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Bare copper wire

Bare wires and bare conductor products refer to conductive wires without insulation and sheath, mainly including three series of products: bare single wire, bare stranded wire and profiled wire.

Copper-aluminum single wire: including soft copper single wire, hard copper single wire, soft aluminum single wire, and hard aluminum single wire. It is mainly used as semi-finished products of various wires and cables, and a small amount is used in the manufacture of communication wires and electrical appliances.

Bare stranded wire: including hard copper stranded wire (TJ), hard aluminum stranded wire (LJ), aluminum alloy stranded wire (LHAJ), steel cored aluminum stranded wire (LGJ) mainly used for connection of electrical equipment and electronic appliances or components For use, the specifications of the above stranded wires range from 1.0-300mm2.

Aerial insulated cables

Aerial cables are also very common, and their characteristic is that they have no sheath. Many people have three misunderstandings about this type of cable, and Dabing corrects it here.

First, its conductors are not only aluminum, but also copper conductors (JKYJ, JKV) and aluminum alloys (JKLHYJ). Now there are also ACSR overhead cables (JKLGY).

Second, it is not only single-core, the common ones are generally single-core, but it can also twist several conductors into a bundle.

Third, the voltage level of overhead cables is 35KV and below, not 1KV and 10KV.

Control cable

The structure of this type of cable is similar to that of a power cable. It is characterized by only copper cores, no aluminum core cables, smaller conductor cross-sections, and more cores, such as 24*1.5, 30*2.5...

It is suitable for stand-alone control or unit equipment control in power stations, substations, mines, petrochemical enterprises, etc. with AC rated voltage 450/750V and below. In order to improve the ability of the control signal cable to prevent internal and external interference, the main measures are to set up a shielding layer.

Common models are KVV, KYJV, KYJV22, KVV22, KVVP. Model meaning: "K" control cable, "V" PVC insulation, "YJ" cross-linked polyethylene insulation, "V" PVC sheath, "P" copper wire shielding.

For the shielding layer, our common KVVP is copper wire shielding. If it is shielded with copper tape, it is expressed as KVVP2. If it is shielded with aluminum-plastic composite tape, it is expressed as KVVP3. The shielding layers of different materials have their own characteristics and functions. In future articles, we will discuss this topic in detail.

Five, cloth wire

It is mainly used in household and power distribution cabinets. Friends who run hardware stores are familiar with it. The BV line that is often said belongs to the wiring line. The models are BV, BLV, BVR, RVV, RVVP, BVVB, etc...

In the model representation of wires and cables, this B is often seen. Different places have different meanings.

Take BVVB as an example, the B at the beginning means wire laying, and it indicates the application classification of the cable, just like JK means overhead cable and K means control cable. The flat type represented by B at the end is an additional special requirement for the cable. The meaning of BVVB is: copper core PVC insulated PVC sheathed flat cable.

Special cables

Special cables are cables with special functions, mainly including flame-retardant cables (ZR), low-smoke halogen-free cables (WDZ), fire-resistant cables (NH), explosion-proof cables (FB), rodent-proof and termite-proof cables (FS), and Water cable (ZS), etc.

Flame-retardant cable (ZR), low-smoke halogen-free cable (WDZ): mainly suitable for important power and control systems. When the line encounters a fire, the cable can only burn to a limited extent under the action of an external flame, with little smoke and very little harmful gas (halogen) in the smoke.

When the external flame disappears, the cable can also extinguish itself, so that the fire damage to human body and property can be minimized. Therefore, this kind of cable is widely used in petrochemical, electric power, metallurgy, high-rise buildings and densely populated places.

Fire-resistant cable (NH): mainly suitable for particularly important power and control systems. When the line encounters a fire, the fire-resistant cable can resist the high temperature of 750~800°C for more than 90 minutes to ensure safe power transmission and gain sufficient time for fire extinguishing and disaster reduction.

In the face of special occasions, new products are continuously derived, such as fire-resistant cables, flame-retardant cables, low-smoke zero-halogen/low-smoke low-halogen cables, termite-proof/mouse-proof cables, oil-resistant/cold-resistant/temperature-resistant/wear-resistant cables cables, radiation cross-linked cables, etc.

Application of cable accessories

In the cable accessories, the cable terminal head and the intermediate joint as part of the cable line, the cable has four main structural layers of conductor, insulation, shielding and sheath. The four structural layers of the cable must be continued separately, and the conductors are well connected, the insulation is reliable, the seal is good and the mechanical strength is sufficient to ensure the quality of the cable terminals and cable joints, so as to ensure the power supply reliability of the entire cable distribution network. Due to the disconnection of the cable metal sheath and shielding layer, at the cable terminal and joint. The electric field distribution is much more complicated than that of the cable body, and there is axial stress in the electric field of the cable terminal. Therefore, it is necessary to use cable accessories to realize the continuity and connection of cables, that is, a connection device that can meet certain insulation and sealing requirements.

Wires and cables are widely used in many industrial fields such as energy, transportation, information communication, construction, railway, urban rail, automobile, aviation, metallurgy, petrochemical industry, etc. At present, my country's wire and cable industry is in a new stage of development. With the development of domestic urbanization and heavy industrialization in the next few years, the demand for wires and cables in the fields of communication network, power grid construction, transportation and other fields is still in a rapid growth trend. (1) Although the low-voltage power cables of 1KV and below are still dominated by PVC cables, the trend of low-voltage cross-linked cables gradually replacing PVC cables will be strengthened, and the application of warm-water cross-linked cables will increase. Low-smoke halogen-free flame-retardant cables will be developed. (2) Cross-linked cables still dominate the 6-35KV medium-voltage power cables, and prefabricated cable accessories are more widely used. (3) The application volume of 110-220KV cross-linked cables will exceed that of oil-filled cables, and it is necessary to improve the supply capacity of complete sets of accessories. The domestic application of oil-filled cables has decreased, and the export quantity of oil-filled cables should be increased.

Introduction of physical and mechanical functions of cold shrinkable cable accessories

The cold shrinkable cable accessories can complete the insulation and straight-through connection of the cable shielding layer according to the needs of users. The conductor in the cable joint is crimped with a hexagonal die according to the specified tonnage, with small contact resistance and strong mechanical strength, which can fully meet the needs of normal operation and short-circuit operation of the line.

Excellent physical and mechanical properties of cold-shrinkable cable accessories: cable accessories generally have low tensile and tear strength of silicone rubber. If the tensile and tear resistance of silicone rubber is to be improved, the elongation will decrease and the hardness will increase; The silicone rubber used to make cold-shrinkable electrical cable accessories needs to have high elongation, low hardness and high tensile and tear strength at the same time, that is, the elongation is 800%, and the Shore A hardness is 45. The tensile strength reaches 10MPa, and the tear reaches 30kN/m.

The tracking resistance of cold shrinkable cable accessories should reach level 1A3.5. Generally, the way to improve the tracking resistance of silicone rubber is to add a lot of aluminum hydroxide; however, after adding a lot of these fillers, the hardness of silicone rubber will definitely be improved and the elasticity will be reduced. The company has developed advanced technology to make the tracking resistance of silicone rubber reach 4.5 without affecting the hardness, elasticity and expansion capacity of silicone rubber. And this technology also makes the silicone rubber have a strong water-repellent ability. When rainwater pours on the cold-shrinkable cable terminal made of it, it can neither adhere to the film nor form a conductive channel; gray layer. That is to say, the electrical performance of the cold-shrinkable cable terminal can be significantly improved, and the application and exploration of the heat-shrinkable cable accessories can achieve a relatively large leakage distance within a relatively short dark group.

Research and analysis of my country's cold shrinkable cable accessories industry

Jilin Province Zhongke Cable Accessories Co., Ltd. invested 130 million yuan and has an annual production capacity of 800,000 sets of Northeast cold-shrinkable cable accessories. The research and development base has been officially established and put into operation in Changchun Jiutai Economic Development Zone. For ordinary readers, this news may not cause them too much attention, but it is a major move for the high and low voltage transmission and distribution and cable accessory manufacturing industries of my country's electric power industry, which is enough to affect the current Even the production, R&D and supply pattern of the cold-shrinkable cable accessories series products that our country urgently needs in the future.

(1) According to authoritative experts, cold-shrinkable cable accessories are mainly used for high-voltage and low-voltage transmission and distribution in the power industry. They are upgraded products of traditional cable accessories. The imported silicone rubber is prefabricated, and there is no fire source during on-site installation, so it saves time and effort, has stable performance, and is less affected by human factors, which can greatly improve the safety and reliability of cable accessories. This type of product was developed by 3M Company of the United States in the 1980s, and was soon promoted and applied to the entire power system. In a short period of time, it accounted for more than 30% of the cable accessories market in the United States. Attachments form a third of the world.

Specifically, the cold-shrinkable cable accessories have the following advantages that other forms of cable accessories do not have: excellent corrosion resistance, anti-aging performance, long service life, especially very good conductivity, which can effectively evacuate the heat caused by conductor resistance and insulating material medium The heat generated by loss can effectively improve the service life of the product; it has the characteristics of inorganic materials, has strong electrical properties such as resistance to electric corrosion and trace leakage, and is more reliable in operation; the applicable temperature range is relatively wide, minus 55 degrees Celsius and zero-up instantaneous It can maintain normal working condition at 250 degrees; it has good hydrophobicity and can be applied to a variety of environments, such as humidity, coastal salt spray, severe pollution, etc.; this type of product still has a lot of compression force on the cable insulation, and the interface pressure The larger the interface, the higher the insulation strength of the interface and the smaller the partial discharge; due to the existence of continuous compression force, the cold shrinkable cable accessories can also avoid the thermal expansion and contraction of the cable during operation, and the breathing phenomenon during bending and soft movement, thus also The phenomenon of creepage breakdown in the air gap generated between the cable and the accessories is avoided.

In addition, the cold-shrinkable cable accessories also have a unique stress cone control unit. The product structure is designed as a built-in stepped stress cone, which is more effective in stress evacuation and more reliable in product operation; The semi-conductive layer is molded by injection rubber and has a corresponding thickness, so as to ensure the performance of the outer semi-conductive layer and the safety of the entire joint. On this basis, the cold-shrinkable cable accessories also have a simple installation process, no special tools are needed, no heating with fire is required, and the installation can be completed by simply following the installation instructions; the universal range is wide, and a product of one specification can be used for a variety of cables Wire size, easy selection and management features.

It is precisely because of the above-mentioned advantages of cold-shrinkable cable accessories that in recent years, domestic major power plants, substations, urban renovations, and industrial and mining enterprises have increasingly chosen to use cold-shrinkable cable accessories, a high-tech environmental protection product. Its share in my country's cable accessories has reached more than 60%, and it is still increasing year by year. But it is regrettable that my country's current production capacity of such products is seriously insufficient, especially for high-voltage products, and half of the demand still depends on imports.

(2) In response to this situation, Jilin Zhongke Cable Accessories Co., Ltd. began to develop and produce such products several years ago. The general manager of the company, Mao Shijie, is the key "promoter": After 2004, Mao Shijie has been to South Korea, Japan, the United Arab Emirates and many other developed countries. The people in the company are busy visiting the scenic spots, but he is busy staring at the local tourist attractions. Look at the telegraph pole. It turned out that he was observing the "mystery" of power transmission in developed countries. When he went to Japan once, all his companions went to Disneyland, but he didn't even enter the gate of the park, and only walked on the main road to look up at the sky. He found that the cable accessories used in developed countries such as the United States, Japan, Germany, and South Korea were all cold-shrinkable, while those used in China at that time were all heat-shrinkable, and then he realized the future development trend of China's cable accessories industry. After returning to China, he immediately contacted his Chinese friends in Japan and bought 5 sets of cold-shrinkable cable accessories for research; then he bought more than a dozen sets in the United States and South Korea. At the same time, he also hired a number of relevant domestic experts and invested millions of dollars to cooperate with top domestic scientific research institutes to develop cold-shrinkable cable accessories. After more than a year of hard work, we finally took the lead in developing cold-shrinkable cable accessories and cable terminals and 10 kV intermediate connection series products in China, and successfully passed the domestic authoritative State Grid Corporation Wuhan High Voltage Research Institute and Shanghai Electric Cable Research Institute. Test and start launching on the market.

The road to success is always full of twists and turns, and this time is no exception. The market's response to new products has been poor. Due to institutional, thinking inertia and other reasons, many users are unwilling to take the risk of product updates. If a new product cannot be recognized by the market, it means that millions of dollars in scientific research funding and long-term labor-intensive efforts may be in vain. Many people in the company were disappointed by this result, but Mao Shijie didn't think so. He firmly believed that his judgment was in line with the development trend of China's cable accessories industry, and he firmly believed that his products would be widely recognized by the market. In order to resolve the current unfavorable situation, Mao Shijie provides new products to users free of charge, and takes the trouble to publicize the advantages of new products to users. Relying on his unremitting efforts, his tenacious will not to admit defeat, and his firm confidence in the product, he is engaged in a protracted "tug of war" with the market. In 2009, all Mao Shijie's persistence finally paid off. In that year, the government's policy on the cable accessories industry changed drastically, and some areas proposed to eliminate heat-shrinkable products and encourage the use of cold-shrinkable products. The change of policy was completely consistent with Mao Shijie's original judgment. The news spread to Jilin Province Zhongke Electric Power Equipment Co., Ltd., and the whole company was very excited, because everyone knew that the day to feel proud had come. At that time, there were very few domestic companies capable of producing cold-shrinkable cable accessories, and Mao Shijie's company was one of them. All of a sudden, users placed orders one after another, the company was in full swing, the employees worked hard and worked overtime 24 hours a day, but the supply of products was still in short supply. As a result, only the series of cold-shrinkable cable accessories created an output value of 30 million yuan for the company.

Under the guidance and promotion of Mao Shijie, the company has continued to grow and become a leading enterprise in the industry. In addition to being widely used in electric power, military industry, aerospace, high-speed railway, airport, nuclear power plant and other industries in China, its products are also exported to the Middle East and Southeast Asia, with an annual export volume of tens of millions of RMB. The company also has a number of self-owned patented technologies, among which the cold-shrinkable cable accessories project is listed in the National Torch Program and the key national Torch Program. The company is also listed as the main drafting unit of the industry standard by the National Wire and Cable Standardization Technical Committee, and participated in the "rated voltage 6kv (Um = 7.2kv) to 35kv (Um = 40.5kv) extruded insulation power cable cold shrinkable accessories" and other three Drafting of industrial standards for accessory products. At present, the company has developed into a key enterprise in the development zone and a key national Torch Plan project undertaking unit. It has 128 employees, including 5 doctors, 6 masters, 46 engineering and technical personnel with college degree or above, and also hired 5 industry authorities. As the company's technical consultant, there are 2 professor-level senior engineers and 3 senior engineers. It is precisely because of strong technical force and unremitting efforts that the company has developed and produced more than 200 types of cold-shrinkable cable accessories of various types. one-third of.

What's more worth mentioning is that the company has already been able to produce high-voltage cold-shrinkable cable accessories with a voltage level of 110,000. At present, there are very few companies in China that can produce products with such a high-voltage level. In the next step, the company will develop cold-shrinkable cable accessories with voltage levels of 220,000 and 500,000. On this basis, the company also actively develops and produces other urgently needed products for power transmission. Now it has formed four series of products: cold shrinkable cable accessories, power transmission and transformation switches, power tower protection devices and substation insulation protection devices. Among the large series of products, the series of cold shrinkable cable accessories have reached the international advanced level, and the other three types of products are all at the domestic first-class level. At present, hundreds of products produced by the company have been sold to all provinces of the country, and the number of users has reached hundreds. In Jilin Province, more than 80% of the major power transmission projects are using the products of Jilin Zhongke Cable Accessories Co., Ltd.

The investment of 130 million yuan this time is to build a Northeast cold-shrinkable cable accessories series product production and research base with an annual production capacity of 800,000 sets. The demand for products should be replaced by imports as much as possible. In order to achieve this goal, Jilin Province Zhongke Cable Accessories Co., Ltd. has selected all the world-class equipment in the base, of which more than 18 million yuan has been invested in testing and testing equipment alone, resulting in a high level of investment from raw materials to production processes and even product testing before leaving the factory. Inspections can all be completed in the base.

Its raw materials are all imported from the United States with the best global quality and performance. The products produced by this base will be further improved in terms of quality, variety and quantity.

(3) According to Mao Shijie's introduction to the reporter of China Industry News, the silicone rubber molding equipment assembly line used to produce cold-shrinkable cable accessories series products in this base is obtained from the world-renowned silicone rubber equipment manufacturer - German Desma Company and 2KM The company purchased the world's top equipment, and the total investment in the entire base equipment exceeded 30 million yuan. It includes 5 sets of flat vulcanizing machines, 2 sets of rubber injection molding machines, 4 sets of pumping machines, 4 sets of weighing machines, and 1 set of horizontal mold clamping machines. The assembly line is fully automatic computer controlled, and has the advantages of segmented temperature control function and small metering error of pump glue. Due to the large investment in the entire base and the selection of internationally leading manufacturing equipment, the vulcanization quality and yield of the cold-shrinkable cable accessories series produced are significantly higher than those of other manufacturers in the same industry, and the impact on product quality due to human factors is reduced. adverse effects, thus providing a reliable guarantee for the production of high-quality products.

It should be noted that because the high and low voltage power transmission and distribution in the electric power industry has high requirements on safety and reliability, the requirements for the experiments and testing capabilities of various product manufacturers supporting it are also very high. In order to meet the high standards of users, Jilin Province Zhongke Cable Accessories Co., Ltd. has spent huge sums of money to build a domestic leading laboratory in the base, of which more than 18 million yuan has been invested in experiment and testing equipment. At present, the company can conduct research including power frequency withstand voltage, partial discharge and thermal cycle of products, electrical properties such as breakdown strength of raw materials, tracking resistance and dielectric loss, as well as hardness, elongation at break, tensile strength, heat aging, etc. Various tests and inspections that can fully meet the needs of users, including physical and mechanical properties, therefore, the hundreds of cable accessories series products produced by the company, whether in the production process or before leaving the factory, all indicators will be After strict testing and control, the product quality is effectively guaranteed. It is understood that currently in China, there are only seven or eight enterprises with such a high level of testing and testing. In the next step, the company will further increase investment and strive to make the testing and testing level of the base's cable accessories series products reach the international advanced level.

(4) While strengthening hardware construction, Jilin Province Zhongke Cable Accessories Co., Ltd. also takes a long-term view and looks at the world, and has established a quality/environment/occupational health and safety integrated management system that can be regarded as a high standard in China. Due to the establishment of the quality management system, the company's management work has become more scientific and legalized, thereby greatly improving the overall quality, quality awareness, management level and work efficiency of the company, and the product quality control level and customer satisfaction have also been significantly improved; The environmental management system has been established, so that enterprises can consciously abide by environmental laws and regulations, reduce the impact on the environment in production and business activities, save energy and reduce production costs, thus obtaining a "green pass" for products to enter the international market "; Due to the establishment of the occupational health and safety management system, the safety quality, safety awareness and operational skills of all employees have been greatly improved in a short period of time, so that employees can consciously prevent safety and health risks in production and operation activities, and then The economic loss and negative impact on the enterprise caused by work-related injuries and occupational diseases are greatly reduced.

In terms of after-sales service, the company adheres to the service principle of "the user is God" and "as long as the user needs it, we will find ways to meet it", and has established a team led by senior engineers and domestic industry experts as technical consultants. Level after-sales service team. All technicians engaged in after-sales service are college graduates or above, have many years of on-site service experience and skills, and have been trained and guided by industry experts. At the same time, they also hold product installation qualifications issued by the domestic industry authority——Wuhan High Voltage Research Institute Certificate. These after-sales service personnel are all-rounders, who can not only carry out technical consultation, but also carry out on-site installation and guidance. In terms of after-sales service, the company also promises: the on-site service time in the province is within 24 hours, and the on-site service time outside the province is within 48 hours.

Mao Shijie is full of confidence in the future development of Jilin Zhongke Cable Accessories Co., Ltd. With the continuous improvement of my country's economic construction speed and the continuous expansion of urban scale, during the "Twelfth Five-Year Plan" period, the state will invest 5 trillion yuan in the power industry, an increase of 68% compared with the "Eleventh Five-Year Plan" investment. Cold-shrinkable cable accessories are indispensable supporting products for the construction and development of the power industry. Therefore, in the next 5 years or even a longer period of time, the market demand for cold-shrinkable cable accessories will be very large and have good development prospects. In response to this industry trend, the company has formulated a corresponding development plan. Its overall goal is to become bigger and stronger as soon as possible, try its best to meet the growing domestic demand for cold-shrinkable cable accessories, and spare no effort to replace imports. It has made outstanding contributions to the great development of the electric power industry.

Troubleshooting and countermeasures for cold-shrinkable cable terminals

With the development of my country's economic construction, XLPE cables are widely used for their convenience and durability. However, failures caused by the production of cable terminal joints occur from time to time. Among the cable faults encountered by the author based on years of work experience, the failure rate of cable terminal joints is as high as 80%. The main reason is that the process of making cable terminals cannot meet the requirements. Now analyze and explain the problems encountered in the acceptance test of the cold shrinkage terminal of XLPE cables, and find out the key links and solutions of the production. (cable accessories, heat shrink cable accessories, cold shrink cable accessories)

1 Fault occurrence process

On September 21, 2013, we conducted an acceptance test on a 35kV#3 capacitor incoming cable in a 220kV substation. The test method used was series harmonic AC withstand voltage, and the three phases were carried out at the same time. The test voltage was 52kV and the test time should be 60 minutes. , when it went to the 14th minute, a high-voltage flashover occurred. At the same time, our guardians heard a strong "pop" discharge sound at the other end of the cable, and then tried to apply the voltage again. When it was added to 31.3kV, it happened again. High-voltage flashover, and the sound of discharge is heard, and then pressurized one by one, A phase rises to 45kV without flashover and retreats to zero, B-phase 36.6kV high-voltage flashover, C-phase 24kV high-voltage flashover, and at the same time, it is heard at the other end of the cable Discharge sound, so at least two phases are problematic. Our first suspicion was a problem with the cable end connectors, as our custodial staff heard a very loud discharge on the other end of the cable.

2 Cause of failure

We disassembled the cable head and found carbon-shaped discharge traces with a diameter of 2 cm at the shielding layer, semiconductor layer and main insulation filter, as shown in Figure 1.

So how did this phenomenon happen? According to the electric field theory, any discharge is caused by the insulation not being able to withstand the electric field strength at the place. Therefore, one is that the insulation is reduced, and the other is that the electric field is too strong. The reduction of insulation is caused by air gaps, moisture and impurities, etc., and the electric field is too strong due to electric field distortion. Therefore, after analysis, there may be the following reasons:

2.1 The main insulation distance is not enough

After inspection, it was found that when the cable head was made, the length of the retained main insulation was not long enough, as shown in Figure 2(a) and (b), which was 5.5 cm different from the matching cold-shrinkable terminal, thus reducing the insulation strength and causing discharge.

2.2 The installation position of the stress cone of the cable head is not suitable

Stress cone refers to: in the cable terminal and joint, the insulation tape (or rubber prefabricated part) is wrapped from the edge of the metal sheath, so that a transition cone is formed between the edge of the metal sheath and the outer surface of the additional winding insulation. Components (in the design, the axial field strength of the cone should be a constant). As one of the key components for homogenizing the electric field of the cable terminal, the performance of the rubber stress cone is very important, which is related to the actual use effect of the cable terminal. The key part of the cable terminal is the interface between the prefabricated rubber stress cone and the cable insulation. The electrical insulation strength of the interface is related to various factors such as the cleanliness and smoothness of the cable insulation surface, interface stress, and rubber materials. The field strength near the stress cone of qualified and correctly installed cables is evenly distributed. However, if the main insulation distance is not enough, the stress cone cannot be installed at the filtering place between the shielding layer and the semiconductor layer, and the conductive rubber part of the stress cone is in a strong position. Under the action of electric field force in the electric field environment, under the action of strong electric field force, the stress distribution state of the rubber stress cone will change, and the interface stress will also change accordingly. Therefore, the stress cone cannot improve the electric field distribution at the end of the metal sheath and reduce the electric field intensity at the edge of the metal sheath. Once the stress cone loses its effect, stress concentration will occur at the cut-off point of the cable head core and the shielding layer, and the electric field intensity will be high. When the voltage reaches a certain height and acts for a certain period of time, the insulation at this place cannot withstand such an electric field intensity. , resulting in flashover or even breakdown. Therefore, the failure of the stress cone is the main reason for this discharge.

2.3 Poor grounding of shielding layer and armor layer

Poor contact between the shielding layer and the armor layer and the grounding braided wire is a possible cause of discharge.

2.4 There are scratches, burrs, semiconductors, etc. on the surface of the main insulation

After the cable semiconductor and shielding layer are stripped and cut, the knife marks are too deep or are not cleaned, and the semiconductor remains on the main insulating layer, or the process requirements are not followed during cleaning, and the cleaning is done back and forth, leaving hidden dangers and causing flashover discharge.

2.5 The crimping between the semiconductor layer, the shielding layer and the insulating layer is not tight, leaving an air gap, generating bubbles and causing discharge.

2.6 The cold-shrinkable silicone sleeve is a prefabricated accessory, which must match the cross-section of the cable

If you do not carefully check whether it is compatible before making the joint, it will inevitably cause the contraction to be not tight and the interface pressure cannot be guaranteed, resulting in impurities, intrusion into the air gap, or moisture.

3 Preventive measures and treatment

3.1 Before making, first check whether the cable accessories match the cable, so as to strictly control the interference of the cold-shrinkable silicone rubber insulating sleeve, and ensure that it has sufficient grip force to make the interface contact tightly without air gaps.

3.2 When the cable head is made, it should strictly follow the requirements of the installation instructions, that is, the length of the copper shield, semi-conductive layer, main insulation and wire core should be cut and intercepted according to the requirements of the installation process, so as to ensure the effect of the stress cone.

3.3 In order to avoid the problem of poor grounding of the shielding layer and armoring layer, the copper shielding and armoring should be kept clean during installation, and the copper braided wire should be clamped on the copper shielding and armoring with a constant force spring, and wrapped with PVC tape Constant force spring, and then wrap a layer of filling tape, clamp the ground copper braided wire in the middle, and wind a layer of rubber tape outside the filling glue.

3.4 During the production process, the force should be controlled, and the main insulation, semiconductor, and shielding layer of the cable should be avoided after the knife marks have been stripped or not cleaned. Therefore, the surface of the insulating layer should be polished with sandpaper to remove scratches, pits or residual semiconductor layers on the surface of the insulating layer, and the burrs on the edges and corners caused by the crimped terminals should be removed, and the insulating surface of the cable should be cleaned with a cleaning towel. Silicone grease is evenly coated on the surface of the insulating layer, which can ensure the elimination of hidden dangers caused by too deep knife marks and semiconductor residues, and avoid flashover discharge.

3.5 In order to avoid the crimping between the semiconductor layer and the shielding layer and the insulating layer is not tight, leaving an air gap and generating bubble discharge, our countermeasure is to place a semi-conductive strip about 1-2 mm thick on the copper shielding strip, and use A small amount of semiconductive tape covers the copper shield and the steps of the outer semiconductive layer. The end of the semiconductive layer is chamfered with a cutter to make a smooth transition between the semiconductive layer and the insulating layer.

After following the above countermeasures, the withstand voltage test passed once.

Discussion on the selection and common faults of power cables and their accessories

Author: Huang Langlang

I. Overview

In the urban rail power supply system, the power cable and its accessories are an important part of the urban power supply system. The correct and reasonable selection of the power cable and its accessories directly affects the economy of the subway investment and the safety and reliability of the power supply system. This paper starts from the power cable The selection of types, the selection of cable accessories and their installation process, the overvoltage of power cables and their grounding methods, as well as the common faults of cables and their preventive measures are briefly introduced.

2. Selection of power cable type

In the subway power supply system, according to the requirements of subway design specifications, low-smoke, halogen-free and flame-retardant cables should be used for power cables and control cables when they are laid underground, and low-smoke, flame-retardant cables can be used when they are laid on the ground.

1 Selection of cable conductors: The conductors of high-voltage cables are mainly divided into copper conductors and aluminum conductors. The selection of conductor cross-section and conductor material mainly considers two factors, one is the carrying capacity, and the other is the allowable short-circuit current. When the cable laying method, environmental conditions and sheath structure are fixed, the current carrying capacity and short-circuit current of the cable mainly depend on the DC resistance of the conductor. For copper and aluminum conductors with the same DC resistance, the weight of the aluminum conductor is different. 50% of the copper conductor, the price is cheaper than the copper conductor. However, due to the low DC resistivity of copper, the current carrying capacity of the copper core cable with the same cross-sectional area is about 1.5 times that of the aluminum core cable, and the copper conductor has a small contact resistance. High mechanical strength, good bending performance and other advantages, so copper is generally used as the conductive material of the cable in the subway power supply system.

2. Selection of cable cross-sectional area: The selection of power cable cross-sectional area mainly includes two methods: selection according to the cable ampacity and selection according to the economic current density. Quantity, and can meet the requirements of dynamic stability, voltage loss, laying temperature, environment, etc. According to the characteristics of the urban rail power supply system, high-voltage power cables and medium-voltage power cables should be selected according to the long-term peak hour power load, low-voltage power cables should be selected according to calculations related to power lighting, and DC traction cables should be selected according to the characteristics of Class VI heavy traction loads .

3 Selection of the cable outer sheath: The main function of the cable outer sheath is to prevent moisture infiltration, mechanical damage and withstand short-circuit current. Copper, aluminum, lead and stainless steel sheaths are commonly used. For the outer sheath of underground cables, the common faults are: a. damage to hard objects next to the cable; b. construction defects; c. termites and other corrosion Therefore, the selection of the outer sheath of the cable should first consider the outer sheath with higher hardness and better anti-ant performance, and secondly choose the metal sheath with corrosion resistance. At present, most of them use copper and aluminum sheaths.

Three. Selection and installation process of power cable accessories

Cable accessories mainly include cable terminal head and cable intermediate head. When selecting, attention should be paid to device type and insulation characteristics. Mechanical strength, metal sheath, grounding method, etc.

The cable terminal is an indispensable accessory when the high-voltage cable is put into the power grid. It is divided into heat-shrinkable type and cold-shrinkable type. The heat-shrinkable cable terminal is mainly made of rubber and plastic, and the linearity of the polymer is made by radiation or chemical methods. The molecular chain becomes a network structure, that is, cross-linked. After being expanded to a specific size, it can be retracted to the size before expansion by proper heating during use. When installing the cable terminal, measure about one meter from the end of the cable, strip off the outer sheath of this section of cable, and wrap a circle of copper woven bag on the shielding layer according to the 30mm opening of the outer sheath section at the lower end, and connect it with the shielding layer. Weld firmly, then fix the stress tube and the crimping terminal, put two terminal liners on the outside of the copper terminal, heat and fix it, wrap the filling glue, and then fix the sealing tube and the rainproof skirt. The cold-shrinkable cable lug is formed by injection vulcanization of elastomer material in the factory, expanded and lined with plastic spiral support. The installation of cold-shrinkable cable terminals is relatively simple and no special tools are required. However, the price of raw materials for cold-shrinkable cable terminals is relatively high, and the cost of manufacturing is relatively high. Therefore, heat-shrinkable cable terminals are still generally used in the market.

When the cable is connected with SF6 fully-enclosed electrical equipment, it adopts a closed terminal, when it wants to connect with a high-voltage transformer, it adopts a trunk-type terminal, when it is connected with an electrical appliance and has an integral plug-in function, it uses a plug-in terminal, and when it is connected with other electrical appliances, it uses an open-type terminal.

When the cable line is long, due to the connection of the cable and the need for the metal sleeves of the cable line to be connected to each other, joints must be used. According to its function, the cable metal sheath, grounding shield and insulation shield are electrically disconnected or continuously divided into insulated joints and straight joints. The construction type of the cable joint should be selected according to the insulation type of the connecting cable, the installation environment, and the operating conditions. The cable joints include insulating joints, T-shaped or Y-shaped branch joints, conversion joints, and straight-through joints.

Four common cable faults and measures

In urban rail power supply system faults, power cable faults are relatively frequent, so understanding the cause of cable faults is very important to reduce cable damage and quickly determine the fault point. Cable faults generally have the following types: (1) ground fault: one or more cores of the cable are grounded; (2) short circuit fault: two or three cores of the cable are short-circuited; (3) disconnection fault: one or more cores of the cable are grounded The fault current is blown or the external force is broken, resulting in complete or incomplete disconnection; (4) Flashover fault: Most of this fault occurs in preventive tests, and most of them appear at the cable joints. When the applied voltage reaches a certain value, it breaks down, and when the voltage drops to a certain value, the insulation recovers; (5) Comprehensive failure. Faults with two or more properties at the same time. The causes of power cable failures can be roughly classified into the following categories:

1 Mechanical damage

Cable faults caused by mechanical damage account for a large proportion of cable accidents. Some mechanical damage is so minor that it did not cause a failure at the time, but it took months or even years for the damage to develop into a failure. The main reasons for the mechanical damage of the cable are as follows: a. Damage during installation: the cable is accidentally bumped during installation, the cable is damaged due to excessive mechanical traction, or the cable is damaged due to excessive bending; b. Directly damaged by external force : Urban construction is carried out on the cable path or near the cable after installation, so that the cable is directly damaged by external force; c. The vibration or impact load of the driving vehicle will cause the lead (aluminum) package of the underground cable to crack; d. Due to natural phenomena The damage caused: such as the expansion of the insulating glue in the middle joint or the terminal head and the expansion of the shell or the cable sheath; the scratch of the cable sheath installed on the nozzle or bracket due to the natural travel of the cable; disconnection of intermediate connectors or conductors.

2 Insulation damp

Insulation caused by damp. The main reasons for the cable to be damp are: a. Water ingress due to unsealed structure or poor installation of the joint box or terminal box; b. Poor manufacturing of the cable, small holes or cracks in the metal sheath;

c. The metal sheath is pierced by foreign objects or corroded.

3 Aging and deterioration of insulation

The air gap inside the cable insulation medium is dissociated under the action of an electric field, which reduces the insulation. When the insulating medium is ionized, chemical products such as ozone and nitric acid are produced in the air gap, which corrodes the insulation; the moisture in the insulation causes the insulating fiber to hydrolyze, resulting in a decline in insulation. Overheating will cause insulation aging and deterioration. Electric dissociation in the air gap inside the cable causes local overheating and carbonization of the insulation. Cable overloading is a very important factor in cable overheating. Cables installed in cable-intensive areas, cable trenches, cable tunnels and other poorly ventilated places, cables passing through drying pipes, and parts close to cables and heat pipes will cause accelerated insulation damage due to their own overheating.

In addition, before the power cable is electrified and put into operation, in order to prevent the occurrence of cable faults, preventive tests should be carried out on the cables in accordance with relevant regulations. The tests mainly include the following contents: measurement of insulation resistance, DC withstand voltage test and leakage current measurement, inspection The phase of the cable line, the insulating oil test of the oil-filled cable, etc.

5. Conclusion

At present, the power cables of the urban rail power supply system are mostly buried underground. Due to the complex and varied cable fault conditions and the buried environment, it brings great inconvenience to cable inspections and power workers at all levels. In order to effectively prevent cables and cables In order to reduce the loss caused by failure, cable manufacturers and cable construction units should standardize production and construction, and do a good job in relevant factory and completion tests. Construction and operation personnel should strengthen the inspection and maintenance of cables and be familiar with cable burial. When a cable fails, it can correctly analyze the cause of the cable failure, accurately determine the nature of the cable failure, and effectively select cable failure measurement methods and instruments to ensure the safety and reliability of the subway power supply system.


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