Study on the Applicability of Aluminum Alloy Cables in Urban Piezoelectric Networks

I. Introduction

As China's economy has entered a new normal development, aluminum alloy cable has occupied a certain share in the electrical market. Relevant media and companies have proposed to fully promote “aluminum to copper” in the field of power cables, and to “the strategic direction” of addressing the shortage of copper resources. In this paper, the eight aspects of aluminum alloy cable and copper cable are compared horizontally, and combined with the actual application situation, a rational and cautious look at the replacement of copper and aluminum.

Second, the actual comparison of copper and aluminum mineral resources

The relative shortage of copper resources in China is an objective fact, but aluminum resources are actually not rich. The so-called "aluminum-rich and copper-deficient" is misleading propaganda that does not meet the national conditions [1]. As we all know, due to a serious overcapacity in the domestic aluminum processing industry, it has long been included in the national limit to eliminate backward production sequences. The alumina-silicon ratio of domestic bauxite ores is relatively low, and alumina production costs are high. The over-exploitation of aluminum mines has resulted in a domestic storage/production ratio of only 6.6 years, much lower than the petroleum industry's 10 year storage/production ratio [2]. Obviously, reserves cannot meet the needs of rapid economic growth. However, in order to maintain a large 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 60% or more at the end of the “Thirteenth Five-Year Plan” period. From a long-term perspective, there is an objective systemic risk of shortage of aluminum resources. In contrast, as a 100% recyclable metal, copper has a domestic storage/production ratio of 16 years. If we can take advantage of the current foreign exchange reserves and low-cost copper mines, we will acquire a large amount of foreign copper resources. More prominent strategic significance.

Third, the performance 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 common aluminum. Although it is an upgraded version of pure aluminum cable, it still has obvious disadvantages compared with copper cables.

(1) Aluminum alloy cables have poor electrical conductivity.

Aluminum alloy cable conductivity is only 61% of copper cable. Under the same cable cross-section, a large resistance will inevitably lead to high line losses and reduce energy efficiency. Under the same current-carrying conditions, the resistivity of aluminum alloy cables is always slightly larger than that of copper cables. Taking a load current of 380 A, annual utilization hours of 4500 hours, and a service life of 30 years as an example, if the cross section of the copper cable is 150 mm2, the aluminum alloy cable cross-section needs 240 mm2, and the electrical resistivity of the two is 0.148/km and 0.150/km, respectively. The consumption is 288,495kwh/km and 292,410kwh/km, and the difference in energy consumption between the two is 117,450kwh/km during the whole life cycle [3]. Obviously, the loss of the aluminum alloy cable during the entire life cycle is relatively large, deviating from the development direction of the country's “energy saving and emission reduction”.

(2) The aluminum alloy cable has a low current carrying capacity.

The power supply reliability requirement of the urban grid reaches 99.99%, and the core area needs to reach a higher level of 99.999%. Since the urban cable network adopts a ring network structure, the protection action is performed in a short time in the event of a fault, and the load is quickly switched to the opposite line to ensure uninterrupted user power supply. But to achieve high reliability of the power grid, perfect network structure, excellent equipment and lines are all essential. The power supply line in the power grid must have high current-carrying capacity, and it can also assume temporary switching load in addition to its own load. The same cross-section of copper core cable than the aluminum alloy cable current carrying capacity of more than 30%, obviously more able to meet the city's power supply reliability requirements.

(3) Aluminum alloy cable has low mechanical tensile strength.

The aluminum alloy cable has a tensile strength of only 46% of the copper cable, allowing the traction force to be 60% lower than the copper cable. A large number of cable distribution networks are used in urban distribution networks. Planning and design consider minimizing the use of cable connectors. In actual use, the length of laying a single copper cable is generally between 600 and 800 meters. It is considered that under the same current-carrying conditions, the length of laying a single Pole 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 an additional large number of intermediate joints will be needed to increase the risk of subsequent operation and maintenance.

(4) Aluminum alloy cable is weak in corrosion resistance.

The corrosion of cable conductors is mainly metal electrochemical corrosion, which means that electrolytic cells are caused by galvanic or stray current disturbances on the metal surface. In the production process of aluminum alloy cables, magnesium, copper, zinc, silicon and other elements are added to improve the creep resistance, and the heat treatment process is added. Due to the complex operating conditions of the cable, in the environment containing the electrolyte, there is an electrode difference between aluminum with a lower electrode potential and other added metal elements, thereby forming a current path, and electrochemical phenomena such as pitting corrosion and crevice corrosion occur. Aluminum alloy cable heat treatment process is also easy to cause the conductor surface is not uniform, increase the possibility of electrochemical corrosion, followed by stress corrosion cracking and intergranular corrosion.

(5) The aluminum alloy cable has poor high temperature performance.

The melting point of copper is 1080, while the melting point of aluminum is only 660. It is clear that copper conductors are a better choice for fire-resistant cables. In the event of a fire, the central ambient temperature can rise to more than 750, and the cable must be able to maintain the basic functions of energization in order to build life support lines. Obviously, when the temperature of the fire field is higher than the melting point of aluminum alloy and aluminum, no matter what insulation measures are taken, the cable conductor will melt in a short time and lose its conductive function, which will seriously affect the safety evacuation of fire personnel.

(6) Aluminum alloy cable joints have a high risk of failure.

Experience with cable operation shows that 80% of all faults occur at the joint. Copper has unparalleled advantages of aluminum and aluminum alloys. The copper oxide produced by the oxidation of copper joints is an excellent conductor and can still guarantee the electrical connection performance of the connector and the terminal. Alumina formed by oxidation of aluminum and aluminum alloy joints is an insulator, and its hard texture and strong adhesive properties make it difficult to form good conductive contacts, which can easily cause contact heating. Most of the electrical equipment terminals are copper joints. The use of aluminum alloy cables will form a copper-aluminum connection. Aluminum alloys have a much higher coefficient of thermal expansion than copper. Since there is always a peak-to-valley difference in the operation of the power grid, when the load changes significantly, the temperature rapidly changes, a large lateral movement occurs in the contact area, the effective connection of the metal contact is cut off, the contact resistance is increased, and the temperature at the connection rises. Upon cooling, thermal stress changes occur again, further forming an interfacial shearing effect. Under the repeated action of long-term hot and cold, when the thermal stress is greater than the yield force of aluminum, it will form irreversible plastic deformation in the contact area, accelerating the degree of loss at the joint, until the final connection failure. Aluminum alloy conductors are more likely to cause poor contact after thermal expansion and contraction, and the vicious circle of contact areas is a great challenge to the safe operation of joints.

(7) Aluminum alloy cables occupy more channel resources.

Under similar energy consumption conditions, the aluminum alloy cable cross-section must be greater than two specifications of the copper cable in order to achieve a similar ampacity. However, the increased conductor cross-section has a serious impact on the cable laying and cable channel structure dimensions. Cable channel resources are an important part of urban cable network construction. Affected by the size of urban roads and traffic organization, most of the cables are laid using pipe and pull pipes. If aluminum alloy cables are used for laying pipes, the hole diameter of the pipes must be enlarged to more than 1.6 times of the diameter of the laid copper cable [4], which obviously increases the cost of cable civil engineering construction. At the same time, the scale of the expansion of civil construction has increased the area. Under the increasingly tense urban underground resources, it is obviously not feasible.

(8) Aluminum alloy conductor installation process requirements are high.

The installation of aluminum alloy cables requires the use of special tools. Joints of different manufacturers even require different tools, which undoubtedly increases the cost of construction and installation. Aluminum alloy cable installation procedures are complex, generally divided into six major steps of stripping the insulation layer, removing the conductor oxide layer, coating the antioxidant, inserting the terminal, crimping the molding, and removing the excess antioxidant. Improper installation can easily result in excessive contact resistance of the connector and abnormal temperature rise until a cable fault occurs. At present, the strength of domestic cable construction is uneven, and the level of site management also lags behind developed countries. In comparison, copper cable has rich application experience, better mechanical performance and fault-tolerant installation, and the construction process is relatively simplified, which is more suitable for the actual situation and development level at this stage.

IV. Conclusion

In summary, the aluminum alloy cable is a product with high investment risk, limited market space, and low degree of safety and reliability for the user. The relative aluminum cable is only partially improved in terms of creep resistance, but it cannot be used for aluminum cables. Other deficiencies provide an effective solution. Through the above comparative analysis, aluminum alloy cables are not suitable for popularization in medium voltage systems and urban distribution networks.