Liquid nitrogen cooling circulation device for magnetic bias superconducting current limiter

Abstract

The invention discloses a liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter, which comprises two pipelines, a refrigeration Dewar, a refrigerator and a control device, and is characterized in that the two ends of the pipelines are respectively communicated with the refrigeration Dewar and the superconducting current limiter Dewar to form a circulation loop for converting nitrogen into liquid nitrogen; a heat exchanger of a refrigerator is arranged in the refrigeration Dewar, a pressure sensor and a heater which are connected with a control device are arranged in the refrigeration Dewar, and the control device controls the heater to regulate and control the temperature of liquid nitrogen according to a gas pressure value actually measured by the pressure sensor; according to the device, liquid nitrogen cooling circulation can be achieved, and the temperature of liquid nitrogen can be accurately controlled without damaging the Dewar structure of the superconducting current limiter.

Description

Technical Field

[0001] The invention relates to a liquid nitrogen cooling cycle, in particular to a liquid nitrogen cooling cycle device for a magnetic bias superconducting current limiter. Background Art

[0002] As an effective short-circuit current limiting device, the superconducting current limiter can quickly limit the short-circuit current to an acceptable level when a short-circuit fault occurs, thereby preventing large short-circuit currents in the power grid from causing serious damage to the safe and stable operation of the power grid and electrical equipment. It can greatly improve the stability of the power grid and improve the reliability and safety of power supply.

[0003] The magnetic biased superconducting current limiter operates in a supercooled liquid nitrogen environment below 77K for a long time. The heating of the current lead and the heat leakage from the outside will cause the temperature of the liquid nitrogen to rise, which will cause the liquid nitrogen to vaporize. This will cause unstable pressure in the supercooled liquid nitrogen container for a long time, which may cause damage. In addition, the temperature increase will also cause the liquid nitrogen temperature to exceed the set value, which does not meet the working requirements. In order to solve this problem in the prior art, an external liquid nitrogen cooling system is used, and a refrigerator is used to cool the nitrogen to cool it into liquid nitrogen and control the temperature of the liquid nitrogen. Since the system needs to run continuously, considering that the vacuum degree of the refrigeration dewar, pipeline and superconducting current limiter dewar will deteriorate, resulting in poor insulation effect, and then the overall heat leakage will increase. Therefore, when selecting the cooling capacity of the refrigerator, a surplus amount is added, which means The actual cooling capacity of the refrigerator is greater than the theoretical cooling capacity required. Under the condition of stable external environment, if the refrigerator is kept working for a long time to liquefy nitrogen, the temperature of the supercooled liquid nitrogen will be too low and will not meet the set requirements. The power of the refrigerator is normally large, which is not convenient for fine-tuning. At this time, a heater is needed to fine-tune the temperature. However, if a temperature sensor is used to measure the temperature of liquid nitrogen, it is necessary to install the temperature sensor in the superconducting current limiter. Due to the low temperature environment in the superconducting current limiter dewar, the temperature sensor needs to be installed in it to overcome problems such as current breakdown. The existing superconducting current limiter dewar needs to be redesigned and modified so that the temperature sensor can work normally. Doing so not only has a poor effect, but also increases the cost and manufacturing difficulty. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter which can realize liquid nitrogen cooling circulation and accurately control the temperature of liquid nitrogen without destroying the Dewar structure of the superconducting current limiter.

[0005] Technical solution: The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter described in the present invention comprises two pipelines, a refrigeration dewar, a refrigerator and a control device, wherein the two ends of the pipeline are respectively connected to the refrigeration dewar and the superconducting current limiter dewar to form a circulation loop for converting nitrogen into liquid nitrogen; a heat exchanger of the refrigerator is arranged in the refrigeration dewar, a pressure sensor and a heater connected to the control device are arranged in the refrigeration dewar, and the control device controls the heater to regulate the temperature of the liquid nitrogen according to the actual pressure value of the pressure sensor.

[0006] Based on the above technical solution, a pressure sensor is arranged in the refrigeration dewar to measure the pressure value of the gas in the refrigeration dewar in real time. Since the refrigeration dewar and the superconducting limiter dewar are connected through a pipeline, the gas pressures therein are equal, and the liquid nitrogen saturation temperature corresponds to the gas pressure one by one. The gas pressure value measured by the pressure sensor can accurately reflect the temperature value of the liquid nitrogen. Compared with directly using the temperature sensor to measure the temperature value to control the temperature heater for heating and temperature control, the pressure value can more accurately reflect the actual temperature of the liquid nitrogen. Controlling the heater to regulate the liquid nitrogen temperature by measuring the gas pressure value can also more accurately make the liquid nitrogen reach the set temperature.

[0007] Preferably, the control device uses a PID algorithm to control the operation of the heater according to the measured gas pressure value.

[0008] The PID algorithm is adopted, and the measured gas pressure value is used as feedback to make the output power of the heater change in real time. When the gas pressure is lower than the "lower pressure limit", the heater will start to work. When the gas pressure is close to the "lower pressure limit", the output power of the heater is large. When the gas pressure gradually approaches the "upper pressure limit", the output power of the heater will gradually decrease. In this way, when the pressure value reaches the set value, the heater will stop working completely and will no longer generate excess heat, causing the final pressure value to deviate, and then the liquid nitrogen temperature to deviate.

[0009] Preferably, the heaters are evenly distributed on the heat exchanger.

[0010] The heaters are evenly distributed on the heat exchanger, which can more quickly dissipate excess cooling capacity in inefficient heat exchangers.

[0011] Preferably, a liquid nitrogen chamber enclosing the heat exchanger is provided in the refrigeration dewar, a pressure sensor is arranged in the liquid nitrogen chamber, and two pipes are connected to the liquid nitrogen chamber.

[0012] The liquid nitrogen chamber can cool and liquefy nitrogen in a smaller enclosed space, thereby improving refrigeration efficiency. The pressure sensor is arranged in the liquid nitrogen chamber to accurately measure the pressure values ​​in the liquid nitrogen chamber and the superconducting current limiter Dewar.

[0013] Preferably, the connection ports of the two pipes and the superconducting current limiter dewar are both located above the liquid nitrogen surface; the two pipes are respectively connected to the bottom and side wall of the refrigeration dewar.

[0014] Preferably, the bottom of the liquid nitrogen chamber is provided with an inclined surface for guiding the liquid nitrogen to the bottom and the pipeline connection port.

[0015] The bottom of the liquid nitrogen chamber is provided with an inclined surface for draining the liquid nitrogen to the pipeline connection port, so that all the liquid nitrogen condensed in the refrigeration dewar can be better drained into the pipeline.

[0016] Preferably, the heat exchanger is a fin-shaped heat exchanger made of copper.

[0017] The use of fin-shaped heat exchangers can improve heat exchange efficiency, and the use of copper can improve heat exchange efficiency because of its good thermal conductivity.

[0018] Preferably, the pipeline adopts a double-layer vacuum tube.

[0019] The use of double-layer vacuum tubes can achieve better thermal insulation and reduce the impact of the external ambient temperature on the temperature of the nitrogen or liquid nitrogen in the pipeline.

[0020] Preferably, the refrigerator is a GM refrigerator, a pulse tube refrigerator or a Stirling refrigerator.

[0021] Preferably, the pipeline, the refrigeration dewar and the superconducting current limiter dewar are all wrapped with multiple layers of thermal insulation materials.

[0022] Wrapping multiple layers of insulation materials can achieve better insulation effect.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant effects: the refrigeration dewar can condense nitrogen into liquid nitrogen for recycling. Only by setting a pressure sensor in the refrigeration dewar, the temperature of the liquid nitrogen is reflected by the measured gas pressure value, and the heater is adjusted accordingly to accurately adjust the temperature of the liquid nitrogen. There is no need to destroy the superconducting limiter dewar for modification, which reduces the manufacturing cost and is easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of the device; Figure 2 Schematic diagram of the relationship between liquid nitrogen saturation temperature and pressure. DETAILED DESCRIPTION

[0025] As shown in the figure, a liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter described in the present invention comprises two pipelines 1, a refrigeration dewar 2, a refrigerator 3 and a control device, wherein the two ends of the pipeline 1 are respectively connected to the refrigeration dewar 2 and the superconducting current limiter dewar 4 to form a circulation loop for converting nitrogen into liquid nitrogen; a heat exchanger 5 of the refrigerator 3 is arranged in the refrigeration dewar 2, a pressure sensor 6 and a heater connected to the control device are arranged in the refrigeration dewar 2, and the control device controls the heater to adjust the liquid nitrogen temperature according to the measured gas pressure value of the pressure sensor 6.

[0026] The pipeline 1 adopts a double-layer vacuum tube, and the connection ports of the two pipelines 1 and the superconducting current limiter dewar 4 are both located above the liquid nitrogen liquid level and can be connected to the top or side wall of the superconducting current limiter dewar 4; the connection ports of the two pipelines 1 and the refrigeration dewar 2 can be arranged at the bottom of the refrigeration dewar 2, and the refrigeration dewar 2 is provided with a liquid nitrogen chamber 7 that wraps the heat exchanger 5, and the liquid nitrogen chamber 7 wraps the heat exchanger 5 as a whole to form a closed space; the pipeline 1 extending into the refrigeration dewar 2 is respectively connected to the bottom and side wall of the liquid nitrogen chamber 7, the pipeline 1 connected to the bottom is used to return the condensed liquid nitrogen to the superconducting current limiter dewar 4, and the pipeline 1 connected to the side wall is used to drain the nitrogen in the superconducting current limiter dewar 4 into the liquid nitrogen chamber 7 for condensation and liquefaction; the bottom of the liquid nitrogen chamber 7 is provided with an inclined surface for draining the liquid nitrogen to the connection port of the pipeline 1, and the pipeline 1 here refers to the pipeline 1 connected to the bottom; liquid nitrogen is stored in the superconducting current limiter dewar 4, and a superconducting magnet is placed in the liquid nitrogen.

[0027] The refrigerator 3 adopts a GM refrigerator, a pulse tube refrigerator or a Stirling refrigerator; the heat exchanger 5 of the refrigerator 3 can be arranged on the top of the refrigeration dewar 2, and the heat exchanger 5 adopts a fin-shaped heat exchanger made of copper; the heater is arranged on the heat exchanger 5, and the pressure sensor 6 is arranged in the liquid nitrogen chamber 7, keeping a certain distance from the heat exchanger 5 and the heater, so that the pressure measured by the pressure sensor 6 can truly reflect the overall gas pressure average in the liquid nitrogen chamber 7, avoiding the difference between the temperature around the heat exchanger 5 and the heater and the overall temperature in the liquid nitrogen chamber 7, which makes the pressure value measured by the pressure sensor 6 inaccurate.

[0028] The control device uses a PID algorithm to control the operation of the heater according to the measured gas pressure value, that is, the measured gas pressure value is used as feedback to control the working state of the heater. The reason why the gas pressure value can be used as feedback to control the temperature is that the liquid nitrogen saturation temperature and the gas pressure are one-to-one corresponding. Figure 2 ; The application of PID algorithm to control heating is a mature existing technology, and its specific principle will not be repeated in this article.

[0029] The operation of the heater is controlled by measuring the gas pressure value. Since the cooling capacity of the refrigerator is greater than the overall evaporation capacity of the system, the system pressure will continue to decrease if the heater does not work, and the corresponding liquid nitrogen temperature will also decrease. In order to maintain the liquid nitrogen temperature stable in a certain range, it is necessary to adjust the heating amount of the heater in real time. When the gas pressure is close to the set pressure upper limit, the heater will reduce the heating amount. When the gas pressure is close to the pressure lower limit, the heater will increase the heating amount, so that the liquid nitrogen temperature is stable in a certain range. That is, the gas pressure value also has a corresponding range. In the following text, the range where the liquid nitrogen temperature needs to be stable is called the temperature range, and the range corresponding to the gas pressure value is called the gas range.

[0030] The operation of the heater is controlled by measuring the gas pressure value. When the gas pressure value is lower than the lower limit of the gas interval, the power of the heater will increase, causing the temperature to rise and the gas pressure value to increase. As the gas pressure value increases, the power of the heater will gradually decrease until the gas pressure value reaches the upper limit of the gas interval. At this time, because the heating capacity of the heater and the cooling capacity of the refrigerator are both stable, the temperature and pressure values ​​will no longer increase but will slowly decrease. After the gas pressure value reaches the upper limit of the gas interval, the gas temperature reaches the upper limit of the temperature interval, but the liquid nitrogen has not actually reached the upper limit of the temperature interval at this time. The liquid nitrogen will gradually absorb external heat, causing the gas pressure value to decrease. At the same time, the refrigerator will gradually cool down the nitrogen and liquid nitrogen. If the gas pressure value is lower than the lower limit of the gas interval, the power of the heater will increase again to increase the temperature. The gas pressure value increases so that it is not lower than the lower limit of the gas interval. If the liquid nitrogen ends its heat absorption and does not cause the gas pressure value to be lower than the lower limit of the gas interval, the power of the heater will not increase until the gas pressure value is lower than the lower limit of the gas interval. The power of the heater will be increased, and this cycle will be repeated to stabilize the liquid nitrogen temperature within the temperature interval.

[0031] Because the cooling and recycling of nitrogen is to be realized, the pipeline 1, the refrigeration dewar 2 and the refrigerator 3 and other devices are indispensable. The temperature control method of liquid nitrogen of the present invention can be realized by setting a pressure sensor 6 in the liquid nitrogen chamber 7 of the refrigeration dewar 2, that is, simply adding a pressure sensor 6 to the existing device in conjunction with the PID algorithm of the controller can realize the precise temperature control of liquid nitrogen, and there is no need to modify the structure of the superconducting current limiter dewar 4 to overcome the current breakdown problem existing in the temperature sensor set therein, which is simpler and easier to realize and reduces the manufacturing cost.

[0032] The pipeline 1, the refrigeration dewar 2 and the superconducting current limiter dewar 4 are all wrapped with multiple layers of thermal insulation materials to enhance the thermal insulation effect and reduce the influence of the outside on the temperature of the liquid nitrogen.

Claims

1. A liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter, comprising two pipes (1), a refrigeration dewar (2), a refrigerator (3) and a control device, wherein the two ends of the pipe (1) are respectively connected to the refrigeration dewar (2) and the superconducting current limiter dewar (4) to form a circulation loop for converting nitrogen into liquid nitrogen; a heat exchanger (5) of the refrigerator (3) is arranged in the refrigeration dewar (2), characterized in that: The refrigeration dewar (2) is provided with a pressure sensor (6) and a heater connected to a control device, and the control device controls the heater to adjust the temperature of the liquid nitrogen according to the gas pressure value actually measured by the pressure sensor (6).

2. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The control device uses a PID algorithm to control the operation of the heater according to the measured gas pressure value.

3. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The heaters are evenly distributed on the heat exchanger (5).

4. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 3, characterized in that: The refrigeration dewar (2) is provided with a liquid nitrogen chamber (7) enclosing a heat exchanger (5), a pressure sensor (6) is arranged in the liquid nitrogen chamber (7), and two pipes (1) are connected to the liquid nitrogen chamber (7).

5. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 4, characterized in that: The connection ports of the two pipes (1) and the superconducting current limiter dewar (4) are both located above the liquid nitrogen surface; the two pipes (1) are respectively connected to the bottom and side wall of the liquid nitrogen chamber (7).

6. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 5, characterized in that: The bottom of the liquid nitrogen chamber (7) is provided with an inclined surface for guiding the liquid nitrogen to the bottom and the connection port with the pipeline (1).

7. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The heat exchanger (5) is a fin-shaped heat exchanger made of copper.

8. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The pipeline (1) adopts a double-layer vacuum tube.

9. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The refrigerator (3) is a GM refrigerator, a pulse tube refrigerator or a Stirling refrigerator.

10. The liquid nitrogen cooling circulation device for a magnetic bias superconducting current limiter according to claim 1, characterized in that: The pipeline (1), the refrigeration dewar (2) and the superconducting current limiter dewar (4) are all wrapped with multiple layers of thermal insulation material.

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