The atomic number of copper is twenty-nine , belonging to transition metal, density is 8.92g/cm3 , melting point one thousand and eighty-three point four ℃, with good thermal conductivity and conductivity, but the copper surface reacts with oxygen in humid air to form Cu2(OH)2CO3 , i.e., copper green. Usually, copper oxidation in cable manufacturers shows that the surface is blackened. It is this phenomenon of copper conductor surface oxidation that puzzles many cable enterprises.
We have observed in the long-term daily production that the quality of cable copper conductor can be effectively controlled, oxidation can be prevented, work efficiency can be greatly improved, and rework frequency can be reduced by means of selecting high-quality copper rods, effectively controlling the drawing process and emulsion concentration and temperature, passivating the stranded or stranded copper wire surface, and optimizing the subsequent processes, So as to achieve the beneficial effect of reducing costs and improving the quality of internal products.
Now let's use Medium voltage power cable Take production as an example, from the arrival of copper rods at the production plant, storage, wire drawing, stranding, inner core storage to insulation extrusion, the effective control measures for each step are analyzed.
▄ Medium pressure power cable Anti oxidation control of copper conductor in production process
In principle, the metallic copper used for cables mainly includes physical methods to block the contact between copper and humid air, cathodic protection redox method to prevent the oxidation of copper conductor, chemical methods to produce passive film on the surface of copper conductor to prevent oxidation, or spraying special liquid on the surface of conductor for protection. Take the production of medium voltage power cable as an example, the main method of anti oxidation control in each process.
▶ Transportation, detection and storage of copper rods before entering the factory
Most cable enterprises in China basically purchase copper, while most of them often ignore the control of the transportation process of copper poles before entering the factory. In the south of the Yangtze River and coastal areas (Jiangsu, Zhejiang, Fujian and Guangdong), the summer is long and rainy, and copper poles are basically transported to the company by heavy trucks. Through the author's understanding, many cable enterprises have not formulated corresponding transportation specifications for copper pole suppliers The standard packaging system in summer rainy days often makes the surface of the copper rod sound without oxidation and blackening when the copper rod supplier delivers the goods, while the surface of the copper rod blackens when it reaches the cable enterprise users, which brings unnecessary trouble.
The incoming inspection of copper rod is basically in accordance with GB/T 3048.2 Or cable enterprise standard, with strict procedures. The storage of copper poles is generally in the warehouse of cable enterprises, which have strict storage management methods. When copper poles are stored, especially in rainy summer days, they must be covered with plastic cloth or plastic film, and the simplest physical barrier method is used to prevent the contact between copper poles and humid air, which is often ignored by cable enterprises. When receiving copper rods in the workshop, it is necessary to visually inspect the copper rod coils one by one for blackening, and control it from the source of production.
▶ Control of Copper Rod Drawing Process
In this stage, the most commonly used power cables 8mm For example, the copper rod is generally drawn by continuous annealing. It needs to go through the process of paying off, drawing and annealing, cooling, drying, and take-up. The mold of the corresponding specification is preferred. It must not be too small. If it is too small, it will force the lattice of the metal copper to change, and increase the sharp rise of the metal temperature.
Before starting the machine, check the concentration of emulsion to ensure that the concentration is acceptable. During the paying off process, keep the paying off tension stable and even without excessive chattering (chattering is inevitable). During the wire drawing process, the operator should pay close attention to the change of annealing temperature. After wire drawing, the conductor hardens, the elongation decreases, and the DC resistivity of the conductor increases, There should be no residual emulsion on the surface of the copper conductor when the wire drawing is waiting to form a circle. Here, the operator needs to identify it with naked eyes. After the lower wall of the wire drawing is sealed with transparent plastic film, it is stored in a dry environment for circulation. However, most cable enterprises omit the film coating process because they suspect that the copper wire coating is troublesome. The surface temperature of the conductor just drawn into a circle is higher than the room temperature, which may lead to potential oxidation risk in rainy season.
▶ Copper wire stranding (bundle stranding) and chemical cross-linking extrusion process
Taking copper wire stranding as an example, in the process of conductor stranding, each single wire is twisted in a regular or irregular way. After each pressing die, the metal copper lattice changes its structure. Under the action of strong external force, the temperature of copper conductor after passing the mold is significantly higher than that before passing the mold, especially for compacted conductors and sector conductors. At present, the frame strander has no online annealing or cooling device, Therefore, the outer layer of copper conductor is easy to oxidize. Before stranding, select the appropriate stranding die, pressing wheel and wire arranger to adjust the tension of take-up and pay off. After the equipment is adjusted, the operator shall visually observe the surface finish of the hanging copper wire and the central copper wire of the pay off rack to ensure that there is no obvious black spot, and then start the machine.
Generally, cable enterprises will apply antioxidant in the stranding process (frame strander, cage strander). The formula of antioxidant is 0.3% For the left and right benzotriazole alcohol solution, the operation method is to melt the antioxidant into industrial alcohol, and drop it into the copper wire with an infusion hose. The dropping standard is based on the standard of just soaking the copper wire, so as to avoid too little local potential oxidation or too much waste. After all the oxidation prevention work is well prepared, start the machine again.
The take-up reel shall be dry at the take-up reel. After the take-up reel is full, it shall be sealed with transparent plastic film. It is the same as copper wire drawing. Many cable enterprises also omit this step. If this step causes copper conductor oxidation, the impact on low-voltage cables is still low, and the impact on ultra-high voltage cables is unimaginable. After the copper conductor is drawn, twisted and stored, the lattice shape of the metal copper is basically stable. Chemical cross-linking means that under the conditions of cross-linking agent, antioxidant, couplant, etc XLPE Evenly extrude and wrap the copper conductor. Before starting the pay off reel full of copper conductors, adjust the tension of the pay off reel to ensure stable operation of the equipment.
Summary
Almost all cable plants in China are faced with the problem of copper conductor oxidation. In terms of the chemical characteristics of metal copper, it is necessary to maintain the reducibility of metal copper. In daily production, the following measures can reduce the probability of copper conductor oxidation:
one Select high-quality metal copper rod, standardize the supplier's transportation and delivery process and system, take transparent plastic film sealing measures after the copper rod arrives at the company, and take transparent plastic film sealing measures after the copper conductor is drawn and twisted;
two . Adopt proper copper wire drawing process, regularly check the annealing parts of the wire drawing machine, give priority to the wire drawing oil containing antioxidant, regularly check the emulsion concentration, and replace it before the expiration of the warranty period;
three Passivation treatment technology is adopted for copper wire stranding and extrusion insulation process;
four Establish quality awareness and strengthen business training to let operators in the above processes know the adverse consequences of copper wire oxidation.
