Comparison between single core parallel type and m

Comparison between single core parallel type and multi-core stranded type of branch cable

1 introduction

the advent of power cable has greatly improved the safety of power generation, transmission, transformation, supply, distribution and consumption, and the single core power cable first appeared. As there are more and more occasions of three-phase four wire power supply, three-phase five wire power supply and multi circuit power supply in actual use, the requirements for occupied space and laying occasions are also higher and higher. When multiple and multi-layer laying is required, and the occupied space and laying conditions are limited, the single core power cable cannot be easily used. Therefore, multi-core power cables have been developed and quickly entered the field of power applications, and have been accepted and used by the majority of users

2 bus duct

with the rapid growth of the use of power cables, even if single core power cables are used in the actual use process, the method of stripping the sheath and insulation on site for their joints and branches, crimping the branches or joints, and then selecting epoxy resin and other insulating materials for sealing treatment is still existing, such as large occupation of construction site, long construction time, high cost, many equipment, high technical requirements Difficulties and other disadvantages, especially the insulation strength, reliability and consistency of joints or branches after on-site construction are difficult to be guaranteed. Therefore, the bus duct has been developed. After the development is put into the market, it will soon be accepted and widely used by the majority of users

with the increase of users' use, it is found that there are also some defects in the bus duct, such as too many parts connected by screws and complex installation and construction; At the same time, the maintenance quantity is large and the maintenance cost is high. In the process of operation, we often encounter the influence of electromagnetic vibration, thermal expansion and contraction, expansion coefficient, external force and other factors, which will cause the loosening of screws. If a screw is loosened, there will be heating and high temperature at the fault point, which will affect the operation stability of the whole bus duct. In particular, the improper use of five wire bus duct will also cause the contact resistance of PE wire to increase, which violates the basic requirement of "PE wire should ensure its continuity (no disconnection)" expressly stipulated in the code for electrical design and construction of buildings. However, in the case of large capacity, bus duct still has its own advantages. Because when the current reaches thousands of amperes, if cables are used, even single core cables must be laid, otherwise the corresponding high current capacity cannot be reached. At this time, the bus duct reflects its own advantages

3 prefabricated branch cables

with the development of technology and the increasing market demand, prefabricated branch cables have been developed, and developed from single core prefabricated branch cables to multi-core prefabricated branch cables, which also includes flame-retardant, fire-resistant, and armored prefabricated branch cables. That is, in the factory, according to the cable specification, model, section and the specific location of each branch specified in the architectural design drawings, special production equipment and molds are used on the professional production line, which can be made at one time. Its advantages are:

high insulation strength, the encapsulated injection molded branch joint connector and the outer insulation sheath of the cable are closely bonded together, which has excellent water tightness and air tightness; Because it is a factory special equipment and mold production, it has excellent reliability and consistency; It is extremely convenient for users to install and use. When installing vertically, only fasten the main cable evenly on the corresponding bracket; It is simpler and more convenient to construct and install in horizontal, embedded, overhead, tunnel, airport runway, port wharf, mine, modern industrial plant and other environmental conditions; During the installation and construction process, the requirements for site, equipment, technical level and cost of construction personnel are much lower than those for dealing with cable branches or bus ducts on the construction site; During the operation, a lot of maintenance costs are saved and the power outage time is reduced; In some cases, maintenance free effect can be achieved. When the cable shaft and cable channel are small, it can show its unique advantages. Therefore, in the power supply occasions with medium and small capacity, branch cable is an ideal product to replace bus duct

4 stranded and twisted prefabricated branch cables

with the acceptance of prefabricated branch cables by the majority of users, with the sharp increase in usage, a variety of prefabricated branch cables have entered the market. This situation has brought certain difficulties to building electrical designers in model selection. The selection of stranded (also known as twisted) branch cables is introduced below

4.1 defects of stranded type

the so-called stranded type and twisted type are multiple single core power cables, but the user must also understand these problems, and the core power cables are "twisted" together. For the core wire of power cable, the purpose of twisting multiple strands of copper wire or aluminum wire is to increase the conductor section. Because they are in the same phase and have the same electromagnetic angle, the distribution and change of capacitance and inductance will not occur due to the difference of any electromagnetic phase angle. However, there is also the influence of heat dissipation and the decrease of current carrying value. Therefore, the larger the section is, the lower the unit interception value is. And it is also on the special equipment that the whole strip is made at one time according to the specified pitch and stranding force. This stranding is consistent in force distance, pitch and uniformity, and there will be no change in pitch or uneven tightness. The following only takes the circular conductor core as an example, and its stranding rules are as follows:

(1) the center is generally one single wire, the second layer is six single wires, and then each layer is increased by six wires relative to the original number of the inner layer. The same wire diameter shall be adopted for single line

(2) the stranding direction of single wires in each layer shall be opposite to that of the previous layer. Generally, the outermost layer adopts left stranding

stranded and twisted prefabricated branch cables, that is, multiple single core power cables are stranded and used as three-phase three wire, three-phase four wire or three-phase five wire power supply circuits. First of all, starting from "stranding", whether it is in the production plant of prefabricated branch cables or in the construction and installation site to complete the "stranding" process, there is neither specialized "stranding" equipment, nor the corresponding regulations or requirements of pitch, strength, tightness and uniformity that can be followed. What is more difficult to deal with is that many branch cables and branch connectors with irregular size and location are enough to determine the "stranded" multi-core cable, which will be unknown in terms of force distance, pitch, density, tightness, uniformity, overall size, etc. Therefore, this "stranded" multi-core cable has become a chaotic "Brownian motion" in terms of the distribution of inductance and capacitance, and some of the most basic parameters and indicators of electromagnetic theory such as electromagnetic vibration, suction and repulsion

in the actual operation after construction and installation, first, the distribution, pitch and tightness of inductance and capacitance are disordered, which will inevitably lead to the disordered heat dissipation and temperature rise of the whole power supply circuit and some parts. Even if several power cables without branches and branch connectors are twisted under the condition that their stranding strength, tightness and pitch can be strictly controlled, under the influence of heat dissipation and temperature rise, their actual allowable ampacity can not be compared with single core power cables laid horizontally or vertically according to the specifications

secondly, under the operation state of balanced or unbalanced current, when the load current, temperature, environment changes or influences, the force and movement of each cable and each cable itself in the aspects of suction, repulsion, extension, contraction, up and down are irregular. This phenomenon is extremely detrimental to the operation stability and reliability of the whole power supply circuit, and even leads to accidents due to sheath and insulation damage at the position where the cable is installed and fastened

again, this kind of power cable, which is "twisted" in a disorderly way, is really unsightly when placed in a cable shaft or cable channel. At the same time, it will also bring a defect to the design of electrical system drawings, the construction of electrical system and the quality of the project, which is not designed and constructed in accordance with the provisions of "horizontal and vertical"

4.2 relevant provisions in the code for electrical design of civil buildings

4.2.1 provisions for parallel laying of cables

9.10.4 of the code for electrical design of civil buildings issued and implemented by the Ministry of construction in 1993 stipulates: "when cables of the same voltage are exposed, the net distance of the cable shall not be less than 35mm, and shall not be less than the outer diameter of the cable." It is stipulated in 9.10.4 of the article: "when cables are laid in parallel, a certain distance should be maintained between cables to ensure the safe operation, maintenance and repair of cables; to avoid burning adjacent cables in case of cable failure; the proximity of cables will affect heat dissipation, reduce current carrying capacity, affect maintenance and cause mechanical damage." In addition, the distribution and influence of inductance and capacitance are also important reasons for the increase of operating temperature and the decrease of ampacity. Obviously, "twisted type" and "twisted type" cannot meet the requirements of polyethylene (PE) pipes for water supply gb/t 13663 (2) 000 specified in this article

4.2.2 regulations on cable selection

7.3.1.1 (9) of code for electrical design of civil buildings stipulates that "four core cables shall be selected for power cables used in three-phase four wire lines." It is stipulated in article 7.3.1.1: "in a three-phase four wire system, if a three core cable plus a conductor or a single core cable is used as the neutral wire, when the three-phase system is unbalanced, it is equivalent to the operating state of the single core cable, which is easy to cause power frequency interference. For bare armored cables, it will also accelerate the corrosion of the metal sheath and armor layer, so four core cables are required." Although this article does not clearly raise the distribution and change of inductance and capacitance, it also indirectly points out their influence. Three core power cable plus one as neutral wire is not enough, let alone the "stranding" and "twisting" of four or five single core power cables? Obviously, "twisted type" and "twisted type" can not meet the requirements of this article

4.2.3 regulations on power cable laying

the laying of power cables, whether indoors or outdoors, in the code for electrical design of buildings, code for electrical construction of buildings, the "specifications" for the design and construction of various power systems, as well as the "specifications" of China, IEC standards, NEC "Regulations", Japanese standards and other national standards, There is an express provision: whether it is installed vertically or horizontally, it must meet the requirements of "horizontal and vertical" in order to ensure the acceptance of project quality. Obviously, "twisted type" and "twisted type" can not meet the requirements of this provision

throughout the development history of power cables, since the advent of single core power cables, we have fully realized in theoretical research and application practice that it is not desirable and feasible to "twist" four and five single core power cables together as three-phase four wire and three-phase five wire power supply circuits in building facilities. China has always followed the practice and regulations of parallel laying, which has its theoretical and practical basis. If in order to reduce or compress its laying and use space, only relying on "stranding" or "twisting" can meet the requirements from theory to practice, then there is no need for the advent of multi-core power cables. Looking at domestic and foreign“