Rutherford cables are transposed single-layer cables with a flattened cross section. Including conductor, metal sheath and multiple superconducting strands. Superconducting power technology can not only significantly improve the quality of electrical energy, improve the stability and reliability of power system operation, but also greatly increase the capacity of individual machines and the transmission capacity of the power grid. Superconducting cable is currently the most promising superconducting power equipment in the field of superconducting power technology.
In mid-June 2020, the "aluminum-based Rutherford superconducting cable" developed by ZGKX high-temperature superconducting technology company passed the project acceptance. The superconducting cable will be used for the pre-research prototype of the EmuS superconducting trapping solenoid, and it is also of great significance for advancing the pre-research work of the superconducting magnet of the CEPC detector. Product quality has reached the international advanced level of similar products. This product is made of Rutherford cable made of 16 strands of NbTi superconducting wire, which is covered with pure aluminum. It is the first domestic production.
Aluminum-based composite material is a material with strong vitality emerging in response to the needs of modern scientific development. It is composed of two or more materials with different properties through various processes. Both aluminum and its alloys are suitable for the matrix of metal matrix composite materials. The reinforcement of aluminum matrix composite materials can be continuous fibers, short fibers, or particles from spherical to irregular shapes. Aluminum-based composite material reinforced particle materials include SiC, AL2O3, BN, etc., intermetallic compounds such as Ni-Al, Fe-Al and Ti-Al are also used as working reinforcement particles. Composite materials can be divided into three categories: polymer matrix composite materials (PMCs), metal matrix composite materials (MMCs), ceramic matrix composite materials (CMCs). The matrix of metal matrix composites is mainly aluminum, nickel, magnesium, titanium, etc. Aluminum has many characteristics in making composite materials, such as light weight, low density, good plasticity, aluminum-based composite technology is easy to master, easy to process, etc.
The superconducting cable is designed and manufactured using the characteristics that the superconducting becomes superconducting state at its critical temperature, the resistance disappears, the loss is minimal, the current density is high, and it can carry large current. Its transmission capacity is far more than the oil-filled cable, and it is also larger than the low-temperature cable, which can reach more than 10000MVA. Since the critical temperature of superconductors is generally below 20K, superconducting cables generally operate in 4.2K of liquid helium.
The structure of superconducting cable has two forms of rigidity and flexibility, the cable core is divided into single core and three cores. When designing, the coefficient of expansion of its constituent materials must be fully considered to prevent the cable from being damaged due to excessive internal stress due to thermal expansion and contraction.
Superconducting cable is an important way to solve large-capacity, low-loss transmission. Because of its attractive potential advantages, scientific and technical workers in various countries are doing a lot of research and development.
Classification of superconducting cables
1. Superconducting cables are classified into low-temperature superconducting cables and high-temperature superconducting cables according to the superconducting materials used.
The conductive layer of the low-temperature superconducting cable is made of low-temperature superconducting wire, usually made of NbTi/Cu or NbsSn/Cu composite superconducting wire. Since the critical temperature of NbTi is 9.5 K and the critical temperature is 18.1 K, low-temperature superconducting cables must be operated in the temperature range of liquid helium.
The conductive layer of the high-temperature superconducting cable mainly uses Bi2223 tape, and its critical temperature is about 110 K, so it can operate in the temperature range of liquid nitrogen, and its low-temperature structure is simpler than the low-temperature superconducting cable.
2. Superconducting cables have DC superconducting cables and AC superconducting cables according to their different forms of power transmission.
The DC superconducting cable has almost no resistance when the superconducting material is in the superconducting state, and only the current lead and low-temperature refrigeration device have power loss during power transmission.
The AC superconducting power transmission cable will cause AC loss and dielectric loss of the insulation layer when the superconductor is energized, so its heat loss is greater than that of the DC cable.
3. According to different insulation methods, superconducting cables can also be divided into room temperature insulated superconducting cables and low temperature insulating superconducting cables.
The electrical insulation layer of the room temperature insulated superconducting cable is in the room temperature area outside the low temperature container of the cable. It can use the electrical insulation materials and technologies of conventional cables.
The electrical insulation layer of the low-temperature insulated 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 the magnetic field on the surrounding environment when the cable is carrying current, a shielding layer is usually added outside the insulating layer.
Organization structure of superconducting cable:
It is mainly composed of cable body, terminal and low temperature refrigeration device.
The superconducting cable body includes a cable core, electrical insulation, and a low-temperature container. The cable core is composed of superconductors. It is installed in a low-temperature container tube that maintains the low temperature required by the cable core. Both ends of the low-temperature container tube are connected to the terminals. The superconducting tape of the cable core is connected to an external power source or load through a 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 the skeleton, and the superconducting tapes are wound with insulating tape between layers to reduce the AC loss caused by the electromagnetic coupling of the cable. The low-temperature container tube of the cable adopts a double stainless steel bellows 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 outside the conductor layer in the same low-temperature environment as the conductor layer. For normal temperature insulated cables, the electrical insulation is outside 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 the external electrical components, and it is also the transitional section between the low-temperature part of the cable and the external room temperature. Therefore, the terminal requires good thermal insulation to ensure that the overall thermal loss of the superconducting cable is minimal. At the same time, the low-temperature cooling device must also 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 bus bar through the current lead, it is required that the terminal has a corresponding level of electrical insulation.