1. Aluminum alloy cable
Aluminum alloy power cable is a new type of material power cable created by advanced technology such as AA8030 series aluminum alloy material as the conductor, using special pressing technology and annealing treatment. The copper cable is a power cable that uses copper as a conductor, which is currently used by the general public.
As my country's economy enters the new normal of development, aluminum alloy cables occupy a certain share in the electrical market. Relevant media and companies have put forward a comprehensive promotion of "aluminum instead of copper" in the field of power cables, and named the future "development direction" as a "strategic measure" to solve the shortage of copper resources. This article mainly conducts a horizontal comparison between aluminum alloy cables and copper cables in eight aspects, and combines actual application conditions to rationally and prudently look at the replacement of copper and aluminum.
2. Realistic comparison of copper and aluminum mineral resources
The relative shortage of copper resources in my country is an objective fact, but aluminum resources are actually not rich. The so-called "rich in aluminum and lack of copper" is misleading propaganda that does not conform to the national conditions. As we all know, the domestic aluminum processing industry has long been included in the national restriction to eliminate backward production sequence due to serious overcapacity. The aluminum-silicon ratio of domestic bauxite ore is relatively low, and the production cost of alumina is relatively high. The over-exploitation of aluminum ore has made the domestic reserve-production ratio only 6.6 years, which is far lower than the 10-year reserve-production ratio of the petroleum industry. Obviously, the reserves cannot meet the needs of rapid economic growth. However, in order to maintain the huge aluminum production capacity, it is necessary to purchase a large amount of aluminum ore resources from overseas. It is expected that the overseas dependence of aluminum ore will reach more than 60% by the end of the 13th Five-Year Plan. From a long-term perspective, objectively there is a systemic risk of shortage of aluminum resources. On the other hand, copper is a 100% recyclable metal, and the domestic reserve and production ratio is 16 years old. If you can take advantage of the current foreign exchange reserves and low-priced copper mines to purchase a large number of foreign copper resources, you will undoubtedly "store copper in the country" It has more prominent strategic significance.
Third, the performance comparison of aluminum alloy cable and copper cable
Aluminum alloy cable is a form of cable that adds trace elements as conductors on the basis of ordinary aluminum. Although it is an upgraded version of pure aluminum cable, it still has obvious comparative disadvantages compared with copper cable.
(1) The conductivity of aluminum alloy cables is poor.
The conductivity of aluminum alloy cables is only 61% of that of copper cables. Under the same cable cross-section, too large resistance will inevitably cause high line loss and reduce energy efficiency. Under the same current carrying capacity, the electrical resistivity of aluminum alloy cables is always slightly higher than that of copper cables. Taking the load current of 380A, the annual utilization hours of 4500h, and the operating life of 30 years as an example, if the copper cable cross-section is 150mm2, the aluminum alloy cable cross-section needs 240mm2. The resistivities of the two are 0.148/km and 0.150/km, respectively. The energy consumption is 288495kwh/km and 292410kwh/km, and the energy consumption difference between the two during the whole life cycle is 117450kwh/km. Obviously, the loss of the aluminum alloy cable during the whole life cycle is too large
(2) The current carrying capacity of the aluminum alloy cable is low.
The power supply reliability of the urban power grid is required to reach 99.99%, and the core area needs to reach a higher level of 99.999%. As the urban cable network adopts a ring network structure, the protection will act in a short time in the event of a fault, and the load will be quickly switched to the opposite line to ensure uninterrupted user power supply. But to achieve high reliability of the power grid, a complete network structure, excellent equipment and lines are indispensable. The power supply lines in the power grid must have a high current carrying capacity, and in addition to its own load, it can also undertake temporary switching loads. The copper core cable of the same cross-section has a current carrying capacity more than 30% higher than that of the aluminum alloy cable, which obviously can better meet the requirements of urban power supply reliability.
(3) Aluminum alloy cable has low mechanical tensile strength.
The tensile strength of aluminum alloy cables is only 46% of that of copper cables, and the allowable traction is 60% lower than that of copper cables. The urban power distribution network adopts a large number of cable ring network structures, and the planning and design considers to minimize the use of cable intermediate joints. In actual use, the laying length of a single copper cable is generally between 600 and 800 meters. Considering that under the same current carrying capacity, the laying length of a single ordinary aluminum cable is only 500 meters. Considering the influence of traction, the laying length of a single aluminum alloy cable is only 350 meters. Obviously, the low tensile strength will inevitably lead to the limitation of the length of a single traction cable, and a large number of additional intermediate joints need to be added, which increases the risk of subsequent operation and maintenance.
(4) Aluminum alloy cable has weak corrosion resistance.
The corrosion of the cable conductor is mainly the electrochemical corrosion of the metal, that is, the electrolytic cell effect caused by the galvanic cell or the interference of stray current on the metal surface. In order to improve the creep resistance of aluminum alloy cables, elements such as magnesium, copper, zinc and silicon are added in the production process, and heat treatment processes are added. Due to the complex operating conditions of the cable, in an environment containing electrolyte, there is an electrode difference between aluminum with a lower electrode potential and other added metal elements, thereby forming a current path, causing electrochemical phenomena such as pitting corrosion and crevice corrosion. The heat treatment process of aluminum alloy cable is also easy to cause uneven physical state of the conductor surface, increasing the possibility of electrochemical corrosion, and then stress corrosion cracking and intergranular corrosion.
(5) Aluminum alloy cable has poor high temperature resistance.
The melting point of copper is 1080, while the melting point of aluminum is only 660. Obviously, copper conductors are a better choice for fire-resistant cables. In the event of a fire, the central ambient temperature can rise above 750, and the cable must be able to maintain the basic functions of energization to build a lifeline. Obviously, when the temperature of the fire site is higher than the melting point of aluminum alloy and aluminum, no matter what heat insulation measures are taken, the cable conductor will melt in a short time and lose its conductive function, which will seriously affect the safe evacuation of personnel on the fire site.
(6) The risk of failure of aluminum alloy cable joints is high.
Cable operation experience shows that 80% of failures occur at the joints. Copper has the superiority that aluminum and aluminum alloy can't match. The copper oxide formed by the oxidation of the copper joint is a good conductor, and it can still guarantee the electrical connection performance of the joint and the terminal. The aluminum oxide produced by the oxidation of aluminum and aluminum alloy joints is an insulator, and its hard texture and strong adhesion make it difficult to form good conductive contacts and easily cause contact heating. The terminals of electrical equipment mostly use copper connectors, and the use of aluminum alloy cables will form a copper-aluminum connection. The thermal expansion coefficient of aluminum alloy is much higher than that of copper. Since the grid operation always has a peak-to-valley difference, when the load changes significantly, the temperature changes rapidly, and a large lateral movement occurs in the contact area, which cuts off the effective connection of the metal contacts, increases the contact impedance, and causes the temperature at the connection to rise. When cooling, the thermal stress changes again, and the interfacial shear action is further formed. Under the repeated action of long-term cold and heat, when the thermal stress is greater than the yield force of aluminum, irreversible plastic deformation will be formed in the contact area, accelerating the loss of the joints, and finally connecting failures. Aluminum alloy conductors are more likely to have poor contact after thermal expansion and contraction. The vicious circle in the contact area has created a huge test for the safe operation of the joint.
(7) Aluminum alloy cables occupy more channel resources.
Under the conditions of similar energy consumption, the cross-section of aluminum alloy cable must be more than two specifications greater than that of copper cable in order to achieve similar current carrying capacity. However, the increased conductor cross-section has a serious impact on the cable laying and the size of the cable channel structure. Cable channel resources are an important part of the construction of urban cable networks. Affected by the scale of urban roads and traffic organization, most cables are laid in the way of arranging and pulling pipes. If aluminum alloy cables are used for laying in the pipe, the pipe diameter must be enlarged to 1.6 times the diameter of the copper cable, which obviously increases the construction cost of the cable civil engineering. At the same time, the expanded scale of civil engineering has increased the land occupation. Under the conditions of increasingly tight urban underground resources, it is obviously not feasible.
(8) The aluminum alloy conductor requires high installation technology.
Special tools are needed to install aluminum alloy cables, and different manufacturers' connectors even need to be equipped with different tools, which will undoubtedly increase construction and installation costs. The installation procedure of aluminum alloy cable is complicated, and it is generally divided into 6 main steps: peeling the insulation layer, removing the conductor oxide layer, coating antioxidant, inserting the terminal, crimping and forming, and erasing excess antioxidant. Improper installation can easily lead to excessive contact resistance of the connector, abnormal temperature rise, and cable failure. The current domestic cable construction strength is uneven, and the level of on-site management is also lagging behind developed countries. In comparison, copper cables have rich application experience, better mechanical properties and installation fault tolerance, and the construction process is relatively simplified, which is more suitable for the actual situation and development level at this stage.