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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Optical cables in a lightning protection cable. Reference data
Encyclopedia of radio electronics and electrical engineering / Reference materials Suspension of fiber optic cables on power lines (TL) is becoming increasingly widespread. A similar line is stretched, for example, between St. Petersburg and Finland. Work is underway to lay a cable parallel to the RRL on the section of the Moscow-Khabarovsk digital highway. So, in this section, about 3600 km of communication lines will be made using an optical cable in a lightning protection cable located on a power transmission line. The article published here, containing a brief story about the design of such cables, responds to readers' requests. Optical communication cables can be laid underground, under water, and also hung on the supports of overhead communication lines (VLAN). The combination of overhead lines with power lines (Fig. 1), which has a number of advantages, is becoming more common.
It is known that each country has an extensive network of high-voltage power lines. Consequently, there is no need to build special poles for overhead lines, but hang the cable on existing (or under construction) pylons of power lines, moreover, more powerful than on overhead lines. In this case, the optical cable is enclosed inside the mandatory element of the power transmission line - a grounded metal lightning protection cable (Fig. 2). In domestic practice, optical cables in a lightning protection cable are designated by the abbreviation OKGT. This cable serves not only as a power element that carries the cable, but also shields it from external electromagnetic influences.
The main advantage of the combined lines of power transmission lines - VLS over underground cable lines of communication is manifested when the route of the line passes through areas that are difficult to access for underground laying, for example, permafrost zones with swelling soil, swamps, rocks. The disadvantage of such lines can reasonably be assumed to be damage to the cable, and hence the cable, when lightning strikes it, which often occurs in thunderstorm areas, as well as due to short circuits on power lines caused by various reasons. To avoid these troubles, leading to interruptions in the operation of communication lines, a special technology was developed for the production of a cable and a cable suspended on power lines. Thanks to this technology, during a lightning strike, the temperature in the cable does not exceed 170...200°C, which is safe for its durability. True, such a cable (and cable) turns out to be significantly more expensive than usual. But at the same time, it is not without interest to note that damage to a cable with an optical cable occurs about five times less often than an underground cable. The basis of the design of an optical cable is the so-called modules. As a rule, these are plastic or metal tubes with a diameter of 2 ... 3 mm, each of which freely accommodates 2 ... 24 optical fibers (in some designs their number reaches 60). The optical fiber consists of a two-layer transparent quartz fiber with a diameter of 125 µm with a protective polymer coating (outer diameter 250 µm). Cables are both single-module and multi-module, containing up to six modules (Fig. 3).
Cables with plastic modules. In single-module cables, a relatively large diameter module is located in the center (Fig. 4). In multi-module modules - peripheral; they are twisted into a twist around a central support element of circular cross section (Fig. 5). The maximum number of peripheral modules is six. If there are fewer of them, then up to six required number of fillers are added to the coil to maintain its cylindrical shape - plastic cordels of the same diameter as those of the modules.
Both the central module and the whole set of twisted peripheral modules and fillers, which is called the core, is enclosed in a polymer or metal shell. The free space inside each module and between the modules (and fillers, if any) in the twisted core is filled with a hydrophobic (water-repellent) compound that prevents moisture from penetrating to the optical fibers. In the case of contact of moisture with a quartz fiber, the losses of the transmitted light communication signals increase and the mechanical characteristics of the fiber deteriorate, up to its destruction. Cable wires are superimposed over the sheath. They can be steel or aluminum with a diameter of 1,5 ... 3,25 mm, but the most widely used are steel, aluminum-clad (aluminized) and Aldreevy - from an alloy of aluminum with magnesium, silicon, iron. The choice of material and diameter of the wires depends both on the size of the optical cable and on the operational requirements for the physical and mechanical parameters of the cable. In cables with plastic modules, i.e. "in plastic version", the cable is single-stranded, but more often double-stranded. In all cases, it consists of a combination of two types of wires: aluminized steel wires, which provide the mechanical strength of the cable, and Aldre wires, which have high electrical conductivity and temperature resistance, which is necessary to protect against lightning strikes and short circuits, when a large current occurs in the cable, a high temperature develops. and unacceptable overheating of the optical cable is possible. So, for example, in one of the variants of a two-stranded cable, the inner layer is formed by a combination of aluminized steel wires 10x2,0 mm and Aldreev wires - 5x2,0 mm, and the outer layer is entirely Aldreev wires of 14x3,25 mm. In another design, on the contrary: the inner layer is formed by 12x3,25 mm Aldreev wires, and the outer one - 13x3,25 mm Aldreev and 5x3,25 steel aluminized wires. The outer diameter of cables, both single-module and multi-module, is 12,5...25 mm. Their weight is 300...1200 kg/km. The total cross section of the cable wires is 80...335 mm2. Estimated breaking load - 40...125 kN. Cables with metal modules. The designs of their cores are significantly different from the cores of cables with plastic modules. The number of metal modules in the cable is less, namely: 1, 2, 3, 4. The module tube is steel or aluminized steel (stainless steel). If there are one or two modules in the cable, then they are located in a layer, which is complemented by five or four aluminized steel wires, respectively. In addition, one of the same wire in the center acts as a support element. In the case of three or four modules, they are twisted together and placed in the center of the cable. In some designs, cable wires are directly superimposed over the entire metal core (Fig. 6) - with one or two layers, for example, steel clad 5x3,0 mm, then Aldreevy 12x3,0 mm and, finally, again Aldreevy 18x3,0 mm.
In other designs, the core is enclosed in a tube of segmented Aldrey wires, on top of which one or two layers of cable wires in combination of aluminized steel and Aldrey wires follow (Fig. 7).
Cable diameter - 10...22 mm, weight - 200...1000 kg/km. The total cross section of metal elements is 70 ... 285 mm2. Estimated breaking load - 40...120 kN. In addition to cables in a lightning wire, there are several types of optical cables designed for overhead lines. These are self-supporting cables, under the sheath of which there is a power-bearing element. It can be a steel or synthetic cable, a fiberglass rod, or a twist of high-strength synthetic threads. These are the so-called twisted cables. They are wound on a lightning protection cable or on a phase wire of a power transmission line. Finally, cables attached to the ground wire, either by a common taping or by frequently spaced bands. According to the information of recent years, in foreign practice, of all the listed types of overhead optical cables, the most widespread (up to 85%) are cables in a ground wire. Author: D.Charlet, Moscow
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