Advantages of high temperature superconducting cable

Compared with traditional power cables, superconducting cables have the advantages of low loss, large transmission capacity, small ABC Cable volume and strong system security and stability. Cold insulated HTS cables have low mutual electromagnetic influence and thermal field influence, and have stable current carrying capacity. They have great development prospects in underground cable systems in densely populated big cities or specific high-capacity transmission applications. There are many underground pipe networks in big cities, various underground pipes (tunnels) are complex, and the space for cable laying is very limited. It is more and more difficult to install and lay traditional cables, and the installation and maintenance cost will be greatly increased. Using the existing pipe or cable tunnel and replacing the existing conventional cable with high temperature superconducting cable can double the transmission capacity of underground power grid and solve the contradiction between the increase of load and the limited underground space.

In terms of loss, the traditional cable transmission loss is mainly conductor loss, dielectric loss and shielding loss. Among them, for general land ACAR Cables, conductor loss accounts for about 95% of transmission loss. The loss of superconducting cable mainly includes: AC loss of cable conductor, Joule loss of cable terminal, heat leakage loss of superconducting cable insulation pipe, cable terminal and refrigeration system, loss of liquid nitrogen overcoming cycle resistance, etc. Considering the efficiency of refrigeration system, the operating loss of HTS cable is about 50% ~ 60% of that of conventional cable when transmitting electric energy of the same capacity.
The interconnection of large power grids is the trend of power development. With the interconnection of major power grids and the increase of power demand, the short-circuit current level of the system will further rise after short-circuit fault. How to solve the problem of fault current has attracted more and more attention of the power department. The transmission conductor of superconducting cable is superconducting material. Under normal working conditions, the transmission density of superconducting cable is large and the impedance is very low; In the case of power grid short-circuit fault and transmission current greater than the critical current of superconducting material, superconducting material will lose its superconducting ability, and the impedance of superconducting cable will be much greater than that of conventional copper conductor; When the fault is eliminated, the superconducting cable will restore its superconducting ability under normal working state. If the high temperature superconducting cable with certain structure and technology is used to replace the traditional cable, the power grid fault current level can be effectively reduced. The ability of superconducting cable to limit fault current is directly proportional to the cable length. Therefore, the large-scale superconducting transmission network composed of superconducting cables can not only improve the transmission capacity of the power grid and reduce the transmission loss of the power grid, but also improve its internal fault current limiting capacity and improve the safety and reliability of the whole power grid.

Internationally, the research and development process of HTS cable can be divided into the following three stages. The first stage is the preliminary exploration of HTS cable technology. With the development of bismuth (BI) high temperature superconducting tape technology, the research on high temperature superconducting cable has attracted extensive attention. The main contents of the research include: the research on the structure of superconducting AAAC Cable, including room temperature insulated (WD) high temperature superconducting cable, cold insulated (CD) high temperature superconducting cable, three-phase coaxial structure, three core in one structure, etc; Carry out research on electrical performance and transmission characteristics of superconducting cable. The second stage is the research and development of CD insulated HTS cable that can truly realize commercial application in the future. At the end of 1999, the 30m, three-phase, 12.5kv/1.25ka cold insulated HTS cable developed by southwire was connected to the grid, which has taken a solid step towards the practicability of HTS technology. The third stage is the demonstration project research of CD insulated HTS cable. In the past 10 years, the United States, Japan, South Korea, China, Germany and other countries have successively carried out a number of demonstration projects on CD insulated HTS cables.

When the traditional cable is running, the heat generated by the transmission loss is directly distributed to the surrounding environment, and its current carrying capacity is very sensitive to the external heat source. Generally, the current carrying capacity will decrease by 8% ~ 10% when the ambient temperature increases by 10 ℃. Therefore, intensive laying will lead to mutual heating of cables and sharp decline of current carrying capacity. According to the calculation, 2 × 4 rows of pipes, 3 × 3 rows of pipes and 4 × The current carrying capacity of traditional cables with dense 4-row pipes will be reduced by about 20%, 30% and 40% respectively. At the same time, because the traditional cable uses combustible organic materials as the main insulation, it is very easy to cause fire in case of failure. The outermost layer of CD insulated HTS cable is a composite vacuum insulation layer, which has good thermal insulation performance. The heat generated by the cable is taken away by circulating liquid nitrogen. Therefore, the thermal field of superconducting cable is relatively independent and the cable has stable current carrying capacity. At the same time, due to the good electromagnetic shielding function of CD insulated superconducting cable, it can completely shield the electromagnetic field generated by the cable conductor in theory, so it will not cause electromagnetic pollution to the environment. Due to these advantages, superconducting cables can be laid in dense ways such as underground pipes, which will not affect the operation of surrounding power equipment, and because it uses non combustible liquid nitrogen as refrigerant, it also eliminates the risk of fire.

Does the fireproof cable really not burn?

Fireproof cable has become an indispensable item in our daily life and production, followed by accidents caused by wires and cables, especially when wires and cables explode after combustion, it is easy to cause casualties. Every year, all kinds of wires and cables cause accidents. The fire accident continues. Most of the shells of ordinary wires and cables are plastic and rubber products. These materials will release a lot of smoke and toxic gases after the fire. Once the ABC Cable burns and explodes, it will also cause serious damage to the surrounding residents and environment.

Why do some cables catch fire or even explode? There are many reasons for AAAC Cable fires. Xiaobian summarized five common situations: short circuit, overload, breakdown, burning of cable head and external fire source. Generally, the insulation layer of the cable is damaged, resulting in cable short circuit. The outer sheath material will burn and severe heating will occur if the cable is overloaded for a long time. If the old cable is running at this time, it is easy to break the line and cause a fire.
Another very common reason is that the cable head is burned. If the surface of the cable head is wet or dirty, the ceramic sleeve of the cable head will break and the conductor is too small, which will cause arcing and fire, thus insulating the surface of the cable head and causing the conductor. Of course, another reason for the burning of the insulation layer is that the external fire source burns the cable, causing the cable to burn or even explode.
Therefore, in order to reduce the loss caused by cable burning, a fireproof cable is designed. Fireproof cable is a general term for the characteristics of flame retardant cable and fireproof cable. They all have certain flame retardancy, but many people can’t distinguish flame retardancy. The concept of cable and fireproof cable. In terms of its characteristics, fireproof cable can replace flame-retardant cable, but flame-retardant cable cannot replace fire-retardant cable. This is because they work differently.

The working principle of flame retardant cable is realized through the flame retardant effect of halogen, as well as halogen-free flame retardant cable. It uses precipitated water to reduce the temperature, but the working principle of fireproof cable is different from that of flame retardant cable. The material of fireproof cable contains a kind of mica material, which has high fire resistance and can be used normally in fire.
After understanding the working principles of the two, it can be seen that the flame-retardant cable is also afraid of fire, but due to the use of flame-retardant materials, it can stifle the fire in the communication base, and the flame-retardant cable is different. It has a fireproof layer structure. Normal operation can be maintained even in a fire, so in many cases, fireproof cables can be used instead of flame retardant cables.

Will flame retardant cables and fireproof cables not burn? Flame retardant ACAR Cables are highly nonflammable, with small combustion distance, and can be extinguished automatically after the fire source disappears, but the flame retardant cables themselves will still burn and cannot burn completely. Burning. Similarly, fireproof cables are the same and will burn. Flame retardant cables reduce the temperature through the flame retardant effect of special materials, and flame retardant cables rely on their unique flame retardant layer to ensure that they can work normally in fire. Therefore, flame retardant cables are better than flame. In some cases, flame retardant cables are used.

Introduction of conductor calculation formula of wire and cable

1、 Wire and AAC Cable material consumption
The conventional calculation method of copper weight without conversion: sectional area * 8.89 = kg / km
For example: 120 * 8.89 = 1066.8kg/km
1. Quantity of Conductor:
(Kg/Km)=d^2 * 0.7854 * G * N * K1 * K2 * C /
D = diameter of copper wire, g = specific gravity of copper wire, n = number of wires, K1 = twisting rate of copper wire, K2 = twisting rate of core wire, C = number of insulated core wires
2. Insulation amount:
(Kg/Km)=(D^2 – d^2)* 0.7854 * G * C * K2
D = outer diameter of insulation d = outer diameter of conductor g = specific gravity of insulation K2 = twisting rate of core wire C = number of insulation core wires
3. Dosage of external coating:
(Kg/Km)= ( D1^2 – D^2 ) * 0.7854 * G
D1 = finished outer diameter d = upper process outer diameter g = insulation specific gravity

4. Amount of wrapping tape:
(Kg/Km)= D^2 * 0.7854 * t * G * Z
D = outer diameter of upper process t = thickness of cladding g = specific gravity of cladding z = overlap ratio (1 / 4lap = 1.25)
5. Winding amount:
(Kg/Km)= d^2 * 0.7854 * G * N * Z
D = copper wire diameter n = number of wires g = specific gravity z = twist in rate
6. Weaving amount:
(Kg/Km)= d^2 * 0.7854 * T * N * G / cos θ
θ = Atan (2 * 3.1416 * (D + D * 2)) * mesh / 25.4/t
D = diameter of braided copper wire t = number of ingots n = number of bars per ingot g = specific gravity of copper
Specific gravity of material:
89; Cu -8; 50; Ag -10; Aluminum – 2.70; Zinc-7.05; 90; Ni -8; 30; tin-7; Steel -7.80; Lead-11.40; Aluminum foil mylar-1.80; 35; Myra -1.37
PVC-1.45; LDPE-0.92; HDPE-0.96; PEF (foaming) – 0.65; FRPE-1.7; Teflon(FEP)2.2; Nylon-0.97; PP-0.97; PU-1.21
55; cotton belt -0; PP rope -0.55; Cotton yarn-0.48
2、 Calculation formula of material outside conductor
1. Sheath thickness: outer diameter before extrusion × 035 + 1 (for power cables, the nominal thickness of sheath of single core ABC Cable shall not be less than 1.4mm, and that of multi-core cable shall not be less than 1.8mm)
2. On line measurement of sheath thickness: sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) / 2 π
Or sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) × zero point one five nine two

3. Thinnest point of insulation thickness: nominal value × 90%-0.1
4. Thinnest point of single core sheath: nominal value × 85%-0.1
5. Thinnest point of multi-core sheath: nominal value × 80%-0.2
6. Steel wire armor: number=
{π ×( Outer diameter of inner sheath + diameter of steel wire) ×λ)
Weight = π × Wire diameter? ×ρ× L × Number of roots ×λ
7. Weight of insulation and sheath = π ×( Pre extrusion outer diameter + thickness) × thickness × L ×ρ
8. Weight of steel strip = {π ×( Outer diameter before wrapping + 2 × Thickness – 1) × two × thickness ×ρ× L}/(1+K)
9. Weight of tape = {π} ×( Outer diameter + number of layers before wrapping × Thickness) × Number of layers × thickness ×ρ× L}/(1±K)
Where: K is the overlap rate or gap rate, if it is overlap, it is 1-k; In case of gap, it is 1 + K
ρ Is the specific gravity of the material; L is the length of cable; λ Stranding coefficient

Why measure the surface temperature of the cable in operation?

The temperature of the cable is closely related to the load, but only checking the load can not ensure that the AAAC Cable is not overheated
(1) the thermal resistance coefficient and side-by-side correction coefficient used in calculating the allowable ampacity of cables may be different from the actual situation.
(2) when choosing cables to determine conductor cross-sectional area, designers may lack sufficient information about the laying conditions and surrounding environment of the whole line.
(3) newly built and rebuilt power lines or thermal pipelines have an impact on the original surrounding environment and heat dissipation conditions.

(4) the excessive density of cables greatly exceeds the regulations of design and operation departments. In addition to measuring the load frequently, the operation Department must also check the actual temperature of the cable surface to determine whether the AAC Cable is overheated. The inspection shall be carried out at the maximum load and the section with the worst heat dissipation condition (generally not less than 10m). Thermocouple or pressure thermometer can be used for measuring instrument. The pressure thermometer is easy to install. Two temperature measuring points shall be installed at each measuring point. When measuring the temperature of the cable, the temperature of the surrounding environment should be measured at the same time, but it must be noted that the temperature measuring point of the surrounding environment should be kept a certain distance from the cable (generally 3M away), and there is no influence of external heat source.
On site detection method
During infrared detection, the cable should be live running for more than 24 hours, and try to remove or avoid the shelter between the cable and the thermometer, such as glass window, door or cover plate, etc; It is necessary to measure all parts of the cable line to avoid missing the measuring parts; It is best to carry out the test in the peak load state of the equipment, generally no less than 30% of the rated load. The infrared detection of the arrester connected with the cable terminal can refer to the requirements of DL / t664.
a) The emissivity of the tested equipment should be selected correctly, especially considering the influence of metal material oxidation on the selection of emissivity. For the selection of emissivity, please refer to Appendix B; Generally, 0.9 for metal conductor and 0.92 for insulator;

b) If the safety distance is allowed, the infrared instrument should be close to the equipment under test as far as possible, so that the equipment under test can fill the field of view of the whole instrument, so as to improve the resolution and temperature measurement accuracy of the instrument to the surface details of the equipment under test. If necessary, medium and long focal length lenses should be used; Generally, medium and long focal length lenses are used for outdoor terminal detection;
c) The compensation parameters such as atmospheric temperature, relative humidity and measurement distance are input and corrected, and the appropriate temperature range is selected;
d) Generally, the infrared thermal imager is used to scan all the tested parts, focusing on the cable terminal and intermediate connector, cross interconnect box, grounding box, metal sleeve grounding point and other parts. After finding the abnormal parts of the thermal image, the abnormal parts and the key tested equipment are measured in detail;
e) In order to measure temperature accurately or track conveniently, several different directions and angles should be set in advance to determine the best detection position and mark it for future retest, so as to improve the comparability and work efficiency;
f) Record the actual load current and voltage of the tested equipment, the temperature of the tested object and the temperature value of the environmental reference body according to the format of Appendix C.

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Discussion of the cable production process: conductor stranding

Discussion on the cable production process: conductor twisting and twisting
: the process of twisting many small diameter monofilaments into a large cross-section conductive core according to certain rules.
1. There are two types of twisted wires: normal twisted wires and irregular twisted AAC Cables.
Ordinary strands can be divided into ordinary concentric single stranded strands and ordinary concentric single stranded strands
(1) Ordinary stranded wire: single wires of the same diameter are twisted layer by layer according to concentric circles, and the direction of each layer is opposite.
(2) Combined stranded wire: made of a single wire of the same diameter, different materials or different diameters and different materials (representative products (such as overhead conductors))
Ordinary concentric stranded wire: a multi-stranded ordinary stranded wire or bundle Stranded wire concentrically stranded.
Irregular stranding (strand): strands formed by multiple single strands in the same direction, which does not comply with the law of twisting. The positions of the single strands are not fixed to each other, and the shape of the strands is difficult to maintain Round.

2. The biggest difference between the bundle wire and the ordinary stranded wire is that each single wire of the ordinary stranded wire has a fixed position and is regularly twisted layer by layer. There is no fixed position between the single wires of the bundle According to the law of twisting, the position will not be twisted together.
3. The characteristics of irregular twisting (bundling) : Since each single wire in the bundle is twisted in one direction, there is a residual amount of sliding between each single wire during bending. The amount is large and the bending resistance is small, so the bending performance of the bundle is particularly good. For wire and cable products that need to be flexible and frequently moved, the wire bundle is used as the conductor core.
4. The characteristics of the stranded core:
(1) Flexibility Good; by using a core composed of several single wires with smaller diameters, the bending resistance of the cable can be improved, and the processing, manufacturing, installation and laying of the wires and cables are convenient.
(2) Good stability; the core is It is composed of multiple single wires twisted according to a certain direction and twisting rules, because each single stranded wire is located in the stretched area above the twisted wire in the twisted wire, and is located in the lower compressed area When the stranded wire is bent in sequence, the stranded wire will not be deformed.
(3) Good reliability; due to material inhomogeneity or defects in twisting, using a single wire as the cable and the conductor of the cable easily affects the reliability of the conductor core. The defects of the conductor core formed by multiple single wires are scattered and will not be concentrated on a certain point of the conductor, so the reliability of the conductor core is much stronger.
(4) High strength; the strength of the single-stranded core is higher than that of the single-stranded core.

5. Explanation of terms:
(1) Pitch: the distance that a single filament advances one circle along the axis.
(2) Pitch diameter ratio: the ratio of the pitch length to the strand diameter.
(3) The relationship between the pitch and the flexibility of the strands: the smaller the pitch, the better the flexibility of the strands; on the contrary, the larger the spacing, the worse the flexibility of the strands.
(4) Stranding factor: the ratio of the actual length of a single wire to the pitch length in the pitch of the twisted wire.
(5) Twisting direction of the stranded wire: right direction (z direction) left direction (s direction)
(6) Compact conductor: common compact conductors are compact round, sector and compact tile-shaped (five-core cables) Semi-circular (two-core cable)
6. Compression purpose:
(1) Compact sector conductor: reduce the outer diameter of the cable, save product costs, and reduce the weight of the cable.
(2) Compact circular conductor: Improve the surface quality of the stranded conductor, reduce the diameter of the conductor, and increase the filling factor of the conductor. The compacted conductor surface is smooth without burrs, and the electric field on the conductor surface is uniform. Save materials and reduce costs (to learn more about cable technology, please click here. A lot of dry goods are waiting for your visit.)
7. Conductor classification:
According to GB/t3956 “Cable Conductor”, there are four types of conductors, namely the first One, the second, the fifth and the sixth. The first is a solid conductor, and the second is a stranded conductor, both of which are suitable for fixed laying of cables. The fifth and sixth types are stranded conductors, which are used for flexible cables and cords. The sixth is softer than the fifth.
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How to calculate the maximum current a wire can withstand

The maximum current that the pvc wire can bear is not calculated, but the result of experiment. The test method is about 20 ° Under the environment of C, the voltage test is carried out on the 1 meter long wire to obtain the safe current value that the wire can continuously pass through. This value will be marked on the product’s nameplate – each brand, each model of wire, only need to be tested once.

But in practice, the current must change. Let’s talk about the factors that will affect the current carrying capacity of the wire in use
1. Temperature
The higher the temperature, the lower the current carrying capacity of the wire. This is the most common problem, which is also the main reason why the cables used in construction need to be thicker than those used in plug-in. And in many cases, the ambient temperature is uncontrollable, ventilation effect, sunshine, cable density, etc., will affect the ambient temperature, and then affect the abc cable ampacity.
2. Cable density
If the cables are laid too densely, not only will the temperature be too high. When multiple wires are laid together, the proximity effect and skin effect will be formed, which makes the charge concentrated in the local section of the wire and reduces the allowable current carrying capacity of the wire.
3. Length
The longer the cable, the lower the current carrying capacity. The difference between the current carrying capacity of a 100 meter cable and that of a 10000 meter cable is not an order of magnitude.
(as my fans pay more attention to the home decoration circuit, I would like to say one more thing here: the above external factors that affect the current carrying capacity of wires are mostly power supply for power transmission, industry and commerce. As the environmental temperature changes little and the distance is short, the influence of external factors on cables can not be considered.)
The internal factors that affect the current carrying capacity of wire are as follows
In addition to some external factors that will reduce the current carrying capacity of the cable in a specific environment, the more important factor that can determine the current carrying capacity of the wire is the internal factors of the wire, mainly including the following three decisions——
1. Core area
That is, we often say “line diameter”, such as 2.5 square mm, 4 square mm and so on, which are commonly used in decoration. It is only emphasized here that the current carrying capacity is determined not by the cross-sectional area of the whole wire, but by the cross-sectional area of the conductor inside the wire. The thicker the line, the greater the current carrying capacity.

2. Material conductivity
It depends on the conductor material, such as common copper wire and aluminum wire. The conductivity of copper material is at least 30% higher than that of aluminum. And when necessary, there may be silver lines. In addition to the substance of the material, it also depends on the purity of the material. Taking copper as an example, the highest purity red copper has much higher conductivity than the second-class brass.
3. Thermal conductivity of insulating layer
In addition to preventing electric shock, the function of insulating layer has the same important function as preventing electric shock flame retardant. The better the thermal conductivity of insulating material, the better the flame retardancy. Therefore, the quality of insulating materials, from another aspect, determines the current carrying capacity of the wire.
Estimation of electric current carrying capacity
Although the current carrying capacity of the wire can not be calculated accurately, the current carrying capacity of the wire can be estimated through the formula, which involves the estimation methods of copper wire, aluminum wire and wire side in each stage. However, as mentioned above, there are too many variables in the wire. To know the accurate current value of the wire, you still need to check the wire nameplate.

Environmental Requirements of PVC wires and cables

The use of non-environmental protection cables is strictly prohibited in developed countries, and China also attaches great importance to this field. Relevant Chinese laws and regulations clearly require important buildings to prohibit the use of polyvinyl chloride wires and cables, and halogen-free and low-smoke cross-linked PVC wires and cables must be used to avoid a large amount of smoke and chlorine that may cause casualties in the event of a fire.

Environmental protection cables will strongly promote the development of the cable industry, enhance the competitiveness of cable manufacturers, and make the cable industry more stable on the road of sustainable development through green practice and innovation. In recent years, my country’s cable companies have gradually worked towards environmental protection cables, low-smoke and halogen-free cables, etc., as far as possible to be free of heavy metals such as lead, cadmium, mercury, bromine-based flame retardants, and no harmful halogen gases. There is no corrosive gas, less heat when burning, no pollution to the soil, etc., environmentally friendly cables will be more favored by the market, and it will be easier to obtain high orders in the bidding of power grid companies.

Users’ requirements for environmentally friendly wires and cables can be summarized as follows:

1. Low lead and heavy metal content is required

It is well known that lead and certain heavy metals will have adverse effects on human health. In PVC formulations, additives such as lead, cadmium, barium and other heavy metal compounds are usually mixed. However, since the 1990s, some developed countries and large electric companies have successively formulated regulations, and in the PVC wires and cables used, the content of eight heavy metals must not exceed the regulations. These new regulations not only bring new business opportunities to wire and cable manufacturers, but also bring serious challenges to PVC cable manufacturers.

2. Low smoke and low hydrogen oxide are required

Good flame-retardant performance is a very obvious advantage of PVC wires and cables. However, when ordinary flame-retardant PVC cables catch fire, they will release thick black smoke and a large amount of HCI gas, which will cause the “secondary” after the fire. “Disasters” seriously affect the evacuation of personnel and the development of fire rescue work. Therefore, while putting forward flame-retardant requirements for wires and cables, “low smoke” and “low HCI” requirements will be further proposed in certain applications.

3. Low toxicity or non-toxicity is required

For some cables used in household appliances, medical and health appliances, and children’s electric toys that may come into contact with food and blood products, in addition to lead-free and heavy-metal-free, they are also non-toxic (or low-toxic). This means that all component materials used in PVC formulations should be non-toxic. Cable material contains a lot of plasticizers, but non-toxic plasticizers must be used to make non-toxic materials. Therefore, this PVC material has higher requirements than lead-free and heavy metal-free materials, and the price is naturally more expensive.

4. For other prohibited items

Currently, the European Union is reviewing regulations that restrict the following items to ensure that wires and cables can meet environmental protection requirements. For example: 1) asbestos; 2) mostly biphenyl and its ethers, polybrominated phenol; 3) polychlorinated biphenyl; 4) short-chain (C10~C13) chlorinated paraffin.

As environmental issues are getting more and more attention, many countries have formulated various environmental protection policies, so people are more inclined to use green cables. Low-smoke halogen-free flame-retardant cable material is one of the most popular environmental protection cables. Although the demand for green cables in the Chinese market is increasing, compared with developed countries, my country still has a long way to go in terms of relevant policy formulation and mandatory application. my country should improve environmental protection cable standards as soon as possible.