Control method of a supercooled liquid nitrogen circulation system

Abstract

The application discloses a control method of a supercooled liquid nitrogen circulating system, which comprises a superconducting cable assembly and a liquid nitrogen circulating assembly; the output end and the input end of the liquid nitrogen circulating assembly are connected with the input end and the output end of the superconducting cable assembly respectively, and are connected to form a closed liquid nitrogen circulating loop; and the liquid nitrogen circulating assembly comprises a cold box, two sets of pump boxes and a cold source assembly. In the application, three sets of cold sources, namely a Stirling refrigerator, a reverse Brayton refrigerator and an evacuation and pressure reduction system, are arranged to provide cold energy for the refrigeration system; the three sets of cold sources are independent of each other and are backup for each other, can realize on-line switching operation and on-line maintenance, improve the operation reliability of the system cold source and save the cost to the maximum extent; two sets of pump boxes are arranged to provide circulating power for the refrigeration system; the two sets of pump boxes are connected in parallel and are backup for each other, can realize on-line switching operation and on-line maintenance, and the liquid nitrogen pumps are arranged in the independent pump boxes respectively, so that the liquid nitrogen pumps are convenient to overhaul.

Description

Technical Field

[0001] This invention relates to the field of superconducting cable cooling technology, and more specifically to a control method for a subcooled liquid nitrogen circulation system. Background Technology

[0002] High-temperature superconducting technology, as a strategically significant cutting-edge technology, is showing broad application prospects in areas such as urban power grid transformation, maglev transportation, and large-scale scientific facilities, driven by rapid urbanization and industrialization. High-temperature superconducting power transmission, in particular, is hailed as a revolution in the power industry this century and is considered one of the most promising electrical technologies of the 21st century. It boasts advantages such as low loss, large capacity, small size, and zero pollution, and is expected to find engineering applications and widespread adoption in urban power grid transformation, narrow-corridor main power grids, and situations with special requirements that are difficult to address with conventional technologies. High-temperature superconducting cables are rapidly approaching commercial reality, with numerous cable demonstration projects showcasing the feasibility of this technology worldwide. A common feature of these projects is that the high-temperature superconducting cables are cooled using supercooled circulating liquid nitrogen. Typically, the circulating liquid nitrogen temperature ranges between 68-75K, and the liquid nitrogen is not allowed to boil while the cable is energized to avoid potential electrical problems and uneven cooling.

[0003] Cooling for high-temperature superconducting cables is divided into open-loop and closed-loop cooling. Open-loop cooling uses a large amount of liquid nitrogen, supplied to a cold box operating under evacuation and depressurization, to cool the circulating liquid nitrogen. The main advantages of open-loop cooling are its relatively simple design, high liquid nitrogen consumption, and high reliability. Closed-loop cooling uses a mechanical refrigeration unit to maintain the operating temperature of the subcooled liquid nitrogen. Compared to open-loop cooling, the main advantages of closed-loop cooling are lower operating costs and the elimination of the need for relatively large liquid nitrogen storage tanks for daily liquid nitrogen replenishment.

[0004] Patent document CN112542271A discloses a liquid nitrogen cooling circulation system for cooling superconducting cables. It includes a control device, a cooling circulation pipeline, a vacuum cooling device, and a liquid nitrogen replenishment device. The vacuum cooling device comprises a supercooled liquid nitrogen Dewar, a refrigeration unit, and a heat exchanger. The supercooled liquid nitrogen Dewar contains supercooled liquid nitrogen, and the heat exchanger is located within it. The control device controls the refrigeration unit to cool the supercooled liquid nitrogen in the supercooled liquid nitrogen Dewar using a vacuum decompression method. The heat exchanger is connected to the cooling circulation pipeline, utilizing the supercooled liquid nitrogen in the Dewar to cool the circulating liquid nitrogen in the cooling circulation pipeline. This system solves the problem of handling nitrogen gas generated during liquid nitrogen evaporation in the circulating liquid nitrogen heat exchange process, achieving cooling without the need for expensive refrigeration units. Therefore, it reduces the construction and maintenance costs of the liquid nitrogen cooling circulation system while simultaneously addressing the nitrogen gas generated during liquid nitrogen evaporation in the circulating liquid nitrogen heat exchange process.

[0005] If the supercooled liquid nitrogen circulation system malfunctions, such as damage to the cold source or liquid nitrogen pump, it will cause cooling failure, making it impossible to cool the superconducting cable and compromising the reliability of the supercooled liquid nitrogen circulation system. Summary of the Invention

[0006] The technical problem to be solved by this invention is how to improve the reliability of cable cooling by a supercooled liquid nitrogen circulation system.

[0007] This invention solves the above-mentioned technical problems through the following technical means: a control method for a subcooled liquid nitrogen circulation system, characterized in that it includes a subcooled liquid nitrogen circulation system, a main / standby switching control, and a cold source switching control. The subcooled liquid nitrogen circulation system includes a superconducting cable assembly and a liquid nitrogen circulation assembly. The output and input ends of the liquid nitrogen circulation assembly are respectively connected to the input and output ends of the superconducting cable assembly, forming a closed liquid nitrogen circulation loop. The liquid nitrogen circulation assembly includes a cold box, two sets of pump boxes, and a cold source assembly. The output end of the superconducting cable assembly is connected to the input end of the cold box through a first connecting pipe, and the output end of the cold box is connected to the input end of the superconducting cable assembly through a second connecting pipe. The two sets of pump boxes are connected in parallel to the first connecting pipe. Each pump box is equipped with a liquid nitrogen pump, which is connected to the first connecting pipe and provides power for the liquid nitrogen circulation. The cold source assembly includes a Stirling refrigerator, a reverse Brayton refrigerator, and a vacuum and depressurization system. The Stirling refrigerator, the reverse Brayton refrigerator, and the vacuum and depressurization system are all connected to the cold box and can each independently cool the medium inside the cold box.

[0008] The cold source switching control includes cold source component switching, which includes the following steps:

[0009] S1: Determine if the currently used cooling source components are faulty;

[0010] S2: If the currently used cold source fails, determine whether the backup cold source is also failing.

[0011] S3: If the backup cold source fails, the emergency cold source device will be forcibly activated, and the currently used cold source will be cut out of the circulation system. The backup cold source will be cut into the refrigeration circulation system. The refrigeration power before the failure of the currently used cold source will be recorded. Based on the conversion formula between the cooling capacity of the backup cold source device and the currently used cold source, the refrigeration power of the backup cold source will be preset. After the cold source is cut out, the system will enter the maintenance standby state and the backup cold source device will start running.

[0012] S4: If the backup cold source fails, an alarm will be triggered. The equipment will be shut down manually or automatically if the program determines that the equipment needs to be shut down and the emergency cooling device will be activated.

[0013] S5: Determine whether the temperature of the entire circulation system is fluctuating within the normal range. If it is fluctuating within the normal range, adjust the cooling capacity of the emergency cooling device and control the temperature. Determine whether the cold source in use and the backup cold source can operate normally. If they can operate normally, reconnect the cold source that can operate normally to the circulation system. If they are operating normally, it indicates that the cold source in use and the backup cold source have been troubleshooted. Manually or forcibly shut down the emergency cooling device.

[0014] If the temperature of the circulation system does not fluctuate within the normal range, further determine whether the temperature of the refrigeration circulation system fluctuates between the warning line and the safety line. If so, issue an alarm and adjust the cooling capacity of the emergency cooling device to control the temperature. If not, determine whether the temperature of the refrigeration circulation system exceeds the safety line. If it exceeds the safety line, issue an alarm and terminate the process.

[0015] The refrigeration system is supplied with cooling capacity by three cold sources: a Stirling chiller, a reverse Brayton chiller, and a vacuum and pressure reduction system. These three cold sources are independent of each other and serve as backups for one another, enabling online switching operation and online maintenance. This improves the reliability of the system's cold source operation while maximizing cost savings. The refrigeration system is powered by two pump boxes connected in parallel, which also serve as backups for one another, enabling online switching operation and online maintenance. In addition, the liquid nitrogen pumps are all housed in separate pump boxes for easy maintenance.

[0016] As a preferred technical solution, the system further includes a pressure control system and a liquid nitrogen storage tank. The pump box is also equipped with a pressure control container, which is connected to the first connecting pipeline. The pressure control container is used to control the working pressure of the circulating liquid nitrogen and to buffer the volume change of the liquid nitrogen during thermal expansion and contraction. The pressure control system includes a gas replenishment pressure control pipeline and a liquid replenishment pressure control pipeline. The gas outlet and liquid outlet of the liquid nitrogen storage tank are respectively connected to the pressure control container through the gas replenishment pressure control pipeline and the liquid replenishment pressure control pipeline. Both the gas replenishment pressure control pipeline and the liquid replenishment pressure control pipeline are equipped with control valves.

[0017] As a preferred technical solution, the pressure control system further includes a heating pressure control component, which includes a heater disposed inside the pressure control container.

[0018] As a preferred technical solution, the liquid nitrogen circulation assembly further includes an air compressor and a pressure-controlled gas replenishment buffer tank. The input end of the gas replenishment and pressure control pipeline is also connected to the output end of the air compressor, and a pressure-controlled gas replenishment buffer tank is provided on the gas replenishment and pressure control pipeline.

[0019] As a preferred technical solution, the gas replenishment and pressure control pipeline includes a first branch, a second branch, and a main pipeline. The liquid nitrogen storage tank is connected to the main pipeline through the first branch, and the air compressor is connected to the main pipeline through the second branch. The first branch is sequentially equipped with a first manual shut-off valve and an ambient temperature vaporizer. The second branch is sequentially equipped with a check valve and a first pressure reducing valve. The main pipeline is equipped with a second pressure reducing valve, which is connected to the pressure control containers in the two pump boxes respectively. The pressure control containers are also connected to an exhaust pipeline, and the exhaust pipeline is equipped with a sixth pneumatic shut-off valve.

[0020] As a preferred technical solution, the input end of the liquid replenishment and pressure control pipeline is connected to the liquid nitrogen storage tank, and the output end of the liquid replenishment and pressure control pipeline is connected to two pressure control containers respectively. The liquid replenishment and pressure control pipeline is equipped with a second manual shut-off valve, a pipeline filter, and a control valve.

[0021] As a preferred technical solution, the liquid nitrogen circulation assembly further includes a replenishment container, the output end of the replenishment pressure control pipeline is connected to the replenishment container through a replenishment input pipeline, the replenishment container is connected to the cold box through a replenishment output pipeline, a fourth pneumatic shut-off valve is provided on the replenishment input pipeline, and a fifth pneumatic shut-off valve is provided on the replenishment output pipeline.

[0022] As a preferred technical solution, the cold box is equipped with a subcooling heat exchanger, the two ends of which are connected to a first connecting pipe and a second connecting pipe, respectively, and the subcooling heat exchanger is located below the liquid level of the cooling medium inside the cold box.

[0023] As a preferred technical solution, the superconducting cable assembly includes a first terminal thermostat, a superconducting cable thermostat, and a second terminal thermostat arranged along the liquid nitrogen circulation direction. The liquid nitrogen circulation assembly is used to provide cooling for the superconducting cable assembly.

[0024] As a preferred technical solution, the reheated liquid nitrogen flowing out from the superconducting cable thermostat is pressurized by the liquid nitrogen pump in the pump box and enters the subcooling heat exchanger in the cold box to obtain cooling capacity and become 70K subcooled liquid nitrogen. The cold source for obtaining cooling capacity in the subcooling heat exchanger is a reverse Brayton refrigerator, a Stirling refrigerator, or a vacuum and depressurization system. Then it flows into the high-temperature superconducting cable thermostat to cool the superconducting cable, and finally returns to the liquid nitrogen pump to start a new cycle.

[0025] The advantages of this invention are:

[0026] (1) In this invention, three cold sources are set up—a Stirling chiller, a reverse Brayton chiller, and a vacuum decompression system—to provide cooling capacity to the refrigeration system. The three cold sources are independent of each other and serve as backups for each other, enabling online switching operation and online maintenance. This improves the reliability of the system's cold source operation while maximizing cost savings. Two pump boxes are used to provide circulating power to the refrigeration system. The two pump boxes are connected in parallel and serve as backups for each other, enabling online switching operation and online maintenance. At the same time, the liquid nitrogen pumps are all set up separately in independent pump boxes for easy maintenance.

[0027] (2) In this invention, a concept of a replenishment container is proposed. The replenishment container is an atmospheric pressure liquid nitrogen Dewar with an exhaust port connected to the atmosphere. After the high-pressure liquid nitrogen in the liquid nitrogen storage tank enters the replenishment container, it becomes 77.2K atmospheric pressure liquid nitrogen. The replenishment container replenishes liquid nitrogen to the negative pressure cold box, reducing the temperature fluctuation of the cold box during the replenishment process, thereby ensuring the temperature stability of the circulating liquid nitrogen in the system.

[0028] (3) In this invention, the replenishment of liquid nitrogen and pressure control of the circulation system are both achieved through a pressure control container. The bottom of the pressure control container is connected to the circulating liquid nitrogen through a pipe, which can replenish liquid nitrogen and transmit pressure to the circulation pipe. The required liquid nitrogen is provided by a liquid nitrogen storage tank. There are three pressure control methods: liquid replenishment and gas release, gas replenishment and gas release, and heating and gas release. The above three pressure control methods are mutually backed up, which can improve the reliability of system pressure control.

[0029] (4) In this invention, all low-temperature automatic valves are pneumatic valves. Pneumatic valves require a stable air source of 4-6 bar to operate. To ensure the reliability of the air source, both nitrogen and air sources are used to supply air to the automatic valves. At the same time, the nitrogen source is dry and sufficient, so the nitrogen source is used first. This goal can be achieved by adjusting the outlet pressure of the pressure reducing valve to be greater than the starting pressure of the air compressor, which further improves the reliability of the system. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the system structure provided in Embodiment 1 of the present invention;

[0031] Figure 2 This is a schematic diagram of the overall structure provided in Embodiment 1 of the present invention;

[0032] Figure 3 This is a schematic diagram of the first pump box structure provided in Embodiment 1 of the present invention;

[0033] Figure 4 This is a schematic diagram of the first pressure-controlled container structure provided in Embodiment 1 of the present invention;

[0034] Figure 5 This is a schematic diagram of the male socket assembly structure provided in Embodiment 1 of the present invention;

[0035] Figure 6This is a schematic diagram of the socket female connector assembly structure provided in Embodiment 1 of the present invention;

[0036] Figure 7 This is a schematic diagram of the first pressure-controlled container cylinder structure provided in Embodiment 1 of the present invention;

[0037] Figure 8 This is a schematic diagram of the second filter screen structure of the pressure control container provided in Embodiment 1 of the present invention;

[0038] Figure 9 This is a schematic diagram of the first filter screen structure of the pressure control container provided in Embodiment 1 of the present invention;

[0039] Figure 10 This is the overall control flowchart provided in Embodiment 2 of the present invention;

[0040] Figure 11 This is a pressure control flowchart provided in Embodiment 2 of the present invention;

[0041] Figure 12 This is a flowchart of level control and flow control provided in Embodiment 2 of the present invention;

[0042] Figure 13 This is a flowchart of the primary / standby switching control provided in Embodiment 2 of the present invention;

[0043] Figure 14 This is a flowchart of the valve box control provided in Embodiment 2 of the present invention;

[0044] Figure 15 This is a flowchart of the liquid level control provided in Embodiment 2 of the present invention;

[0045] Figure 16 This is a flowchart of the refrigeration device switching process provided in Embodiment 2 of the present invention;

[0046] Figure 17 This is a flowchart of the fault diagnosis process provided in Embodiment 2 of the present invention;

[0047] Figure 18 This is an alarm flowchart provided in Embodiment 2 of the present invention;

[0048] Figure 19 This is a temperature control flowchart provided in Embodiment 2 of the present invention;

[0049] Figure 20 This is a flowchart of data acquisition and analysis provided in Embodiment 2 of the present invention;

[0050] Reference numerals: 1. Air compressor; 2. Check valve; 3. Valve air source storage tank; 4. First pressure reducing valve; 5. Second pressure reducing valve; 6. Ambient air vaporizer; 7. First manual shut-off valve; 8. Pipeline filter; 9. Second manual shut-off valve; 10. Liquid nitrogen storage tank; 11. First pneumatic shut-off valve; 12. Second pneumatic shut-off valve; 13. Third pneumatic shut-off valve; 14. Fourth pneumatic shut-off valve; 15. First pipeline filter; 151. Filter tube; 152. Filter screen; 16. Liquid replenishment container; 17. First pump box; 171. Socket female connector assembly; 1711. First socket female connector tube body 1712. Second female socket pipe body; 1713. Female socket connecting pipe; 1714. Female socket flange; 172. Male socket assembly; 1721. Male socket pipe body; 1722. Male socket pressure ring; 18. Fifth pneumatic shut-off valve; 19. Sixth pneumatic shut-off valve; 20. First pressure control vessel; 201. Copper rod; 202. Heat exchanger rod; 203. Heat exchanger rod fixing pipe; 204. First filter screen of pressure control vessel; 205. Second filter screen of pressure control vessel; 206. First pressure control vessel cylinder; 2061. Outer cylinder of first pressure control vessel; 2062. First pressure control... Container inner cylinder; 21. Stirling refrigeration unit; 22. Seventh pneumatic shut-off valve; 23. Eighth pneumatic shut-off valve; 24. First ambient air vaporizer; 25. Ninth pneumatic shut-off valve; 26. Tenth pneumatic shut-off valve; 27. Eleventh pneumatic shut-off valve; 28. First vacuum pump; 29. ​​Second vacuum pump; 30. Twelfth pneumatic shut-off valve; 31. Second ambient air vaporizer; 32. Cold box; 33. Subcooling heat exchanger; 34. Thirteenth pneumatic shut-off valve; 35. Fourteenth pneumatic shut-off valve; 36. Fifteenth pneumatic shut-off valve; 37. Reverse Brayton refrigeration unit; 38. Sixteenth pneumatic shut-off valve 39. First terminal thermostat; 40. Seventeenth pneumatic shut-off valve; 41. Superconducting cable thermostat; 42. Eighteenth pneumatic shut-off valve; 43. First flow meter; 44. First liquid nitrogen pump; 45. Nineteenth pneumatic shut-off valve; 46. Twentieth pneumatic shut-off valve; 47. Second pressure control container; 48. Second flow meter; 49. Second liquid nitrogen pump; 50. Second pipeline filter; 51. Second terminal thermostat; 52. Second pump box; 53. Twenty-first pneumatic shut-off valve; 54. Twenty-second pneumatic shut-off valve; 55. Twenty-third pneumatic shut-off valve; 56. Pressure control and gas replenishment buffer tank. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] See Figure 1 and Figure 2 A supercooled liquid nitrogen circulation system includes a superconducting cable assembly and a liquid nitrogen circulation assembly. The output and input ends of the liquid nitrogen circulation assembly are connected to the input and output ends of the superconducting cable assembly, respectively, forming a closed liquid nitrogen circulation loop. The superconducting cable assembly includes a first terminal thermostat 39, a superconducting cable thermostat 41, and a second terminal thermostat 51 arranged along the liquid nitrogen circulation direction. The liquid nitrogen circulation assembly is used to provide cooling for the superconducting cable assembly. The warm liquid nitrogen (<77K) flowing out from the second terminal thermostat 51 is pressurized by the liquid nitrogen pump in the pump box 17 and enters the supercooling heat exchanger 33 in the cold box 32 to obtain cooling and become 70K supercooled liquid nitrogen (the cold source is a reverse Brayton refrigerator 37 or a Stirling refrigerator 21 or a vacuum and depressurization system). Then it flows into the first terminal thermostat 39 of the high-temperature superconducting cable to cool the superconducting cable, and finally returns to the liquid nitrogen pump to start a new cycle.

[0053] The liquid nitrogen circulation assembly includes a cold box 32, two sets of pump boxes, a cold source assembly, a pressure control system, a liquid nitrogen storage tank 10, a liquid replenishment container 16, an air compressor 1, and a pressure control and gas replenishment buffer tank 56. The output end of the superconducting cable assembly is connected to the input end of the cold box 32 through a first connecting pipe. The output end of the cold box 32 is connected to the input end of the superconducting cable assembly, i.e., the first terminal thermostat 39, through a second connecting pipe. The two sets of pump boxes are connected in parallel to the first connecting pipe. The first connecting pipe is equipped with an insulating section, which allows liquid nitrogen to flow through the first connecting pipe and also provides insulation. The insulating section is located below the second terminal thermostat 51. Since all connecting pipes are metal pipes to prevent them from conducting, each pump box is equipped with a liquid nitrogen pump and a pressure control container. The liquid nitrogen pump is connected to the first connecting pipe and provides power for the liquid nitrogen circulation. The pressure control container is connected to the first connecting pipe and is used to control the working pressure of the circulating liquid nitrogen and buffer the volume change of the liquid nitrogen during thermal expansion and contraction. The cold source components include a Stirling refrigerator 21, a reverse Brayton refrigerator 37, and a vacuum and pressure reduction system. The Stirling refrigerator 21, the reverse Brayton refrigerator 37, and the vacuum and pressure reduction system are all connected to the cold box 32 and can each independently cool the medium (i.e., saturated liquid nitrogen) in the cold box 32.

[0054] The system employs three high-capacity cold sources: a reverse Brayton chiller 37, a Stirling chiller 21, and a vacuum and pressure reduction system. These three cold sources are mutually redundant and can be switched and maintained online at will. The liquid nitrogen circulation power source, the liquid nitrogen pump, is located in an independent pump box. The system uses two sets of pump boxes for mutual backup, and the pump boxes can be switched and maintained online. The system pressure control adopts three methods: liquid replenishment pressure control, gas replenishment pressure control, and heating pressure control. Each pressure control method is independent and mutually redundant. The system's automatic valves are pneumatic valves, and the valve air source comes from either the liquid nitrogen storage tank 10 or the air compressor 1, with one in use and one on standby to ensure the reliability of the air source.

[0055] See Figure 1 The pressure control system includes a gas supply pressure control pipeline, a liquid supply pressure control pipeline, and a heating pressure control component. The gas outlet and liquid outlet of the liquid nitrogen storage tank 10 are connected to the pressure control container through the gas supply pressure control pipeline and the liquid supply pressure control pipeline, respectively. The heating pressure control component includes a heater installed in the pressure control container. Both the gas supply pressure control pipeline and the liquid supply pressure control pipeline are equipped with control valves, and all control valves are pneumatic shut-off valves.

[0056] See Figure 1 The two pump boxes include a first pump box 17 and a second pump box 52. The first pump box 17 and the second pump box 52 have the same structure. The first pump box 17 is equipped with a first pressure control container 20, a first pipeline filter 15, a first liquid nitrogen pump 44, and a first flow meter 43. The second pump box 52 is equipped with a second pressure control container 47, a second pipeline filter 50, a second liquid nitrogen pump 49, and a second flow meter 48. The first pump box 17 and the second pump box 52 are connected in parallel to the first connecting pipeline. The first pump box 17 is equipped with a first pneumatic shut-off valve 11 and an eighteenth pneumatic shut-off valve 42 located upstream and downstream of the first connecting pipeline, respectively. The second pump box 52 is equipped with a twenty-first pneumatic shut-off valve 53 and a twenty-sixth pneumatic shut-off valve 46 located upstream and downstream of the first connecting pipeline, respectively. The first pressure control container 20 and the second pressure control container 47 are both equipped with heaters.

[0057] See Figure 3 , Figure 4 , Figure 5 , Figure 6 The first pipeline filter 15 and the second pipeline filter 50 are both detachable structures. The original pipeline filter is directly welded to the pipeline, that is, the first connecting pipeline in this embodiment. However, it is not convenient for maintenance. Once a problem occurs, the metal pipeline can only be dismantled, which makes maintenance difficult.

[0058] In this embodiment, taking the socket structure on the first pipeline filter 15 as an example, a female socket assembly 171 is fixedly connected to the top of the first pump box 17, a male socket assembly 172 is fixedly connected to one end of the first connecting pipeline that is inserted into the female socket assembly 171, and the first pipeline filter 15 is fixedly connected to the bottom of the male socket assembly 172. The male socket assembly 172 includes a male socket tube body 1721 and a male socket pressure ring 1722, with the male socket pressure ring 1722 fixedly connected to the male socket tube body 1721. The top of the socket-female assembly 171 includes a first socket-female tube body 1711, a second socket-female tube body 1712, a socket-female connecting pipe 1713, and a socket-female flange 1714. The second socket-female tube body 1712 is coaxially fixed inside the first socket-female tube body 1711. The first socket-female tube body 1711 is fixedly connected to the first pump box 17. The second socket-female tube body 1712 is slidably fitted with the socket-male tube body 1721. The bottom of the second socket-female tube body 1712 is fixedly connected to the socket-female connecting pipe 1724. 713, the socket female connector 1713 is part of the first connecting pipeline. The top of the first socket female connector 1711 is fixedly connected to a socket female flange 1714. The inner diameter of the second socket female connector 1712 is slightly larger than the outer diameter of the socket male connector 1721. The bottom of the socket male connector 1721 has a threaded interface, which is threadedly connected to the first pipeline filter 15. The first pipeline filter 15 includes a filter pipe 151 and a filter screen 152. The filter pipe 151 has multiple sets of circular through holes circumferentially. Each set of through holes includes multiple... The filter tube 151 has multiple through holes distributed at equal angles around its circumference. The purpose of the side openings is to ensure the flow area and not to cause resistance to the circulation system. There is a gap between the socket female connector tube 1713 and the filter tube 151. A filter screen 152 is fixed in the gap. The filter screen 152 and the socket female connector tube 1713 form a closed cavity structure, namely a dense wire mesh, which can play a filtering role. The filter screen 152 can be fixed at the bottom of the second socket female tube body 1712.

[0059] Nitrogen gas enters the male connector tube 1721 from the first connecting pipe and then enters the filter tube 151. It passes through the dense wire mesh through the through hole on its side and enters the sealed cavity structure formed between the filter mesh 152 and the female connector tube 1713, and then enters the subsequent first connecting pipe.

[0060] See Figure 1The pressure-controlled container is connected to the first connecting pipe, meaning both the first pressure-controlled container 20 and the second pressure-controlled container 47 are connected to the first connecting pipe. In this embodiment, taking the first pressure-controlled container 20 connected to the first connecting pipe via a cooling pipe as an example, a copper rod 201 is provided inside the cooling pipe to transfer redundant cooling capacity in the first connecting pipe to the first pressure-controlled container 20 for cooling. This reduces the heat difference between the circulating nitrogen in the first connecting pipe and the saturated liquid nitrogen in the pressure-controlled container, ensuring heat balance. If there is heat leakage (i.e., the heat leakage of saturated liquid nitrogen in the pressure-controlled container is greater than the cooling input), the liquid nitrogen will vaporize. Vaporization increases the pressure inside the pressure-controlled container, requiring the opening of the sixth pneumatic shut-off valve. To ensure stable pressure, copper rods are used to transfer the subcooled liquid nitrogen from the circulation pipeline to the saturated liquid nitrogen in the pressure control container. This balances the cooling capacity input to the pressure control container with its heat leakage. However, since the heat leakage value of the pressure control container cannot be accurately calculated, copper rods 201 of different sizes need to be replaced to ensure optimal cooling performance. Therefore, copper rods 201 are not directly fixed in the cooling pipeline but are designed as detachable structures. Furthermore, since the insertion of copper rods 201 into the cooling pipeline affects the replenishment rate of the entire circulation pipeline, a replenishment pipeline is added between the first pressure control container 20 and the first connecting pipeline to ensure that the insertion of copper rods 201 does not affect the replenishment rate of liquid nitrogen from the liquid nitrogen storage tank to the circulation system.

[0061] See Figure 7 , Figure 8 , Figure 9The first pressure-controlled container 20 includes a first pressure-controlled container cylinder 206, a first pressure-controlled container filter 204, a second pressure-controlled container filter 205, a heat exchange rod fixing tube 203, and a heat exchange rod 202. The first pressure-controlled container cylinder 206 includes a first pressure-controlled container outer cylinder 2061 and a first pressure-controlled container inner cylinder 2062 coaxially fixed. A socket head cap 2063 is fixedly connected to the first pressure-controlled container outer cylinder 2061, and a detachable fixed connection is made to the socket head cap 2063. The male socket head cap 2064, female socket head cap 2063, and male socket head cap 2064 operate on the same principle as the male socket head cap assembly 172 and female socket head cap assembly 171, all using a sliding fit and being connected via flange bolts. The female socket head cap 2063 is fixedly connected to the top of the outer shell 2061 of the first pressure vessel. The male socket head cap 2064 is snapped onto the female socket head cap 2063. The two flanges of the male socket head cap 2064 and the female socket head cap 2063 are detachably fixed by bolts. The male socket head cap 2064... 4. The bottom is fixedly connected to the copper rod 201 via the heat exchanger fixing pipe 203 and the heat exchanger 202. The heat exchanger fixing pipe 203 is fixedly connected to the heat exchanger 202, and the bottom of the heat exchanger 202 is threadedly connected to the copper rod 201 for easy replacement of the copper rod 201. The first filter screen 204 of the pressure control container is fixed inside the inner cylinder 2062 of the first pressure control container and located outside the heat exchanger fixing pipe 203 and the heat exchanger 202. A [missing information - likely a device or component] is fixedly connected to the inner wall of the bottom of the inner cylinder 2062 of the first pressure control container. The second filter 205 of the pressure control container, the first filter 204 of the pressure control container, and the second filter 205 of the pressure control container can all prevent impurities from entering the circulation pipe. This allows the first pressure control container 20 to be connected to the first connecting pipe through the liquid replenishment pipe. The first pressure control container 20 is also connected to the first connecting pipe through the cooling pipe. The vertical side wall of the cooling pipe has an input port of the first connecting pipe. The first connecting pipe is connected to one end of the cooling pipe and one end of the liquid replenishment pipe through a T-junction.

[0062] See Figure 1The input end of the air replenishment and pressure control pipeline is also connected to the output end of the air compressor 1. The air replenishment and pressure control pipeline is equipped with a pressure-controlled air replenishment buffer tank 56. The air replenishment and pressure control pipeline includes a first branch, a second branch, and a main pipeline. The liquid nitrogen storage tank 10 is connected to the main pipeline via the first branch, and the air compressor 1 is connected to the main pipeline via the second branch. The first branch is sequentially equipped with a first manual shut-off valve 7 and an ambient temperature vaporizer 6. The second branch is sequentially equipped with a check valve 2, a valve air source storage tank 3, and a first pressure reducing valve 4. The valve air source storage tank 3 is used to supply air to all pneumatic valves. The pipeline is equipped with a second pressure reducing valve 5, which is connected to the first pressure control container 20 and the second pressure control container 47 in the first pump box 17 and the second pump box 52 respectively. The first pressure control container 20 and the second pressure control container 47 are both connected to exhaust pipes. The exhaust pipe is equipped with a sixth pneumatic shut-off valve 19. The main pipeline is connected to the first pressure control container 20 through the first air supply pipeline. The main pipeline is connected to the second pressure control container 47 through the second air supply pipeline. The first air supply pipeline is equipped with a third pneumatic shut-off valve 13. The second air supply pipeline is equipped with a twenty-third pneumatic shut-off valve 55.

[0063] See Figure 1 The input end of the liquid replenishment and pressure control pipeline is connected to the liquid nitrogen storage tank 10. Two of the output ends of the liquid replenishment and pressure control pipeline are connected to the first pressure control container 20 and the second pressure control container 47, respectively. The liquid replenishment and pressure control pipeline is equipped with a second manual shut-off valve 9, a pipeline filter 8, a second pneumatic shut-off valve 12, and a twenty-second pneumatic shut-off valve 54. The liquid replenishment and pressure control pipeline is connected to the first pressure control container 20 through the first liquid replenishment pipeline and to the second pressure control container 47 through the second liquid replenishment pipeline. The second pneumatic shut-off valve 12 and the twenty-second pneumatic shut-off valve 54 are respectively configured to control whether the liquid replenishment and pressure control pipeline is connected to the first pressure control container 20 and the second pressure control container 47. The other output end of the liquid replenishment and pressure control pipeline is connected to the liquid replenishment container 16. The liquid replenishment container 16 is connected to the cold box 32 through the liquid replenishment output pipeline for replenishing liquid nitrogen to the cold box 32. The liquid replenishment input pipeline is equipped with a fourth pneumatic shut-off valve 14, and the liquid replenishment output pipeline is equipped with a fifth pneumatic shut-off valve 18.

[0064] By setting up the replenishment container 16, which is an atmospheric pressure liquid nitrogen Dewar with its exhaust port connected to the atmosphere, the high-pressure liquid nitrogen in the liquid nitrogen storage tank 10 becomes 77.2K atmospheric pressure liquid nitrogen after entering the replenishment container 16. The replenishment container 16 replenishes liquid nitrogen to the negative pressure cold box 32, reducing the temperature fluctuation of the cold box 32 during the replenishment process, thereby ensuring the temperature stability of the circulating liquid nitrogen in the system.

[0065] See Figure 1The cold box 32 is a double-layered vacuum insulated container. A subcooling heat exchanger 33 is installed inside the cold box 32. The two ends of the subcooling heat exchanger 33 are connected to the first connecting pipe and the second connecting pipe, respectively. The subcooling heat exchanger 33 is located below the liquid surface of the cooling medium inside the cold box 32, so that the subcooling heat exchanger 33 is immersed in negative pressure liquid nitrogen and has the same temperature as the negative pressure liquid nitrogen. The high-pressure liquid nitrogen circulating in the system exchanges heat with the negative pressure liquid nitrogen through the subcooling heat exchanger 33 to obtain cooling capacity, thereby ensuring that the liquid nitrogen entering the superconducting cable is subcooled liquid nitrogen. The cold box 32 is a double-layered vacuum insulated container, which stores negative pressure saturated liquid nitrogen. Its function is to cool the high-pressure liquid nitrogen in the circulation pipe and keep it in a subcooled state. The cooling capacity of the negative pressure liquid nitrogen comes from the reverse Brayton refrigerator 37 or the Stirling refrigerator 21 or the first vacuum pump 28 or the second vacuum pump 29.

[0066] See Figure 1 The first pump box 17 adopts a single-layer structure and uses vacuum multi-layer thermal insulation. The first liquid nitrogen pump 44 and the second liquid nitrogen pump 49 both provide circulation power for subcooled liquid nitrogen. The circulation flow rate is adjusted by speed regulation through a frequency converter. The first pressure control container 20 and the second pressure control container 47 are used to control the working pressure of the circulating liquid nitrogen and also serve as buffer containers for the circulation system to buffer the volume changes of the liquid nitrogen during thermal expansion and contraction. System replenishment and pressure control are both achieved through the pressure control containers. The bottoms of the first pressure control container 20 and the second pressure control container 47 are connected to the circulating liquid nitrogen through pipes to transmit pressure. There are three ways to control the system working pressure:

[0067] (1) Gas replenishment and pressure control: When the pressure is lower than the lower limit of the set value, the third pneumatic shut-off valve 13 opens, and the pressure control gas replenishment buffer tank 56 replenishes gas to the pressure control container (first pressure control container 20 or second pressure control container 47). When the target pressure is reached, the gas intake stops. When the pressure is higher than the upper limit of the set value, the sixth pneumatic shut-off valve 19 is opened to exhaust gas. After the set value is reached, it is closed. The gas source of the pressure control gas replenishment buffer tank 56 comes from the gas phase space of the liquid nitrogen storage tank 10. After passing through the ambient temperature vaporizer 6, it becomes room temperature nitrogen. After being depressurized by the second pressure reducing valve 5, it enters the pressure control gas replenishment buffer tank 56.

[0068] (2) Liquid replenishment and pressure control: When the pressure is lower than the lower limit of the set value, the second pneumatic shut-off valve 12 is opened, and the liquid nitrogen storage tank 10 replenishes the pressure control container. When the target pressure is reached, the liquid inlet stops. When the pressure is higher than the upper limit of the set value, the sixth pneumatic shut-off valve 19 of the pressure control container is opened. After the set value is reached, it is closed. Since the gas-liquid two-phase flow comes in when the second pneumatic shut-off valve 12 is first opened (for liquid replenishment), the purpose of pressurization can be achieved.

[0069] (3) Heating and pressure control: When the pressure is lower than the lower limit of the set value, the heater arranged in the pressure control container is opened. When the target pressure is reached, heating is stopped. When the pressure is higher than the upper limit of the set value, the first pneumatic shut-off valve (for exhaust) 19 of the pressure control container is opened. After the set value is reached, it is closed.

[0070] The above pressure control methods can be selected independently and are mutually redundant. The pressure control process is automatically controlled by the program and does not require personnel to be on duty. A liquid replenishment port is set on the top of the pressure control container. The required liquid nitrogen is provided by the liquid nitrogen storage tank 10. The pressure control container is equipped with a liquid level gauge to monitor its liquid level in real time. When the liquid level is lower than the set lower limit, the third pneumatic shut-off valve 13 is opened to replenish the system. When it is higher than the set upper limit, the liquid replenishment valve is closed to stop the liquid replenishment. In order to protect the liquid nitrogen pump impeller from being damaged by solid particles, a pipeline filter is installed upstream of the liquid nitrogen pump. A first flow meter 43 is installed at the outlet of the liquid nitrogen pump to monitor the system flow in real time. The pump rotation frequency can be controlled by the frequency converter to adjust the liquid nitrogen circulation flow rate, thereby adjusting the temperature difference between the inlet and outlet of the superconducting cable. The liquid nitrogen pump is a moving part and requires regular maintenance. In order to improve the reliability of the refrigeration system, the liquid nitrogen pump and the pressure control container are placed in a pump box, and the pump box is designed with redundancy. The system is equipped with two pump boxes, the first pump box 17 and the second pump box 52, one for use and one for backup. The backup cold pump box can be quickly switched to the working state at any time.

[0071] When the refrigeration system is running, the cold box 32 contains saturated liquid nitrogen at approximately 68K, with a saturation pressure of approximately 17kPa. At this time, the liquid nitrogen storage tank 10 contains saturated liquid nitrogen at approximately 0.8MPa, which is at a higher temperature. To prevent the liquid nitrogen temperature in the cold box 32 from being impacted when replenishing liquid, a normal pressure replenishment container 16 is provided for the cold box 32. The high-pressure liquid nitrogen in the liquid nitrogen storage tank 10 enters the replenishment container 16 and becomes normal pressure liquid nitrogen, corresponding to a liquid nitrogen temperature of 77.2K. The replenishment container 16 always maintains a certain level of liquid nitrogen. When the cold box 32 needs to be replenished, the fifth pneumatic shut-off valve 18 is opened, and liquid is replenished to the cold box 32 by gravity and pressure difference.

[0072] See Figure 1The Stirling refrigerator 21 is connected to the cold box 32 via its input and output pipes. The input and output pipes are equipped with a seventh pneumatic shut-off valve 22 and an eighth pneumatic shut-off valve 23, respectively. A first connecting pipe is connected to the cold box 32 via its input pipe, which is equipped with a seventeenth pneumatic shut-off valve 40. The cold box 32 is connected to a second connecting pipe via its output pipe, which is equipped with a thirteenth pneumatic shut-off valve 34. The first and second connecting pipes are also connected via an intermediate connecting pipe, which is sequentially equipped with a sixteenth pneumatic shut-off valve 38, a reverse Brayton refrigerator 37, and a fifteenth pneumatic shut-off valve 36. This intermediate connecting pipe allows the entire circulation system to bypass the cold box 32 and directly pass through the reverse Brayton refrigerator 37. The reverse Brayton chiller 37 directly provides cooling capacity. The cold box inlet pipe is also connected to the intermediate connecting pipe via a third connecting pipe. The end of the third connecting pipe connected to the cold box inlet pipe is located downstream of the seventeenth pneumatic shut-off valve 40, and the end of the third connecting pipe connected to the intermediate connecting pipe is located upstream of the fifteenth pneumatic shut-off valve 36 and downstream of the reverse Brayton chiller 37. The fourteenth pneumatic shut-off valve 35 is installed on the third connecting pipe. By setting the third connecting pipe as a bypass pipe, cooling capacity can be provided to the cold box 32 through the reverse Brayton chiller 37. The advantage of this setting is that the cold box 32 acts as a cold storage device. Even if the turbine fails, the cooling capacity of the cold box 32 can be maintained for a certain period of time to ensure the overall operation of the system. The end of the third connecting pipe connected to the intermediate connecting pipe is located upstream of the fifteenth pneumatic shut-off valve 36.

[0073] See Figure 1 The vacuum and pressure reducing system includes two sets of parallel vacuum and pressure reducing pipelines with identical structures, namely the first vacuum and pressure reducing pipeline and the second vacuum and pressure reducing pipeline. The first vacuum and pressure reducing pipeline is equipped with a first ambient air vaporizer 24, an eleventh pneumatic shut-off valve 27, and a first vacuum pump 28 in sequence. The second vacuum and pressure reducing pipeline is equipped with a second ambient air vaporizer 31, a twelfth pneumatic shut-off valve 30, and a second vacuum pump 29 in sequence. The first ambient air vaporizer 24 and the eleventh pneumatic shut-off valve 27 are also connected to a first maintenance and repair pipeline, which is equipped with a ninth pneumatic shut-off valve 25. The second ambient air vaporizer 31 and the twelfth pneumatic shut-off valve 30 are also connected to a second maintenance and repair pipeline, which is equipped with a tenth pneumatic shut-off valve 26.

[0074] It should be noted that the reverse Brayton chiller 37 has a long maintenance cycle, stable temperature control, and does not consume liquid nitrogen during operation. It can be used detached from the cold box 32. When maintenance is required, closing the fifteenth pneumatic shut-off valve 36 and the sixteenth pneumatic shut-off valve 38 will disconnect it from the system. When other cold sources need maintenance, this chiller can also be used to ensure system operation. The Stirling chiller 21 is connected to the cold box 32 through the Stirling chiller input pipe and the Stirling chiller output pipe. Cold nitrogen enters the Stirling chiller 21, cools down, and returns to the cold box 32, continuously reducing the vapor pressure of the cold box 32 to provide cooling capacity. It does not consume liquid nitrogen in the cold box 32 during operation. When maintenance is required, closing the seventh pneumatic shut-off valve 22 and the eighth pneumatic shut-off valve 23 will disconnect it from the refrigeration system. The vacuum and pressure reduction system is connected to the cold box 32 through the first and second vacuum and pressure reduction pipelines. When the vacuum and pressure reduction system is activated, taking the first vacuum and pressure reduction pipeline as an example, the evaporated nitrogen gas passes through the pipeline and is heated to room temperature by the first ambient temperature vaporizer 24. Then, it is extracted by the first vacuum pump 28 and discharged to the atmosphere. During the evaporation process, the liquid nitrogen provides cooling for the remaining liquid nitrogen in the cold box 32. The vacuum and pressure reduction system adopts a redundant design, with one system in use and one in standby. This method requires the extraction of nitrogen gas, so the consumption of liquid nitrogen is relatively large. However, it has the advantages of simple structure, low cost, and fast cooling speed. When maintenance is required, the ninth pneumatic shut-off valve 25, the tenth pneumatic shut-off valve 26, the eleventh pneumatic shut-off valve 27, and the twelfth pneumatic shut-off valve 30 can be closed to disconnect the vacuum pump from the system.

[0075] In this embodiment, all pneumatic shut-off valves (i.e., the first pneumatic shut-off valve 11 to the twenty-third pneumatic shut-off valve 55) are pneumatic valves. Pneumatic valves require a 4-6 bar air supply to operate. To ensure a stable and reliable air supply, both nitrogen and air are used, one for operation and one for backup. The nitrogen source comes from the gas valve in the liquid nitrogen storage tank 10. The nitrogen is heated to room temperature by the ambient temperature vaporizer 6, and after being depressurized by the first pressure reducing valve 4, it enters the gas source storage tank 3 to supply the valves. The air source comes from the air compressor 1, and enters the valve gas source storage tank 3 via the check valve 2 to supply the valves. The nitrogen source is dry and sufficient, and is used preferentially. This can be achieved by adjusting the outlet pressure of the first pressure reducing valve 4 to be greater than the starting pressure of the air compressor 1.

[0076] Example 2

[0077] The difference between this embodiment and Embodiment 1 is that this embodiment provides a control method based on the subcooled liquid nitrogen circulation system in Embodiment 1.

[0078] See Figure 10 A control method for a subcooled liquid nitrogen circulation system includes a terminal connector control system, a pump box control system, a valve box control system, a cold box control system, a replenishment container control system, data acquisition and analysis, local and remote switching, fault diagnosis, alarm control, and temperature control. The terminal connector control system includes monitoring its operating status, monitoring data, and safety alarms.

[0079] See Figure 10 The pump box control includes the following steps: operation control of the first pump box 17 and the second pump box 52, which includes pressure control, liquid level control, flow control, and main / standby switching control; in this embodiment, the structure of the first pump box 17 and related connecting pipelines are taken as an example.

[0080] See Figure 11 Pressure control includes the following steps:

[0081] S1: Beginning;

[0082] S2: Set up pressure monitoring points throughout the entire circulation system, and use sensors to collect data to detect the pressure of the entire circulation system.

[0083] S3: If the sensor detects that the pressure at the monitoring point of the circulation system is too high, open the exhaust valve (i.e., the sixth pneumatic shut-off valve 19) in the open circulation to exhaust and reduce pressure, or increase the cooling capacity of the refrigeration unit in the closed circulation. If the pressure returns to the safe operating pressure range of the system, the process ends. If the sensor still detects that the pressure at the monitoring point of the circulation system is too high, repeat the above steps. When the pressure exceeds the warning line but is below the safety upper limit, issue a warning. If the pressure exceeds the warning line but is not below the safety upper limit, the process ends. When the pressure exceeds the safety upper limit, shut down the gas replenishment, heating, and liquid replenishment pressurization operations and sound an alarm. When the pressure does not exceed the safety upper limit, the process ends.

[0084] If the sensor detects that the pressure at the monitoring point of the circulating system is too low, the exhaust valve in the open circulation will be opened to replenish air and reduce pressure, or the heater in the closed circulation will be opened to evaporate and pressurize. If the pressure returns to the safe operating pressure range of the system, the process ends. If it does not return to the safe operating pressure range, the exhaust valve in the open circulation will continue to replenish air and reduce pressure until the pressure returns to the safe operating pressure range of the system. If the pressure is lower than the warning line but higher than the safety lower limit, an alarm will be issued. If the pressure is not lower than the warning line but higher than the safety lower limit, technical procedures will be followed. If the pressure is both lower than the warning line and lower than the safety lower limit, the gas replenishment, heating, and liquid replenishment pressurization operations of the device will be shut down and an alarm will be triggered.

[0085] See Figure 12 Liquid level control includes the following steps:

[0086] S1: Beginning;

[0087] S2: Install a liquid level sensor in the circulation system to operate and collect data;

[0088] S3: When the liquid level is lower than the expected lower limit, the liquid replenishment valve (second manual shut-off valve 9, second pneumatic shut-off valve 12) is opened in the open cycle to start liquid replenishment or reduce the cooling capacity of the refrigeration unit in the closed cycle. During this process, it is detected whether the liquid replenishment causes excessive pressure. If it causes excessive pressure, pressure control is performed. Pressure control is as described above and will not be repeated here. When the liquid level reaches the expected setting, the process ends. If the liquid level does not reach the expected upper limit, the liquid replenishment valve is opened again to start liquid replenishment. If the liquid level is lower than the expected lower limit and the warning line, an alarm is triggered.

[0089] Flow control includes the following steps:

[0090] S1: Start (Calculate the cycle determination period based on the pipe volume and the expected flow rate);

[0091] S2: The circulation system is equipped with a flow sensor to operate and collect data;

[0092] S3: Determine whether the flow rate has reached the expected setting. If it has not reached the expected setting, calculate the required flow rate according to the formula of heat exchange efficiency, flow rate and temperature difference, and adjust the working frequency of the first liquid nitrogen pump 44. The process ends after the expected setting is reached. If the flow rate has not reached the expected setting and is lower than the warning line but higher than the safety lower limit, a warning will be issued. If the flow rate is lower than the safety lower limit, an alarm will be triggered.

[0093] See Figure 13 The main / standby switching control includes the following steps: (taking the switching from the first pump box 17 to the second pump box 52 as an example). If the first pump box 17 malfunctions, it is determined whether the refrigeration unit needs to be switched out of the refrigeration cycle system. If it needs to be switched out, the refrigeration unit switches out of the cryogenic cycle system and forcibly opens the exhaust valve of the second pump box 52, closes the first liquid nitrogen pump 44 of the first pump box 17, and closes the outlet valve of the first pump box 17. If the refrigeration unit does not need to be switched out of the refrigeration cycle system, the step of switching out of the cryogenic cycle system is skipped, and the exhaust valve of the second pump box 52 is forcibly opened, the first liquid nitrogen pump 44 of the first pump box 17 is closed, the outlet valve of the first pump box 17 is closed, and the outlet valve of the second pump box 52 is opened. The inlet is normally open for pre-cooling, and the outlet pipe is naturally cooled. When the cooling temperature reaches the expected value or the cooling time reaches the expected value, or the cooling temperature and cooling time both reach the expected value, and the second pump box 52 has no fault alarm, the second liquid nitrogen pump 49 of the second pump box 52 is turned on, the refrigeration unit runs and switches in. If the second pump box 52 has a fault alarm, the system alarms.

[0094] See Figure 14 Valve box control includes cold source switching, cold source switching includes refrigeration unit in / out and refrigeration unit switching, refrigeration unit in / out includes starting, calculating the average switching rate according to the preset time, adjusting the opening and closing ratio of the proportional valves of different branches to ensure that the diameter of the circulation pipeline remains unchanged, and ending.

[0095] In this implementation, the switching of refrigeration units, or cold source switching, refers to the first refrigeration unit, the second refrigeration unit, and the third refrigeration unit, which can be a Stirling refrigerator 21, a reverse Brayton refrigerator 37, or a vacuum and pressure reduction system. The switching of refrigeration units includes the following steps:

[0096] If the first refrigeration unit is running and malfunctioning, while the second refrigeration unit is running normally, there is no need to forcibly activate the emergency refrigeration unit. Instead, the first refrigeration unit is switched out of the circulation system, and the second refrigeration unit is switched into the circulation system. The refrigeration power of the first refrigeration unit before the malfunction is recorded. Based on the conversion formula between the refrigeration capacity of the first and second refrigeration units, the refrigeration power of the second refrigeration unit is preset. Once the first refrigeration unit is switched out and enters the maintenance state, the second refrigeration unit is started and running.

[0097] If the second refrigeration unit is running and malfunctioning, while the first refrigeration unit is running normally, there is no need to forcibly activate the emergency refrigeration unit. Instead, the second refrigeration unit is switched out of the circulation system, and the first refrigeration unit is switched into the circulation system. The refrigeration power of the second refrigeration unit before the malfunction is recorded. Based on the conversion formula between the refrigeration capacity of the second and first refrigeration units, the refrigeration power of the first refrigeration unit is preset. Once the second refrigeration unit is switched out and enters the maintenance standby state, the first refrigeration unit is started and running.

[0098] If the first refrigeration unit is operating and malfunctioning, the system checks if the second refrigeration unit is malfunctioning. If so, an alarm is triggered, and the emergency refrigeration unit is manually or forcibly activated. If the refrigeration cycle system temperature fluctuates within the normal range, the emergency refrigeration unit's cooling capacity is adjusted to control the temperature. If the second refrigeration unit operates normally after adjustment, the first refrigeration unit is switched off and enters a maintenance-ready state, and the second refrigeration unit is started. If the second refrigeration unit fails to operate normally, but the first refrigeration unit operates normally, the second refrigeration unit is switched off and enters a maintenance-ready state, and the first refrigeration unit is started. The system is then manually or forcibly shut down (manual shutdown or automatic program determination requires forced shutdown). If the refrigeration cycle system temperature fluctuates between the warning line and the safety line, an alarm is triggered, and the emergency refrigeration unit's cooling capacity is adjusted to control the temperature, and the first and second refrigeration units are checked as described above. If the cycle system temperature exceeds the safety line, an alarm is triggered directly.

[0099] See Figure 14 , Figure 15 , Figure 16 The same applies to cold source switching control, liquid replenishment container control, and liquid level control in cold box control, including the following steps:

[0100] S1: Both the cold box 32 and the liquid replenishment container 16 are equipped with liquid level sensors, which collect data.

[0101] S2: Determine if the liquid level is below the expected lower limit;

[0102] S3: If the liquid level is lower than the expected lower limit, open the liquid replenishment valve to start replenishing the liquid (initiating circulation). If the liquid level is not only lower than the expected lower limit but also lower than the warning line, issue a warning.

[0103] S4: After replenishing the liquid in step S3, determine whether the liquid level has reached the expected upper limit. If it has not been reached, continue replenishing the liquid; otherwise, end the process. At the same time, determine whether replenishing the liquid has caused excessive pressure. If it has not caused excessive pressure, continue replenishing the liquid; if it has caused excessive pressure, open the vent valve to reduce the pressure and adjust it according to the pressure.

[0104] See Figure 20 Data acquisition and analysis includes data interaction and parameter acquisition of the execution equipment. Parameter acquisition includes the entire circulation system, terminal connector control, and equipment safety monitoring system, including temperature sensors, pressure sensors, flow sensors, liquid level sensors, current transformers, gas concentration analyzers, smoke alarms, mechanical dry contact feedback, and NPN contact feedback installed in the entire circulation system.

[0105] The data interaction of the equipment includes the interaction of data on the operating status, measurement and control data, and safety alarms of the first refrigeration unit, the second refrigeration unit, the third refrigeration unit, the cooling unit, and the frequency converter.

[0106] Local-to-remote switching includes the following steps:

[0107] S1: Beginning;

[0108] S2: Local / remote switching uses a dual confirmation operation. The switching operation signals of the hardware (local knob) and the human-machine interface must be switched to local or remote simultaneously to achieve the switching of local and remote control operations. Switching the hardware to local permission will, if successful, switch the human-machine interface platform to local permission. If successful, the operation can begin. If unsuccessful, it is considered as another operation combination. Switching the hardware to remote permission will, if successful, switch the human-machine interface platform to remote permission. If unsuccessful, it is considered as another operation combination. If successful, the switching can begin. Other operation combinations are invalid operation combinations and the operation will not take effect.

[0109] See Figure 17 Fault diagnosis includes sensor faults, valve faults, actuator faults, and functional faults. Sensor fault diagnosis includes the following steps:

[0110] S1: Periodic start;

[0111] S2: Whether the hardware diagnostic feedback line of the acquisition unit is faulty;

[0112] S3: If there is a line fault, a fault error will be reported. If there is no line fault, the software will determine whether the sensor itself has a measurement fault. If the measurement exceeds the range, a fault error will be reported. If there is no fault, the judgment will end.

[0113] Valve malfunctions include valve opening signals. If the valve action is delayed, the valve will report an open status. If it is already open, it is in normal working condition, and this judgment ends. If the valve reports that it is not open, the valve function and corresponding measurement and control sensor parameter data will be combined to determine whether the valve has acted. If it has acted, the feedback device is faulty, and this judgment ends. Otherwise, it is a valve malfunction, and this judgment also ends. The valve closing signal follows the same control steps as the valve opening signal.

[0114] The equipment fault execution steps are as follows: periodic start, equipment action, equipment action delay, and judgment of whether there is a feedback fault in components such as thermal relays or the equipment itself. If there is no fault, the equipment is working normally; if there is a fault, it indicates that the equipment is faulty, and this judgment ends.

[0115] The functional fault diagnosis steps include the following (taking a function that requires 1 valve and 1 sensor (with 1 backup) as an example): First, determine if the valve is faulty. If the valve is faulty, it indicates that the valve function is lost. If the valve is not faulty, determine if the sensor is faulty. If the sensor is faulty, switch to the backup sensor. If the backup sensor is faulty, it also indicates that the function is lost. According to the importance of the function to the system operation, alarm according to importance. If the backup sensor is not faulty, it indicates that the equipment is working normally.

[0116] See Figure 18 The alarm includes four operating conditions: open flame in the environment, accident operation, hardware system damage, and overload operation. Open flame in the environment is detected by fire alarm. Accident operation and hardware system damage should be determined by combining the equipment function and the corresponding measurement and control sensor parameter data to determine whether the equipment can support the system operation. If it is normal aging and wear (which does not affect the system operation), a prompt will be issued. If it is a common functional failure (which needs to be stopped in time), a warning will be issued. If it is a major functional failure (which cannot maintain operation), an emergency stop or load switch will be issued. Overload operation will directly trigger an emergency stop or load switch.

[0117] See Figure 19 Temperature control involves determining whether the temperature difference between the inlet and outlet of the virtual system reflects the effective heat exchange / cooling capacity and whether it can maintain the superconducting state. This includes the following steps:

[0118] S1: Data is collected via a temperature sensor;

[0119] S2: Determine whether the temperature measurement and control point data (temperature difference) is stable within the normal range. If so, determine whether the temperature exceeds the warning line but is below the safety limit. If so, issue a warning; if it exceeds the safety limit, issue an alarm.

[0120] S3: Determine if the current refrigeration unit is faulty; if it is faulty, use the backup refrigeration unit. First, determine if the backup refrigeration unit is faulty. If the backup refrigeration unit is faulty, an alarm will be triggered. If the backup refrigeration unit is not faulty, the refrigeration unit switching control will switch the refrigeration unit. If the current refrigeration unit is not faulty, determine if the current circulation unit is faulty. If the current circulation unit is faulty, use the backup circulation unit. It is necessary to determine if the backup circulation unit is faulty. If the backup circulation unit is faulty, an alarm will be triggered. If there is no fault, the refrigeration unit switching control will switch the refrigeration unit.

[0121] S4: If the current circulation device is fault-free, first determine whether the temperature control-related equipment is normal through temperature control. Then select flow control or temperature control, and determine whether to adjust the flow rate. If so, maintain the cooling capacity unchanged, calculate the adjustment flow rate according to the formula relating temperature, load, cooling capacity, heat exchange efficiency (100% for immersion type), and flow rate, and determine whether the flow rate adjustment is effective. If effective, end the process. If flow rate adjustment is not prioritized, when the flow rate reaches the expected flow rate or the actual operating limit, calculate and adjust the cooling capacity according to the formula relating load, cooling capacity, heat exchange efficiency (100% for immersion type), and temperature. It should be noted that if the flow rate does not reach the expected flow rate or the actual operating limit, determine whether the flow rate adjustment is effective. If so, end the process.

[0122] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method of a supercooled liquid nitrogen cycle system, characterized by, The supercooled liquid nitrogen circulating system, the main and standby switching control, and the cold source switching control, the supercooled liquid nitrogen circulating system comprises a superconducting cable assembly and a liquid nitrogen circulating assembly, the output end and the input end of the liquid nitrogen circulating assembly are connected with the input end and the output end of the superconducting cable assembly respectively, and are connected to form a closed liquid nitrogen circulating loop, the liquid nitrogen circulating assembly comprises a cold box, two groups of pump boxes and a cold source assembly, the output end of the superconducting cable assembly is connected with the input end of the cold box through a first connecting pipeline, the output end of the cold box is connected with the input end of the superconducting cable assembly through a second connecting pipeline, the two groups of pump boxes are connected in parallel on the first connecting pipeline, the pump boxes are each provided with a liquid nitrogen pump, the liquid nitrogen pump is connected on the first connecting pipeline and provides power for liquid nitrogen circulation, and the cold source assembly comprises a Stirling refrigerator, a reverse Brayton refrigerator and an evacuation and pressure reduction system, the Stirling refrigerator, the reverse Brayton refrigerator and the evacuation and pressure reduction system are connected with the cold box and can independently refrigerate the medium in the cold box. The cold source switching control comprises cold source assembly switching, and the cold source assembly switching comprises the following steps: S1: judging whether the currently used cold source assembly is faulty; S2: if the currently used cold source is faulty, judging whether the standby cold source is faulty; S3: if the standby cold source is faulty, forcibly starting an emergency cold source device, cutting out the currently used cold source from the circulating system, cutting in the standby cold source into the refrigeration circulating system, recording the refrigeration power before the fault of the currently used cold source, calculating the refrigeration power of the standby cold source according to a refrigeration capacity conversion formula of the standby cold source and the currently used cold source, and starting the standby cold source to operate after the cutting out of the currently used cold source is completed; S4: if the standby cold source is faulty, alarming, manually operating to shut down the equipment or automatically judging to execute this state to forcibly shut down the equipment and start the emergency refrigeration device; S5: judging whether the temperature of the entire circulating system fluctuates within a normal range, adjusting the refrigeration capacity of the emergency refrigeration device and controlling the temperature if the temperature fluctuates within the normal range, judging whether the currently used cold source and the standby cold source can normally operate, re-cutting the normally operating cold source into the circulating system, and manually or forcibly shutting down the emergency refrigeration device if the currently used cold source and the standby cold source are normally operated, which indicates that the currently used cold source and the standby cold source have been excluded from faults; if the temperature of the circulating system does not fluctuate within the normal range, further judging whether the temperature of the refrigeration circulating system fluctuates within a pre-warning line to a safety line, pre-warning and adjusting the refrigeration capacity of the emergency refrigeration device to control the temperature if the temperature fluctuates within the pre-warning line to the safety line, and judging whether the temperature of the refrigeration circulating system exceeds the safety line, alarming and ending if the temperature exceeds the safety line.

2. The control method of a subcooled liquid nitrogen cycle system according to claim 1, wherein, The supercooled liquid nitrogen circulating system further comprises a pressure control system and a liquid nitrogen storage tank, the pump boxes are further provided with a pressure control container, the pressure control container is connected with the first connecting pipeline, the pressure control container is used for controlling the working pressure of the circulating liquid nitrogen and buffering the volume change in the process of thermal expansion and cold contraction of the liquid nitrogen, and the pressure control system comprises a gas supplementing pressure control pipeline and a liquid supplementing pressure control pipeline, the gas outlet and the liquid outlet of the liquid nitrogen storage tank are connected with the pressure control container through the gas supplementing pressure control pipeline and the liquid supplementing pressure control pipeline respectively, and the gas supplementing pressure control pipeline and the liquid supplementing pressure control pipeline are each provided with a control valve.

3. The control method of a subcooled liquid nitrogen cycle system according to claim 2, wherein, The pressure control system further comprises a heating pressure control assembly, which comprises a heater arranged in the pressure control container.

4. The control method of a subcooled liquid nitrogen cycle system according to claim 2, wherein The liquid nitrogen circulation assembly further comprises an air compressor and a pressure control air supplement buffer tank, and the input end of the air supplement pressure control pipeline is connected with the output end of the air compressor.

5. The control method of a subcooled liquid nitrogen cycle system according to claim 4, wherein The air supplement pressure control pipeline comprises a first branch, a second branch and a main pipeline, the liquid nitrogen storage tank is connected with the main pipeline through the first branch, the air compressor is connected with the main pipeline through the second branch, the first branch is sequentially provided with a first manual stop valve and an air temperature vaporizer, the second branch is sequentially provided with a check valve and a first pressure reducing valve, the main pipeline is provided with a second pressure reducing valve and connected with the pressure control containers in the two pump boxes, and the pressure control containers are further connected with an exhaust pipeline, and the exhaust pipeline is provided with a sixth pneumatic stop valve.

6. The control method of a subcooled liquid nitrogen cycle system according to claim 5, wherein, The input end of the liquid supplement pressure control pipeline is connected with the liquid nitrogen storage tank, the output end of the liquid supplement pressure control pipeline is connected with the two pressure control containers, and the liquid supplement pressure control pipeline is provided with a second manual stop valve, a pipeline filter and a control valve.

7. The control method of a subcooled liquid nitrogen cycle system according to claim 6, wherein The liquid supplement pressure control pipeline is further connected with a liquid supplement container through a liquid supplement input pipeline, and the liquid supplement container is connected with the cold box through a liquid supplement output pipeline.

8. The control method of a subcooled liquid nitrogen cycle system according to claim 7, wherein, The cold box is provided with a supercooling heat exchanger, and the two ends of the supercooling heat exchanger are connected with a first connecting pipeline and a second connecting pipeline respectively, and the supercooling heat exchanger is located below the liquid level of the cooling medium in the cold box.

9. The control method of a subcooled liquid nitrogen cycle system according to claim 7, wherein, The superconducting cable assembly comprises a first terminal thermostat, a superconducting cable thermostat and a second terminal thermostat arranged along the liquid nitrogen circulation direction, and the liquid nitrogen circulation assembly is used for providing cold energy for the superconducting cable assembly.

10. The control method of a subcooled liquid nitrogen cycle system according to claim 9, wherein, The temperature returning liquid nitrogen flowing out of the superconducting cable thermostat is pressurized by the liquid nitrogen pump in the pump box, enters the supercooling heat exchanger in the cold box to obtain cold energy and becomes 70K supercooling liquid nitrogen, the cold source for obtaining cold energy of the supercooling heat exchanger is an inverse Brayton refrigerator or a Stirling refrigerator or an air exhaust and pressure reduction system, then the temperature returning liquid nitrogen flows into the superconducting cable thermostat to cool the superconducting cable, and finally returns to the liquid nitrogen pump to start a new cycle.

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