Aluminum alloy power cable is a new material power cable with aa8030 series aluminum alloy material as conductor and advanced technologies such as special compaction process and annealing treatment. Copper cable is a power cable with copper material as conductor, which is widely used now.

 

1、 Introduction

 

As China's economy enters the new normal of development, aluminum alloy cables occupy a certain share in the electrical market. Relevant media and enterprises put forward the "strategic measures" to comprehensively promote the "substitution of aluminum for copper" in the field of power cables, crown it with the "development direction" in the future and solve the shortage of copper resources. This paper mainly makes a horizontal comparison between aluminum alloy cable and copper cable in eight aspects, and looks at the substitution of copper and aluminum rationally and prudently combined with the actual application.

 

2、 Realistic comparison of copper and aluminum mineral resources

 

The relative shortage of copper resources in China is an objective fact, but aluminum resources are not rich. The so-called "rich aluminum and lack of copper" is misleading propaganda that is not in line with national conditions. As we all know, the domestic aluminum processing industry has long been listed in the national restriction and elimination of backward production sequence due to serious overcapacity. The aluminum silicon ratio of domestic bauxite ore is low, and the production cost of alumina is high. The over exploitation of aluminum ore makes the domestic reserve production ratio only 6.6 years, which is far lower than the reserve production ratio of the petroleum industry for 10 years. Obviously, the reserves can not meet the needs of rapid economic growth. However, in order to maintain the huge aluminum production capacity, a large number of aluminum resources must be purchased from overseas. It is expected that the overseas dependence of aluminum will reach more than 60% by the end of the 13th five year plan. In the long run, there is a systematic risk of shortage of aluminum resources. In contrast, copper, as a 100% recyclable metal, has a domestic reserve production ratio of 16 years. If we can make use of the advantages of current foreign exchange reserves and low-cost copper mines to acquire a large number of foreign copper resources, "storing copper at home" will undoubtedly have more prominent strategic significance.

 

3、 Performance comparison between aluminum alloy cable and copper cable

 

Aluminum alloy cable is a cable form in which trace elements are added 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 cable is poor

 

The conductivity of aluminum alloy cable is only 61% of that of copper cable. Under the same cable section, too large resistance will inevitably lead to high line loss and reduce energy efficiency. Under the same current carrying capacity, the resistivity of aluminum alloy cable is always slightly higher than that of copper cable. Taking the load current of 380A, the annual utilization hours of 4500h and the operation life of 30 years as an example, if the copper cable section is 150mm ²， The section of aluminum alloy cable shall be 240mm ²， The resistivity of the two are 0.148/km and 0.150/km respectively, the annual energy consumption is 288495kwh / km and 292410kwh / km, and the energy consumption difference between the two in the whole life cycle is 117450kwh / km. Obviously, the loss of aluminum alloy cable in the whole life cycle is too large, which deviates from the development direction of national "energy conservation and emission reduction".

 

2. The current carrying capacity of aluminum alloy cable is low

 

The power supply reliability of urban power grid shall reach 99.99%, and the core area shall reach a higher level of 99.999%. As the urban cable network adopts the ring network structure, the protection acts in a short time in case of fault, and the load is quickly switched to the opposite line to ensure uninterrupted power supply to users. However, in order to achieve high reliability of power grid, perfect network structure, excellent equipment and lines are essential. The power supply line in the power grid must have high current carrying capacity and can bear temporary switching load in addition to its own load. The current carrying capacity of copper core cable with the same section is more than 30% higher than that of aluminum alloy cable, which can obviously meet the requirements of urban power supply reliability.

 

3. Low mechanical tensile strength of aluminum alloy cable

 

The tensile strength of aluminum alloy cable is only 46% of that of copper cable, and the allowable traction force is 60% lower than that of copper cable. Cable ring network structure is widely used in urban distribution network, and the use of cable intermediate joints is considered to be minimized in planning and design. In actual use, the laying length of a single copper cable is generally in the range of 600 ~ 800m. Considering that the laying length of a single ordinary aluminum cable is only 500m under the same current carrying capacity. 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 limited length of single traction cable, and a large number of additional intermediate joints are required to increase the risk of subsequent operation and maintenance.

 

4. The corrosion resistance of aluminum alloy cable is weak

 

The corrosion of cable conductor is mainly metal electrochemical corrosion, that is, the effect of electrolytic cell caused by galvanic cell or stray current interference on the metal surface. In order to improve the creep resistance of aluminum alloy cable, magnesium, copper, zinc, silicon and other elements are added in the production process, and the heating treatment process is added. Due to the complex operating conditions of the cable, in the environment containing electrolyte, there is an electrode difference between aluminum with lower electrode potential and other added metal elements, resulting in current path, pitting corrosion, crack corrosion and other electrochemical phenomena. The heat treatment process of aluminum alloy cable is also easy to cause uneven physical state on the conductor surface, increase the possibility of electrochemical corrosion, and then stress corrosion crack and intergranular corrosion.

 

5. Poor high temperature resistance of aluminum alloy cable

 

The melting point of copper is 1080, while that of aluminum is only 660. Obviously, copper conductor is a better choice for fire-resistant cables. In case of fire, the central ambient temperature can rise above 750, and the cable must be able to maintain the basic function of power on to build a life support line. Obviously, when the fire site temperature is higher than the melting point of aluminum alloy and aluminum, no matter what thermal insulation measures are taken, the cable conductor will melt in a short time and lose the conductive function, which will seriously affect the safe evacuation of personnel in the fire site.

 

6. High risk of aluminum alloy cable joint failure

 

The cable operation experience shows that 80% of the faults occur at the joint. Copper has incomparable advantages over aluminum and aluminum alloys. Copper oxide produced by copper joint oxidation is an excellent conductor, which can still ensure the electrical connection performance of the joint and terminal. Aluminum oxide formed by oxidation of aluminum and aluminum alloy joints is an insulator. The characteristics of hard texture and strong adhesion make it difficult to form good conductive contacts and easy to cause contact heating. Copper connectors are mostly used for electrical equipment terminals, and copper aluminum connections will be formed by using aluminum alloy cables. The coefficient of thermal expansion of aluminum alloy is much higher than that of copper. Because there is always peak valley difference in power grid operation, when the load changes significantly, the temperature changes rapidly, and there is a large lateral movement in the contact area, which cuts off the effective connection of metal contacts and increases the contact impedance, resulting in the rise of temperature at the connection. The thermal stress changes again during cooling, and the interfacial shear effect is further formed. Under the repeated action of cold and heat for a long time, when the thermal stress is greater than the yield stress of aluminum, irreversible plastic deformation will be formed in the contact area, accelerating the loss degree at the joint until the final connection failure occurs. Aluminum alloy conductors are more prone to poor contact after thermal expansion and cold contraction, and the vicious cycle in the contact area poses a great test for the safe operation of the joint.

 

7. Aluminum alloy cables occupy more channel resources

 

Under the condition of similar energy consumption, the section of aluminum alloy cable needs to be more than two specifications larger than that of copper cable to achieve similar ampacity. However, the increased conductor section has a serious impact on cable laying and the structural size of cable channel. Cable channel resources are an important part of urban cable network construction. Affected by the scale of urban roads and traffic organization, most cables are laid in the way of pipe laying and pipe pulling. If aluminum alloy cable is selected for laying in the pipe, the pipe diameter must be enlarged to more than 1.6 times that of copper cable, which obviously increases the construction cost of cable civil engineering. At the same time, the expanded civil engineering scale increases the land occupation, which is obviously not feasible under the condition of increasing shortage of urban underground resources.

 

8. The installation process of aluminum alloy conductor is high

 

The installation of aluminum alloy cables requires the use of special tools, and the joints of different manufacturers even need to be equipped with different tools, which undoubtedly increases the construction and installation cost. The installation procedure of aluminum alloy cable is complex, which is generally divided into six main steps: stripping the insulating layer, removing the conductor oxide layer, coating antioxidants, inserting terminals, crimping molding and erasing excess antioxidants. Incorrect installation can easily lead to excessive contact resistance of the connector, abnormal temperature rise and even cable failure. At present, the domestic cable construction force is uneven, and the on-site management level also lags behind developed countries. In comparison, copper cable has rich application experience, better mechanical performance 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.

 

4、 Conclusion

 

To sum up, aluminum alloy cable is a product with high investment risk, limited market space and low guarantee for user safety and reliability. Compared with aluminum cable, it only has partial improvement in creep resistance, but it can not provide effective solutions for other deficiencies of aluminum cable. Through the above comparative analysis, aluminum alloy cable is not suitable for popularization in medium voltage system and urban distribution network.

 

(from: electromechanical network)