With the rapid development of urban rail transit in China, the demand for DC traction cables for rail transit is also increasing. The traction power supply system of the subway is divided into several independent power supply sections by contacting the segmented insulator installed on the network. In order to improve the reliability of traction power supply and reduce the electric corrosion caused by stray currents to the environment, each power supply section uses bilateral power supply. That is, each of the two traction substations has one power supply branch and supplies power to the same contact network area through a DC power cable.

The direct-current traction cable for rail transit refers to the low-voltage positive cable, connection cable and negative cable of 3000V and below 3000V that directly supply the traction locomotive in the power supply system of subway and light rail.

    There are many advantages of HVDC transmission. The cost of paper lines, low loss, no reactive power, easy power connection, easy control and adjustment, especially in the long-distance transmission of DC power systems have been widely used.

    The DC power cable has the following advantages: high strength of the insulated working electric field, thin insulation thickness, small outer diameter of the cable, light weight, good flexibility, and easy manufacture and installation; low dielectric loss and conductor loss, large current carrying capacity; no AC magnetic field, Environmental advantages. The characteristics of DC cables are essentially different from those of AC cables. In addition to the loss of the core wire resistance, the latter has the dielectric loss and the magnetic induction loss of the metal sheath. The former basically has only the core wire resistance loss and the insulation aging is also slower than the latter. And therefore, the operating costs are lower. Under comparable conditions with the same transmission power and comparable reliability indices, the investment in DC cable transmission lines is lower than that in AC lines, and the transmission system technology can improve the operational reliability and scheduling flexibility of the power system.

With the rapid development of rail transit systems, more and more DC cables have been put into use.

    Recently, with some track companies, design institutes, construction companies, electrical technicians of cable manufacturers, and related senior cable experts, they conducted some in-depth discussions and exchanges on DC traction cables for rail transit. As a result, it was found that DC cables appeared to be common. There are problems in many aspects. These problems are technical and non-technical; there are material and technical aspects; there are production and installation aspects; there are application and design aspects. To solve these problems, rail transit operation and management companies, related design institutes, construction units, and cable manufacturers need to work together.

    1, about the service life

    Shanghai Metro Line 1 is a German-made cable that has been operating for 20 years so far. There is no cracking on the surface of the cable and no cables have been replaced. The cable of the No. 3 line is a domestic brand. The service life of the cable is not long and it has been replaced. The main reason is that the surface of the cable is cracked. The reason for cracking is due to ultraviolet light in the atmosphere.

The DC cable used in Guangzhou Metro Line 1 is made in the United Kingdom. It has been replaced by 7 cables in 8 years. The reason for the replacement is that there is damage to the cable laying process; second, the cable is exposed to light for a long time; and third, some cables are soaked in water for a long time ( Sometimes soaked in water for up to three months).

    The use environment of the DC cable is sometimes worse than that of the AC cable.

Subways and light rails, as important transport vehicles for urban transportation, will cause many problems once they fail, and even lead to traffic congestion in local cities. Since the replacement of faulty cables requires the metro and light rails to be shut down, the replacement work is generally scheduled to take place at night.

    2. Laying of cables

    The environment for installing DC traction cables for rail transit is rather harsh. First, when laying cables, the construction is savage. Most of the people who lay cables are temporary personnel. These personnel also did not conduct professional training before laying cables. They only manage progress, regardless of quality. In the country, there is not a rail transit project where the installation and installation of cables is a civilized construction; second, the site for laying cables is uneven; industrial waste can easily scratch the outer jacket; and the third is the cable through the pipe, when the cable pipe is handled Inadvertently, it will cause cable sheath strain. After such a cable is laid and buried in operation, it is intermittently short-circuited until it is blown. Such failures often cause multiple cables to burn at the same time. Fourth, the cabling space is small and the cables are not easy to bend or cause excessive bending.

Many manufacturing companies rarely provide customers with training, guidance, and service before cable laying, nor do they provide guidance documents and precautions for installation and installation, and they do not even notice that necessary and special The tools will protect the cable installation and cause unnecessary, hidden cable faults.

     3, on the waterproof cable

     At present, many design units require the DC cable to have waterproof performance, but the qualitative ambiguity of the waterproof performance makes the waterproof structure, performance and requirements of the DC cable multifarious.

In fact, the cable waterproof performance requirements are nothing more than two, one is longitudinal waterproof; the second is radial waterproof. Vertical waterproofing generally uses expanded materials. Radial waterproofing generally uses expanding belt wraps, metal clad tubes, or polymer materials. Nowadays, in the cable used in many subways and light rails, the so-called waterproof performance requirement is only that the cable has a vertical waterproof function, and this function does not achieve waterproof effect at all.

Due to the influence of the underground environment and the atmosphere, the cables are laid in a humid environment for a long time. The water molecules penetrate the inside of the cable through rubber or plastic layers, causing the insulation electrical properties to decline, and even causing safety accidents. Therefore, the DC traction cable should be waterproof and moisture-proof. The cable waterproofing is generally based on radial waterproof, using a layer of material that is not water permeable or difficult to permeate, blocking the moisture outside the insulation, thereby achieving the purpose of protecting the insulation. Usually aluminum/plastic adhesive integrated sheath can be used; a layer of linear low density polyethylene material can also be separately designed as a waterproof layer between insulation and sheath, because linear low density polyethylene has better toughness and wear resistance. And lower water permeability. In addition, it is also possible to use an intumescent water blocking tape wrapped around the outer surface of the insulating layer so as to act as a longitudinal or radial waterproof.

     In the design of subway projects, the general design requires that the cable trench has a drainage function, but the actual situation is that by the flood season and the rainy season, the drainage system of the cable trench will not play any role at all and is merely a decoration. Therefore, there must be a distinction between the structural and performance requirements for the overhead and laying of DC cables. In addition, for the DC cable laid in the cable trench, if radial waterproof requirements are applied, the bending performance of the cable must meet the 6D requirement.

     4, the cable's flame retardancy

     At present, DC cables have higher requirements for flame retardancy, and they all use Class A flame retardant, and require halogen-free, low-smoke and other properties. The insulation and sheathing of DC traction cables are made of plastic or rubber. The cables are single-wired. The conductors are of Category 5 or Category 6. The cable is characterized by its flexibility and flexibility.

     The number of DC cables laid at the exit of the traction substation cable in the metro can reach up to 20. Calculated according to domestic cable data, the content of the cable combustible at the exit can reach 11.9L/m. Therefore, the fire-retardant category of the flame-retardant cable used in the subway DC cable belongs to Category A, and other external fire prevention measures are also required.

     Because the direct current cables for rail transit use a single structure, a single cable must meet Class A flame retardant requirements. The structural design and material selection of the cable must be relatively complicated. Usually, the integrated oxygen fingering method is difficult to apply in the technical design of cables. Many manufacturers in the design of the cable, increase the high fire-retardant oxygen barrier layer or design metal shielding layer (high flame-retardant oxygen barrier layer is divided into two kinds of extrusion type and wrapping type, extrusion type will affect the bending performance of the cable The result is that the cable's flame retardant performance is improved while the cable's other properties are reduced. In addition, because the cable also requires halogen-free, low smoke and other properties, the protective layer of the cable waterproof performance, mechanical strength decreased.

In fact, whether the high index of Class A flame retardancy is needed remains to be discussed, especially if the DC cables exposed to the atmosphere need to be halogen-free and low-smoke. B type flame retardant not? Low halogen, low smoke can not you?

    5, on the cable anti-ultraviolet performance

Due to the fact that DC traction cables are sometimes exposed to the atmosphere under long-term exposure to sunlight, the outer sheath of the DC cable in this case is easily cracked. The main reason for this is that the sheath is made of flame-retardant, halogen-free, low-smoke material. The material has low mechanical strength and elongation, and its electrical properties and water resistance are reduced.

In order to solve this problem, many rail companies have added awnings to the cables exposed to the atmosphere. The effect is also not ideal. The high flame-retardant performance of the cable brings about a decline in the UV-resistance of the cable, while the traditional HYA-type cable does not crack on long-term exposure to the cable surface below the atmosphere.

The ultraviolet resistance of the cable is generally achieved by adding an ultraviolet light absorber, a light shielding agent, and the like to the jacket material of the cable.

In addition, another cause of cracking of the cable sheath is the clamp used to fix the cable. When the cable is sub-fixed, the long-term thermal expansion and contraction of the cable in the longitudinal direction will lead to longitudinal cracking of the sheath. Materials and laying methods put forward higher requirements.

    6. Anti-rat and termite resistance of cable

      DC cables are sometimes rodents (rats) in places where they are laid

      And destruction of termites, and various other underground insects.

      Rats will actually bite anything that shortens their teeth because their incisors need to bite continuously to shorten their incisor length. And they can also be attracted by the smell or color of plasticizers.

      Termites are smaller than mice. These gregarious insects believe that if they are together, they will die. A single termite may not cause incalculable damage, but the combined forces have the power to overthrow your house overnight.

      The best way to solve cable rat and termite resistance is to use metal armor, usually 0.10mm copper tape, 0.10mm ordinary steel tape, 0.10mm stainless steel tape, but the flexibility of this type of cable Decline, the cost rises. In addition, the cable with metal strips will have a certain impact on the flame retardant properties of the cable, and it is easy to form a "chimney effect" during the combustion process, which will make the combustion more intensified.

In addition, a DC cable with a metal layer structure is of great benefit in finding and resolving faults or hidden faults in the cable. Sometimes technological innovation often requires solving many contradictory problems.

The rodent and termite resistance performance requirements of DC cables are concentrated on the outer jacket of the cable. It usually increases the surface hardness of the cable jacket to prevent rodents and prevent termites. The DC cable jacket must have flame retardant, halogen-free low smoke and UV, waterproof, oil resistance and other properties. When all the requirements are focused on a unit structure, this is a very contradictory problem in itself.

In the past, cable predecessors conducted extensive research on cable rat-proof performance, and considered that when the cable's outer diameter exceeds 28 mm, the destructiveness of the cable by rats will be greatly reduced.

7, on the voltage level

     At present, the voltage rating of DC traction cables used in many domestic projects is 3000V or 1500V, and part of the operating voltage is 750V. In fact, the actual use voltage of some cables is less than 500V. Therefore, it is necessary to develop a DC cable with a thin insulation structure type.

The DC power supply is generally a bipolar transmission with two wires, one positive electrode and the other negative electrode. The positive cable is insulated and sheathed. The negative cable is a jacket and has an insulation function. In addition, during the engineering design, production, and use, the positive and negative polarities of the DC cable must be distinguished. The large-section cable is basically used for the negative electrode.

8, the wear resistance of the sheath

For cables used outdoors, sometimes the cables need to be put on the rods. The cables on the upper poles are prone to wind blows. As the wind blows, it is easy to cause constant friction between the cable and the iron frame. The friction eventually leads to short circuit of the cables. While the line is out of service, the point of failure of such failures is difficult to eliminate unless the cable line has a monitoring system. Such accidents have occurred many times in cities that have already operated subways and light rails.

Due to the laying conditions, this laying method can also cause long-term stress on the cable head, which can cause cable failures.

To solve the wear-resistance of this type of laying sheath, additional protective measures can be added on the outer surface of the cable.

9, the cross section of the cable

Currently used cable conductor cross-section: 95mm2, 150mm2, 240mm2

300mm2, 400mm2, 630mm2, 800mm2.

Due to the limited space for DC cabling, DC cables are required to be lightweight, flexible, and have a small turning radius. Therefore, most of the conductors of DC cables use Category 5 conductors.

10、Conclusion

10.1 Judging from the current requirements for the use, characteristics, and technical specifications of DC traction cables for rail transit, people do not have a uniform standard for the characteristics and specifications of DC traction cables. Many performances are proposed on a cable and a sheath. Requirements are feasible and worth discussing. Because, the performance, cost, and service life of cables have always troubled the cable industry.

10.2 DC cables are an important part of traction power supply systems. For DC transmission cables for rail transit, cables with different performance requirements should be selected and designed according to the relevant special requirements for different occasions.

10.3 The assessment and research work on the reliability and service life of DC cables shall be strengthened.

10.4 It is recommended to accelerate the development of corresponding industry or national standards as criteria for design, production, and selection so as to meet the rapid development of rail transit in China. In the preparation of standards, the use of units, production units, design units must participate.

10.5 It is recommended that design and production units use flexible cables with thin armoring as much as possible to improve the accuracy and reliability of the insulation measurement data. In addition, the on-line monitoring technology of urban rail transit DC cables is still blank at home and abroad. Qualified enterprises must strengthen their research work, and they can also establish an expert evaluation system through on-line inspections to assess the merits of insulation and to warn their aging trends. In addition, it is also an effective means to improve the manufacturing process of DC cables, improve the quality of DC cable laying, and improve the operating environment of DC cables.

10.6 The research and development work on the flame-retardant properties and sheathing materials of DC traction cables should be strengthened.

10.7 It is necessary to develop a DC cable with a monitoring and control function based on the characteristics of the DC cable.