Applicability of Aluminum Alloy Cable in Urban Medium Voltage Network

Absrtact: "Aluminum-rich and copper-deficient" is not in line with the national conditions, while "copper in China" has more prominent strategic significance. Compared with copper cable, aluminum alloy cable has obvious differences in electrical conductivity, current carrying capacity, mechanical properties, corrosion resistance, high temperature resistance and other hard indicators. There are also comparative disadvantages in joint failure, channel resource occupation, construction technology and so on. Aluminum alloy cables are not suitable for medium voltage systems and urban distribution networks because of the life cycle of comprehensive assets and the need for safe operation of power grids.

[Key words] Aluminum-rich and Copper-deficient Cables with Aluminum as Substitute for Copper and Aluminum Alloys

I. Introduction

With China's economy entering a new normal state of development, aluminum alloy cable occupies a certain share in the electric market. Relevant media and enterprises have put forward a "strategic measure" to comprehensively promote "replacing copper with aluminium" in the field of power cables and crown it with the future "development direction" to solve the copper resource shortage. This paper mainly compares aluminum alloy cable with copper cable in eight aspects, and considers the substitution of copper and aluminum rationally and prudently combined with practical application.

II. A Realistic Comparison of Copper and Aluminum Mineral Resources

The relative shortage of copper resources in China is an objective fact, but the aluminum resources are not rich. The so-called "aluminium-rich and copper-deficient" misleading propaganda is not in line with the national conditions [1]. As we all know, the domestic aluminium processing industry has long been listed in the national restriction and elimination of backward production sequence due to the serious overcapacity. The alumina-silicon ratio of domestic bauxite ore is lower and the production cost of alumina is higher. Overexploitation of bauxite has resulted in a domestic reserve-production ratio of only 6.6 years, which is much lower than the 10-year reserve-production ratio of petroleum industry. Obviously, the reserves can not meet the needs of rapid economic growth. However, in order to maintain the huge capacity of the aluminium industry, it is necessary to purchase large quantities of aluminium resources from overseas. It is expected that the overseas dependence of the "13th Five-Year Plan" aluminium ore will reach more than 60%. In the long run, there is a systematic risk of shortage of aluminium resources objectively. On the contrary, as a 100% recyclable metal, the domestic storage and mining ratio is 16 years old. If we can take advantage of the current foreign exchange reserves and low-price copper mines, and buy a large number of foreign copper resources, it will undoubtedly have more prominent strategic significance to "store copper at home".

III. Performance comparison between aluminium alloy cable and copper cable

Aluminum alloy cable is a kind of cable which adds trace elements as conductor on the basis of ordinary aluminium. Although it is an upgraded version of pure aluminium cable, it still has obvious disadvantage compared with copper cable.

(1) The conductance of aluminium alloy cable is poor.

The conductivity of aluminium alloy cable is only 61% of that of copper cable. Under the same cable section, the larger resistance will inevitably lead to higher line loss and lower energy efficiency. The resistivity of aluminium alloy cables is always slightly higher than that of copper cables under the same current carrying capacity. Taking load current 380A, annual utilization hours 4500h, operation life 30 years as an example, if 150 mm2 is used for copper cable section, then 240 mm2 is needed for aluminum alloy cable section. Their resistivity is 0.148/km and 0.150/km respectively. Their annual energy consumption is 28845 kwh/km and 292 410 kwh/km, and the energy consumption difference between them is 17450 kwh/km in the whole life cycle. Obviously, the loss of aluminium alloy cable in the whole life cycle is on the high side, which deviates from the development direction of "energy saving and emission reduction" of our country.

(2) The current carrying capacity of aluminium alloy cable is low.

The reliability requirement of power supply in urban power grid is 99.99%, and the core area is 99.999%. Because the city cable network adopts ring network structure, the protection action in short time in case of failure can quickly switch the load to the opposite line to ensure uninterrupted power supply for users. But in order to achieve high reliability of power grid, perfect network structure, excellent equipment and lines are indispensable. The power supply lines in the power grid must have high current carrying capacity and can bear temporary switching load in addition to their own load. Copper core cables with the same cross-section are 30% higher than aluminium alloy cables in carrying capacity, which obviously can better meet the requirements of urban power supply reliability.

(3) The mechanical tensile strength of aluminium alloy cable is low.

The tensile strength of aluminium alloy cable is only 46% of that of copper cable, and the allowable traction force is 60% lower than that of copper cable. The cable ring network structure is widely used in urban distribution network, and the use of cable intermediate joints should be minimized in planning and design. In practice, the laying length of a single copper cable is generally between 600 and 800 meters. Considering the same current carrying capacity, the laying length of a single ordinary aluminium cable is only 500 meters. Considering the influence of traction force, the laying length of a single aluminum alloy cable is only 350 meters. Obviously, the low tensile strength will inevitably lead to the limited length of single traction cable, which requires a large number of additional intermediate joints and increases the risk of subsequent operation and maintenance.

(4) The corrosion resistance of aluminium alloy cable is weak.

The corrosion of cable conductor is mainly metal electrochemical corrosion, that is, the electrolytic battery action caused by the interference of primary battery or stray current on metal surface. In order to improve creep resistance of aluminium alloy cable, magnesium, copper, zinc, silicon and other elements are added, and heat treatment process is added. Because of the complex operation conditions of cable, in the environment containing electrolyte, the electrodes of Al with lower electrode potential are different from those of other added metal elements, resulting in current path, pitting corrosion and crack corrosion and other electrochemical phenomena. The heat treatment process of aluminium alloy cables is also prone to cause non-uniform physical state of conductor surface, which increases the possibility of electrochemical corrosion, followed by stress corrosion cracking and intergranular corrosion.

(5) Aluminum alloy cable has poor high temperature resistance.

The melting point of copper is 1080, while that of aluminium is 660. Obviously, copper conductor is a better choice for refractory cables. In case of fire, central environment

(6) Aluminum alloy cable joint has high risk of failure.

The cable operation experience shows that 80% of the faults occur at the joint. Copper has incomparable advantages over aluminium and aluminium alloys. Copper oxide produced by oxidation of copper joints is a good conductor, which can still guarantee the electrical connection performance of joints and terminals. Alumina produced by oxidation of aluminium and aluminium alloy joints is an insulator. Its hard texture and strong bonding force make it difficult to form good conductive contacts and easily cause contact heating. The terminal of electrical equipment is mostly made of copper joints, and copper-aluminium connections will be formed by using aluminium alloy cables. The thermal expansion coefficient of aluminium alloy is much higher than that of copper. Because there is always peak-valley difference in the operation of power grid, when the load changes significantly, the temperature changes rapidly and the contact area moves laterally, which cuts off the effective connection of metal contacts and increases the contact impedance, leading to the temperature rise at the connection. Thermal stress changes occur again during cooling, further forming interfacial shear. When the thermal stress is greater than the yield force of aluminium, irreversible plastic deformation will be formed in the contact area under the repeated action of long-term heat and cold, accelerating the wear and tear at the joint, and eventually leading to joint failure. Aluminum alloy conductors are more likely to have poor contact after thermal expansion and cold contraction. The vicious circle of contact zone also poses a great test for the safe operation of joints.

(7) Aluminum alloy cable occupies more channel resources.

Under the condition of similar energy consumption, the section of aluminium alloy cable needs to be larger than two specifications of copper cable to achieve the same carrying capacity. However, the increase of conductor cross-section has a serious impact on cable laying and cable channel structure size. Cable channel resources are an important part of urban cable network construction. Influenced by the scale of urban roads and traffic organization, most cables are laid by piping and pulling. When aluminium alloy cable is used to lay in the row pipe, the hole diameter of the row pipe must be enlarged to more than 1.6 times of the hole diameter of the copper cable laying, which obviously increases the construction cost of the cable civil engineering. At the same time, the expansion of the scale of civil construction has increased the occupation of land, which is obviously not feasible under the increasingly tense urban underground resources.

(8) The installation technology of aluminium alloy conductor is highly demanded.

Installation of aluminium alloy cables requires special tools, and different manufacturers even need to equip different tools, which undoubtedly increases the cost of construction and installation. The installation procedure of aluminium alloy cable is complex, which is generally divided into six main steps: stripping insulating layer, removing conductor oxide layer, coating antioxidant, inserting terminal, pressing and forming, erasing redundant antioxidant, etc. Incorrect installation can easily lead to excessive contact resistance, abnormal temperature rise until cable failure occurs. At present, the domestic power of cable construction is uneven, and the level of field management lags behind that of developed countries. Comparatively speaking, copper cable has rich experience in application, better mechanical performance and installation fault-tolerance, simplified construction technology, and more suitable for the actual situation and development level at this stage.

Four, conclusion

In summary, aluminium alloy cable is a product with high investment risk, limited market space and low security and reliability for users. Compared with aluminium cable, it only has partial improvement in creep resistance, but it can not provide effective solutions to other shortcomings of aluminium cable. Through the above comparative analysis, the aluminium alloy cable is not suitable for medium voltage system and urban distribution network.