Characteristics and mechanical properties of aluminum

Although aluminum has good corrosion resistance in the atmosphere, the atmospheric temperature, salinity and other impurities have a great impact on it. For example, the corrosion rate is 0.0011 mm / year in rural areas, 0.11 mm / year at sea and 0.08 mm / year in industrial areas. In neutral aqueous solutions such as carbonate, chromate, acetate and sulfide, the corrosion resistance is good, but it deteriorates in aqueous solutions of chloride. In acidic aqueous solution, the corrosion accelerates with the increase of hydrogen ion concentration. In sulfuric acid, dilute nitric acid and phosphoric acid, the corrosion resistance is poor, especially in hydrochloric acid, the corrosion is faster. In concentrated nitric acid (more than 80%), it is almost free from corrosion due to the formation of dense and firm oxide film. In acetic acid and other organic acids, it generally has good corrosion resistance. In alkaline aqueous solution, it is corroded due to the destruction of oxide film, but in ammonia water, it is not corroded due to the regeneration of oxide film.


Industrial aluminum alloys are divided into two categories, namely, deformation pot alloy and cast aluminum alloy. Deformation ship alloys can be divided into non heat treatment strengthening and heat treatment strengthening. According to the common use, performance and process characteristics, aluminum alloys can be divided into high-strength, medium strength and low-strength aluminum alloys, heat-resistant aluminum alloys, low-temperature aluminum alloys, forged aluminum alloys, rivet aluminum alloys, welded aluminum alloys, aluminum alloys with special physical properties and decorative aluminum alloys.

Basic knowledge of aluminum alloy materials

Aluminum plate refers to the rectangular aluminum plate sheet made of pure aluminum or aluminum alloy through pressure processing (shearing or sawing) to obtain rectangular cross section and uniform thickness Internationally, it is customary to call aluminum materials with thickness of more than 0.2mm, less than 500mm, width of more than 200mm and length of less than 16m as aluminum plate or aluminum sheet, less than 0.2mm as aluminum foil and width of less than 200mm as row or strip (of course, with the progress of large equipment, there are more rows with width of 600mm).

aluminium sheet

Aluminum plate is usually divided into the following two types:

1 According to the alloy composition, it can be divided into: high-purity aluminum plate (rolled by high-purity aluminum with a content of more than 99.9), pure aluminum plate (basically rolled by pure aluminum), alloy aluminum plate (composed of aluminum and auxiliary alloys, usually aluminum copper, aluminum manganese, aluminum silicon, aluminum magnesium, etc.) composite aluminum plate or welding plate (aluminum plate materials for special purposes are obtained by means of multiple material composites) aluminum clad aluminum plate (thin aluminum plate is coated on the outside of aluminum plate for special purposes)

2. According to the thickness, it is divided into: (unit mm) thin plate 0.15-2.0, conventional plate 2.0-6.0, medium plate 6.0-25.0, thick plate 25-200, five rib pattern aluminum plate and ultra thick plate more than 200

3.Common aluminum plate grades include 5083, 5086, 5754, etc.

Fire retardant cable classification

At present, power cable industry is used to refer to cables with certain fire resistance properties such as Fire Retardant, Low Smoke Halogen Free (LSOH) or Low Smoke Fume (LSF), etc., collectively referred to as fire resistant cables.
Fire retardant cable
The characteristic of fire retardant cable is to delay the spread of flame along the cable to prevent the fire from spreading. Because of its low cost, it is a large number of cable varieties used in fire-resistant cables. Regardless of whether it is a single cable or a bundle laying, the flame spread can be controlled within a certain range when the cable is burned, so it can avoid major disasters caused by the cable fire and prolonged combustion, thereby improving the fire protection level of the cable line .

fire retardant cable
Halogen-free low-smoke fire-retardant cable (LSOH)
The characteristics of halogen-free low-smoke cables are not only excellent flame-retardant properties, but also that the materials constituting the low-smoke halogen-free cables are halogen-free, have low corrosiveness and toxicity during combustion, and produce a very small amount of smoke, thereby reducing the impact on the human body. , The damage of instruments and equipment is conducive to timely rescue in the event of a fire. Although halogen-free low-smoke fire-retardant cables have excellent flame retardancy, corrosion resistance and low smoke concentration, their mechanical and electrical properties are slightly worse than ordinary cables.
Low-halogen, low-smoke flame-retardant cable (LSF)
low-halogen, low-smoke, flame-retardant cables have a hydrogen chloride release and smoke density index between flame-retardant cables and halogen-free low-smoke fire-retardant cables. Low halogen cable material will also contain halogen, but the content is lower. The characteristics of this cable are not only flame retardant, but also less smoke and hydrogen chloride release during combustion. This kind of low-halogen, low-smoke flame-retardant cable is generally made of polyvinyl chloride (PVC) as the base material, and then processed with high-efficiency flame-retardant, HCL absorbent and smoke suppressant. Therefore, this fire-retardant material significantly improves the combustion performance of ordinary flame-retardant PVC materials.

Reasons for the poor price of cables of the same model

Whether it is home improvement needs wires or large quantities of overhead  cables for engineering, everyone has such a doubt during the inquiry process when purchasing wires and cables. The prices of products of the same model and specifications sometimes vary a lot, as the saying goes Distribution of goods, this principle is applicable to any industry, the profit of the wire and cable industry is very thin, and the price is almost transparent. If you encounter a quotation with too much price difference, you should pay attention. Be sure to confirm whether it is a national standard product and whether the quality is up to standard. If the price is too cheap, the manufacturer also says that the national standard product is a national standard or a non-standard product. Everyone must carefully distinguish. Today, I will explain the difference between the national standard and the non-standard.

Pay attention to the raw materials, the most important material is copper conductor. Disaster relief may be different here. The copper conductor used in a good national standard wire and cable is high-quality oxygen-free copper, but the non-standard is not. When the weight of the copper scrap used in the non-standard wire and cable is the same as the high-quality oxygen-free rod used in the national standard ABC cable, the copper scrap and the oxygen-free rod are the same. The price difference is 10%. Look at whether the auxiliary materials are qualified, and whether there is no cutting corners on the auxiliary materials. Recently, the country has increased the resistance inspection of wires and cables, and non-standard cables have begun to work hard on auxiliary materials. If copper accounts for 80% of the cost of the cable, then the cost of auxiliary materials occupies about 20%, which is the 20% of the cost. The material is slightly inferior, and the price difference with the national standard can be 5%. Recently, many cable quality problems have been exposed, and a large proportion of them have problems with accessories. Let’s see if the quantity is enough, such as 95-meter, 98-meter, or 90-meter noodles. This will make a big difference. I hope everyone will pay attention to the 100-meter reduction by 1 meter, and 1% of the profit will come out. The most difficult thing to notice is that the two sides are thick and the middle is thin. You probably have not noticed it. That is, the cables at both ends are national standard, and the middle part is non-standard. The department cut corners and cut corners, so a lot of costs are saved, and the price of the corresponding cable will be much lower. Everyone who purchases wire and cable products must look for regular manufacturers, and not be exploited by some illegal businessmen.
This is why the price of wires and cables of the same model and specifications are so different. All products of Henan Shenghua Cable Group strictly implement the national standards GB, International Electrotechnical Commission (IEC) and other advanced standards of developed countries, and have obtained national production license, 3C certification, weapon equipment quality management system certification, weapon equipment research and production unit qualification certificate, CQC Certification, Tyre certification, TUV Rheinland certification and flame retardant certification, etc., and establish a comprehensive management system certification for quality, environment, and occupational health. It must be the safest and safest wire and cable product.

Matters needing attention in the production of ACSR Cable

1. Greasy dirt on the wire surface
Overhead ACSR cables used for overhead transmission lines are often questioned by customers or even returned due to surface quality problems. This phenomenon is mainly caused in the drawing process. The main reasons are as follows:
(1) The size of the finished wire drawing die is too large, the compression is relatively small, and the working area of ​​the wire drawing die is short, which is not enough to remove the oil on the surface of the wire;
(2) The temperature of the lubricating grease is too low, the viscosity is large, and it is not easy to play, causing the drawing oil to adhere to the surface of the aluminum wire;
(3) The thread rubbing felt at the exit of the finished mold is too dirty;
(4) When stranding the wire, the stranding machine body or the traction wheel is too dirty;
(5) There is too much water under the traction wheel.
For the above reasons, in order to avoid the generation of oil stains on the wire surface, select a suitable wire drawing die and adjust the temperature of the lubricating grease during wire drawing; frequently replace the wire rubbing felt, and keep the stranding equipment and mold clean and free of oil when stranding the wire.

2. The wire surface is scratched
Since there are sharp metal objects in the wire drawing equipment and  ACSR Cable drawing die or the wire stranding equipment and the wire stranding die, it is easy to cause surface scratches when the wire is drawn or the wires are stranded. This kind of defect firstly affects the surface quality of the wire, and secondly, corona occurs due to the unevenness of the conductor when the wire is running. Therefore, once the surface scratches are found, the cause should be investigated immediately from the equipment or mold.
3. Wire scratches
Continuous scratches on wires are the most common cause of corona, and corona is one of the most important causes of power loss in power systems. In order to avoid the occurrence of wire scratches, the following aspects should be controlled in production: firstly, consider the conductor itself. First, ensure that the aluminum rod is not damp. The aluminum rod is damp, causing aluminum chips to stay in the working area of ​​the drawing die hole, and the lubricating oil cannot be used. All enter the work area, resulting in the drawn aluminum wire is not round and not smooth. Second, ensure that the drawing oil is clean and free of moisture. In the process of drawing, the drawing oil is in a high temperature state when it enters the work area from the lubricating area. If it contains a certain amount of moisture, the lubricating oil will be diluted and the lubricating effect will be greatly reduced, resulting in wire scratches and aluminum wire breakage.

Secondly, from the perspective of production, the following issues should be paid attention to:
In the drawing process:
(1) Check whether the mold of the wire drawing machine is placed accurately;
(2) Check whether the drawing drum has a groove after friction, and whether there is a crimping phenomenon;
(3) Check whether the surface finish of the lubrication area of ​​the drawing die meets the requirements, and whether there is blockage by aluminum chips.
In the stranding process:
(1) Check the sensitivity of the top of the spool;
(2) Check whether the tension of the spool is uniform;
(3) Check whether the single wire is slightly crimped or the wire is uneven;
(4) Check whether the single wire is in the groove of the guide wheel during twisting.
4. Loose wires and serpentine bends
(1) Adjust the pay-off tension of the wire reel when twisting the wire. The tension depends on the accumulation of experience in normal work. Generally, experienced operators should be able to adjust the tension to the appropriate requirements;
(2) When selecting the parallel mode first, the diameter of the parallel mode is required to be 0.3-0.5mm smaller than the outer diameter of the stranded wire;
(3) Adjust the back-twisting device to make the single wire “S” before twisting, so as to eliminate the internal stress generated by the single wire during the twisting process;
(4) Minimize the number of stops during the stranding process. It is best to produce the same stranded wire at the same fixed speed to prevent uneven speeds or parking/driving.
(5) Check in advance to use the steel core for stranding to ensure that there is no looseness; when the wire is stranded, the steel core should maintain a sufficient constant pay-off tension.

Can PVC be used as a cable?

PVC cable material is made of polyvinyl chloride as the basic resin, adding stabilizers, lubricants and inorganic fillers, etc., through mixing, kneading and extrusion. Although its dielectric point performance is general and not environmentally friendly, its price is low and the process is simple; PVC cable material is still one of the most used cable materials.
one. Cable material air hole problem
There are two main reasons for this problem, one is the problem of moisture, the other is the problem of degradation.
1. The moisture in the raw material is high. The raw materials that may exceed the standard moisture content are PVC resin, plasticizers, fillers and stabilizers. Due to the relatively large amount of addition, PVC resin and fillers should be the focus of inspection. This situation is generally manifested in the kneading process and the vacuum of the extruder.
2. Poor stability of the formulation system or too long residence time of the material at high temperature will cause the material to decompose and cause pores to appear. When this problem is serious, it is usually accompanied by a color change.
two. Rough surface of cable material

There are two types of surface roughness, one is pimple and the other is pitting.
1. Pimple phenomenon
Mainly: some powder aggregates that are unevenly dispersed during mixing can not be plasticized during the extrusion process. The plasticized PVC material is wrapped together and extruded from the die, which is formed in the cable material.
The fish eyes of the PVC resin and the PVC resin particles that do not fully absorb the plasticizer can also cause lumps, but they are generally relatively small.
Today’s formulations pursue finer and more fillers. If the surface of the filler is not treated well, the effect will not be good during mixing, and the probability of agglomeration will be higher. It’s just the size of the degree of reunion and whether the performance in the cable material is serious? Has it become a problem?
2. The pitting problem
The pitting is relatively more complicated, and it is generally believed to be related to the small molecules that escape from the material. These small molecules come from the resin itself, plasticizers, and lubricants.
Since it is necessary to vacuum when extrusion pelletizing to produce cable material, it is reasonable that these small molecules should be extracted away, but why do they still appear? Through analysis, it is found that, in fact, most of these small molecular substances are produced in the materials after vacuuming, and many of them come from lubricants that are not very large, and of course there are also plasticizers.
three. PVC cable material has poor insulation
Due to the limitation of PVC material and the influence of plasticizers and other additives, the insulation of PVC  insulated cable material has a certain limit. For ordinary cable materials, if the insulation is obviously deviated, the main reasons are as follows:
1. Too many impurities
The mixing of impurities will have an adverse effect on the cable material, and excessive impurities will cause insulation problems. These impurities may come from PVC resin and various additives, or from mixing and feeding.
2. The powdery particles are too coarse
The powdery additives in the cable material are generally used after grinding. If it saves trouble or some mechanical failure, the added powdery substance particles are too thick, which will adversely affect the insulation of the cable material.
3. The colorant heavy metal problem
Many pigments are heavy metal salts, these heavy metal ions will increase the electrical conductivity of the cable material and reduce its insulation. So the choice of cable material pigment is very important.

4. Cable material damp problem
Because the cable material has a certain proportion of fillers, and some have a certain proportion of low-grade plasticizers (or plasticizer substitutes), cable materials that are not prone to moisture will also have such problems in certain seasons.
The dampness of the cable material has a lot to do with the packaging process and the packaging. Drying should be strengthened, cooled to a certain temperature and then sealed and sealed. In addition, the packaging should be improved and moisture-proof measures should be added.
At the same time, attention should be paid to false moisture caused by potential degradation and surface adhesion.
5. The cable material is brittle
The problem of cable material brittleness is generally related to formula components such as PVC resin model, plasticizer, lubricant, and filler.
1. Choose a higher model
If a higher type of PVC resin is used, the performance of the cable material will be brittle due to the short molecular chain of PVC. The amount of plasticizer added is small, and the cable material is hard and sometimes brittle.
2. The amount of filler added is too large
It is more because the filler is added too much, and the performance of the cable material is reduced, and the strength is not good.
3. Lubricant
Lubricant is another important point. If the external lubrication is excessive, it will often cause poor plasticization (low plasticization temperature is another main reason for poor plasticization). At this time, the cable material will obviously have poor strength and become brittle.

What should I do if the cable’s fire protection effect is not good?

Is power cable’s fireproof effect poor? Have you tried mineral insulated fireproof cables! Mineral insulated fireproof cables not only have the characteristics of fireproof, high temperature resistance and corrosion resistance, but also good waterproof effect, light weight and long life.
(1) In order to prevent electrical fires caused by the quality of the wires and cables themselves, qualified wires and cables must be selected;
(2) The aaac conductor joints are prone to temperature rise and poor contact due to oxidation. Therefore, copper core wires and cables should be selected as a priority to reduce fire hazards.

(3) For cross-linked polyethylene insulated overhead cable, its electrical and physical properties are better than other wires and cables. It has the advantages of long life, good water resistance, good corrosion resistance, and less harmful combustion products, so it is insulated with polyethylene. Wire and cable (YJV) can be used as a priority;
(4) Because flame-retardant wires and cables can effectively slow down or even prevent the expansion and spread of fire, the use of flame-retardant wires and cables can control the fire at the initial stage of the fire to avoid greater losses, so flame-retardant wires and cables should be selected;
(5) In order to ensure that the electrical circuits can be energized normally during a period of fire, fire-resistant wires and cables should be selected;
(6) Try to use halogen-free low-smoke flame-retardant wire and cable or low-halogen low-smoke flame-retardant wire and cable to avoid generating a large amount of smoke and corrosive gas;
(7) On the one hand, mineral insulated cables provide excellent low grounding resistance and save a grounding wire than other cables. On the other hand, they can effectively ensure the safe operation of workers; for fire-fighting electrical equipment, it should be preferred Mineral insulated fire-resistant cable

Typical failure of overhead cable line

1. Common faults of empty overhead  cable lines
1. Overhead line failure
①Short circuit and damage of wires. The reasons that cause short circuit and damage to the wire are:
a. The distance between the wires and wires is small, and the flatness of the three-phase wires is different. It is very easy to cause color collision and short circuit when the wind blows, and the wires are burned.
b. Tree technology and internal metal objects falling on the line; accidentally touching the line when the crane is working also cause the wire to be short-circuited and damaged.
c. Long-term erosion by harmful substances in the surrounding environment, with a reasonable cross-section reduction. The damage to the wire is not very serious, and the method of laying wire maintenance can be considered as appropriate to solve it.
2. The wire connector is overheated. The reasons that lead to the over-temperature of the wire connector are:
a. The load current of the wire exceeds the larger specified value of the wire cross section.
b. The quality of the construction work of the wire connector is not good.
c. The circuit has been blown by rain and wind for a long time, and air oxidation and erosion have become more and more serious: the resistance of the connector loop increases: the hazard is hot. Department is
If the wire is overheated, you can try to reduce the load. Monitor the operation again, and allocate and solve it according to the development trend.

2. ABC Cable line failure
In terms of conditions, common cable faults include mechanical equipment damage, lead (aluminum) cracking, bursting, environmental pollution of terminal equipment heads, explosion of terminal equipment heads or intermediate connectors, insulation penetration, and metal material protection wire Common faults such as sleeve erosion holes. For this reason, common cable faults include damage by external force, chemical corrosion or electric erosion, lightning strikes, water immersion, improper construction of plant diseases and insect pests ↓Improper maintenance and other common faults. Common cable faults and avoiding methods are as follows:
① Water seepage of the plastic cable: Once the plastic is invaded by water, it is very easy to cause embrittlement of the insulation layer, especially when the temperature of the conductor is high, the infiltration and embrittlement caused by the water in the conductor is more serious. Therefore, no water seepage is allowed in the transportation, storage, laying and operation of plastic cables.
②Cable overload operation: The safety factor of cable operation is not so much related to the current carrying capacity of the cable. Overloading may increase the failure rate of the cable, and at the same time, it will continue to reduce the service life of the cable. The cable damage caused by overload is mainly in the following aspects:
a leads to the destruction of the wire contacts;
b. Speed ​​up the embrittlement of the insulation layer of the cable maintenance;
c. Make the lead bag of the cable swell and even crack, such as the fatigue, cracking, and cracking of the lead bag caused by the long-term overload of the cable with poor quality of production and poor installation standards;
d. The cable terminal equipment head is swollen and cracked by the bituminous insulating rubber.
③Destroyed by external force: Some of the safety accidents of the cable itself are caused by the destruction of mechanical equipment under external force. According to the investigation, the damage of mechanical equipment caused by external force caused by improper municipal management methods and improper construction of the project is about It accounts for 50% of cable safety accidents. Therefore, when cables are transported, hoisted, and laid through regenerated buildings, care must be taken to avoid the harm of external forces. During the construction of the surrounding engineering of the cable line, it is necessary to remind the construction workers to pay attention to this point and implement safeguard measures when necessary.
④Environmental pollution of the waterproof casing of the cable terminal equipment head: The key method is to clean the waterproof casing on time, and it is best to carry out complete cleaning under the power-off standard: in the area with serious pollution, the waterproof casing of the cable terminal equipment head should be coated Stain-resistant architectural coatings, or moderately improve the flame retardant grade of waterproof casing.
⑤The water seepage of the outdoor terminal equipment head explodes: mainly because of unreasonable operation and maintenance, the condensed water of the terminal equipment head will accumulate in the cable head, and finally cause the insulation layer to penetrate back and cause an explosion.
⑥The cable’s middle connector exploded: most of the cases were caused by the over-loading of the insulating rubber in the connector box and the shell swelling, or the poor conductor connection caused the connector to overheat and explode.
⑦The inspection of outdoor cable terminal equipment head has the following aspects:
a. Whether the waterproof casing at the end of the cable is cracked, and whether the electrode connecting wire clamp of the grounding wire of the transformer is heated:
b. Whether the insulating glue in the cable terminal equipment head becomes soft, overflows, lacks, and whether there is moisture on the surface:
c. Whether each sealing part of the cable terminal equipment head leaks oil;
d. Is the wire connector good?

Is it still clear to distinguish between fire-resistant cables and flame-retardant cables?

1. Fire-resistant cable
On the basis of improving the manufacturing process of the refractory layer and adding the refractory layer, the Class A refractory cable has been developed. It can withstand combustion for at least 90 minutes in a flame at 950℃~1000℃ and at a rated voltage without the cable being broken down ( That is, the 3A fuse does not blow).
2. Flame-retardant cable
Flame-retardant cable is a kind of fire-resistant cable with better performance. It is processed by copper core, copper sheath, and magnesium oxide insulation material, referred to as MI cable.
The cable is completely composed of inorganic materials as the fire-resistant layer, while the fire-resistant layer of the ordinary fire-resistant cable is composed of inorganic and general organic materials. Therefore, the MI cable has better fire resistance than ordinary fire-resistant cables and will not decompose due to combustion to produce corrosiveness. gas.
MI cable has good fire resistance characteristics and can work at a high temperature of 250 ℃ for a long time. At the same time, it also has explosion-proof, strong corrosion resistance, large current carrying capacity, radiation resistance, high mechanical strength, small size, light weight, long life, and smokeless specialty.

However, the price is expensive, the process is complicated, and the construction is difficult. In oil-irrigated areas, important wooden public buildings, high-temperature places and other occasions with high fire resistance requirements and acceptable economic efficiency, this kind of cable with good fire resistance can be used.
Note: Fire-resistant cables can maintain normal power supply for a period of time when a fire occurs, but flame-retardant cables do not have this feature
Three, the problems that should be paid attention to when designing and applying
1. When fire-resistant cables are used in cable tunnels, cable interlayers with dense cables, or in flammable places such as oil pipes, oil depots, etc., Class A fire-resistant cables should be selected first. In addition to the above cases and when the number of cables is small, Class B fire-resistant cables can be used.
2. Fire-resistant cables are mostly used as power supply circuits for emergency power supplies, which require normal operation in the event of a fire. As the ambient temperature rises sharply during a fire, in order to ensure the transmission capacity of the line and reduce the voltage drop, for circuits with long power supply lines and strictly limited allowable voltage drops, the cross-section of the fire-resistant cable should be enlarged by at least one level.
3. Fire-resistant cables cannot be used as high-temperature cables.
4. In order to reduce the failure probability of cable joints in fire accidents, the number of joints should be reduced as much as possible during installation to ensure that the lines can work normally in a fire. If branch wiring is needed, fire-proof the joints should be done.

Grounding method of shielded cable

1. Shielded cable technical characteristics
With the increase in the capacity of the power system and the continuous improvement of the level of automation, the secondary equipment of the power system has now widely used integrated circuit or micro-type protection devices. The application of these protection devices is very useful to improve the stable operation of the system. of. But correspondingly, some new problems have been raised. For example, because the microcomputer protection device is composed of electronic components and single-chip microcomputers, and it runs in a high-voltage environment, there is a problem of how to resist electromagnetic interference. However, it is not obvious that the previous conventional electromagnetic protection devices are affected by this aspect.
Therefore, in the high-voltage substation, all currents, voltages and DC trips used to connect the relay protection equipment in the control room from the switch field may introduce interference voltage from the switch field to the secondary circuit of the relay protection based on microelectronic devices. , Should use shielded control cables. In the high-voltage substation, shielded cables are used to suppress electromagnetic interference. How to properly ground the shielding layer plays an important role in reducing the interference level of the external electromagnetic field to the secondary equipment of the microcomputer.
The magnetic flux generated by the interference source of the shielded cable and the current in the outer wire is represented by dashed concentric circles. A part of these magnetic fluxes surrounds the shielded cable core and its shielding layer (it can be approximated as the magnetic fluxes surrounding the two are equal), which is called interference Magnetic flux, as shown in Figure 1(a). It induces an electric potential Es(.) in the cable core and the shielding layer, and produces the shielding layer current Is(.), as shown in Figure 1(b). The potential Es(.) is equal to the voltage drop of the shielding layer current on the shielding layer resistance Rs and self-inductance Xs, namely: Es(.)= Is(.)Rs+jIs(.)Xs
The magnetic flux generated by the shielding layer current surrounds the shielding layer and all overhead cable cores. These magnetic fluxes are opposite to the interference magnetic flux generated by the outer conductors, so they are called reverse magnetic fluxes, as shown in Figure 1(a) Concentric circles are indicated by solid lines. According to the principle of electromagnetic induction, under ideal circumstances, if the shielding layer resistance is zero, this reverse magnetic flux can cancel all the interfering magnetic flux, that is, the mutual electromotive force Er(.) and the reverse magnetic flux generated in the cable core. The electromotive force Es(.) induced by the interference magnetic flux in the cable core is equal in magnitude and opposite in direction. Suppose the mutual inductance of the shielding layer to the cable core is Xm, then: Er(.)=-jIs(.)Xm
Because the shielding layer completely surrounds the cable core, Xm=Xs. It can be seen from the above formula that if the shielding layer resistance Rs=0, then Es(.)=- Er(.). However, the shielding layer cannot be without resistance, so the part of the electromotive force induced in the cable core by the interference magnetic flux that cannot be cancelled is Es(.) + Er(.) = Is(.) Rs, which is proportional to the resistance of the shielding layer. Therefore, in order to effectively eliminate the interference of electromagnetic coupling, it is necessary to use materials with small resistivity such as copper and aluminum to make the shielding layer.
2. Grounding of shielded cables
Shielded cables have poor balance characteristics, so good shielding integrity and good grounding are very important for shielded cables. Shielding grounding is the grounding of shielding equipment to prevent electrical equipment from being affected by electromagnetic interference, affecting its work or causing electromagnetic interference to other equipment.
The use of shielded control cables, and the shielding layer is grounded at both ends of the switchyard and the control room at the same time, is an effective secondary loop anti-electromagnetic interference measure used internationally. In the document “Selection and Installation of Control and Low-Voltage Cable System in Substations” proposed by the working group of the IEEE Substation Special Committee and the Relay Environment Subcommittee, there is a special section “Metal Shielding Capability of Control Cables” “Reducing induced transient voltage” talks about related issues: “It is recommended that shielded cables should be grounded at both ends. The integrity of the shield must be particularly maintained. Disconnecting or separating the shield will greatly reduce the shielding efficiency; if the shield is grounded at only one end, The foreskin on the non-grounded end may have a very high transient voltage to the ground.”

The advantages of grounding both ends of the shielding layer of the control shielded cable are:
① When the control cable is surrounded by the magnetic flux generated by the busbar transient current, the shielding current will be induced in the cable shielding layer. The magnetic flux generated by the shielding current will offset the magnetic flux generated by the busbar transient current to the cable core. The influence of the line. Assuming that the shielding effect is ideal, the result of the two interactions will make the magnetic flux in the cable core completely surrounded by the shielding layer to be zero, and the shielding layer forms an ideal Faraday cage. This is also the same as an ideal transformer with a secondary short-circuit coil, the magnetic flux in the iron core will be zero. Of course, the shielding effect of the shielding layer cannot be completely ideal due to various reasons. Therefore, the shielded core wire will still induce a certain voltage under the effect of the transient current of the busbar.
② Both ends of the shielding layer are grounded, which can reduce the transient induced voltage caused by the rise of ground potential.
When lightning is injected into the ground grid through the arrester, and the inrush current in the ground grid of the substation increases, transient potential fluctuations will occur, and the apparent grounding resistance of the ground grid will temporarily increase. The measurement results of the ground potential rise of the substation show that the ground resistance is often increased by more than 10 times compared with the normal AC resistance. When the low-voltage control cable is laid near the above ground potential rise, the fluctuation of the cable potential will be disturbed. Therefore, the ground potential rise introduced by the ground surge current may have a serious impact on the insulation coordination of the low-voltage control loop.

In order to quantitatively estimate the amount of transient induction caused in the control cable core when lightning is injected into the ground grid of the substation, it is measured when a small inrush current (100~4000A) is injected into the ground grid manually in 30 substations. The voltage situation. The transient voltages under two cable shielding conditions are measured, one is the cable without metal shield, and the other is the cable with metal shield and grounded at both ends. The test proves that the use of shielded cables with grounding at both ends can suppress the transient induced voltage to less than 10% of the original value, which is an effective measure to reduce the interference voltage.
3. Shielded cable conclusion
There are two grounding methods for the shielding layer of a shielded cable, namely, grounding at both ends and grounding at one end. When one end is grounded, the voltage of the shielding layer is zero, which can significantly reduce the electrostatic induced voltage; the grounding of both ends causes electromagnetic induction to generate an induced longitudinal current on the shielding layer, which generates a secondary field opposite to the main interference and cancels the main winding field. It can significantly reduce the magnetic field coupling induced voltage, which can reduce the induced voltage to less than 1% of the induced voltage when it is not grounded.
The two ends of the shielding layer of the shielded cable are grounded with the following two problems: ① When a short-circuit current or lightning current occurs on the grounding grid, due to the different potentials of the two points of the cable shielding layer, the current flowing in the shielding layer will cause additional impact or Interference voltage. ② When current flows in the shielding layer, interference signals will be generated for each core wire, but the circuit where the cable core is located is a strong current loop, so the interference signal generated by the shielding layer current has less impact.
However, corresponding to shielded cables used for relay protection and automatic device circuits, because the input and output have one end in the high-voltage or ultra-high voltage environment of the switching field, electromagnetic induction interference is the main contradiction to prevent transient overvoltage, so the relay The protection and automatic device regulations stipulate that the shielding layer should be grounded at both ends.