The stranding methods include concentric stranding, compression stranding, profile stranding, bundle stranding, bundle wire re stranding, fan-shaped conductor, separation conductor, ring conductor, stranded conductor in the same direction, etc.
The purpose of conductor stranding is to facilitate laying, forming and connection, and consider the influence of proximity effect and skin effect on AC resistance.
We can mainly understand the questions such as “why should conductors be stranded? What are the ways of conductor stranding?”.

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3.6 stranding
large section solid conductors cannot be laid, formed and connected smoothly because they are not easy to bend. Stranded conductors solve this problem. The method of stranding depends on which metal is selected and the characteristics of the metal. Copper conductors are usually stranded in #6awg (metric 13.3mm2) or larger, and for semi hard aluminum solid conductors, the maximum is #2 / 0awg (metric 67.4mm2).
(Note: conductor of GB / T 3956-2008 cable: the nominal cross-sectional area of compact stranded circular aluminum conductor or aluminum alloy conductor shall not be less than 10 mm2. The nominal cross-sectional area of stranded copper conductor, aluminum conductor or aluminum alloy conductor shall not be less than 25 mm2.)
3.6.1 concentric stranding
this is a typical stranding method for power cable conductors. It is composed of a center line or a core surrounded by one or more layers of helix. For each additional layer, there are six more cores than the previous layer. Except for unidirectional stranded conductors, each layer is stranded in the opposite direction to the lower layer. For the conductor of power cable, the core is a single wire, and the stranded wires are all wires of the same diameter. As shown in Figure 3-1, the first layer outside the core is 6; The second layer, 12; The third layer, 18; and so on. The distance at which a single wire of a conductor completes a complete spiral in one layer is called pitch. The requirements for pitch are specified in ASTM standard part IV
It is specified that the pitch shall be 8 ~ 16 times of the outer diameter of the corresponding layer.
In power cables, the stranding standard is class B. The standard requires that the outermost twist direction should be left. That is, when you look along the axial direction of the cable conductor, the outermost strand is twisted to the left when leaving the observer. Better flexibility is obtained by increasing the number of wires in the conductor. Level C has one more single line than level B, and level D has one more single line than level C. Level setting up to level M (usually used for welding cables, etc.). These are included in ASTM standards (Note: American Society for testing and materials, abbreviated as ASTM, ASTM has nearly 34000 members, of which about 4000 are from hundreds of countries outside the United States. ASTM has formulated more than 10000 standards).
the weight of class C and class D conductors is similar to that of class B conductors, and the outer diameter is within 3mil larger than that of class B conductors.
the following formula can be used to calculate the number of single wires of concentric stranded conductors.
n=1+3N(N+1)  (3-3)
where, n is the total number of single wires in the stranded conductor; N is the number of layers outside the central single line.

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   3.6.2 compression stranding
this concept describes the tight combination of twisted layers through slight deformation of twisted layers. The conductor area is not reduced. Finally, the diameter of the compacted conductor can be reduced by no more than 3% compared with the equivalent diameter of the ordinary concentric stranded core conductor. The general reduction was 2.5%.
reducing the outer pitch of the conductor can solve this problem, but it will lead to high impedance and consume more conductor materials.
compression stranding is a more common stranding method, because the pitch set in concentric stranding causes a small gap between the middle layers of the conductor. Low viscosity materials “fall” into these gaps after extrusion. This will lead to irregular surface, increase the voltage to be borne, and make it more difficult to peel this layer.
3.6.3 type wire stranding
this method is similar to compression stranding, except that the additional forming process can reduce the conductor diameter by 9% compared with the diameter of ordinary concentric stranding conductor. Causes the conductor diameter to approximate that of a solid conductor. However, the air gap still exists as the migration channel of moisture. The main advantage of profile stranded conductor is to reduce the conductor diameter.
3.6.4 stranding
this concept is used to describe the centralized twisting of single lines in a unified direction without considering the geometric arrangement.
this structure is used for cables with high flexibility requirements for small-size conductors of American wire gauge, such as portable cables, which are used in vacuum cleaners, lawn mowers and other occasions.
in class k and M conductors, the single wire diameter is constant. Increase the number of single wires to meet the required conductor cross-sectional area.
3.6.5 bundle rewinding
this concept is used for concentric stranded conductors, in which each single wire is also stranded. That is, a collection of concentric conductors and stranded conductors. The final conductor is composed of a group of bundle stranded or coaxial stranded conductors. Each group is composed of a certain number of single wires, rather than a single wire stranded. For the description of bundle stranded conductor, the number of stranded groups and the number of single wires in each group shall be given.
grade g and H cables are usually used for mining portable cables. 1. Grade L and M cables are concentric cables composed of bundle cables with the same single line size. The required cross-sectional area size can be achieved by increasing the number of lines. Grade I cable uses #24awg (0.020in) single wire, grade L cable uses #30awg (0.010in) single wire, and grade m cable uses #34awg (0.0063in) single wire. Class I cables are usually used in railways, and class L and M cables are used in more portable occasions, such as welding cables and mobile power lines.

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3.6.6 sector conductor
a conductor whose cross section is approximately fan-shaped is called a fan-shaped conductor. A typical three conductor cable has three 120 ° sectors, which form the basic shape of a circular cable. Such a cable has a smaller outer diameter than the corresponding concentric conductor cable, and can show a lower AC impedance due to reducing the proximity effect.
for paper insulated cables, most of the sector conductors are twisted and pressed to obtain the highest ratio of conductor cross-sectional area to cable cross-sectional area. The exact shape and size of products vary slightly from manufacturer to manufacturer.
To a certain extent, solid rather than stranded sector conductors have been used in low-voltage cables. Some people have tried to use this kind of conductor in medium-voltage cables, but it has not been realized due to economic considerations at that time.
  3.6.7 split conductor
the split conductor is circular and is twisted by three or more strands, which are separated by a thin insulating layer. Each block carries a smaller current than the whole conductor, and the current is transmitted through the inner and outer positions of the whole conductor. The advantage of this structure is that the skin effect is low, and the smaller skin effect makes the AC impedance lower than the traditional stranding method.
this form of conductor should be considered for cables carrying large current, such as 1000kcmil (corresponding to metric 507mm2) and larger cross-sectional area. The diameter of the quarter conductor is approximately equal to the diameter of class B concentric stranded conductor.
Note: GB / T 18890.2-2015 states that “conductors with nominal cross-sectional area above 800mm2 shall adopt split conductor structure”
3,6.8 ring conductor (hollow conductor)
the ring conductor is a circular stranded conductor, which is to twist each single wire around the central fiber rope, threaded metal pipe or I-shaped beam. The advantage of this conductor is that for a conductor with a certain cross-sectional area, the influence of skin effect on the central position can be eliminated, so that the overall AC impedance becomes smaller. When space permits, the ring conductor is more economical for cables with cross-sectional areas of 60Hz, 1000kcmil and above, and cable conductors with cross-sectional areas of 1500kcmil (corresponding to metric 760mm2) and above under low frequency conditions (such as 25Hz).

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  3.6.9 stranded conductors in the same direction
like its name, all single wires of the same direction conductor face the same direction. Low voltage power cables are usually designed in the same direction, and some of the outer single wires are single wires with a slightly smaller diameter than other single wires. The outer diameter of this stranding method can be close to that of the profile stranded conductor. The common co directional conductor is tightly pressed, which is commonly used in 8000 Series aluminum alloy.
3.7 stranding barrier
moisture in insulated conductors can cause some problems. Aluminum will hydrolyze in the presence of water and oxygen. Therefore, if water enters the insulated cable with aluminum conductor, aluminum and water will react chemically to produce aluminum hydroxide and hydrogen. This situation is more serious when the insulated conductor is in an oxygen deficient environment.
aluminum hydroxide is a white powder material and a high-quality insulator. Many users of Stranded Aluminum Conductors now require the conductor to be water resistant for this reason. Water blocking parts, such as water blocking tape, water blocking yarn and other seals, are used as longitudinal water blocking materials to delay the formation of damage. Copper conductors can of course block water in the same way.
regardless of the conductor material and degree of tightness, air will exist in the voids of the conductor, which will become a storage place for water vapor and provide water for the generation of water trees. In order to reduce the possibility of water tree, water blocking stranded conductors are often used for underground cables. Of course, solid conductors are also used for conductors of #2 / 0 AWG and below for this reason.

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