The corrugated aluminum sheath in high-voltage cables serves to withstand short-circuit current, provide radial waterproofing, and resist lateral pressure. Its manufacturing process includes two forms: longitudinal wrapping, argon arc welding, and continuous extrusion.
1. Argon arc welding process for aluminum sheaths
1. The argon arc welding process for aluminum sheathing utilizes rolled aluminum sheets of uniform thickness, followed by cleaning, precision cutting, longitudinal wrapping, welding, online inspection, and texturing. Under the protection of argon and helium, the aluminum sheet acts as the negative electrode, and the tungsten electrode as the positive electrode. Welding is completed using low voltage and high current. The tungsten electrode welding head is only 2mm in diameter, and the protective gas is continuously blown onto the weld point, rapidly carrying away heat and ensuring uniform and rapid cooling of the welded area. This prevents any adverse effects on the cable structure and avoids high-temperature oxidation of the aluminum sheath.
2. Advanced argon arc welding technology is employed, along with online testing devices such as ultrasonic sensors, ensuring the weld's airtightness. To further check for any leaks, the manufacturer has added an intermediate inspection device to conduct an airtightness test on the entire reel of welded cable, performing 100% inspection. Through several years of production, use, and operation, this production process technology has proven to be stable and reliable.
3. The Shanghai Cable Research Institute conducted mechanical strength tests on the welded aluminum sleeve and found that the tensile strength of the weld (78 N/mm²) was slightly higher than that of the surrounding aluminum metal (76 N/mm²), and also slightly higher than that of the aluminum sleeve itself (75 N/mm²). After discussion and consultation with metallographic experts from Xi'an Jiaotong University, this phenomenon was deemed reasonable; the strength of the welded material is higher than that of the original material because the metallographic structure of the welded component has changed. Lateral pressure tests were also conducted using hollow aluminum sleeves, performed on the weld, at 90 degrees and 180 degrees from the weld, and the load-deformation curves were basically consistent.
4. Temperature distribution tests on corrugated welded aluminum-sheathed cables were also conducted at the Shanghai Cable Research Institute. When the weld temperature reached 700℃, thermocouples were used to measure the temperatures at three points 90 degrees apart on the water-blocking layer inside the aluminum sheath: 69, 43, and 37℃. Below the water-blocking layer, the temperatures were 34, 26, and 27℃ respectively. This is because the aluminum sheath is welded at a single point; although the temperature is high, the energy is not large, and aluminum dissipates heat quickly, resulting in a very low temperature on the cable insulation. Meanwhile, the Insulation Research Laboratory of Xi'an Jiaotong University conducted numerical calculations of the welding temperature field of the cable's aluminum sheath, finding that the temperature near the insulation layer was approximately 40℃. The test results from the two research institutions are essentially consistent.
5. Due to the very low temperature of the corrugated welded aluminum-sheathed cable, there is no possibility of damage to the insulation or the water-blocking layer on the insulation. The gap between the aluminum sheath and the cable is very small, thus achieving a water-blocking structure for the cable. At the same time, the water-blocking layer of the cable can also be used as a buffer layer. It is not difficult to design and calculate that the expansion of the insulation radius of a domestic 220kV cable is about 1.5mm, which can be completely absorbed by the water-blocking layer. In this way, the cable structure is very compact and reasonable. Even if the aluminum sheath is damaged, the cable will not be able to get water in due to the effect of the water-blocking layer.
II: Continuous Extrusion Aluminum Sheath Process Technology
The continuous extrusion aluminum sheathing process uses aluminum ingots to continuously extrude aluminum onto the cable insulation core in a semi-molten state using an aluminum pressing machine. The extrusion temperature can reach as high as 460℃, which may adversely affect the internal structure of the cable and reduce its service life.
1. Extruding aluminum sheaths requires a large extrusion unit, which consumes a lot of energy. Extrusion at a high temperature of around 600℃ may damage the insulation, affecting the quality of the cable product. Except for some old companies, new companies rarely add aluminum extrusion machines. The production of aluminum extrusion machines is decreasing in foreign countries. The most stringent American cable standard, AEIC CS7-93, recommends the use of corrugated welded aluminum sheaths or welded copper and steel sheathed cables, but does not mention the requirement for extruded aluminum sheathed cables.
2: In order to reduce the possibility of heat damage to extruded aluminum-sheathed cables, copper wire woven cloth is generally used to block the heat source during the processing between the aluminum sheath and the insulation. However, it is also necessary to leave room for the expansion of the insulation. There is a large gap between the aluminum sheath and the insulation. Once water enters the cable, the cable insulation will be completely immersed in water, causing great damage to the power supply line.
3: Extruded aluminum sheaths are produced using an extrusion process. If not properly controlled, this can easily lead to uneven thickness of the aluminum sheath, resulting in micropores, pinholes, and other defects. This causes the strength of the aluminum sheath to be extremely uneven, reducing its radial waterproof performance.
With the development of welding technology, the reliability of corrugated welded aluminum sheathed cables has become increasingly recognized, and in the near future, they are trending towards replacing extruded aluminum sheathed cables.
