Countermeasures for cable failure

Strengthen cable construction management.

The following points should be done during cable construction: a. Strengthen the handover and acceptance. Before laying the cable, verify whether the type, specification, and quantity of the cable are consistent with the design drawings, and conduct insulation tests, and do not use unqualified cables; b. Strengthen the management of cable laying facilities. In order to facilitate the installation and reduce errors, hang up the cable laying cross-section diagrams at appropriate places such as cable supports, pipe channels, shafts, and turns. Cable laying is strictly forbidden to twist, flatten the armor, break the protective layer and severely scratch the surface; for direct buried laying, parallel laying on and below the pipeline is strictly prohibited. C. Strengthen the management of cable head production and construction. When installing the cable head, avoid installing it in windy, rainy days or in a humid environment, and take measures to prevent dust when installing outdoors. The ambient temperature for installation must be above 0°C. The relative humidity is below 70%. For long-running cables, moisture and small impurities are very harmful, and are likely to cause water treeing and partial discharge. Therefore, attention must be paid to environmental humidity and dust during joint construction. Pay attention to cleaning the environment before construction. In summer, people who construct joints should wear gloves. If the humidity in the environment is too high, they should be dehumidified (increasing the ambient temperature or using a dehumidifier).Use a blower to dry the insulating surface before inserting the stress cone. When installing the cable accessories, insulation treatment is carried out on the spot. Except for the influence of climate on the installation quality, dust and debris in the environment will have adverse effects. Special attention should be paid to cleaning when installing cross-linked cables. When using a heat-shrinkable tube with a lamp, soot particles that are not fully burned in the flame will be attached to the surface of the tube, causing poor contact between the insulating layers, lowering the insulation level, and increasing the leakage current. Therefore, it is required to scrub the surface with solvent for every shrinkage of a pipe. The construction of the cable head should be continuous and the time should be shortened as much as possible. After production, the cable head is tightly closed, the filler is filled with full, no bubbles, and no oil leakage; the core wires are tightly connected, the insulating tape is tightly wrapped, and the moisture-proof paint is evenly brushed; the lead sealing surface is smooth, free of blisters and cracks, and ensure the correct phase sequence .

Strengthen the completion acceptance and data management. After the completion of the cable project, strict inspection and acceptance. Production management, operation and other departments shall carry out acceptance inspections in accordance with the “Code for Construction and Acceptance of Cable Lines of Electrical Installation Engineering” and other relevant standards.(For example:Aerial Bundled Cable ASTM B231 Standard)

With the further development of the construction and transformation of urban power grids, the utilization rate of power cables has increased greatly, construction quality has been improved, project completion acceptance and operation management of power cables during normal operation have been strengthened, and potential accidents have been reduced, which is of great significance to improving the reliability of power supply . The majority of installation and operation personnel have accumulated experience in practice and summarized them in time, which also greatly promotes the improvement of the level of installation and operation.

Application of XLPE cable in urban network

Actively develop the ABC cable distribution network to meet the needs of load growth and urban construction (improvement of the city appearance and reduction of land occupation). In recent years, with the extensive use of cables in urban network construction and transformation, cable failures have gradually increased.

It is an important task for electric power workers to discuss the problems that should be paid attention to in the use of cables in urban networks.

1. Analysis of common cable faults The most direct cause of cable faults is breakdown due to insulation degradation.

Analyze the causes of cable failures in recent years, mainly include:

(1) Overload operation. Long-term overload operation will cause the temperature of the power cable to rise and the insulation aging, which will lead to insulation breakdown.

(2) Construction quality. Electrical aspect: the construction process of the cable head fails to meet the requirements, the sealing of the cable head is poor, moisture invades the inside of the cable, and the insulation performance of the cable is reduced; protective measures are not taken when the cable is laid, the protective layer is damaged, and the insulation is reduced. In terms of civil engineering: poor drainage of pipe trenches in industrial wells, long-term soaking of cables in water, impairing the insulation strength; too small industrial wells, insufficient cable bending radius, and long-term extrusion.

(3) Damage by external force. Mainly, it was the brutal construction of machinery in municipal construction, digging wounds and cutting cables.

(4) Corrosion. The protective layer suffers from chemical corrosion or electric corrosion for a long time, which causes the protective layer to fail and the insulation decreases.

(5) The quality of the cable itself or the cable head accessories is poor, and the insulating glue dissolves and cracks, resulting in insulation degradation and accidents.

2. Response measures

(1) The design model should have a proper margin. On-site operating experience shows that for the important load cables that are continuously produced, a proper margin should be left in the design and selection. Although the investment is slightly larger, it can ultimately reduce cable failures and extend cable life.

(2) Laying methods should be adapted to local conditions. Different laying methods should be adopted for different areas. Commonly used laying methods are: direct buried laying, pipe laying, trench laying, laying in tunnels, etc. In urban areas and rainy and humid areas, direct burial should not be used; cable tunnels or cable wells should be used in areas where the number of cables is relatively concentrated; when trenches are not suitable, pipe laying methods can be used.

3) Choose good quality cables and cable head accessories. The quality of the cable is very important to prevent the deterioration of water branches. When choosing a cable, you should have a certain understanding of the production process and management of the cable, so that you can buy a good quality cable and lay the foundation for reducing failures. The quality problems of the cable head accessories will dissolve and crack the cable head insulation, causing the cable to be damp, the insulation drop, and cause accidents; in recent years, we have begun to adopt cold shrinkage on the cable heads and middle heads on both sides of the main cable (especially outdoors) Head craft. Because the processing and installation are more convenient, and it is easy to store and not easy to deform, the construction quality is less affected by human factors, so it is gradually promoted.

Cable quality defects and troubleshooting methods

The degree of crosslinking (hot extension) is unqualified
If the degree of cross-linking does not meet the standard, the thermal-mechanical properties of the cable are unqualified and cannot meet the requirement of 90°C working temperature.
The reasons for the unqualified cross-linking degree are the unqualified formula and the improper proportion of the compounding agent. The formula should be adjusted to solve it; the second is the improper vulcanization process, such as low air pressure, fast line speed, high cooling water level and other factors.
The solution is to first find out what is the reason, which may be one or several reasons at the same time, and eliminate the reasons.
Unqualified structure and appearance
(1) The thinnest point of the insulating layer thickness is lower than the minimum value specified in the standard, or the average thickness is lower than the nominal value. The reasons for the unqualified insulation thickness are the fast line speed, the small amount of glue from the extruder, and the improper mold selection.
The solution is to reduce the linear speed or increase the speed of the extruder and adjust the size of the die.

(2) Eccentricity
The reason for the eccentricity is that the mold is not adjusted or the suspension control has changed after the recruitment is adjusted.
The solution is to control the drape as stable as possible during driving.
Bamboo-shaped
The reason for the appearance of the bamboo-shaped outside of the cable is electrical and mechanical systems that cause the traction speed to be unstable, and the second is that the core is too small, or the conductor outer diameter is uneven.
The troubleshooting method is to check the mechanical and electrical system and eliminate the fault. Properly adjust the size of the core, control the outer diameter of the suburbs and counties as uniform as possible, and the stranded wire that exceeds the process regulations can be used.
Surface scratches
(1) The cable touches the upper or lower wall or foreign matter in the vulcanizing tube. Therefore, it is required to adjust the degree of suspension, try to make the wire core move in the middle of the cross-linked tube, and clean up in time if foreign matter is found.
(2) There is burnt on the outer edge of the mold sleeve. The solution is to adjust the temperature of the mold sleeve when starting the car to prevent overheating. Once it is found to be burnt, it should be stopped immediately to remove it, or it will not get better by itself.

Impurities
Most of the impurities in insulating materials and semi-insulating materials are brought in during mixing and during the feeding process of the extruder. Strict attention should be paid to the cleanliness of materials during operation to prevent the mixing of external impurities. Another kind of impurity is scorch, which affects the performance and service life of the cable. Therefore, the temperature is strictly controlled during mixing and extrusion to prevent the occurrence of scorching.
bubble
There may be two reasons for bubbles generated in the insulation, one is caused during extrusion. The solution is to select appropriate molds. There are bubbles in the shielding layer. The main reason is that there is water in the material. It should be dried before extrusion. The second is insufficient cooling. At this time, the following will appear. A circle of bubbles will appear on the circumference equidistant from the core.
The elimination method is to strengthen cooling, raise the water level and lower the temperature of the cooling water.
Unqualified ABC cable performance

(1) Unqualified free discharge and dielectric loss
The causes of dissociation discharge and dielectric loss unqualified are complex, and they are shielded from the outside. Whether the insulation contains bubbles and impurities is closely related to the performance of the raw materials. The elimination method is to keep the raw materials clean and strictly follow the production process.
(2) Cable insulation breakdown
The main cause of cable insulation breakdown is the mixing of insulating materials and external damage. Due to strict avoidance of external damage, the mixing of impurities should be avoided as much as possible. The semi-finished products are strictly managed to prevent stumbling.
(3) Cushion breakdown
The main reason for the cushion layer breakdown is the steel with burrs, curling, and puncture the cushion. The method of elimination is the use of different quality requirements for the unqualified steel belt. The cushion shall be made of plastic tape with higher hardness, and ensure The thickness of the cushion.If you want to buy cables, you can learn about our Huaxing cables

Overhead lines need “nine checks”

The inspection of overheadcable is one of the basic contents of the operation and maintenance of overhead lines. Defects can be found in time through inspections so that preventive measures can be taken to ensure the safe operation of the line. Usually, line inspectors should do “nine inspections” when inspecting overhead lines.
Check the pole tower. Check whether the tower is collapsed, tilted, deformed, decayed, damaged, whether the foundation is cracked, and whether the iron components are bent, loose, skewed or rusted. Check whether the wire length of the iron bolts or iron screw caps of the tower is insufficient, the screws are loose, the binding wires are broken and loose. Check whether there are bird nests and other objects on the tower.
Second, check the crossarm and fittings. Check whether the cross arm and fittings are displaced, whether they are firmly fixed, whether the weld seam is cracked, whether the nut is missing, etc.
Three check the situation along the line. Check whether flammable, explosive or strongly corrosive substances are piled on the ground along the line, whether there are illegal structures near the line, whether there are buildings and other facilities that may harm the line during thunderstorms or strong winds; check the poles and towers Whether to erect other power lines, communication lines, broadcast lines, and install broadcast speakers, etc.; check whether the lines are connected to electrical equipment without authorization.

Four check the route. Check the wires and lightning protection wires for broken strands, back flowers, corrosion, damage from external forces, etc.; check whether the distance between the wires, the ground and adjacent buildings or adjacent trees, sag, etc. meet the requirements, and whether the sag of the three-phase wire is unbalanced Phenomenon: Check whether the wire connector is in good condition, whether there are signs of overheating, severe oxidation, and corrosion.
Five check insulators. Check the insulator for cracks, dirt, burns and flashover marks; check the deflection of the insulator string and the damage to the iron parts of the insulator.
Six check lightning protection devices. Check whether the size of the protection gap is qualified and whether the auxiliary gap is intact. Check whether the external gap of the tubular arrester changes and whether the grounding wire is intact. Check whether the porcelain sleeve of the valve-type arrester is cracked, dirty, burned, or flashover marks, and the sealing is good. Check whether the down conductor of the arrester is intact, whether the grounding body is exposed by water washing, and whether the connection between the grounding down conductor and the grounding body is firm.

Seven check pull lines. Check the power cable for rust, slack, broken strands and uneven force on each strand. Whether there is any decay or damage to the cable pile and protection pile Whether the cable anchors are loose, lack of soil and sinking of soil irrigation. Whether the wire rod, wedge-shaped wire clamp, UT-shaped wire clamp, and wire-holding hoop are corroded, whether the nut of the UT-shaped wire clamp is missing, and whether the stop device of the turnbuckle is in good condition. Whether the pull cord is pulled into the wood pole at the binding place.
Switch equipment on eight check poles. Check whether the switchgear is installed firmly, whether there is any deformation, damage or discharge traces, whether the operating mechanism is intact, and whether the distance between the leads and the ground meets the regulations.
Nine check crossing points. Check whether there are new crossing points, whether the crossing distance meets safety requirements, and whether the original crossing points endanger the safe operation of the line. Whether the protective measures are perfect.

Wire and cable commonly used plastic

Commonly used plastics for wires and cables include polyethylene, cross-linked polyethylene, polyvinyl chloride, polypropylene, polyolefin, fluoroplastics, nylon, etc.

Polyethylene is currently the most widely used and most used plastic. From the data in the table, it can be seen that polyethylene has low meson loss, high electrical resistivity, high breakdown field strength, good weather resistance, and good manufacturability. It is currently the best electrical Insulation Materials. However, due to its low operating temperature, it is mainly used as insulation for communication cables. Medium-density and high-density polyethylene have high strength and hardness, and their water permeability is low, and they are mostly used as cable sheaths. However, polyethylene has the biggest disadvantage, that is, it is easy to burn and has strong black smoke, so its application has brought many hidden dangers to the environment.

Cross-linked polyethylene is an excellent thermosetting insulating material formed by adding a cross-linking agent to low-density polyethylene. On the basis of inheriting many excellent properties of polyethylene, it has improved mechanical properties, weather resistance and allowable working temperature, thus becoming the best insulating material for power cables.

Due to the different cross-linking agents added, different cross-linking processes are formed. At present, there are three kinds of chemical cross-linking, warm water cross-linking, and radiation cross-linking that are most used. Chemical crosslinking is mainly used for medium and high voltage cables (such as 10KV and above); warm water crosslinking and radiation crosslinking are mainly used for low voltage cables (1kV and below).

The insulation performance of cross-linked polyethylene is closely related to its purity. High-voltage and ultra-high voltage cables above 35KV must be insulated with ultra-clean cross-linked polyethylene, which not only requires high purity of raw materials, but also requires high cleanliness of cross-linking process equipment and environment, and the process is stable and reliable.

It should be particularly pointed out that the insulation performance of polyethylene and cross-linked polyethylene has a “quirk”, that is, it is suitable for AC insulation, not DC insulation, especially DC high voltage will reduce its insulation life. Therefore, the DC cable insulation is mostly rubber insulation or oil-paper insulation. In addition, polyethylene and cross-linked polyethylene insulation have “hydrophobia”, and their breakdown is often related to the presence of water, that is, the formation of “water branches” under high voltage, leading to insulation damage. Therefore, when polyethylene and cross-linked polyethylene are used for the insulation of high-voltage and ultra-high-voltage cables, they are particularly “water-proof” during their processing, storage and transportation, and insulation extrusion, and there should be a water-blocking structure outside the cable insulation shield, such as metal jacket.

Polyvinyl chloride has good physical and mechanical properties and excellent process performance. It is the most used plastic in the 20th century. It is also the main insulation material and sheath material for low-voltage wires and cables. But entering the 21st century, PVC cable will gradually shrink or even fade out in the cable market. There are two reasons for this. On the one hand, people’s safety awareness has increased and they hope to adopt halogen-free materials, so many halogen-free materials have emerged. There is no doubt that it will become the new favorite of the 21st century cable industry and squeeze the market. On the other hand, PVC has five weaknesses: one is its high density, which is about 1.5 times that of cross-linked polyethylene, and its insulation cost is high; the other is its low operating temperature; and the third is its higher dielectric loss than cross-linked polyethylene. One hundred times higher; fourth, poor cold resistance (brittle at -15 degrees); fifth, toxic gas (HCL) is released during combustion. In recent years, the mechanical properties, electrical heating properties, and insulation resistance of cross-linked polyvinyl chloride developed in recent years have been greatly improved. Some small cross-section cables have been introduced into the market by irradiation technology, and they have been used in equipment and installation wires, high-voltage lead wires, automotive wires and building wiring. Application, but its shortcomings of halogen cannot be changed.

Smart grid spawns smart cables, experts may have temperature sensing function

What should a “smart cable” look like? Some experts gave an example of whether the wire and cable can understand the conveying capacity through the color of the surface, can have a temperature sensing function, and the color display can be different at different temperatures. For example, if the wire is transported more than 1000 A, the color of the wire will change from white to other colors if the transmission capacity is exceeded. If this can be achieved, it will be of great help to the operation of the line. Another example is insulation, which has a self-recovery function in the case of discharge, and the defect can be recovered by adding other materials. It is hoped that enterprises can work hard to develop such products, which will also play a great role in the upgrading and innovation of enterprises.

Some experts believe that the purpose of my country’s construction of a strong smart grid is to ensure the safety and reliability of the grid, so that people can use it more conveniently and at ease. This requires more reliable grid-related equipment. “The measure to reduce the occurrence of cable failures is to use qualified cables, set up safe cable channels and lay them in accordance with regulations, check them regularly, and conduct state assessments.” said Chen Peiyun, chief engineer of Qingdao Hancable Co., Ltd., currently laying in ultra-high voltage cables Some fiber optic cables can detect its partial discharge and temperature through the fiber optic cable, which is equivalent to having a certain perception effect. These are smart cables.

The double-ring network power cable  supply in big cities, the increasingly narrow space corridors, and the increase in the rate of underground cables in the city center have brought great demand for cables. The maintenance-free requirements of cables and the attention to the life of insulation withstand voltage put forward higher requirements on the insulation medium, performance indicators, and brand reputation of distribution cables.

With the increasing application of cables, coupled with the improvement of the power supply reliability requirements of the smart grid, cable faults occur from time to time, and there is an urgent need to study new and smarter cable detection technologies. “Diagnosing and evaluating the status of cables is an important technical means for rationally arranging cable replacement and ensuring the safety and reliability of power supply. It is also an extremely important part of effective cable management in smart grids.” National Wire and Cable Quality Supervision and Inspection Center Director Wu Changshun said.

Some cable companies have seen the business opportunities brought by smart cables and want to dig money in the field of smart cables. Recently, the intelligent ultra-flexible fireproof cable successfully developed by Far East Smart Energy has passed the full performance type test of the “National Fireproof Building Material Quality Supervision and Inspection Center” and the “National Wire and Cable Quality Supervision and Inspection Center”. The comprehensive performance has been appraised by relevant experts. As an internationally advanced level, the products have obtained national invention patents and were selected into the list of “the first batch of Jiangsu high-tech products in 2014”.

Cable Outer Sheath Types

The selection of cable sheath should meet the following requirements:
1 For AC system single-core power cables, when it is necessary to increase the resistance of the cable to external forces, non-magnetic metal armor layers should be used, and steel armor without non-magnetic effect treatment should not be used.
2 For cables in humid, chemically corrosive environments or easily immersed in water, the metal layer, reinforcement layer, and armor should have a polyethylene outer sheath, and the thick steel wire armor of the underwater cable should have an extruded outer sheath.
3 In crowded public facilities and places with low-toxicity, flame-retardant and fire-proof requirements, halogen-free outer protective layers such as polyvinyl chloride or ethylene-propylene rubber can be selected.
When the fire protection has low toxicity requirements, it is not suitable to use PVC outer sheath.
4 Except for low-temperature environments below -150C or places immersed in medicinal chemical liquids, as well as cables with low-toxicity and flame-retardant requirements, polyethylene should be used as the outer sheath of cables, and the outer sheath of polyvinyl chloride can be selected.
5 Cross-linked polyethylene(XLPE) insualted cables of 6~35kV or above 35kV used in places where water or chemical liquids are immersed should have radial waterproof structures such as metal-plastic composite water-blocking layer and metal jacket that meet the requirements of use. Medium and high voltage XLPE cables laid underwater should have a longitudinal water-blocking structure.

For the reinforced layer type of self-contained oil-filled cable, the height difference between the highest and lowest point when the line is not equipped with a plug-type connector should meet the following requirements:
1 When there is only a radial reinforcement layer such as copper tape, the allowable height difference should be 40m; but it should be 30m when used in important circuits.
2 When there are reinforcing layers such as copper tape in the radial and longitudinal directions, the allowable height difference is 80m; but it should be 60m when used in important circuits.

The selection of the outer sheath of the cable for direct burial laying shall meet the following requirements:
1 When the cable is under high pressure or has a danger of mechanical damage, it should have a reinforced layer or steel tape armor.
2 In the soil where displacement may occur, such as the quicksand layer and backfilled land belt, the cable should be armored with steel wire.
3 For extruded cables used in areas where termites are seriously harmed, a higher-hardness outer sheath should be selected, or a thin outer sheath with higher hardness can be extruded on the ordinary outer sheath.
The material of the protective layer can be nylon or special polyolefin copolymer, etc., or metal sheath or steel tape armor.
4 In areas with high groundwater level, polyethylene outer sheath should be used.
5 In addition to the above, an outer protective layer without armor can be selected.

The selection of cable sheath when laying in the air should meet the following requirements:
1 When the small cross-section extruded insulated cable is directly laid on the arm support, it should be armored with steel tape.
2 In places with high safety requirements and severe rodent damage, such as underground passenger transportation and commercial facilities, plastic insulated cables should be armored with metal tape or steel tape.such as 1-35kV middle voltage underground armoured power cable .
3 When the cable is in a high drop stress condition, the multi-core cable should have steel wire armor, and the AC single-core cable should comply with Article 3.5.1 of this code.
Provisions.
4 Cables that are laid on bridges and other densely supported cables may not contain armor.
5 When it is clear that it is necessary to coordinate with environmental protection, the outer protective layer of polyvinyl chloride should not be used.
6 Except for cables with heat-resistant outer sheaths such as polyethylene, which should be used in accordance with the provisions of paragraphs 3 and 4 of Article 3.5.1 of this code and paragraph 5 of this article, and cables with heat-resistant outer sheaths such as polyethylene should be used in places with high temperature above 60℃ Outer protective layer.
Cables that require frequent bending or loops with higher flexibility requirements, such as mobile electrical equipment, should use rubber outer sheaths.
The cables in the place where radiation is applied should have outer sheaths such as polyvinyl chloride, neoprene rubber, and chlorosulfonated polyethylene suitable to withstand the intensity of radiation radiation.
The cable laid in the protective tube shall have an extruded outer protective layer.

The selection of the sheath for laying cables under water shall meet the following requirements:
1 In ditches, small rivers that are not navigable, cables that do not need an armored layer to withstand tension can be armored with steel tape.
2 For cables in rivers, lakes and seas, the steel wire armor type selected should meet the stress conditions. Optional when laying conditions have mechanical damage and other preventive requirements
Use an outer protective layer that meets the requirements of protection and corrosion resistance enhancement.
3.5.9 The selection of cable sheath when the path passes through different laying conditions shall meet the following requirements:
1 When the total length of the line does not exceed the length of the cable manufacturing, it is advisable to select the same or different ones that meet the conditions of the entire line.

Factors Affecting XLPE Insulation Thermal Shrinkage

The cross-linked polyethylene cable material (XLPE) can make the aggregate structure of the insulating material in a reasonable state through the cross-linking process, and can increase the long-term working temperature of the power cable to 90 ℃, and the instantaneous short-circuit temperature is 170 ℃ ~ 250 ℃. The performance remains unchanged while other performances are improved and enhanced. Therefore, the use of cross-linked polyethylene (XLPE) insulated cables is becoming wider and wider. However, the author found in the experiment that due to the relatively small contact area between the insulating material and the conductor of the small-area cable, especially when the surface of the single-core conductor is smooth and rounded and the adhesion is insufficient, the thermal shrinkage of the insulation is large, and it is difficult to reach the national standard GB/ T12706-2008 “Rated Voltage 1kv (Um=1.2kV) to 35kV) Extruded Insulated Power Cable,such as 1-35kV underground armoured power cable , and Accessories” stipulates the requirement of not more than 4%, and the cable with larger area and higher voltage level is due to insulation and conductor It is easier to pass the test if the contact area is larger and the insulation thickness is larger.
Cables that fail the insulation heat shrinkage test are prolonged during use. Because of the excessive shrinkage caused by the insulation, the conductors may be exposed, which may cause the danger of telephone calls. Therefore, we must try our best to solve the problems in the production and improve the product quality of the cable.
So what factors will affect the insulation thermal shrinkage, and what causes the insulation thermal shrinkage test to fail?

PE is a crystalline polymer that is subjected to shearing and traction stretching under a heated environment (melting temperature), which makes the crystal grains of PE molecules increase in size along the stretching direction (longitudinal) and decrease in lateral size. The ordering is improved, that is, PE molecules are oriented, which increases the number of crystal nuclei, shortens the crystallization time, and increases the crystallinity and strengthens the orientation. However, when the finished XLPE insulated cable is placed at room temperature, the internal stress (shrinkage stress) generated during the extrusion of the XLPE insulation increases, which makes the crystallized XLPE molecules easy to de-orientate (the trend of shrinking). These factors, and Mainly related to the melting temperature and time, cooling rate, and external force (traction and stretching) in these three aspects:

1) Melting temperature and time
At high melt temperature, the crystalline polymer is a melt containing crystal nuclei, and the longer the melting time, the fewer the number of crystal nuclei. Therefore, during the cable insulation extrusion process, the higher the heating and melting temperature of the XLPE insulation material, the longer the residence time (holding time) at the heating temperature, the less the number of crystal nuclei, and the lower the crystallization performance of PE Cable, which is beneficial to reduce The crystallinity of the insulation can make the insulation thermal shrinkage meet the standard requirements;

2) Cooling rate
The temperature decrease rate of the polymer melt from above the melt temperature to below the glass transition temperature is called the cooling rate, and the cooling rate is the key to the crystallization of the polymer. The cooling rate is not only related to the melt temperature and room temperature, but also related to the crystallization rate and thermal properties of the polymer itself. The crystallization rate of PE itself is very large, and PE insulation can also get a higher crystallinity under extremely fast cooling conditions. Therefore, this situation is especially obvious in winter, and special attention should be paid to the control of the cooling rate during the extrusion process of XLPE cable insulation. The specific heat capacity of PE is large and the thermal conductivity is small. If the PE melt has a slow cooling rate and sufficient cooling is obtained, the relaxation process of the PE molecules will be prolonged, the orientation can be easily de-orientated, the degree of orientation can be reduced, and the generation of PE nuclei can be controlled and the grains can be delayed Grow up;
In addition, the conductor temperature also affects the cooling rate of the XLPE insulation. The conductor temperature is too low. When the high-temperature PE melt at the die of the extruder is coated on the surface of the conductor, the LPE insulation will shrink and shrink due to contact with the low-temperature conductor to produce shrinkage stress, and reduce the adhesion between the XLPE insulation and the conductor. Focus on reducing the resistance to heat shrinkage, and ultimately affect the heat shrinkage performance of the insulation layer of the XLPE insulated cable.


3) External force (traction and stretching)
In the cable insulation production process, the molecules are oriented along the force side under the action of external force (traction and stretching), which will promote the formation of nuclei, increase the speed of crystal nucleus generation, increase the number of crystal nuclei, and shorten the knot time and crystal Degree increases. Extrusion tube extrusion is commonly used in current electric winding production enterprises. Compared with extrusion molds, the insulation heat recovery of Si-XLPE insulated electric windings produced by extrusion tube molds that must be stretched during the extrusion process It is much larger. The disadvantage of the poor compactness of the plastic layer of the extrusion tube extrusion is likely to cause the marginal heat shrinkage test to fail, but because the process inspection and final inspection do not do the type test item, the insulation heat shrinkage test is ignored, even if it is extrusion In order to improve the production speed and the smoothness of the extrusion surface, the compression mold is generally several millimeters larger than the insulation outer diameter of the cable. In this way, the insulation outer diameter is ensured during the insulation production process, and insulation is inevitable. It will be stretched, and the molecules are still affected by external forces during the stretching process, resulting in orientation, which makes the insulation thermal shrinkage of the produced cable also larger, even far exceeding the standard requirements.

Repair Method of Poor Insulation Sheath

Scope of application
When the PVC insulation cable and sheath layer of the wire and cable have local defects, they are allowed to be repaired, such as broken glue, collapsed pits, disconnections, wrinkles, bumps, ears, edges, breakdown, joints, etc.

Materials and equipment used:
The raw materials use plastic strips, skins, blocks and tubes of the same plastic. The raw materials should be smooth, clean and free of other defects.
The instruments used are fine wooden files, knives, scissors, pliers, screwdrivers, copper sheets or flat and smooth cable paper. The power of hot-air plastic welding torch, electric soldering iron and welding torch for plastic welding is above 300W.

Repair method of local defect
Repair methods such as breakdown points, holes, pits, etc.
Use a knife to trim the defect and cut into a plastic block with a slope of 45° angle and the same size, place it on the repair area, fix it with pliers or a screwdriver, and then use a hot air speed welding gun to continuously weld it, compact and press it with a copper sheet Tighten and flatten. When welding plastic, pay attention to the hot air temperature of the welding gun not to be too high, so as not to burn the plastic in the repaired area. The repaired defect is tested by a spark machine, and it is qualified if it does not break down.
Use a knife to cut the defect of the plastic layer into a slope of 45°, remove the plastic block or strip with the same shape, color and thickness, fix it with pliers or a screwdriver, connect it with a hot air speed welding gun, and then compact it with a copper sheet , Compacted, flattened, and finally tested by spark machine, it is qualified without breakdown.
Scrape the plastic defect flat with a knife, fill in the recessed part with the same plastic strip under the action of a hot-air plastic welding gun, and then flatten, compress and compact the defect repair place with a copper sheet. After the spark machine test, no breakdown Is qualified.


Repair Method of Large joint:
1) Repair of general large joints: cut both sides of the broken glue on the plastic layer along the circumference with a knife.
Take a 45° angle slope, take a clean plastic tube with the same color and thickness, and the same length and outer diameter as the glue break. Cut one side of the tube along the axis to form a 45° angle with each other. Use thin copper cables to tie them at equal distances, then use the same plastic strip to bond and weld them under the hot-air plastic welding gun, and then use copper sheets to compact, compact and flatten. It is qualified if the spark machine does not break down.

 


2) Repair of large joints in the production process: During the production process, due to other reasons, the temporary stop, the sheath is disconnected, and the joint can be continuous. The method is to cut the plastic sheath into a circular slope with a 45° angle, retreat to the nose, extend into the mold core mouth for 30mm, and then run the glue. After the glue is run, the crew will cooperate with each other and drive the car. Connect the plastic layers by hand, and then reshape and repair.

3) Quality defects appear on the longer length of the PVC sheath power cable from one end, and most of the sheath of the other broken end is good. The length of the cable is fixed, and the repair method of large joints in the production process can also be used. Only after removing the defective end of the sheath, choose a larger mold on the extruder, according to the process, first extrude the end of the sheath, and gradually increase the traction speed to the large joint so that the sheath at the interface gradually It is thinned and covered on the original sheath that is cut into a slope shape, and then reshaped and repaired after the machine is off.

Performance Comparison of Copper and Copper Clad Aluminum Cable

Cables are divided into different inner conductors. There are two main types, one is pure copper material and the other is copper clad aluminum material. The English name of copper-clad aluminum is Copper Clad Aluminum, so copper-clad aluminum conductors are also often called: CCA conductors. Copper-clad aluminum composite wire was first introduced by Germany in the 1930s, and then promoted in the United Kingdom, the United States, France and other countries, and is widely used in various fields. CATV cables in the United States began to trial copper-clad aluminum wires as early as 1968, and consumed 30,000 tons/year. Now American countries have replaced pure copper cables with copper-clad aluminum (steel) cables.
Copper-clad aluminum is formed by concentrically cladding a copper layer on the surface of aluminum or aluminum/steel alloy core material, and the thickness of the copper layer is above 0.55mm. Due to the skin effect characteristics of high-frequency signal transmission on the conductor, the cable TV signal is transmitted on the surface of the copper layer above 0.008mm. The copper-clad aluminum inner conductor can fully meet the signal transmission requirements, and its signal transmission characteristics are comparable to those of copper with the same diameter. The body is consistent.

Compare copper-clad aluminum and pure copper in the following three aspects:

1. Mechanical characteristics:
The strength and elongation of pure copper conductor cable are larger than copper-clad aluminum conductor, which means that pure copper is better than copper-clad aluminum in terms of mechanical properties. From the perspective of cable design, pure copper conductors have the advantage of better mechanical strength than copper-clad aluminum conductors, which are not necessarily required in actual applications. Copper-clad aluminum conductors are much lighter than pure copper, so the overall weight of copper-clad aluminum cables is lighter than pure copper conductor cables, which will bring convenience to cable transportation and power cable erection. In addition, copper-clad aluminum is a little softer than pure copper, and cables made of copper-clad aluminum conductors are a little bit better in flexibility than pure copper cables.

2. Electrical performance:
Because the conductivity of aluminum is worse than that of copper, the DC resistance of copper-clad all aluminum conductors is greater than that of pure copper conductors. Does this affect the main 2014 high school entrance examination sprint comprehensive review guidance Beijing area test questions Guangdong area test questions Jiangsu area test questions to see if the cable will be damaged Using power supply, such as providing power to the amplifier, if it is used for power supply, the copper-clad aluminum conductor will cause additional power consumption and voltage drop. When the frequency exceeds 5MHz, there is no obvious difference in the attenuation of AC resistance under these two different conductors. Of course, this is mainly due to the skin effect of high-frequency current. The higher the frequency, the closer the current flows to the surface of the conductor. The surface of the copper-clad aluminum conductor is actually pure copper. When the frequency is high, the entire current is plated. It flows in the copper material. In the case of 5MHz, the current flows in a thickness of about 0.025 mm near the surface, and the thickness of the copper layer of the copper-clad aluminum conductor is about twice this thickness. For coaxial cables, because the transmitted signal is above 5MHz, the transmission effect of copper-clad aluminum conductor and pure copper conductor is the same. The attenuation of the actual test cable can prove this point.
Copper-clad aluminum is softer than pure copper conductors and is easy to straighten during the production process. Therefore, to a certain extent, it can be said that cables with copper-clad aluminum have better return loss indicators than cables with pure copper conductors.

3. Economy:
Copper-clad aluminum conductors are sold by weight, and pure copper conductors are also sold by weight. Copper-clad aluminum conductors are more expensive than pure copper conductors of the same weight. But the copper clad aluminum of the same weight is much longer than the pure copper conductor, and the cable is calculated according to the length. The copper clad aluminum wire of the same weight is 2.5 times the length of the copper wire, and the price is only a few hundred yuan per ton. Taken together, copper-clad aluminum is very advantageous. Since copper-clad aluminum cables are relatively light, the transportation cost and installation cost of the cable will be reduced, which will bring certain convenience to the construction.

4. Ease of maintenance:
The use of copper-clad aluminum can reduce network failures and prevent network personnel from “cutting the core in winter and cutting the skin in summer” during maintenance (aluminum strip or aluminum tube products). Due to the large difference in thermal expansion coefficient between the copper inner conductor and the aluminum outer conductor of the cable, in the hot summer, the aluminum outer conductor stretches greatly, and the copper inner conductor is relatively retracted, and cannot fully contact the elastic contact piece in the F header; in severe cold In winter, the aluminum outer conductor shrinks greatly, causing the shielding layer to fall off. When the coaxial cable uses a copper-clad aluminum inner conductor, the thermal expansion coefficient between it and the aluminum outer conductor is small. When the temperature changes, the cable core failure is greatly reduced, which improves the transmission quality of the network.


In general, the overall performance of copper-clad aluminum conductors is better than pure copper conductors, which will save users’ costs.
The industry believes that the use of copper-clad aluminum wire in the wire and cable industry is also a good way to relieve the current pressure on enterprises. The aluminum wire is coated with a layer of copper and made of bimetallic wire. Because of its small specific gravity and good transmission performance, it is especially suitable for the inner conductor of radio frequency coaxial cable. Compared with pure copper wire, its density is About 40% pure copper. The transmission characteristics are better than pure copper wire, and it is the most ideal internal conductor of radio frequency coaxial cable branch line.