What is the full-process control method for anti-oxidation of cable copper conductors?

Copper has an atomic number of 29 and belongs to transition metals. Its density is 8.92g/cm³ and its melting point is 1083.4℃. It has good thermal and electrical conductivity. However, in humid air, the surface of metal copper reacts chemically with oxygen to generate Cu2(OH)2CO3, i.e. verdigris. Usually, copper oxidation in cable manufacturers often shows black surface. It is the existence of this copper conductor surface oxidation phenomenon that troubles many cable companies. What is the method for anti-oxidation of cable copper conductors?

We have observed in long-term daily production that through the selection of high-quality copper rods, effective control of wire drawing process and emulsion concentration and temperature, passivation treatment of copper conductor stranding or bundled copper wire surface, and subsequent process optimization treatment, the quality of cable copper conductors can be effectively controlled and prevented from oxidation, which will greatly improve work efficiency and reduce the frequency of rework, thereby achieving the beneficial effect of reducing costs and improving the quality of internal products.

Let’s take the production of medium-voltage power cables as an example, from the arrival of copper rods at the production plant, storage, wire drawing, stranding, inner core storage to insulation extrusion, and analyze the effective control measures for each step.

Anti-oxidation control of copper conductors in the production process of medium-voltage power cables

In principle, the main methods of metal copper used in cables are physical methods to block copper from contact with humid air, cathodic protection oxidation-reduction method to prevent copper conductor oxidation, chemical methods to produce a passivation film on the surface of the copper conductor to prevent oxidation, or spraying special liquids on the surface of the conductor for protection. Taking the production of medium-voltage power cables as an example, the main methods of anti-oxidation control in each process.

Transportation, inspection and storage of copper rods before entering the factory

Most cable companies in my country basically purchase copper from outside, and most companies often ignore the control of the transportation process before copper rods enter 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 rods are basically transported to the company by heavy trucks.

According to the author’s understanding, many cable companies have not formulated corresponding copper rod supplier transportation specifications, summer rainy day packaging specifications and other systems, which often results in the copper rod supplier shipping the copper rod surface intact without oxidation and blackening, but when it arrives in front of the cable company’s users, the copper rod surface is black, which brings unnecessary trouble.

The inspection of copper rods entering the factory is basically in accordance with GB/T 3048.2 or the cable company’s enterprise standard, with strict procedures. The storage of copper rods is generally placed in warehouses by cable companies.

Generally, companies have stricter storage management methods. When storing copper rods, especially on rainy days in summer, they must be covered with plastic cloth or plastic film to prevent the copper rods from contacting humid air with the simplest physical barrier method, which is often ignored by cable companies.

When using copper rods in the workshop, it is necessary to visually check whether there is blackening on each copper rod roll, and control it from the source of production.

Control of copper rod drawing process

In this stage, the most commonly used 8mm copper rod for power cables is taken as an example. Nowadays, continuous annealed copper is generally used for drawing. It needs to go through the processes of wire release, wire drawing and annealing, cooling, drying, and wire winding.

The mold of the corresponding specification should be selected first, and it must not be too small. If it is too small, it will force the lattice of the metal copper to mutate and aggravate the sharp rise in metal temperature.

Check the concentration of the emulsion before starting the machine to ensure that the concentration is qualified. During the pay-off process, the pay-off tension should be kept stable and uniform, and no excessive vibration (vibration is inevitable).

During the wire drawing process, the operator should pay close attention to the changes in the 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 drawing into a coil. The operator needs to identify it with the naked eye. After the wire is drawn, it is sealed with a transparent plastic film and stored in a dry environment for circulation.

However, most cable companies omit the film wrapping process because they find it troublesome to wrap the copper wire with film. The surface temperature of the conductor that has just been drawn into a coil is higher than the room temperature. In the rainy season, there is a potential risk of oxidation.

Copper wire twisting (bundle twisting) and chemical cross-linking extrusion process

Take copper wire twisting as an example. During the conductor twisting process, each single wire is twisted in a regular or irregular manner. After each die pressing, the metal copper lattice changes its structure.

Under the action of strong external force, the temperature of the copper conductor after the die is much higher than that before the die, especially for the compact conductor and fan-shaped conductor. In addition, the frame twisting machine currently has no online annealing or cooling device, so the outer layer of the copper conductor is easy to oxidize. Before twisting, select a suitable twisting die, compacting wheel, and wire arrangement device, and adjust the wire collection and pay-off tension.

After the equipment is adjusted, the operator observes the surface finish of the upper coil copper wire and the copper wire in the center of the pay-off frame with the naked eye to ensure that there are no obvious black spots before starting the machine.

Generally, cable companies will apply antioxidants in the stranding process (frame stranding machine, cage stranding machine). The antioxidant formula is about 0.3% benzotriazole alcohol solution.

The operation method is to dissolve the antioxidant in industrial alcohol and drip it into the copper wire with an infusion hose. The dripping standard is based on just soaking the copper wire to avoid too little local potential oxidation or excessive waste. After all anti-oxidation work is well prepared, start the machine.

The wire reel should be kept dry at the wire reeling place. After the reel is full, it is sealed with a transparent plastic film. Similar to copper wire drawing, many cable companies also omit this step. If this step causes oxidation of the copper conductor, the impact on low-voltage cables is still low, but for ultra-high voltage cables, the impact is unimaginable.

After wire drawing, stranding, and storage, the metal copper lattice morphology is basically stable. Chemical cross-linking means that XLPE is evenly extruded on the copper conductor under the conditions of cross-linking agent, antioxidant, coupling agent, etc. Before starting the pay-off reel filled with copper conductors, adjust the pay-off and take-up tension to ensure smooth operation of the equipment.

Summary

Almost all cable factories in my country are faced with the problem of copper conductor oxidation. From the perspective of the chemical properties of metal copper, it is necessary to maintain the reducibility of metal copper. In daily production, what is the full-process control method for anti-oxidation of cable copper conductors? the following measures can reduce the probability of copper conductor oxidation:

Select high-quality metal copper rods, standardize the supplier’s transportation, delivery process and system, and take transparent plastic film sealing measures after the copper rods arrive at the company. Transparent plastic film sealing measures should be taken after the copper conductors are drawn and twisted;

Use appropriate copper wire drawing process, regularly check the annealing parts of the wire drawing machine, give priority to wire drawing oil containing antioxidants, regularly check the concentration of emulsion, and replace it in advance before the expiration of the shelf life;

The copper wire twisting and extruded insulation process adopts passivation treatment technology;

Establish quality awareness and strengthen business training to make the operators of the above processes aware of the adverse consequences of copper wire oxidation.