PVC Cable

On the service life of control cable

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The service life of control cable is determined by the air oxidation induction period of sheath raw materials. Generally, the design scheme of control cable is applied for 20 years, which is the most applied life of control cable. Although the specific daily life is likely to be longer, from the perspective of safety, the design scheme period is the most life of control cable.

Life of control cable
Elements endangering the service life of control cables and ways to increase the service life of control cables:
1. The control cable does not have to return to moisture to avoid heat and corrosion. Because the sheath of the cable has a certain maintenance effect on the cable, the corrosion is more harmful to the cable in case of heat.
2. Control cables do not have to be overloaded. Overload application of the cable will cause immediate damage to the insulation and sheath of the cable, endanger the service life of the cable or destroy the cable immediately.
3. For the route of old engineering buildings, if it is found to be swallowed or wet by water, especially if the route is worn and brittle, electric welders should be asked to repair it immediately.
4. The control cable shall prevent common faults of cable connector, which will lead to ABC Cable short-circuit fault, which will not only damage the cable, but also destroy household appliances.


5. Control cable commodity itself: select national and industrial standard cables as much as possible. Cost-effective non-standard cables are very easy to cause short-circuit fault, and the service life of non-standard AAC Cables is short.
6. Control the natural environment and temperature of the cable. The external natural environment and heat source where the control cable is located will also lead to excessive temperature of the control cable, penetration of the insulation layer, and even explosion and fire.

What is the economic optimization of cable section?

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The International Electrotechnical Commission Standard IEC 287-3-2 / 1995 puts forward the viewpoint of economic optimization of cable size, that is, conductor section: the selection of cable conductor section should consider not only the initial cost of cable line, but also the power loss cost of AAAC Cable during its economic life, which should comply with the principle of minimizing the sum of the two costs. Expressed mathematically, i.e
CT=CI+CJ


Where CT is the total cost; Ci is the initial investment cost; CJ is the present value of Joule loss during the economic life of N years, that is, it is converted into present value according to the discount rate (I). The economic optimization of cable conductor section is called “economic conductor section”, which can be obtained by calculation.
The section determined according to the maximum allowable temperature and current carrying capacity of cable insulation is actually the minimum allowable conductor section. At this time, only the initial investment is calculated without considering the conductor loss cost during the economic life of the cable. If the conductor section is increased, the line loss cost is reduced and the initial investment is increased. However, the initial investment increased by increasing the conductor section can be compensated from the reduced ACSR Cable loss during long-term operation, so as to reduce the total cost of power supply and improve the economic benefits of the power sector.
Using economic conductor section, the cable operating temperature is much lower than the maximum allowable temperature of cable insulation, which can prolong the service life of cable line and improve the safety of cable power supply.
IEC 287-3-2 / 1995 points out that the temperature difference between the operating temperature of the cable and the average ambient temperature shall be taken as 1 / 3 of the temperature difference between the maximum allowable temperature of the cable and the average ambient temperature, i.e


For example, the maximum allowable temperature of XLPE cable is 90 ℃, and if the average ambient temperature is 24 ℃, its operating temperature should be
℃
The cable section meeting this operating temperature is in line with the principle of economic optimization. According to the calculation, the section of the economic conductor shall be enlarged by 2 grades than the section standard determined by the maximum allowable temperature. For example, if the section determined according to the maximum allowable temperature is 120, the economic conductor section should be 185.

How to train cable line operators?

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The operation of cable line is a highly professional work. The technical training of AAAC Cable operators should focus on cultivating various basic skills. The learning contents mainly include:
1) basic knowledge of electrical theory;
2) structure and characteristics of power cable;
3) cable laying and manufacturing methods of joints and terminals;
4) understand the cable line design, power system operation diagram and assembly drawing of joint process;
5) performance, processing and storage methods of various common insulating materials;
6) high altitude operation skills on poles and towers;
7) cable test technology;
8) be familiar with safety and quality management procedures and systems (including urban construction, public utilities, transportation and other relevant regulations);
9) computer and other related skills.
in addition to the necessary classroom explanation and on-site practice, these basic skills should be carried out in combination with the on-site conditions. Cable line operators must be trained in the above basic skills, have certain on-site work experience, and then learn the professional knowledge of ACSR Cable operation in order to be competent for the operation of cable line.

The above text is excerpted from the technical Q & A on power cable installation and operation)
reference:
[1] Shi Chuanqing, editor in chief. Q & A on power cable installation and operation technology. Beijing: China Electric Power Press, 2007
Requirements for technical training in DL / T 1253-2013 operation specification for power cable lines
7.8 technical training
7.8.1 cable operators shall receive special technical training and pass the examination before they can carry out various operation work, and post assessment shall be carried out at least once a year.
7.8.2 operators shall have the following basic knowledge and skills:
a) Relevant laws and regulations, systems, plans, standards, etc;
b) Basic theoretical knowledge of power cable line;
c) Drawing of cable route map;
d) Finding method of cable fault;
e) Cable test and live detection technology;
f) Various laying methods of ABC Cable lines;
g) Key points of process construction methods of various cable accessories;
h) Work at height on Tower:
i) State evaluation and state management requirements.


Relevant training discussion in the three-year action plan for improving the professional level of high voltage cable 2019-2021:
3. The contradiction between the high requirements for the whole process control of cables and the low level of personnel and equipment is becoming increasingly prominent
With the rapid growth of equipment scale and assets, the business volume of operation and maintenance, maintenance and engineering cooperation of cables and channels is growing rapidly. In order to ensure high reliability power supply of urban power grid, the requirements for the whole process control of high-voltage cables are becoming higher and higher. However, some cable professionals have insufficient skills in inspection, test, maintenance and project acceptance, lack of civil engineering and other professionals, insufficient equipment allocation for test and detection, and there is still a gap with the actual work requirements. At the same time, the training of professionals is still lack of the support of practical training system, and the talent training mode needs to be improved.

Take you to quickly understand the types of flame retardant cables

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Flame retardant wires and cables have unique flame retardant, low smoke emission and harmful smoke suppression properties. What flame retardant properties should AAC Cable materials have?


Type and performance of flame retardant wire and cable
1. General flame retardant wires and cables
Generally, the insulating layer and protective sleeve of flame-retardant wires and cables are generally made of polymer materials containing halogen bulbs (or modified materials added with halogen bulb flame retardant), and polyethylene (PVC) materials are common. The general PVC epoxy resin has higher electrolytic strength, acid resistance, wear resistance and anti-aging properties. The quality and cheap price of the epoxy resin become the raw material of AAAC Cable material which is in great demand in China. However, when PVC burns, hydrogen chloride, carbon monoxide, carbon dioxide, all kinds of fatty hydrocarbons, effective chlorine chemicals and other harmful substances will be released.


2. Low smoke halogen-free flame retardant wire and cable
Low smoke halogen-free cable flame retardant wire and cable can add inorganic nickel hydroxide flame retardant such as aluminum chloride and aluminum hydroxide to the insulating layer and protective wire sleeve materials. The basic principle of flame retardant is the basic principle of condensation phase flame retardant: the water released by the decomposition reaction of aluminum chloride and aluminum hydroxide, at the same time, digest and absorb the heat, reduce the specific temperature of insulating layer and protective wire sleeve materials, and inhibit the dissolution and release of combustible gases. The converted hydroxide is also a fireproof material, which is covered on the surface of the material to improve the working ability of the insulating layer and protective wire sleeve to resist fire, and has the purpose of blocking gas and blocking combustion. Aluminum chloride and aluminum hydroxide are the advantages of flame retardant: no poison, no evaporation, high quality and low price, flame retardant and destructed opium. However, the compatibility with the epoxy resin of the base material is poor. When it is used as a flame retardant, it must be added a lot to achieve a certain flame retardant grade. In addition, it is easy to reduce the forming process performance and structural mechanical properties of the material.


3. Low smoke halogen-free flame retardant wire and cable
Thermosetting polyurethane elastomer materials are often used for the insulation layer and protective sleeve of low smoke halogen-free flame retardant wires and cables. The materials do not contain halogen elements such as fluorine, chlorine, bromine and iodine. Mercury, chromium, cadmium, lead and other heavy metal elements that cause great environmental pollution to the natural environment are removed in the manufacturing process. Acrylic resin is often used. The flame retardant principle of this kind of ABC Cable is the flame retardant principle of final break interchange. Final flame retardant refers to taking away part of the calorific value caused by the combustion of flame retardant materials, resulting in that the materials can not maintain the decomposition reaction temperature, so it can not continuously cause flammable gases, so the combustion will extinguish itself. For example, when the flame-retardant material is subjected to strong heat or burning, it can melt, and when the molten material drops, it can take away most of the calorific value, resulting in slow combustion, which is likely to stop combustion.

What are the advantages of overhead cables over underground cables?

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(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?

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“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

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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?

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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

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(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?

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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