Conductive core is an important part of power cable, and its function is to transmit current. my country has formulated a national standard for LSOH cable conductors equivalent to IEC228 "Conductors of Insulated Cables"-GB/T 3956-1997 (Conductors of Cables". As a widely used and large amount of plastic insulated power cables with a rated voltage of 1kV, silane cross-linked Polyethylene insulated power cable occupies a considerable proportion of it. Its conductor structure and performance use stranded conductors that meet the requirements of GB/T 3956-1997 Type 2 conductors.
In the production process of the stranded conductive core, the outermost twisting direction is left. In some authoritative articles and textbooks in my country, such as "Electrical Insulation Structure Design Principles", it is pointed out that the outermost twisting direction of the stranded wire should be left. Because the cable formation direction is stipulated to adopt the right direction (except for SZ cable formation), and the outermost twisting direction of the conductor of each core of the cable adopts the left direction, so that the cable structure can be more reasonable and stable.
The author once encountered such a situation in production, that is, in the process of producing copper core 1kV, 4×240 mm silane-polyethylene insulated power cable, in order to save raw materials and reduce the outer diameter of the cable, the conductor uses 9O. Pressing the sector-shaped conductor structure, when forming a cable, sometimes one of the insulated cores may break the insulation by the conductive core inside the insulation before entering the paralleling die of the cable forming machine, and the outermost conductive core The layered stranded wire is stretched like an umbrella, which causes the entire cable to be scrapped at a fixed length and can only be processed in separate trays. Moreover, this situation occurs from time to time, indicating that it is not caused by accidental factors, it is inevitable, and there may be problems in the process. This requires analysis, find out the cause of the problem, and make process improvements to avoid this situation from happening again.
Cause analysis
It is analyzed that the real reason for the expansion of the conductive core is not due to the improper pitch of the strands or the poor compression of the layers, and the insufficient toughness of the conductor. Adjusting the pitch of the strands and the degree of compaction of each layer, and increasing the degree of toughening of the single wire, can only reduce the probability of this occurrence, but the main reason for the problem should be found to fundamentally eliminate this occurrence.
The scalloped cores and round cores have different cabling conditions. The circular cores need to be twisted to eliminate the internal stress of the wire when they are cabled, while the sectored cores need to be pre-twisted when they are cabled. Taking a four-core sector cable as an example, the four insulated cores should be circular after being cabled, and the cross-section of each insulated core should be 1/4 circle, consisting of two sides at right angles to each other and one arc side constitute. After the cable is formed, the arc edges are outside, the arc edges of the four cores form a circle, and the right angle edges of the four cores are closely attached to the right angle edges of the other cores. In order to meet this requirement, it is required that each insulated wire core should enter at a fixed angle when entering the die opening of the parallel die, so that the vertex of the right angle side of each insulated wire core is at the center of the circle. The pre-twisting angle of the right-angled edges must be appropriate so that the faces of the right-angled edges coincide exactly, and the cable can become circular after being cabled. Otherwise, the insulated wire core rotates or the angle is wrong, which may cause the intersection of the right-angle side of the wire core and the arc to move to the center position; or the arc side turns to the center position, so that the cable out of the cable will not be rounded, commonly known as " The core turns over." In order to fix the entry angle and position of each core, a torsion force must be applied to each insulated core to forcibly control the entry position of the core. The torque is achieved by pre-twisting the core, and its size is adjusted by adjusting the number of turns of the pay-off reel To adjust, that is, to adjust by adjusting the pre-twist pitch. The magnitude of the torque is mainly related to the cross section of the conductive core. The larger the cross section, the greater the required torsion; it is also related to the toughness of the copper wire and the tightness of the conductor.
The cable forming direction is specified as right, and the pre-twisting direction can only be right, otherwise the balance cannot be achieved. The force generated by the pre-twist acts on the insulated conductor, and the outermost layer of the conductive core is twisted to the left, so that a force is applied to the outermost strand of the conductor to loosen the outermost strand of the conductor. The outer surface of the conductor is an insulating layer, and the outermost layer of the AAC conductor is not loosened mainly due to the pressing effect of the insulating layer. When the conductor cross-section is large, the required pre-twisting force is also large, and the force acting on the insulating layer is also large. The insulation thickness of the lkV silane cross-linked cable is relatively thin. When the insulation squeezed outside the conductor is not enough to withstand, the insulation will be squeezed at the weakest part of the insulation to loosen the outer core of the conductor. When the conductor cross-section is small, the force is also small, and the insulation will not be crushed under normal circumstances. Therefore, this situation basically only occurs in cables with large cross-section conductors.
way of solving the problem
To find out the cause of the problem, the next step is to find a solution. First of all, according to the conventional thinking, improve the toughness of the copper single wire to make it softer; adjust the pitch of each layer and the height of the pressing wheel during the pressing process. These measures only reduce the reverse twisting stress of the twisted wires during cable formation, thereby reducing the occurrence of the above-mentioned problems, but they have not been fundamentally solved. If another method is adopted, that is, to adjust the twisting direction of the outermost layer of the twisted wire to the right, the twisting directions of the other layers are also adjusted accordingly. Since the cable formation direction (right direction) is the same as that, the reverse torsion stress generated by the twisted wire during cable formation can be fundamentally improved, thereby avoiding the above problems.
In GB/T 3956—19976 Cable Conductor, there is no provision for the twisting direction of the conductor. Therefore, the right-handed outermost layer of the conductor does not conflict with the national standard and the IEC standard, and is in compliance with the national standard and IEC requirements.
As for the cable performance, the change is the twisting direction of each layer of the conductive core, which has no effect on the conductor, the insulation outside the conductor and the outer structure, so it has no effect on the cable performance. What may affect the appearance of the cable is the appearance of the cable, and for the sector-shaped cores, the cores are tightly bonded after the cable is formed, and the cores do not twist when the cable is formed, so we think it has no effect. Of course, these need to be verified through product trial production. In addition, there is no question about whether it is feasible in the process, just adjust the rotation direction of the cage when twisting the wire.
Trial production and verification
At the beginning, we first choose a four-core 1 kV silane cross-linked cable with a length of 280 m for trial production. The outermost layer of the conductor is 4×240, and the outermost layer of the conductor is right. The twisting direction of each layer of twisted wires remains unchanged for the rest of the cable manufacturing process. The trial production results show that the trial production situation is very good. The appearance of the cable after the cable has not changed compared with before, and it has no effect on the cable performance. Then increase the length and quantity of cable trial production, and conduct batch trial production. 240 mm in three cores and four cores. A total of more than 30 kilometers of cables have been trial-produced on sector-shaped conductor cables, and the results are very good. There is no case that the conductive core is stretched and the insulation is broken, and it has no effect on the performance and appearance of the cable, so it is fundamentally solved. The problem occurred.
in conclusion
(1) In two-core, three-core, and four-core 1 kV silane cross-linked polyethylene insulated cables with a sector-shaped conductor structure (185 mm and above), the outermost layer of the conductor should be considered Right-hand process; sector-shaped conductor cross-section felt less than 185 mm can be considered for this structure.
(2) For multi-core cables with sector-shaped conductor structure in 1 kV PVC insulated cables, when the conductor section is large, the outermost layer of the conductor should also be considered to be twisted to the right. Although its insulation thickness is thicker than that of silane cross-linked insulation, the above situation may rarely occur in actual production, but it may also occur when the conductor cross-section is large, and this process can avoid its occurrence.