Motor Engineering Handbook: Transmission and Distribution Equipment Volume

Author: Chief Editor: Liu Siyuan
Publisher:
Publish Date: 1997-06-01
Features:
Fragment: The protective sheath of a cable generally does not determine its transmission parameters. However, it plays a very important role in ensuring the environmental adaptability of the cable, extending its service life, and improving its reliable operation. The structure, performance, and key selection points of cable sheaths will be introduced in Chapters 4, 5, and 6 of this section.
Electrical cables also have high requirements for installation and laying. In a certain sense, electrical cables are only semi-finished products and are a type of electrical material. The cable body itself is only a component of an electromagnetic wave transmission system or an engineering system. It, together with cable accessories and terminal equipment, forms a complete engineering system. The quality of the entire system and its reliable operation depend not only on the product quality of the cable itself but also closely on the quality of the construction and laying of the cable route. When selecting cables, special attention should be paid to their compatibility with cable accessories, and strict control must be exercised over the construction and installation process. In the statistical analysis of actual cable line failure rates, failures caused by factors such as construction, installation, and splicing are often much higher than those caused by inherent defects in the cables themselves.
In international patent literature (H02G), "Installation of Electrical Cables" is a category independent of (H01B) "Cables." Moreover, the number of patents in the former category is even higher than in the latter. This indicates that the "innovation density" of cables in terms of installation is higher. The geometric dimensions of the cross-section of electrical cables are also an important parameter and are a key component of the installation requirements mentioned above. Since, by definition, electrical cables are longitudinally continuous, equal-sectioned cable-like electrical products capable of transmitting electromagnetic waves, their geometric dimensions are often characterized by two-dimensional dimensions on the cross-section. For circular cross-sections, only one parameter—the diameter—is needed to describe them. Although all nominal diameter specifications in cable products require the use of recommended preferred number systems as specified in GB321 "Preferred Numbers and Series," the total number of specifications for electrical cables has now exceeded 100,000 due to different series and size specifications.
### 3. Processing and Manufacturing of Electrical Cables and Their Specialized Equipment
The processing technology of electrical cables differs from that of other structurally complex electrical products. They cannot be processed using general machine tools such as lathes, drills, planers, or milling machines, and even modern flexible machining centers are ineffective in processing them. The processing methods of electrical cables can be simply summarized as three major specialized processes: drawing, wrapping, and stranding, which are characterized by low material consumption and energy consumption.
The drawing process is typically used to stretch thick conductors into thin ones. To ensure sufficient flexibility, the cores or conductors must be twisted into cable form. "Wrapping" refers to a general term encompassing multiple processes such as winding, extrusion, coating, braiding, and longitudinal wrapping, which are often used for insulation processing and sheath production.
Actual cable production equipment and assembly lines are divided into six major categories: drawing lines, stranding lines, cabling lines, extrusion lines, coating lines, and braiding lines. A significant number of common auxiliary components are used in this type of equipment. These include four basic auxiliary components: reelers, payers, pullers, and winders. In JB/T5812～5820, detailed specifications for the types, dimensions, technical requirements, and basic parameters of the aforementioned equipment are provided.
Cable reels are a common component of specialized cable equipment and an indispensable packaging for cable products. In China, GB4004 and GB4005 have been separately established for the machine reels (PN type) and delivery reels (PL type) of electrical cables. In GB4006.1～5—83, specific regulations are also provided for the finished winding reel (PC, PCZ type). Practical modern specialized cable equipment is essentially a production line that combines the six major categories of equipment as reasonably as possible [2].
### 4. Development Trends in Electrical Cables
Over the past two and a half centuries, electrical cables have seen significant advancements in theory, materials, structure, variety, technology, installation, and manufacturing. Some industrialized countries had already established relatively complete international electrical cable industries as early as the 1970s. However, at the same time, the infiltration of optical cables (detailed in Chapter 6), which use quartz as the "light conductor," into the cable industry has brought profound changes to this traditional electrical industry. It is an important industry in the future "information society."
Although the existing generalized transmission line theory can be applied to the design of various types of cables, including optical fibers (whose essence is dielectric waveguides) and future superconducting cables made from high-temperature superconducting materials, the structure and variety of cables will continue to innovate. For example, recently, Rosec et al. proposed utilizing the self-suspension Meissner effect of superconducting wires to create coaxial cables without insulation. When two superconducting coaxial lines enter a superconducting state, they automatically maintain a certain distance from each other without touching, forming a coaxial pair.
This superconducting coaxial cable, due to the absence of solid dielectric insulation, can significantly reduce the attenuation of electromagnetic waves in the insulation. At the same time, since the conductor is also in a superconducting state, its resistance is extremely low. Therefore, this coaxial superconducting cable will be a transmission line with extremely low attenuation. According to Reference [3], a superconducting coaxial pair with an inner conductor diameter of 0.22 mm and an outer conductor inner diameter of 0.79 mm has an attenuation of only 0.001 dB/km at 10,000 MHz. Its prospects for application in ultra-long-distance communication without repeaters are highly promising.