Advantages of copper-clad aluminum power cable

1.DC resistivity. The resistivity of copper clad aluminum wire is larger than that of pure copper wire, about 1.5 times that of pure copper wire. When the cathode value is the same, the weight of copper clad aluminum wire is about 1/2 of that of pure copper wire. According to the calculation of the skin effect, when the high frequency is above 5MHz, compared with the acsr conductor of the same cross section, its resistivity is the same. In the use of power cables with a frequency of 50 Hz, the skin effect and proximity effect of copper conductors gradually become prominent above 150mm2. At the same time, due to the continuous development of science and technology, high-order harmonic currents and energy will be injected into the power supply system. , The high-order harmonic voltage of the corresponding frequency is generated on the impedance of the system, causing the voltage waveform to be distorted, increasing the loss of the power supply system, and increasing the heat of the conductor; in addition, the cable amplifies the harmonics and generates overvoltage at the joint. Damage to the cable head. The use of copper-clad aluminum conductors can reduce the AC impedance (resistance) generated by high-order harmonics. In other applications, by increasing the volume of copper in the copper-clad aluminum monofilament and corresponding technological measures, the copper-clad aluminum/copper composite conductor is within the upper limit of the outer diameter of the existing conductor of the same specification and meets the conductor’s DC resistance requirement.
Connection method. The use of copper-clad abc conductors can meet the habit of product selection, design, use, installation, etc. of wires and cables that will be continued for many years, and it is also beneficial to the pressing and tin welding of the cable terminals.

2.Reduce AC resistance. AC resistance is the main basis for the current carrying capacity. According to the principle of skin effect, the current per unit area on the surface of a single wire is larger than the current per unit area of ​​the center of the wire, that is, the center of a large cross-section conductor In the area of ​​a circle formed by the same conductor, the current passing through the center of the circle is smaller than that of the circle. Therefore, it is the most reasonable and economical to use different metals for the center conductor and the circumferential conductor. In addition to the DC resistance and the skin effect, the AC resistance index also has the proximity effect. Compared with the copper conductor with the same DC resistance, after the copper composite conductor is used, the aluminum is in the center of the aac conductor and the copper is on the outer edge; In the composite conductor, the inner layer is copper-clad aluminum and the outer layer is pure copper. Aluminum is not sensitive to the skin effect and proximity effect of copper. At the same time, the copper composite conductor will increase the total cross-section of the conductor by a part, so it also increases the conductor. The surface area of ​​the cable improves the heat dissipation conditions of the cable and increases the heat dissipation area. The thermal conductivity of aluminum is similar to that of copper. Under the same material cost condition, the AC resistance index is much more economical.

3.It has good corrosion resistance: aluminum is more corrosive than copper, but because the copper-clad aluminum material has been completely metallurgized, aluminum is completely covered by copper and will not be in contact with water or air. It can fully achieve the same performance as copper. Copper-clad aluminum/copper composite conductors are also more useful to prevent the cable from being corroded, bruised, or due to tight pressure, poor soldering, and poor contact between the conductor and the terminal, heating and causing the copper layer to fall off and copper and aluminum. A potential difference is formed between metals, which accelerates galvanic corrosion and causes the hidden danger of burning at the end of the cable. For aluminum conductors, especially in coastal areas, the chloride ions contained in the salt spray in the atmosphere will condense on the surface of aluminum, which will easily cause local corrosion around impurities and defects on the surface, forming holes, cracks and micro-batteries, and aggravate the damage of aluminum conductors. corrosion.
4.Low cost and light weight. Compared with the copper core cable with the same technical index, the copper clad aluminum conductor cable can save more than 40% of the cost, and the copper clad aluminum/copper composite conductor cable can save more than 20% of the cost. The proportion of copper clad aluminum wire is only 37%-40% of pure copper wire. When the wire diameter and weight are equal, its length is 2.5 times that of pure copper wire.
5.Good weldability. The copper-clad aluminum wire is concentrically coated with a layer of pure copper, so it has the same solderability as pure copper wire and is convenient for production.

Causes of power cable failure

1.The insulation has deteriorated due to aging. The insulation of power cables is subject to thermal, chemical and mechanical actions that are accompanied by electrical action, so that physical and chemical changes occur in the insulating medium, and the insulation level of the medium is reduced. The insulation is damp. Intermediate joints or terminal heads are damp due to the top and bottom sealing of the structure or poor installation quality; defects such as blisters or cracks are left when the abc cable is covered with lead, which will also make the cable damp.


2.The cable is overheated. There are many reasons for the overheating of the cable. The internal cause is that the internal air gap of the cable insulation causes local heating, which causes the insulation to be carbonized. The external cause is the cables installed in densely-cable areas, cable tunnels, etc. The aac cables passing through the drying pipes and the cables close to the pipes will cause accelerated insulation damage due to cable overload or poor heat dissipation.
3.mechanical injury. Mainly refers to cable damage caused by external force. This is mainly due to mechanical action such as vehicle vibration, which deforms the cable. Deformation of the cable leads to excessive bending, damaging the internal insulation or causing an air gap inside the insulation.


4.Corrosion of the protective layer. The lead bag of the cable corrodes due to electrolysis or chemical action. Due to the different nature and degree of corrosion, the lead bag has red, yellow, orange and light yellow compounds or fine pores like sponge.
5.Overvoltage causes breakdown. Atmospheric overvoltage and internal overvoltage cause the stress on the cable insulation to exceed the allowable value and cause breakdown.
6.Moreover, analysis of actual failures shows that many outdoor terminal head failures are caused by atmospheric overvoltages.
7.Problems in the design and manufacturing process of the intermediate joint and terminal head. When the semiconductor is peeled off, the internal insulation is damaged or there are particles, dust and other impurities on the insulation surface; poor sealing of the cable head causes moisture inside the insulation, resulting in damp insulation; non-standard cable joint technology and non-standard sealing, resulting in grounding; high humidity in the production environment , Causing the insulation integrity of the production part (cable head) to be damp; the cable grounding error occurs, causing the grounding wire to form a circulating current or break.

Various types of cables

Because cables are used in many different environments, they look very different in appearance. But regardless of any cable type, they are used as a conductor for signal transmission. These acsr conductor size also have different quality performances in transmitting different signals. Except for some special applications, the cables currently used for audio and video transmission are mainly single wires, twisted pairs, coaxial wires and optical fibers.


1.1. Single wire
A single wire is the most basic type of cable (such as a wire). It consists of one or a group of wires surrounded by a plastic protective layer. This kind of cable is commonly used to transmit low-frequency signals, such as power, audio, and computer ID code.
1.2. Twisted pair
Twisted pair is a general term. There are no restrictions on the number of wires and the type of twisting, but there are only two types in the structure of the cable: twisted pair with shielded net (STP: ShieldedTwistedPair) and without shielded net Twisted pair (UTP: UunshieldedTwistedPair). Twisted-pair cables are commonly used in telecommunications, the Internet, and professional audio. This aaac conductor full form is composed of two or more independent and mutually insulated cables that are continuously twisted. Two of the cables that are twisted to each other are called a group. , The transmission impedance is generally 100_, and the diameter of a single wire is between 20AWG (American wireline standard: 0.91mm) and 24AWG (0.61mm). Twisted pair is a relatively inexpensive cable. Each set of wires has the same anti-interference ability, which can effectively suppress external electromagnetic interference (EMI) and effectively shield the transmission signal from electromagnetic interference to the outside world.
UTP cables are most commonly used in telecommunications transmission and computer network environments. According to the type of twisting, they are divided into five, super five and six types of cables. Generally, they can reach a transmission rate of 100Mbps (100 million bits per second). The STP cable adds a layer of woven metal mesh or tin foil to the periphery of the wire group, which is more conducive to improving the signal and suppressing the impact of external radio waves. The metal shell of each connector of the STP cable must maintain good contact with the shielding net.

1.3. Coaxial cable
Coaxial cable (Coaxial) is a composite composed of two conductors. The center wire of the coaxial cable is used to transmit signals. The metal shielding net plays two roles: one is to provide current for the signal as a common ground wire for the signal. The second is as a shielding net for the signal, which suppresses the interference of electromagnetic noise on the signal. The center wire and the shielding net are between the semi-foamed polypropylene insulation layer. The insulation layer determines the transmission characteristics of the aac cable and effectively protects the wires in the middle.
Coaxial cables are widely used in audio, video or radio frequency transmission, and the transmission impedance is generally 75_, which has become the standard impedance of video (in the early days, 50_ impedance characteristics were also used for video transmission). High-quality standard coaxial cables are generally more expensive than twisted pairs because of the reliable physical characteristics of coaxial cables, which can provide excellent audio and video performance. The frequency and resolution of the signal and the effective transmission distance of the cable play a decisive role in the audio and video system.


1.4. Optical fiber
Optical fiber cable (OpticCable) is the best choice for long-distance signal transmission. Optical fiber transmission is a technical method based on photoelectric conversion instead of electronic transmission. The simple principle of optical fiber transmission is: the analog electric signal is transmitted to the optical transmitter, and the input voltage signal is converted into a current signal through the signal buffer circuit and the driving circuit, and the light-emitting tube or laser is driven. In this way, the input electrical signal is converted into an optical signal, which is coupled into the optical fiber through precise optical alignment and guidance.
After the optical signal is transmitted through the optical fiber, at the receiving end the optical signal is converted into the original electron source by a wavelength-matched photodiode, and then output after being amplified by a low-noise linear amplifier.
Optical fiber signal transmission avoids many shortcomings of traditional cable transmission, and has many advantages that cable transmission can’t match; Excellent anti-electromagnetic interference: In long-distance cable transmission, the cable itself is a huge antenna, which picks up electromagnetic signals in the surrounding space. , Especially in the display system, this kind of interference signal shows the granular noise that cannot be eliminated in the image display. The core of the fiber optic cable is glass, and it transmits optical signals, which is not easily interfered by external electromagnetic waves.
Very small volume: Most optical fibers are as thick as human hair.
Very low attenuation: Because the optical fiber transmission is completed by the glass conduit, there is no signal attenuation caused by the cable resistance and capacitive reactance. The optical fiber greatly improves the transmission bandwidth and transmission distance. High security of optical fiber: The signal content transmitted by optical fiber is not easy to be eavesdropped on.
Although optical fiber seems to be the ultimate method of signal transmission, there are some disadvantages;
Higher prices: optical cables, transmitters, and receivers are expensive
High labor: In the process of arranging and routing optical cables, a lot of human resources and special tools are required. Although the transmission loss of the optical signal in the optical fiber is very low, the attenuation of the signal by the electro-optical and photoelectric conversion at the transmitting end and the receiving end is very severe. Therefore, to ensure no insertion loss transmission, it is necessary to add a high-gain multi-stage amplifier to the transmission, and to ensure that the circuit can work stably.

What are the selection methods of high-voltage cables

The ultimate principle for power supply lines in selecting cables is to ensure the safety of power supply, reduce costs to the utmost extent, and be reasonable in technical design.

The selection of AAAC conductor is mainly based on the following principles:

  1. Choose according to the duration and the maximum allowable load current. The selected cable will not generate heat after a long time of work, and the heat of the cable cannot exceed its allowable long-term temperature.
  2. Choose according to the voltage loss of the cable. The cable at the end that accepts the voltage has enough voltage, and the voltage loss must be small enough to ensure the quality of the power supply.
  3. Choose according to economic principles. The selected ABC  cable should be able to minimize the operating cost and achieve the purpose of economical power supply.
  4. Choose according to the mechanical strength of the cable. This is mainly to avoid cable damage or breakage due to insufficient strength during transportation or installation. Only when the strength meets the requirements can the safety of power supply be guaranteed.
  5. Choose according to whether the cable is stable enough when it is short-circuited. The cable must not exceed its short-circuit allowable temperature when it is short-circuited.

The difference between 3 + 2 and 4 + 1 on wires and cables

There are single core cables and multi-core cables. Some users who don’t understand have doubts. 3 + 2 core cables and 4 + 1 core cables are all 5-core cables. Is there any difference between them? The following small series to introduce the wire and cable 3 + 2 and 4 + 1 core meaning and difference.

1. The difference between wire and cable

The wire is composed of one or several soft wires with light and soft protective layer;

The cable is composed of one or several insulated wires, which are wrapped with a tough outer layer made of metal or rubber.

2. 3 + 2 and 4 + 1 on the wire represent the number of cores of a cable. There are 2 or 3, 3 + 2, 4 + 1 and 5 mutually insulated wires in a cable, which are called 2-core or 3-core, 3 + 2, 4 + 1 and 5 core wires respectively.

3.Take power cable YJV as an example

YJV 3 + 2 core

3 + 2 cable 3 is three phase lines (L1, L2, L3 phase lines, the three lines with the largest diameter), the diameter of the line is the same, 2 is the zero line and PE line, (learn more cable technology, please click here, a large number of dry goods waiting for you) one line is used for connecting the zero line (n neutral line, the diameter is smaller than L1, L2, L3), one line is used for grounding wire (PE grounding protection line, the diameter is smaller than L1, L2, L3 are also small).

4.YJV 4 + 1 core

 

In 4 + 1 cable, 4 means that the diameter of three phase wires is the same as that of zero wire, and 1 means that PE grounding wire is a single diameter; 4 + 1 means that a cable consists of four wires, one of which is used for grounding wire (L1 phase wire), one for neutral wire (n neutral wire) and one for grounding wire (PE grounding protection wire).

How to put out a fire when wires and cables are on fire

It’s common knowledge that soldiers come to cover water and earth, and water and fire are inherently contradictory. The use of water to extinguish fires is something everyone knows, but in fact, not all fires are suitable for extinguishing fires with water, and we need to consider fires. The source of the problem.  If wires and cables are on fire, especially if all aluminum alloy conductor  can they be extinguished with water? Under normal circumstances, wire and cable fires cannot be extinguished with water, because wires and cables are conductive. When water is used to extinguish a fire, water is also conductive, which will cause the risk of electric shock and easily cause short circuits in surrounding equipment. This led to aggravation of the fire.

 

Therefore, when a wire or cable catches a fire, the most important thing to do to put out a fire is to cut off the power supply, and then use a carbon dioxide fire extinguisher or dry powder fire extinguisher to extinguish the fire. When extinguishing a fire, try to be more than 2 meters away from the wire to ensure better protection. Your own personal safety.

Taking precautions before they happen, in the face of fires caused by low-voltage cables, it is better and more effective to take protective measures than emergency fire extinguishing afterwards. Wires and cables are also a high-prone place for fires because they need to transmit electricity.

Tips for ACSR cable fault location

The occurrence of ACSR Cable 336.4 MCM  faults is accompanied by the laying and use of cables. The location of cable faults varies with cable laying methods, and the difficulty in locating is gradually increasing. Among them, the positioning and searching of bridges, tunnels, and trenches are relatively simple, while the direct-buried method is the most difficult to locate. When the nature of the fault is simple, a dedicated cable fault location device can be used to locate the fault within tens of minutes. When the fault is special, it often takes 4-5 days or even longer to locate the fault.
When using the echo method to locate cable faults, sometimes through the transfer of faulty phases and wiring methods, complex faults are often transformed into simple faults, and the fault location can be quickly determined to gain time for on-site line repairs. This is important for power supply departments. Significant.


Low-voltage power cables are generally multi-core cables. After a fault occurs in continuous use after laying, they generally show two-core and multi-core phase-to-phase or relative-to-ground short-circuit faults. Sometimes when it is detected that the fault waveform collected by a certain core is not ideal, consider switching the wiring to other faulty cores for fault waveform detection. Unexpected effects will often occur, and the collected and detected waveforms will become More typical and regular, so you can quickly determine the specific location of the cable fault point.
In the long-term on-site measurement process of cable customers, it is found that after the failure of small cross-section copper core direct-buried power cables (35mm2 and below) and aluminum core cables, there may be short-circuit and disconnection faults at the same time. During on-site detection, according to the nature of the failure of each faulty core The difference between the short-circuit fault and the disconnection fault measurement will often get twice the result with half the effort.


For low-voltage cable and  direct-buried power cables with extruded armored inner lining, most of the faults are caused by external mechanical damage. When the insulated core fails, the inner lining may have been damaged. When encountering a special cable insulation fault, it is difficult to use a professional cable fault meter to collect waveforms. Consider using the acoustic measurement method to directly apply high-voltage pulses between the steel strip and the copper shielding layer of the cable, which will often quickly fix the point.
During the on-site measurement process, we also found that when the low-voltage cable fault point is determined by the acoustic measurement method, when the high-voltage wire and the ground wire are connected between the bad phase and the metal shield or armor, the insulation resistance of the two shows a low-resistance metallic connection. State, the sound is very small, the probe cannot be used to listen to the fixed point, and the effect is not ideal. Through the actual listening side many times, it was found that the distance between the discharge ball gaps was appropriately increased, and the high voltage and grounding wires were reconnected between the two phases where the fault occurred. Often the discharge sound will become louder, and the fault point will be quickly determined. .

There are many dangers of cable construction accidents in winter

When Low voltage cables and middle voltage Cables are constructed in winter, because the outdoor temperature is below minus 10 ℃, the low temperature polyvinyl chloride and cross-linked polyethylene insulation and sheath materials show hard and brittle characteristics, such as accidental accidents during the laying process If bumped, the insulated core and copper shield are easily damaged. Even if the on-site completion test can pass smoothly after the cable is laid, after the cable is continuously energized, partial discharge will occur after the capacitive current flows in the damaged part, accelerating the insulation aging, and leading to breakdown of the line. This situation is not prominent in low-voltage cables, but is prominent in medium-voltage cables. Sometimes there is no problem in the cable factory test. Because of the restrictions of various conditions, the cable is directly constructed in winter without any measures. After the cable is laid, the on-site completion test is all normal, but it appears after the power-on operation in a short period of time. Failure, anatomy of the failure point, there is no trace of mechanical damage in appearance, but serious breakdown and ablation occurred in the internal insulation.


In recent years, some newly laid medium-voltage 10kV and 35kV faulty cables have not been heated on site due to various conditions, and they have been directly constructed and installed at minus 10°C in winter. In a short period of time when the cable was energized and operated, there was a phase-to-phase and relative-to-ground insulation breakdown fault (the incident time was as short as 10 days, and the length was about 1 month). After the accident investigation, the cable production and factory tests showed no abnormalities. The completion test is also smooth (26/35kV power cable AC 52kV/1h passed). However, the failure occurred in a short period of time (as short as 10 days, and as long as 1 month) after power-on operation. This is undoubtedly related to the fact that the power cables were not heated during construction in0 winter.

Finding method for broken core of flame retardant cable

Power cables are known as the nerves of modern industry. Modern human life and production cannot do without the help of wires and cables. Since the founding of the People’s Republic of China, China has realized the important role of wires and cables in the future economic development, so it has spent a lot of manpower, material resources and financial resources. With investment in construction and research, China has become the world’s largest cable manufacturer.


In the past few decades, the productivity formed by my country’s cable manufacturing industry has made the world look at it with admiration. With the continuous expansion of my country’s power industry, rail transit industry, data communications industry, automobile industry, and mines, the demand for the cable market is also increasing. This is one reason why China’s wire and cable industry has developed so fast.
With the continuous improvement of people’s awareness of fire protection, in project construction, people increasingly prefer to use flame-retardant cables with fire-retardant properties. The market demand for flame-retardant cables is also increasing, but the cables may be used when they are in use. There will be some faults, such as cable breakdown, cable conductor damage, broken cores and other faults. The conductor of the flame-retardant cable has a small cross-sectional area and may be broken during the production of the cable or during the extrusion process. So how to check the disconnection of flame-retardant cables?


In the cable industry, there are usually three ways to check the broken core of flame-retardant cables. The first is the energized capacitance method, which uses continuous electricity to accurately determine the disconnection point of the cable. This method is extremely inefficient, and one is often checked. It takes several hours to break the line, so this inefficient method has been abandoned by many people.
There is also a method of combining capacitance method and induction method, which is to use the capacitance to find the approximate location of the disconnection, and then use the induced voltage to find the accurate disconnection point. This double combination method can locate faster than the pure capacitance method. The disconnection point has also become a more commonly used method in the industry.
Another method is the combination of constant current source and bridge method. This method is also often used. The principle is to burn and puncture the insulation at the broken core of the flame-retardant cable with a constant current source. Using the bridge method fault locator to locate, its efficiency is very high and accurate.

Analysis of Operation Failure of Armored Cable

At present, there are various on-site treatment methods for the armor layer of low-voltage direct-buried cables, including single-ended grounding and two-end grounding. There are also those that are not grounded at both ends. According to the different treatment methods of the steel tape armor at both ends of the field cable, after the cable fails, the appearance of the fault point will be different.
The steel strips at both ends of the cable are all suspended and not grounded. After a short-circuit fault occurs in the cable, the breakdown point may only be a burn-through hole in a local location of the cable line, which will not cause long-distance and large-area burning and carbonization. Because when the cable is partially damaged by accidental mechanical damage and the sheath insulation is damaged, the system may not immediately trip and power off. The damaged point will cause intermittent flashover discharge on the earth due to the action of moisture and moisture in the soil, which will eventually develop into Permanent grounding and short-circuit between phases cause a trip and power failure. Since the discharge current of the live wire to the ground is limited to the location of the damaged point of the cable, the discharge current does not form a branch circuit to the ground through the steel belt, so there is generally only one point in the entire cable after the cable fails. malfunction. However, the surface of the armor layer will be electrified at this time. In consideration of safe use of electricity, the armor layer exposed at both ends of the cable must be insulated and sealed.


The steel belt of the cable line adopts single-ended grounding or double-ended grounding. After a short-circuit fault occurs in the cable, the fault may be a section of the cable, and the local area of ​​the cable may have long-distance surface burnt and carbonized adhesion. Because after the steel belt adopts this connection method, when a single-phase ground fault occurs in the cable, a relatively large ground short-circuit current will flow in the steel belt of the cable; at the same time, the three-phase load current of the cable will also appear unbalanced. Eddy current may also occur in the belt. After the two currents flow through the steel belt, the steel belt will be like a high-power electric furnace, heating the cable sheath and insulation, and the customer’s switch selection is not appropriate, and the soil is localized. Poor heat dissipation, excessive thermal resistance, partial accumulation of cable coils, poor heat dissipation and other unfavorable reasons may cause long-distance and large-area burning and carbonization of the cable insulation and sheath. The burned area is relatively random. It may be near the fault point, or in another section. It is often the section with the most difficult heat dissipation and the section with the largest thermal resistance that burns the most. The system may not trip until the single-phase grounding develops into a two-phase short circuit and cannot be reclosed to transmit power.


For low-voltage cable armored cables, it is necessary to strengthen the real-time online detection and monitoring of the three-phase current of the cable. At the same time, after the armor layer is grounded, an armor layer current transformer should be installed to monitor the current of the steel strip from time to time. The single-phase grounding short-circuit fault of the cable should be detected and dealt with in advance to avoid the long-distance burning of the cable and cause unnecessary power economic loss, and to ensure the economy, reliability, stability and safety of the grid operation.
According to normal analysis, after a short-circuit fault occurs in a directly buried low-voltage cable, there should generally be only one fault point. However, during the actual field excavation and processing of cable fault points, it is found that low-voltage cable faults may have two or more fault points, and may also be accompanied by long-distance insulation sheath heating and burning and carbonization adhesion. The author believes that the difference in the failure of low-voltage armored cables may be related to the grounding or non-grounding of the cable armor, and the opinions and opinions may not be correct. It is hoped that professionals who have sincere insights into such phenomena can provide more scientific and authoritative analysis and opinions. To uncover the underlying cause of this phenomenon.