Superconducting cable technology refers to a physical phenomenon in which the material resistance becomes zero at a certain temperature. According to the different working temperature zones, superconducting materials are divided into high and low temperature types. Those working in liquid helium environment (4.2K, -268.8℃) are called low-temperature superconducting materials, and those working in liquid nitrogen environment (77K, -196℃) are called high-temperature superconducting materials.
Heat-resistant and high-temperature wires and cables are generally determined by two needs. The first is that the ambient temperature of the wires and cables is high, and the cables can normally transmit signals or electric energy under long-term high temperatures; the other is power transmission cables, which are mainly used to increase the interception capacity.
Working in a high-temperature environment, ordinary cables are prone to insulation aging and scorching at high temperatures, and the used cables lose their performance and are damaged and cannot be used. High-temperature cables can work normally and stably at rated high temperatures, signal or power transmission is not affected, and the cable can also be guaranteed to have a long service life. This type of functional cable is the most common and most common type of high-temperature cable.
Load-increasing high-temperature cables are mainly used to reduce the outer diameter and weight of cables under the premise of carrying current, and develop towards lightweight. Generally speaking, the higher the working temperature of the cable, the greater the current carrying capacity of the cable with the same cross-section. In occasions such as airplanes and automobiles, weight reduction is of great significance, and the use of high-temperature cables greatly reduces the cross-section. When the working temperature rises from 90℃ to 155℃, the current carrying capacity increases by 50%. Under the same current carrying capacity, the weight of the cable should be reduced by half, and the cost is also reduced. Of course, at the same time as the high current cutoff, the power loss of most insulating materials will also increase.
At present, according to rough statistics, the annual sales of special cable products in my country are about 40 billion yuan, accounting for about 30% of the total sales of the domestic cable market, among which high-temperature cables are in great demand. As an important component of special cables, high-temperature cables have extremely strong “vitality” and are currently in short supply in the market. On the basis of high-temperature cables, the relevant national plans also set up support for major projects of “high-temperature superconducting power transmission”. Superconducting cables have also achieved fruitful results and basically meet market demand.
Superconducting cables are designed and manufactured based on the characteristics of superconducting at their critical temperature, disappearing resistance, extremely low loss, high current density, and the ability to carry large currents. Its transmission capacity far exceeds that of oil-filled cables and cryogenic cables, and can reach more than 10,000 MVA. It is a new type of cable that is being vigorously researched and developed. Since the critical temperature of superconductors is generally below 20K, superconducting cables generally operate in liquid helium at 4.2K.
Superconducting cables are an important way to solve large-capacity and low-loss power transmission. Because its potential advantages are so attractive, scientific and technological workers from various countries are conducting a lot of research and development work for this.
Superconducting cable HTS principle
The basis of superconducting cable technology is the superconducting phenomenon. Simply put, after reaching below the critical temperature, superconductors will show zero resistance and perfect magnetic field shielding effect. This means that when current passes through a superconductor, no resistance, heat, or electromagnetic waves will be generated. Moreover, superconductors can exclude external magnetic fields from their interior to achieve perfect magnetic field shielding.
Superconducting cable HTS structure
The structure of superconducting cables has two forms: rigid and flexible. The cable core is divided into single-core and three-core. The expansion coefficient of its constituent materials must be fully considered during design to prevent the cable from being damaged by excessive internal stress due to thermal expansion and contraction.
Advantages of Superconducting Cable HTS
Compared with conventional cables, superconducting cables have obvious advantages, which are mainly manifested in the following points.
(1) Superconducting cables transmit large currents without resistance at the vaporization temperature of liquid nitrogen (about -196℃). The conductor loss is less than one-tenth of that of conventional cables. Together with the energy loss of refrigeration, its total operating loss is only 50% to 60% of that of conventional cables. Therefore, the loss is low and energy is saved.
(2) The current transmission capacity of superconducting cables with the same cross-section is 3 to 5 times that of conventional cables. Therefore, the use of superconducting cables can save the floor space and space of the transmission system and save a lot of precious land resources.
(3) Superconducting cables with the same transmission capacity use less metal and insulating materials.
(4) Conventional oil-filled cables are at risk of oil leakage and environmental pollution, while superconducting cables have no possibility of causing environmental pollution.
(5) Superconducting cables have the characteristic of low noise.
Superconducting transmission technology is the most ideal transmission technology in principle, but due to its high construction cost and immature technology, its widespread application will take a long time.
Classification of superconducting cables HTS
- Superconducting cables are divided into low-temperature superconducting cables and high-temperature superconducting cables according to the different superconducting materials used.
The conductive layer of low-temperature superconducting cables is made of low-temperature superconducting wires, usually NbTi/Cu or NbsSn/Cu composite superconducting wires. Since the critical temperature of NbTi is 9.5 K and the critical temperature of is 18.1 K, low-temperature superconducting cables must operate in the liquid helium temperature range.
The conductive layer of high-temperature superconducting cables mainly uses Bi2223 tape, which has a critical temperature of about 110 K, so it can operate in the liquid nitrogen temperature range, and its low-temperature structure is simpler than that of low-temperature superconducting cables.
- Superconducting cables are divided into DC superconducting cables and AC superconducting cables according to their different forms of power transmission.
Since DC superconducting cables have almost no resistance when the superconducting material is in the superconducting state, only the current lead and the low-temperature refrigeration device have power loss during power transmission.
AC superconducting transmission cable has greater heat loss than DC cable because superconductors will generate AC loss and dielectric loss of insulation layer when powered on.
- According to different insulation methods, superconducting cables can also be divided into normal temperature insulation superconducting cables and low temperature insulation superconducting cables.
The electrical insulation layer of normal temperature insulation superconducting cable is in the normal temperature zone outside the cable low temperature container. It can use the electrical insulation materials and technology of conventional cables. The electrical insulation layer of low temperature insulation superconducting cable is directly wrapped around the conductor and is in the low temperature zone together with the conductor, so that the cable size will be more compact. In order to prevent the influence of magnetic field generated by the cable on the surrounding environment when carrying current, a shielding layer is usually added outside the insulation layer.
Composition of superconducting cable HTS
Superconducting cable mainly consists of cable body, terminal and low temperature refrigeration device.
The superconducting cable body includes cable core, electrical insulation and low temperature container. The cable core is composed of superconductor. It is installed in a low temperature container tube that maintains the low temperature required for the cable core. Both ends of the low temperature container tube are connected to the terminal. The superconducting tape of the cable core is connected to the external power supply or load through the current lead at the terminal. For high-temperature superconducting cables, the cable core is composed of multiple layers of high-temperature superconducting tapes wound on a skeleton, and insulating tapes are wound between the superconducting tape layers to reduce the AC loss of the cable caused by electromagnetic coupling. The low-temperature container tube of the cable adopts a double stainless steel corrugated tube structure with high vacuum and super insulation. This structure ensures the flexibility of the high-temperature superconducting cable and maintains the high vacuum degree of the interlayer. For low-temperature insulated cables, the electrical insulation is wrapped on the outside of the conductor layer and is in the same low-temperature environment as the conductor layer. For normal-temperature insulated cables, the electrical insulation is on the outside of the low-temperature container, and a cable protective layer is added outside the insulation layer.
The terminal is the port where the superconducting cable is connected to external electrical components, and it is also the transition section between the low-temperature part of the cable and the external room temperature. Therefore, the terminal is required to have good thermal insulation to ensure that the overall heat loss of the superconducting cable is minimized. At the same time, the low-temperature cooling device also needs to cool the superconducting tape of the superconducting cable core through the terminal to ensure that the superconductor can operate at the designed operating temperature. In addition, since the conductor layer of the superconducting cable will be connected to the external high-voltage busbar through the current lead, the terminal is required to have a corresponding electrical insulation level.
Application fields of superconducting cable HTS
- Power transmission
Superconducting cable technology is widely used in the field of power transmission and can be used to transmit high-voltage and high-power electricity. The advantage of superconducting cable is that its capacity will far exceed that of traditional cables when the cable length remains unchanged. Moreover, since superconducting cables will not have resistance, there will be no problem of cable heating, and the power transmission efficiency is high. In addition, since there is no need for power conversion steps during power transmission, superconducting cables can also improve the energy utilization rate of the system and reduce the waste of power transmission.
- Maglev train
Maglev trains need to generate current through electromagnetic induction during high-speed driving to maintain the power of vehicle suspension and operation. Traditional maglev trains need to lay current rails, while superconducting cables can directly transmit current to the suspension system of maglev trains, making the electromagnetic suspension effect better, energy consumption lower, and noise less. In addition, for maglev trains, superconducting cables can also increase the running speed and driving distance of trains.
- Medical field
Superconducting cable technology is also increasingly used in the medical field. For example, MRI (magnetic resonance imaging) technology uses superconducting cables to transmit super-strong magnetic fields to achieve human body imaging. Superconducting cables play the role of carrying super-strong magnetic fields in magnetic resonance imaging, so that the magnetic field strength can reach several Teslas, thereby improving the resolution of imaging.
Future development of superconducting cable HTS
Although superconducting cable technology has been widely used in many fields, its own problems also need our attention. The preparation of superconductors, the research and development of insulating materials, and the manufacture of cables are all difficult problems that superconducting cable technology needs to overcome in the future. In addition, since the cost of superconducting cables is still relatively high, it is necessary to further reduce the cost before it can be more widely used in low-weight and medium-voltage fields.
In addition, the current superconducting cables still have temperature limitations and need to be maintained below a very low critical temperature, which brings certain limitations to practical applications. Therefore, future research should be to find new superconducting materials, reduce the critical temperature, and improve the transmission capacity and reliability of superconducting cables.
The application of superconducting cable technology will be a trend. It can not only improve the energy efficiency and quality of power transmission, but also be applied to future green transportation, medical imaging and other fields. In the future, TST cables will continue to innovate, promote the rapid development of superconducting cable technology, and contribute to the development of various fields. If you have any needs or questions about customized superconducting cable, you are welcome to send an email to TST CABLES technical experts.
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