stranded conductor core is like a solid core, which significantly reduces the outer diameter of the core and improves the conductivity. Therefore, when the ampacity is the same, the outer diameter of the cable after aluminum alloy cable replaces the copper core cable does not increase much.
2、 Mechanical properties of aluminum alloy cables
The elongation of hard pure aluminum is very low, and it is easy to be damaged or broken during repeated bending. The elongation of soft pure aluminum is much higher than that of hard pure aluminum, but its yield strength is only half that of copper, and its creep resistance is poor. After installation for a period of time, the connection is easy to relax, resulting in increased contact resistance, forming a hidden danger for safe operation. Through the comparison in Table 1, it can be concluded that the mechanical properties of aluminum alloy conductor have the following advantages:
（1） High extension. The elongation of aluminum alloy conductor after annealing can reach 30%, which is close to copper but much higher than aluminum. This also shows that aluminum alloy conductor can withstand greater external forces than aluminum conductor.
（2） Strong and flexible, easy to bend. During installation, aluminum alloy conductor has smaller bending radius than aluminum conductor, which makes it easier to connect terminal blocks.
（3） Good creep resistance. The creep resistance and compactness of the alloy elements in aluminum alloy have been greatly improved after special process treatment. When the conductor encounters extreme conditions such as cold flow, overload and overheating, stable connection can also be ensured.
3、 Corrosion resistance of aluminum alloy cable
The rare metals added in the aluminum alloy conductor further improve the corrosion resistance of metal materials with aluminum as the conductor in terms of chemical properties, and reduce the potential difference between different metals. The smaller the potential difference, the lighter the corrosion will be. The rare earth elements in alloy materials can play a role in filling surface defects, refining grains, and eliminating local corrosion of conductors. At the same time, they also bring negative electrode potential shift of aluminum, which has an anodic effect, thus greatly improving the corrosion resistance.
4、 Economic performance of aluminum alloy cable
Compared with copper cable, the price advantage of aluminum alloy cable is also obvious. The price of aluminum alloy cable with the same ampacity is only 60% – 70% of that of copper cable. In addition, due to the light weight, the installation cost is indirectly reduced.
Selection of 35kV cable conductor in wind farm
For 35kV high-voltage cables used in the wind farm, in order to save the project cost, aluminum core is preferred as the conductor. On the premise of meeting the voltage drop and the corrected ampacity, the economic current density is used to calculate the appropriate cable cross-section.
With the increase in the number of wind farms and the passage of operation time, the disadvantages of aluminum core cables gradually emerge, mainly in the following aspects:
1、 Poor mechanical strength, easy to break. The area where the wind farm is located is in a harsh environment with high wind speed, and the wind load on the cable after it is mounted on the tower is large. In particular, due to the influence of umbrella groups, the wind surface at the cable terminal is larger, so it is easier to have failures. The connection of cable and wire is shown in Figure 1.
2、 The creep resistance is poor. After the wind farm cable is installed on the tower, when the current passing through the conductor is too large, the aluminum conductor will become hot and creep, and the cable joint is easy to become loose and deformed, resulting in poor line contact, which will cause line and equipment failure accidents.
3、 High resistivity and high loss. The cable conductor section is selected for the wind farm. The aluminum core section is much larger than the copper core section. For example, if the cable section is too large and the cable length far exceeds the maximum production reel length of the cable manufacturer, the number of cable sections and intermediate joints will increase, which is unfavorable for the long-term operation of the collector line.
Based on the above reasons, the power cables in wind farms in recent years mainly use copper cores. Copper core cables can solve various problems of aluminum core cables. However, in recent years, China’s demand for copper materials has gradually increased, and copper prices have also been rising, which has virtually increased the cost of many projects and increased the risk of stolen cables at the construction site. In addition, due to the heavy weight of copper core cables, the requirements for cable supports are also high.
Compared with aluminum core and copper core cables, aluminum alloy cables do have many advantages and can replace them in many occasions. In addition, aluminum alloy conductors have been added to the GB/T3956-2008 Conductor of Cables. Therefore, it is theoretically feasible to use aluminum alloy cables in the collection lines of wind farms.
However, there are still some problems to be solved in the massive promotion of aluminum alloy cables in wind farms:
Aluminum alloy conductors have various models according to the chemical composition added, but there is no description of the added chemical composition in the domestic specifications, so the products are easy to be different in bidding and actual supply.
Due to the different expansion coefficients of aluminum alloy, aluminum and copper conductors, different conductors should not be directly connected. Transition terminals are required to ensure the reliability of the connection between aluminum alloy conductors and copper and aluminum conductors, which increases the risk of use.
There are few application cases of aluminum alloy cables used in wind farms. Whether aluminum alloy cables are more reliable than aluminum core and copper core cables in applications in special areas such as high temperature, cold, high altitude, high salt spray, and high temperature difference also requires time to test.
Epilogue
After the above analysis, aluminum alloy cable has good conductivity and excellent mechanical properties, which improves the unreliable connection, poor mechanical properties and easy creep of aluminum core cable. However, considering that there are still some problems of aluminum alloy cables that need to be improved and solved, it is suggested that the wind farm should try out aluminum alloy cables in a part of the collector line or in a certain transmission circuit according to the site conditions, especially in the technical transformation of the early wind farm. Through comparison under the same operating environment, we can know whether its line loss, failure rate and other aspects are better than copper core and aluminum core cables, It will provide practical experience for future promotion.
We have reason to believe that the use of aluminum alloy cables in specific occasions is the development trend of the cable industry in the future after time inspection and gradual improvement of relevant regulations and specifications.