What are the advantages of overhead cables over underground cables?

(1) The power transmission is not easily affected by the surrounding environment and has high reliability.
(2) The insulation distance between wires is small, the land occupation is small, and there is no interference with radio waves.
(3) When laying underground, it does not occupy the ground and space, which is not only safe and reliable, but also not easy to expose the target.
Therefore, in densely populated urban areas, large factories, power plants, traffic congestion areas and power grid intersection areas require small floor area, safety and reliability, reduce the impact of power grid on transportation and urban construction, and generally use AAC Cable power supply; In severely polluted areas, cables are often used to improve the reliability of power transmission; For the river crossing and river crossing lines with large span, it is not suitable to erect overhead lines, or cables are also used to avoid the interference of overhead lines to ship navigation or radio; Some national defense and military projects use cables to avoid exposing targets; Some also use cables due to the needs of architectural beauty.
Whenever encountering stormy weather, the high-voltage lines exposed in the air will be affected, resulting in power failure, and the power grid staff will nonstop start to repair and maintain the “injured” power grid. Therefore, some people will ask: since it is easy to be damaged when erected in the air, why not “hide” them underground?

Because if all the high-voltage lines are buried underground, once there is a fault, the power grid staff should first apply for the formalities of digging the ground channel, then work overtime to dig the channel, check the fault section by section, and finally find the fault point before repair. The whole process is time-consuming and labor-consuming. So, is it more appropriate to lay the high-voltage line underground or in the air?

Cost

Underground cables: AAAC Cables need to be wrapped in expensive and thick insulating shells.
Overhead wire: high enough to expose the conductor.

technically

Underground cable: difficulty in manufacturing, construction and maintenance.
Overhead wire: it can quickly find and solve faults.

Safety

Underground cable: if electric leakage occurs, it will spread around and cause accidents. Therefore, the voltage level will be limited when laying.
Overhead wire: the exposed wire can be erected in the air according to a certain safe distance, because the air is a natural insulating medium. In this way, it can not only reduce the consumption in transportation, but also reduce the risk of electric shock.

To sum up, overhead high-voltage lines have obvious advantages! Therefore, the use of overhead lines in remote mountainous areas has the advantages of fast construction and low cost. However, there is no space for building overhead lines in many parts of the city, so we can only go underground, which saves space and gives consideration to the beauty of the city.

What is the content of periodic test for high voltage cable?

“Power cable operation regulation” stipulates that under the condition of not losing normal oil pressure, it is not necessary to conduct DC withstand voltage test regularly for high voltage oil filled AAAC Cable. As long as the oil pressure is normal and the dielectric loss and breakdown strength of oil are qualified, it can be considered that the cable insulation is reliable. Therefore, oil filled cable should be tested regularly to measure its dielectric loss and breakdown strength. The test period is generally sampled one year after putting into operation. If the results of two consecutive tests do not change, the test period can be gradually extended. In addition to the oil sample test, the following tests should also be carried out.


1. Measure the insulation resistance of outer sheath
the outer sheath of oil filled cable not only insulates the metal sheath from the ground, but also has anti-corrosion effect on the lead sheath and reinforcing tape. The insulation resistance of the sheath of oil filled cable shall be measured once a year by insulation resistance meter or other methods to check the integrity of the sheath insulation and the degree of water seepage. The high pressure oil filled cable is under the positive oil pressure, and the moisture in the air cannot invade people. Periodic measurement of outer sheath insulation can find out the early mild damage of sheath. When measuring the insulation of protective layer, the transposition bar and protector on transposition board in cross interconnection box shall be disassembled, and the insulation resistance of protective layer shall be measured by sections. Long lines shall be measured with electric insulation resistance meter.


2. Measure the grounding resistance
in the sheath grounding system with two ends grounded, one end grounded or cross interconnection connection, the value of grounding resistance will affect the operation of the protector. GEOX type grounding resistance measuring instrument can be used to measure the grounding resistance of the cable sheath connected with the grounding electrode or the grounding connecting line of the protector.
3. Measure the DC resistance of conductor
by measuring the DC resistance of the conductor and comparing it with the test values in previous years, we can check the change of the contact resistance between the outgoing stem or connecting pipe and the cable core, and judge whether the conductor connection is good.

Difference between low voltage cable and medium voltage cable

There are many kinds of AAAC Cables. According to voltage, they can be divided into low-voltage cable, medium voltage cable and high-voltage cable. What we can often touch and use in our life are low-voltage cable and medium voltage cable. How can we distinguish them? Some people say it’s 1kV, others say it’s 10kV. How to distinguish between low voltage cable and medium voltage cable? Now let Dr. Bao popularize science for you.
1、 Voltage level
According to different voltage levels, cables are habitually divided into low voltage power cables (3KV and below), medium voltage power cables (6kV ~ 35kV), high voltage power cables (66kV ~ 220kV) and ultra-high voltage power cables (above 220kV).
2、 Application and laying
Low voltage cable
Low voltage cables are used to transmit and distribute electric energy, mainly used in urban underground power grid, outgoing lines of power stations, internal power consumption of industrial and mining enterprises, and power transmission under river and sea water; It has the characteristics of reliable operation, no pole, no occupation of the ground, no hindrance to the view, less external influence and so on. The common laying methods include direct buried laying, AAC Cable trench laying, tunnel laying and wall laying.


■ medium voltage cable
MV cables are mostly produced as part of power generation and distribution networks for many applications such as utilities, petrochemicals, transportation, wastewater treatment, food processing, commercial and industrial markets. The common laying methods are underground direct buried laying; Laying of protective pipe (steel pipe, plastic pipe, row pipe, etc.); Laying of cable structures (cable trench, tunnel or working shaft, etc.); Laying in other public facilities (public facilities such as roads, railway bridges, tunnels or underground shopping malls, wharves, trestle roads, etc.); Underwater laying, etc.
3、 Observe the appearance
■ insulating materials
Low voltage power cable insulation material can be: PVC, cross-linked polyethylene;
Medium voltage power cable insulation material: cross linked polyethylene.
■ number of cable cores
The number of low-voltage power cable core is divided into: 1, 2, 3, 4, 5 core, 3 + 1 core, 3 + 2 core, 4 + 1 core
The number of medium voltage power cable core is divided into: 1 core, 3 core (so this kind of cable is mainly used for trunk transmission, 3-phase balance).
Minimum specification
The minimum specification of low voltage power cable is 1.5mm ²;
Minimum specification of medium voltage power cable: 3.6/6kv: 10mm ², 6/6kV, 6/10kV:16mm ², 8.7/10kV, 8.7/15kV:25mm ², 12/20kV:35mm ², 18/30kV:50mm ², 21/35 kV、26/35 kV: 50mm ²。
Color discrimination
Low voltage power cable cores are distinguished by different insulation colors, color lines or digital marks;
The core of medium voltage power cable shall be distinguished by ribbon.


Conductor structure
The conductor of low voltage power cable can be round non compact, round compact and irregular compact;
The conductor of medium voltage power cable must be round and tightly pressed( At present, the medium and low voltage power cables produced by our company all adopt circular compact structure
Basic structure of cable
The basic structure of low voltage power cable is conductor + insulation + filling + wrapping + outer sheath;
The basic structure of medium voltage power cable is conductor + three-layer coextrusion (conductor shielding, insulation, insulation shielding) + copper tape shielding + filling + wrapping + outer sheath.
Low voltage cable and medium voltage cable have different uses due to different voltage levels, composition, structure, etc. when choosing, you can choose according to your own actual situation.

How to prevent cable corona discharge?

Corona discharge occurs in the air gap of indoor heat shrinkable, cold shrinkable or nylon AAC Cable terminal due to the action of electric field at the junction of semiconductor shielding layer and insulation layer of three core or single core cable.
When the terminal position of the cable installed in the indoor switch cabinet or metering cabinet of the distribution station is low, the ponding in the cable trench or the poor ventilation of the distribution station, and the indoor air humidity is high, the corona discharge phenomenon will also occur.
In order to prevent the corona discharge of indoor terminal, the filling material of trident of three core cable terminal must be filled. In addition to the filling material, the heat shrinkable or cold shrinkable terminal should also pay attention to the elimination of gas in the pipe during the heat shrinkable or cold shrinkable process without leaving air gap. Special attention should be paid at the junction of the two media.
The experimental results show that after the heat shrinkable terminal is moved, the discharge amount of partial discharge test will increase, and the re heating shrinkage partial discharge will decrease. Therefore, if corona discharge occurs at the heat shrinkable terminal, it will disappear after reheating and shrinking to eliminate the air gap.

If the cable trench in the station is flooded and the ventilation is poor, measures such as drainage, improving ventilation and keeping the station environment clean shall be taken( The above text is excerpted from “technical questions and answers for power cable installation and operation”)
reference:
[1] Shi Chuanqing, chief editor. Question and answer of power ABC Cable installation and operation technology. Beijing: China Electric Power Press, 2007
Extended reading:
Corona discharge, the most common form of gas discharge, refers to the partial self-sustaining discharge of gas medium in non-uniform electric field. In the vicinity of the tip electrode with a small radius of curvature, the local electric field intensity exceeds the ionization field intensity of the gas, which makes the gas ionize and excite, resulting in corona discharge. When corona occurs, light can be seen around the electrode, accompanied by a hissing sound. Corona discharge can be a relatively stable discharge form, or it can be an early development stage in the process of gap breakdown in uneven electric field.

Methods to prevent cable corrosion

(1) The cable manufacturer is required to use the protective layer outside the metal sheath of the AAC Cable when manufacturing the cable, and the aluminum sheathed cable must have polyethylene or PVC outer sheath.
2) When selecting the cable route, it is necessary to make full investigation, collect the soil data near the area where the line passes, and carry out chemical analysis, so as to judge the degree of soil and groundwater erosion. If necessary, measures should be taken, such as changing the route of the line, partially replacing the bad soil, or adding special protective measures, such as putting the cable through the corrosion-resistant pipe.


Prevent external damage and theft of facilities
Design and infrastructure stage
The setting of cable route, auxiliary equipment and facilities (ground grounding box, entrance and exit, ventilation Pavilion, etc.) shall be approved by the planning department. It should be avoided that the cable passage is adjacent to the thermal pipeline, flammable and explosive pipeline (oil and gas) and corrosive medium pipeline.
Note: it is a new clause. It is proposed that the route, auxiliary equipment and facilities should be approved by the planning department, and the requirements of avoiding the cable passage adjacent to thermal pipelines, flammable and explosive pipelines (oil and gas) and pipelines with corrosive media should be avoided.
It is difficult to understand the corrosion degree of the cable on the running cable line. In the area where the corrosion of the cable has been found or the leakage of some chemicals piled on the ACSR Cable line, the soil should be dug to check the cable, and the nearby soil should be chemically analyzed to determine the damage degree
2. Methods to prevent electrolytic corrosion
(1) strengthen the insulation between the metal sheath of the cable and the huge metal objects nearby.
(2) installation of drainage or forced drainage, polarity drainage equipment, cathode station, etc.
(3) for areas with serious electrolytic corrosion, shielding pipes should be installed.
stray current is related to the distribution of electrical track, the maintenance of joints and the layout of other underground metal pipelines. In order to protect the underground pipelines and communication cables, the departments of underground oil and gas pipelines and local communication cables sometimes adopt cathodic protection. Therefore, the cable operation Department must systematically measure the potential difference between the lead sheath and the earth and other pipelines, the stray current density of the lead sheath and the current density flowing into the earth, so as to accumulate data. When we know which area is in danger of corrosion, we should take preventive measures in time, and work together with relevant units to try to eliminate the source of stray current and do a good job of corrosion prevention.


the operation experience shows that when the current density from the cable metal sheath reaches the average value of one day and night, the metal sheath will be corroded. This paper introduces a method of measuring stray current density auxiliary electrode method,
the auxiliary electrode is made of a section of cable similar to the tested cable, and its length should be such that the contact surface between the electrode and the earth is not less than 500. Peel off the outer protective layer of the electrode surface, wipe off the armored surface, weld the connecting wire, and pour asphalt or other insulating materials on the solder joint insulation and both ends of the electrode.
In addition to cable corrosion, there are also corrosion phenomena such as metal bracket, metal hoop and grounding wire in cable channel.

How to make a perfect cable identification?

The most commonly used label for ACSR Cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label. Common material types include vinyl, polyester and polyfluoroethylene, as shown in the figure.

For bundled cables, it is recommended to use identification plates for identification. This kind of sign can be printed by printer, fixed with nylon tie or felt belt and AAC Cable bundle, and can be placed horizontally or vertically. The sign itself should have good tear resistance and conform to ROHS standards.

The most commonly used label for cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label, as shown in the figure.
5.3 label of wiring panel / outlet panel
The wiring panel identification is mainly plane identification, which requires that the material can withstand the test of the environment, meet the environmental requirements of RoHS, maintain good image quality in various solvents, and can be pasted to various surfaces including low surface energy plastics. The label should be printed and not filled in by hand. It should be clear and easy to read. All labels shall be clear and complete and meet the requirements of the environment, as shown in the figure

Why should a certain length of cable be reserved

The environment of AAAC Cable laying is complex and diverse, such as bridge, direct burial, cable trench, pipe laying and so on. Therefore, when laying, operators often reserve a certain length in the planned length. There are many advantages in doing so. On the one hand, for the convenience of future maintenance, on the other hand, the environment is complex when laying, in order to cope with emergencies, It is also necessary to reserve the length in advance.

Generally, the reserved length of cable is reserved in the cable trench of feeder cabinet, and the end should also be reserved; In case of laying through pipe or cable trench, a certain length of manhole shall be reserved in the middle.
There are many advantages to reserve the head and tail sections. Firstly, when the wire and cable are connected to the feeder circuit or equipment, the copper nose needs to be crimped, and the outer protective layer of the cable also needs to be pulled away. Generally, the exposed live parts at the head and tail ends of the ACSR Cable are prone to problems; Moreover, when the power distribution cabinet and control cabinet have problems, it is easy to affect the cables, so a certain length should be reserved at both ends of the cable to cope with the subsequent maintenance.
And one of the advantages of reservation is convenient for future maintenance. For example, when laying, a manhole will be set nearby, and the manhole can be used for cable reservation. When the maintenance personnel check and repair in the future, if there is a problem with a certain section of cable, they can use the reserved cable in the manhole for operation, which is convenient and time-saving.

There are also cable laying. The path is often extremely complex, so reserving a certain length can also cope with the sudden situation. For example, in the actual cable laying, the cable in the slot box or bridge is not completely straight, with bending and sag, so it is necessary to consider these laying redundancy. In addition, there is still a little distance between the cable head and the cable box of the motor, and a part of the cable should be cut off when making the cable head, so more redundancy should be considered in the design of cable laying.

For more information on cables, click Joy ’cable Blog

Seemingly the same national standard cable, the difference is not just the price!

Often hear AAC Cable practitioners complain: it is too difficult to sell national standard. You say you are national standard, and those who sell non-standard also say that he is national standard. As a result, the price is more than 10% cheaper than yours, so it is helpless. Here, we will review and summarize the four rounds of the national standard and non-standard confrontation.

The first round of confrontation is calculated by the weight of copper. At first, it worked, but the national standard ignored a problem. Your copper is oxygen free rod, but not standard one. When the weight of waste copper and oxygen free rod is the same, the price of waste copper and oxygen free rod is 10%. So the national standard cable spent great efforts in propaganda, and finally failed, the difference between national standard and non-standard guarantee bottom was 10%.
The second round of confrontation, auxiliary materials on the work and materials. The state has increased resistance inspection, and also increased the inspection of conventional products. Non-standard cables began to work on auxiliary materials. If copper accounts for 70% of the cost of cables, the cost of auxiliary materials takes up about 20%, which is 20% of the cost. If the materials are used a little bit, the price is 5% different from that of national standard. We also see that many of the cables exposed recently have problems with accessories.

The third round of confrontation, the wire is short to the cable short meter. In fact, the short meter has always existed, this is the most difficult to check. It used to be common on BV, such as 95 meters, 98 meters, or 90 meters. Now, because of the increase of inspection items, all the products inspected have been included, so non-standard ones have begun to work in this direction again. I hope you can draw attention, 100 meters reduced by 1 meter, 1% of the profit came out.
The fourth round of confrontation, both sides thick and thin. That is to say, both ends of the cable are national standard, the middle part is non-standard, so it saves a lot of costs, the corresponding cable price will be much lower. You can’t cut the cable from the middle, so the leak is just drilled.
Huaxing Cable Factory has always adhered to the business philosophy of “innovation technology +, continuous improvement +”. Strictly control the product quality, select the high-quality raw material supplier as the cooperation unit, and 99.99% and above oxygen free copper is used for copper core. The products are superior to our standards, have better conductivity, lower consumption, safer and environmental protection. Is many consumers more assured choice!

For more information on cables, click Joy ’cable Blog

Technical requirements of explosion proof combined switch and cable in coal mine

Technical requirements of explosion-proof intelligent vacuum combined switch
1. certificate and certificate with relevant qualification.
2. inspection report of qualified unit inspection.
3. the switch shall be provided with coal safety mark and explosion-proof sign.
4. the switch has explosion-proof and explosion resistance.
5. the altitude is 2000 meters.
6. switch protection is sensitive and reliable, with high measurement accuracy and high anti-interference capability; It has overload, over-current, phase break or leakage blocking protection.
7. fully digital, LCD display, fault self diagnosis, memory function, fault simulation test, can be inquired and modified online, with event sequence record, communication transmission, remote operation of shearer or mining face scraper conveyor.
8. when the scraper conveyor of mining face stops running, the shearer must be locked.
9. there is program control on and off operation, and three machines are matched, and full automatic centralized control is implemented.
10. it can be used for the AC 50Hz, 1140V to 3300v power grid, and interface with other automation equipment or system in a centralized control, protection, monitoring and communication.

11. it can realize single machine control, multi loop program control, single machine double speed control and dual speed control.
12. the integrity standard of flameproof joint surface must meet the following table: ① when the diameter of the control rod is d < 6.0mm, l must not be greater than 6.0mm; When d < 25mm, l must be no less than D; d> At 25mm, l must not be greater than 25mm. ② The maximum unilateral clearance m between the shaft and the shaft hole of the rolling bearing structure shall not be greater than 2/3 of the W value.
13. the surface roughness of flameproof joint surface shall not be greater than 6.3; The surface roughness of the control lever shall not be greater than 3.2.
14. the defects of the flameproof joint surface or the raised parts on both sides of the mechanical scar higher than the surface without damage shall not exceed the following provisions after grinding:
③ The local sand holes with diameter no more than 1mm and depth no more than 2mm shall not exceed 5 for each 25px2 on the explosion-proof surface with width of 40, 25 and 15mm; The explosion-proof surface with a width of 10 mm shall not exceed 2. ④ The mechanical scars, width and depth shall not be greater than 0.5mm; The length shall ensure that the effective length of the remaining non injury flameproof surface is not less than 2/3 of the specified length.
15. the explosion-proof surface shall not be rusted or oil stained, and rust proof oil or phosphating shall be carried out. If there is rust, after wiping with cotton yarn, it is qualified to leave the traces of ferrous oxide in green brown color, and those who feel no sense by hand are qualified.
16. the bolt fixed flameproof joint surface shall be tightened to the extent that the flat spring washer is not loose and the clearance is not more than the specified.
17. observe the window hole with good sealing and transparency, without damage and crack.
18. screw explosion-proof structure: thread accuracy shall not be lower than grade 3; The pitch shall not be less than 0.7mm; The minimum number of thread engagement threads and the minimum screw in concentration meet the requirements.
19. the length of the diameter of the control rod and the flameproof joint surface must meet the requirements.
Technical requirements for mine cables
1. flame retardant shielded cable which has passed the inspection by qualified units and obtained safety signs of coal mine products must be selected.
2. qualified cable unit inspection and test report.
3. relevant certificates and certificates with qualification.
4. the number and section of cable cores shall be determined by the supporting manufacturers of three machines according to the power supply rules, and the length shall be determined according to the control location.
5. it is strictly forbidden to use aluminum clad cable and aluminum core cable.
6. aluminum core cable shall not be used for grounding.
7. rubber sleeve soft cable conforming to mt818 standard must be used for high and low voltage cables laid.
8. the number of cores of special control and communication cables shall be selected and reserved with a certain number of spare lines according to the actual requirements of communication, signal and control system.
9. the cable grounding core wire shall not be used for other purposes except for monitoring the grounding circuit.
10. according to the maximum working current actually passed by the cable, it shall not exceed the allowable current for the long time of the AAC Cable.

11. cable section shall be selected according to mechanical strength, and its core section shall not be less than the minimum section specified by various electrical equipment according to the mechanical strength requirements.
12. insulation shake test of cables of different voltage withstand grades must meet the insulation shake test standard.
13. the connection between ABC Cable and electrical equipment must be made with junction box consistent with the performance of electrical equipment. The cable core must be connected to the electrical equipment by means of a tooth shaped crimping plate (claw) or a nose.
14. rubber sleeve cables of different sections are strictly prohibited to be directly connected, and must be connected through the junction box, connector or bus box meeting the requirements.
15. the low-voltage rubber sleeve cable is connected with electrical equipment. The sealing ring is made of rubber with shore hardness of 45-55 degrees, and aging treatment shall be carried out according to the regulations, and it shall meet the requirements of intact.
16. the inlet and outlet nozzles are connected tightly. The fastening degree of fasteners after wiring is qualified that the crimp type nozzle is qualified to take the pulling cable without string operation, and the screw thread nozzle shall screw the compression nut into no more than half circle with five fingers of one hand. The flattening amount of the cable after pressing the cable with the pressure plate type nozzle shall not exceed 10% of the cable diameter.
17. the connection of cable core wire shall be crimped or welded, and binding is strictly prohibited. The joint resistance after connection shall not be greater than 1.1 times of the resistance of the same length of the core, and the tensile strength shall not be less than 80% of the original core.
18. when connecting the shielded cables, the shielding layer of the main wire core must be stripped.

Is the cable conductor detected by the cable nose in accordance with the national standard

The AAC Cable nose is often used for the end connection and continuation of the cable, which can make the connection between the cable and electrical appliances more firm and safer. Cable companies often receive customer complaints: with the corresponding specifications of the wire nose through the corresponding specifications of the conductor, found that the wire nose is larger than the outer diameter of the cable conductor! If it is suspected that the cable is a non-standard product, let the cable enterprise send someone to explain and deal with it on site immediately; In particular, with the use of “special-shaped wire stranded cable conductor and its production method” by cable enterprises, the compression coefficient of cable conductor has increased from 0.90 to more than 0.98, and the outer diameter of cable conductor has decreased by more than 5%. In this way, cable users worry about whether the cable is non-standard or wrong cable specification after receiving the goods? Today, I want to explain to you: why is the cable nose larger than the outer diameter of the cable conductor?

The reason why the cable nose is larger than the outer diameter of the cable conductor is mainly to reduce the contact resistance of the connection and strengthen the local heat dissipation capacity. For a cable, the resistance of the whole length is evenly distributed, but no matter how much pressure you use to install the connection of the cable nose, the resistance of the contact part is always greater than that of the non connection part of the cable, resulting in serious local heating, The solution is to increase the contact area and reduce the contact resistance. At the same time, the larger the nose, its heat dissipation capacity will be higher. Generally, the area of the cable nose is 30-50% larger than the corresponding specification of the cable conductor, because the cable conductor is sometimes rolled into a fan or tile shape, which is not a pure circle. Generally, it needs to be compressed by a hydraulic press and then turned into a circle. If the inner hole of the cable nose is designed according to the nominal outer diameter of the cable conductor, some special-shaped conductors cannot be penetrated.

For more information on cables, click Joy ’cable Blog