Are aluminum alloy cables any good?
In a large public building, electrical investment accounts for about 10% of the total project investment, while cables account for almost 10% of the total investment in the project. accounting for more than half of the budget. The use of low-smoke halogen-free copper cables will significantly increase the already high cable costs. From a national level, China is a copper-poor country. The stock of copper ore resources is about 26 million tons. The import dependence on refined copper ore exceeds 70%, which is higher than the external dependence on iron ore and oil. The sharp rise in copper prices has caused social problems. Increase in costs. On the other hand, large public buildings consume a lot of electricity and have high electricity levels. When selecting cables, multiple large cables of 185mm2 and 240mm2 often appear. For high-rise buildings, in vertical shafts Laying large cross-section cables is difficult. In the construction budget quota, the cost of laying cables vertically is 2.6 to 4 times that of laying cables horizontally. Therefore, how to choose cables and what kind of cable materials to choose is a big problem. Finding a safe, reliable, and replaceable product is a common wish of owners, contractors, and designers.
In developed countries such as the United States and Canada, a kind of low-voltage cable with aluminum alloy as conductor, cross-linked polyethylene insulation and interlocking aluminum alloy tape armor is widely used. This kind of metal armored cable is widely used in North America. It has been installed and operated in the market for nearly 40 years. It is widely used due to its reliable connection, light weight, small bending radius, easy installation, good heat dissipation, saving bridges, saving resources, low smoke and halogen-free, and obvious price advantage compared with WDZR-YJY. Suitable for use in airports, military bases, offices, residences, hotels, supermarkets, schools, stadiums, hospitals, industrial plants, data processing centers, prisons and other places.
The following is a brief analysis of the advantages of aluminum alloy power cables:
1. Conductor materials
Common cable conductors on the market now include copper and aluminum. . Many people believe that the resistivity of aluminum conductors is greater than that of copper conductors, so if aluminum is used as a cable conductor, the energy loss during transmission should be greater than that of copper materials. In fact, this view has no theoretical basis. Because energy loss during transmission depends on conductor resistance rather than resistivity. According to GB/T 3956-2008 "Conductors of Cables", at 20°C, the DC resistance value of a certain copper conductor cross-section is equivalent to the DC resistance value of an aluminum conductor corresponding to one or two specifications. In other words, considering the energy loss during the transmission process, a certain copper conductor power cable can be completely replaced by an aluminum conductor cable corresponding to two sizes larger. At this time, the carrying capacity of the aluminum cable is greater than that of the copper cable.
As early as the 1960s, our country began to implement the national policy of "replacing copper with aluminum". Aluminum core cables are widely used, from high-voltage transmission lines to household entrance lines, all using aluminum cores. Article 3.1.4 of the interpretation of the 94th edition of GB50217 "Code for Design of Electric Power Engineering Cables" clearly states that "it is obviously appropriate to continue to adopt the technical and economic policy of replacing copper with aluminum." However, in practical applications, aluminum wires have poor mechanical strength and are easy to break. Aluminum wires used for a long time will become brittle, which is not conducive to maintenance. The cross-section of early residential aluminum wires was small and could not meet the increase in electricity consumption. The newly purchased copper wires purchased by residents themselves were directly twisted with the aluminum wires. Electrochemical corrosion would occur in humid air, causing the contact resistance to increase. Over time, Short circuits, open circuits, etc., coupled with the poor insulation materials and low melting points of the original wires and cables, increase the probability of fires. According to statistics from the Fire Bureau of the Ministry of Public Security, fires caused by electrical lines have always been relatively high, accounting for about 15% of the total number of fires. (There are three main reasons for electrical fires: 1. The electrical design drawings do not meet the fire safety requirements; 2. The construction does not respect the drawings and the construction personnel do not operate according to the regulations; 3. The electrical manufacturers use fake and shoddy materials.)
< p> The same experience also occurred in North America. Failure to handle the contact points of hard pure aluminum wires caused a large number of fires, circuit breaks and other accidents. After two years of careful research and analysis, it was found that the problem mainly lies in the conductor material and connection points. The aluminum conductor of residential lines is made of hard pure aluminum (AA1350). During mechanical connections, under pressure, irreversible deformation will occur after long-term operation, which is the so-called "creep". After creep occurs, the original compression is not enough, the pressure at the contact point decreases, causing the contact resistance to increase rapidly, and the current flows causing the joint to overheat. If maintenance is not performed regularly, the risk of accidents greatly increases.In response to problems that arise during the application of aluminum cables, the United States researched and developed aluminum alloy conductors in the early 1960s, and successively developed a series of aluminum alloy materials, forming the AA8000 series of aluminum alloy materials; my country We began to develop this alloy in 2005, and obtained patented technology and passed the U.S. UL certification. All performance indicators meet the performance requirements of AA8000 series aluminum alloy materials.
These aluminum alloy conductor materials solve many of the shortcomings of pure aluminum and possess some of the characteristics of copper conductors. These performances will be analyzed in detail below.
1. Electrical properties
Compared with copper, the electrical conductivity of aluminum alloy is 61.8% of copper and the current carrying capacity is about 78% of copper when the cross section and length are the same. . According to the specification distribution of the cable cross-section, if the cross-section of the aluminum alloy cable is increased by about 1.5 times, the electrical parameters such as current carrying capacity and voltage drop will be equivalent to those of copper. As the U.S. National Electrical Code NEC330.14 stipulates: "Solid conductors of 8, 10, and 12AWG cross-sections (equivalent to domestic 8.37mm2, 5.26mm2, 3.332mm2) should be made of AA8000 series electrical grade aluminum alloy materials. Stranded conductors are made from 8AWG (equivalent to 8.37mm2 in China) to 1000kcmil (equivalent to 506.7mm2 in China) should be made of AA8000 series electrical grade aluminum alloy conductor materials.
Use aluminum alloy cables one or two sizes larger to replace copper cables. People will worry about whether enlarging the conductor will increase the outer diameter of the conductor too much, thus affecting the installation connection. In order to solve the problem of increasing the outer diameter of the conductor due to the increase in cross-section, we can use advanced compression technology, with a compression coefficient of 0.93. , the outer diameter of the replaced aluminum alloy cable is only increased by about 10% compared to the copper cable, so it will not affect the installation connection at all.
2. Connection performance
Aluminum. The alloy material has greatly improved its connection performance due to the addition of iron, magnesium, rare earth and other elements. In particular, the added iron element when the conductor is annealed produces high creep resistance, which can ensure high creep resistance even when overheated for a long time. Connection stability.
In the past experience and understanding, copper-aluminum connection has always been the most critical issue. If copper and aluminum (or aluminum alloy) are in direct contact, there are different metal materials. There is a potential difference, and electrochemical corrosion will occur in humid air. In power systems, this corrosion will increase the contact resistance and cause heat at the joints, which is a hidden danger to the safe operation of the system.
Electrochemical corrosion. It is the result of the simultaneous action of the following factors:
(1), different electrode potentials (>0.5V), different resistances between metals, and polarization characteristics of metals.
(2) The conductivity of the electrolyte and the contact area (current density) of the cathode and anode surfaces.
In addition, the expansion coefficients of copper and aluminum (or alloys) are different. If rigid connection is used, metal will be formed. Fatigue will cause the connection to loosen after long-term use, and it will also increase the contact resistance.
The processing of aluminum alloy power cables mainly relies on the following methods:< /p>
First, tin plating. Tin plating of copper bars or tin plating of aluminum terminals is feasible. Tin plating of copper bars and tin plating of aluminum terminals has been widely used in the power industry. Proven to be safe and reliable.
Second, a special transition gasket is used, which is between copper and aluminum to effectively prevent corrosion problems caused by copper and aluminum contact.
Third, apply antioxidants. When the copper contacts of electrical equipment are not tinned, and the aluminum terminals are not tinned, you can use the method of applying antioxidants between copper and aluminum. It can effectively prevent the entry of electrolyte, thereby avoiding electrochemical corrosion.
When using the first and third methods above for wiring, you need to install disc-shaped gaskets to absorb the slack caused by the different expansion coefficients of copper/aluminum. After tightening with a torque wrench, there is no need to modify the entire connection system. Perform any repairs and secondary tightening.
3. Mechanical properties
(1) Excellent tensile properties: Compared with pure aluminum, aluminum alloys are greatly improved due to the addition of special ingredients and special processing techniques. The tensile strength is improved, and the elongation is increased to 30%.
(2) Super bendability: The minimum bending radius of aluminum alloy cables during installation can be greater than 7 times the outer diameter of the cable, which is far better than the "minimum bending radius during cable installation" of GB12706 - —12 times to 20 times the outer diameter of the cable.
(3) Excellent low rebound performance: Aluminum alloy cables have 40% less rebound performance than copper cables.
(4) Excellent creep resistance: The special alloy and heat treatment process of aluminum alloy conductors greatly reduce the "creep" tendency of the metal under heat and pressure. Compared with pure aluminum, it has better creep resistance. Performance improved by 300%.
4. Corrosion resistance
Aluminum quickly forms a dense oxide film with a thickness of about 2×10-4mm in the air. Prevent the metal inside from further corrosion. Copper cannot form an oxide film, so pollutants will stay on the surface and corrode further inward. After a period of corrosion, rainwater will wash away all the oxides, and new pollutants will accumulate and corrode new metal again, while copper There is no oxide film, which causes copper to oxidize quickly and cause a lot of metal loss.
Due to the addition of rare earth metals to aluminum alloy conductors, in terms of chemical properties, the corrosion resistance of metal materials using aluminum as a conductor is further improved, and the potential difference between different metals is reduced, because the potential difference The larger it is, the more severe the corrosion will be. The other metal elements added are allowable metal pairs to reduce contact corrosion caused by potential difference.
2. Flame retardant properties
When fires occur in high-rise buildings now, thick smoke is the fatal killer in fires. Inhaling a large amount of thick smoke into the human body will cause death, and inhaling a small amount of smoke will cause death. Thick smoke may cause fainting and affect escape, so the relevant national fire departments and building management departments have clearly stipulated the use of low-smoke, halogen-free and flame-retardant cables.
YJHL8, YJHL8Y type (aluminum alloy belt interlocking armored cable) armored cable uses aluminum alloy conductor, flame retardant silane cross-linked polyethylene insulation, aluminum alloy belt interlocking armor structure (or add low smoke and no smoke Halogen polyolefin sheath), can achieve flame retardant level IA, fire resistance level IA, and is low smoke and halogen-free. It has excellent safety performance and economic benefits.
3. Installation characteristics
Compared with copper cables, aluminum alloy interlocking armored cables are more like metal conduits and cables integrated together. In practical applications, the work of threading pipes is omitted. When the electrical design is exposed, the bridge can be omitted, which reduces the cost for the owner, saves working hours and shortens the construction period for the general contractor, saves steel for the country, and reduces pollution and energy waste.
Since the density of aluminum is about 1/3 of copper, aluminum alloy cables are half lighter than copper cables at the same carrying capacity. For high-rise buildings, the difficulty and workload of vertical laying are greatly reduced, which reduces the labor intensity of construction workers, shortens the construction period, saves labor costs, and also reduces potential losses caused by excessive cable weight.
Aluminum alloy armor makes bending of the cable easier. The bending radius of armored cables can reach 7 times the outer diameter of the cable. Such a small bending radius makes the internal wiring of the building very convenient and flexible in turning. Especially for some old building renovation and other projects where the original space is very narrow, power can be easily transmitted to more rooms. For PVC-sheathed armored external cables, obstacles can be easily bypassed, and construction with a smaller bending radius is easier, saving cable length. Aluminum alloy armor is a non-magnetic material. Even if there are three-phase unbalanced currents, eddy currents will not be generated, which can reduce line losses.
IV. Energy Saving and Environmental Protection Performance
Energy conservation and emission reduction are the only way to promote economic structural adjustment and transform the growth mode, and are the inevitable choice for building a resource-saving and environment-friendly society. Aluminum alloy is used as the conductor material. Under the same current carrying capacity, the weight of the cable is only half of that of copper cables. Carbon emissions during transportation and installation are reduced. YJHL8 and YJHL8Y armored cables are laid without bridges and steel pipes, saving a lot of steel.
On the other hand, the recycling rate of aluminum alloys is very high.
Recycled aluminum is an energy-saving and environmentally friendly industry. The comprehensive energy consumption of recycled aluminum is only 5% of that of electrolytic aluminum, while the comprehensive energy consumption of recycled copper is 18% of that of smelting copper.
5. Comparison of material costs and installation costs
On the premise of achieving the same performance, the direct cost of aluminum alloy cables, that is, the cost of the cable itself, is about 35% less than that of copper cables. Because aluminum alloy cables have better bending properties and are lighter, they can eliminate the need for bridges or reduce the amount of bridges, and the installation cost can be saved by 20% to 50%.
It can be seen from the above comparison that aluminum alloy cables make up for the shortcomings of pure aluminum. On the basis of electrical properties reaching those of copper, they are lighter, more flexible, more energy-saving, mechanical, safe and economical. Much more than copper cable. The designed service life of domestic copper core cables is 30 years, while the service life of aluminum alloy cables is 40 years. The products and experience of foreign applications for 40 years can be copied to our country. Utilizing my country's abundant aluminum resources to manufacture aluminum alloy cables to meet the needs of industrial and civil power distribution is in line with the needs of scientific development and is a good thing that benefits the country and the people.