Three-phase coaxial high-temperature superconducting cable thermal balance monitoring device and thermal balance optimization method

Abstract

Three-phase coaxial high-temperature superconducting cable thermal balance monitoring device and thermal balance optimization method, including a three-phase coaxial superconducting cable, cable terminals, a liquid nitrogen circulation monitoring unit, a thermal resistance temperature measurement unit, thermal resistors, an optical fiber temperature measurement host, cryogenic temperature measurement optical fibers, three-phase current transformers, shield current transformers, a thermal balance monitoring unit, and a main circuit operating parameter adjustment unit. The thermal balance monitoring unit realizes the measurement and protection functions of all operating parameters of the entire superconducting cable. The present invention ensures that the entire test system can monitor the current-carrying and temperature distribution of the three-phase coaxial high-temperature superconducting cable under operating conditions, and dynamically adjusts the loop current according to the monitoring results, enabling the superconducting cable to operate in a thermal balance state, improving the current-carrying capacity and resistance to external thermal disturbances, and ensuring the safe and stable operation of the superconducting cable system.

Description

Technical Field:

[0001] The present invention belongs to the field of superconducting cables, and particularly relates to a thermal balance monitoring device and an optimization method for a three-phase coaxial high-temperature superconducting cable. Background Art:

[0002] With the rapid growth of urban power grid load, there are problems such as cable tunnel space saturation, insufficient cable current-carrying capacity, and high land acquisition costs for new or expanded existing power transmission lines, resulting in a dilemma that the distribution capacity of some urban load centers cannot meet the actual development needs. The application of superconducting cables has great technical advantages in underground cable systems in urban load centers or in achieving large-capacity power transmission under specific environments. Replacing existing conventional cables with high-temperature superconducting cables can multiply the power transmission capacity of the underground power grid, thus solving the contradiction between load growth and limited underground space and breaking through the bottleneck of urban power transmission.

[0003] Due to the structural differences of the three phases in a three-phase coaxial superconducting cable, the electromagnetic coupling between the three-phase conductors is uneven, and the inter-phase imbalance problem often occurs. When a short-circuit current impact or an asymmetric fault occurs in the line, due to the normal resistance generation and heat accumulation caused by the quenching of the superconductor, the current in each phase of the three-phase coaxial cable shows a transfer distribution phenomenon, which causes changes in the shield induced current, cable voltage, and equivalent parameters. For example, under the action of a short circuit, the structure of the three-phase coaxial cable will show certain deficiencies. The inner-phase conductor and the outer-phase conductor are closer to the cooling medium, with a short heat transfer distance, and can obtain good cooling. Even when passing through a fault current, the heat will quickly be transferred to the cooling medium. However, the middle-phase conductor is far from the cooling media on both sides. Once a single-phase short-circuit fault occurs in this phase, the generated heat can only be dissipated by the other two phases, which takes a longer time and will also cause a certain temperature rise in the other two phases. When the low-temperature cooling environment and the cooling medium also fail or change, due to the inability to remove the accumulated heat in time, the continuously rising temperature will cause the superconducting cable to lose stability, and in severe cases, even cause cable damage. At present, there is no good method to monitor the temperature of the middle-phase conductor of the three-phase coaxial superconducting cable structure and the shield shunt situation under fault conditions. Therefore, it is impossible to control the cable operating parameters to make the cable work in a more efficient thermal balance stable state. Summary of the Invention:

[0004] Aiming at the deficiencies and improvement requirements of the prior art, the purpose of the present invention is to provide a thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable, and another purpose is to provide a thermal balance dynamic optimization method for a three-phase coaxial superconducting cable. The present invention ensures that the entire set of devices can monitor the current-carrying and temperature distribution during the operation of the three-phase coaxial high-temperature superconducting cable, and dynamically adjust the loop current according to the monitoring results, so that the superconducting cable operates in a thermal balance state, improving the current-carrying capacity and the ability to resist external thermal disturbances, and ensuring the safe and stable operation of the superconducting cable system.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable, comprising a three-phase coaxial superconducting cable 1, a cable terminal 2, a liquid nitrogen circulation monitoring unit 3, a thermal resistance temperature measurement unit 4, a thermal resistance 5, an optical fiber temperature measurement host 6, a temperature measurement optical fiber 7, a three-phase current transformer 8, a shield layer current transformer 9, and a thermal balance monitoring unit 10; characterized in that:

[0007] The liquid nitrogen circulation monitoring unit 3 is connected to the cable terminal 2 on one side of the three-phase coaxial superconducting cable, and is used to provide circulating liquid nitrogen as a cooling medium for the three-phase coaxial high-temperature superconducting cable 1 and the cable terminal 2;

[0008] Two thermal resistances 5 are respectively arranged on the liquid nitrogen outflow channel and the liquid nitrogen return channel of the liquid nitrogen circulation monitoring unit 3, and the thermal resistance temperature measurement unit 4 is connected to the thermal resistance 5 for monitoring the temperature of the cooling medium in the outflow channel and the return channel of the liquid nitrogen circulation monitoring unit 3;

[0009] The optical fiber temperature measurement host 6 is connected to the ultra-low temperature measurement optical fiber 7 installed inside the three-phase coaxial superconducting cable 1 for simultaneously collecting and analyzing the temperature signals of each measurement point within the entire length of the three-phase coaxial high-temperature superconducting cable, and obtaining the temperature distribution of the entire length and between layers of the cable;

[0010] A three-phase current transformer 8 is arranged on the connection line between the cable terminal on the other side of the three-phase coaxial superconducting cable and the main circuit;

[0011] A shield layer current transformer 9 is arranged at the cable terminal on the other side of the three-phase coaxial superconducting cable for measuring the shunt current of the superconducting cable shield layer;

[0012] The liquid nitrogen circulation monitoring unit 3, the thermal resistance temperature measurement unit 4, the optical fiber temperature measurement host 6, the three-phase current transformer 8, and the shield layer current transformer 9 all transmit measurement signals to the thermal balance monitoring unit 10 through the measurement signal line 12.

[0013] The present invention further includes the following preferred solutions.

[0014] The thermal balance monitoring unit 10 evaluates and determines the operating state of the three-phase coaxial high-temperature superconducting cable 1 by combining all temperature and current monitoring data.

[0015] The thermal balance monitoring device for the three-phase coaxial high-temperature superconducting cable further includes a main circuit operating parameter adjustment unit 11, and the main circuit operating parameter adjustment unit 11 adjusts and executes a new operating strategy according to the operating state of the three-phase coaxial high-temperature superconducting cable 1.

[0016] The temperature-measuring optical fiber 7 refers to a temperature-measuring optical fiber with a temperature measurement range of -200 to -180 °C;

[0017] The temperature-measuring optical fiber 7 is installed between two superconducting tapes of each phase conductor layer of the superconducting cable and is laid and installed in a form of co-wrapping with the semiconductive layer.

[0018] The temperature-measuring optical fiber 7 uses a bare optical fiber coated with high-performance materials such as polyimide, and the surrounding gaps are filled with acrylate adhesives for fixation and protection.

[0019] For superconducting cables with a length less than or equal to 10 m, a distributed optical fiber sensor is used for detection; for superconducting cables with a length greater than 10 m, a fiber Bragg grating sensor is used for detection.

[0020] If the thermal balance monitoring unit 10 determines that the change in loop parameters does not affect the normal operation of the three-phase coaxial high-temperature superconducting cable, the main loop operating parameter adjustment unit 11 does not adjust the operating state of the three-phase coaxial high-temperature superconducting cable;

[0021] If the thermal balance monitoring unit 10 determines that the change in loop parameters will not cause permanent damage to the superconducting cable system, the main loop operating parameter adjustment unit 11 will adjust the three-phase coaxial high-temperature superconducting cable to enter a derated state and continue to monitor the operation;

[0022] If the thermal balance monitoring unit 10 determines that the change in loop parameters will cause permanent damage to the superconducting cable and it cannot continue to operate, the main loop operating parameter adjustment unit 11 will cut off the loop where the three-phase coaxial high-temperature superconducting cable is located, so that the three-phase coaxial high-temperature superconducting cable exits the operation.

[0023] If the thermal balance monitoring unit 10 determines that the change in loop parameters does not affect the normal operation of the three-phase coaxial high-temperature superconducting cable, the main loop operating parameter adjustment unit 11 does not adjust the operating state of the three-phase coaxial high-temperature superconducting cable;

[0024] If the thermal balance monitoring unit 10 determines that the change in loop parameters will not cause permanent damage to the superconducting cable system, the main loop operating parameter adjustment unit 11 will adjust the three-phase coaxial high-temperature superconducting cable to enter a derated state and continue to monitor the operation;

[0025] If the thermal balance monitoring unit 10 determines that the change in loop parameters will cause permanent damage to the superconducting cable and it cannot continue to operate, the main loop operating parameter adjustment unit 11 will cut off the loop where the three-phase coaxial high-temperature superconducting cable is located, so that the three-phase coaxial high-temperature superconducting cable exits the operation.

[0026] Further, the process flow of the thermal balance dynamic optimization method under the cable operating state is as follows:

[0027] (1) Measure the three-phase current of the circuit. Under the initial temperature condition, obtain the curve of the current varying with time through the ordinary differential equations of the circuit.

[0028] (2) Calculate the heat generation rate of each conductive layer under the initial isothermal condition.

[0029] (3) Use the obtained heat generation rates of each conductor layer and the heat conduction equation to obtain a new temperature distribution.

[0030] (4) Use the new temperature distribution as the temperature load, substitute it into the ordinary differential equations of the circuit, and calculate the new heat generation rate.

[0031] (5) Repeat the above steps and iterate repeatedly until the set time ends.

[0032] Furthermore, the operation strategies after the dynamic optimization of the cable thermal balance include reducing the load of the cable circuit and cutting off the cable circuit.

[0033] Furthermore, in the operation strategies after the dynamic optimization of the cable thermal balance, the priority of abnormal cooling medium circulation is the highest. When the monitored values of the liquid nitrogen mass flow rate, pressure, refrigeration power, and inlet and outlet temperatures obtained by the liquid nitrogen circulation monitoring unit show related abnormalities, the main circuit operation parameter adjustment unit will directly cut off the cable circuit.

[0034] The present invention has the following beneficial technical effects compared with the prior art:

[0035] The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device and optimization method provided by the present invention are based on the combined use of traditional thermal resistance temperature sensors and optical fiber temperature measurement technology. By pre-installing cryogenic sensors into the high-temperature superconducting cable during its manufacture, it can innovatively carry out the temperature monitoring of the intermediate-phase superconductor and the distributed temperature monitoring of the entire line of the high-temperature superconducting cable system; through multiple current transformers, the three-phase main circuit current and the shielding layer shunt are monitored to form a complete cable system operation monitoring system. This system can accurately and real-time grasp the temperature distribution along the high-temperature superconducting cable, thereby realizing the dynamic analysis and optimization adjustment of the cable balance state according to the current-carrying capacity and operating temperature of the high-temperature superconducting cable, being able to timely detect cable operation defects related to thermal disturbances and loop faults related to overcurrent, and timely adjust the loop operation mode to ensure the safe operation of the high-temperature superconducting cable. The high-temperature superconducting cable temperature measurement system provided by the present invention can be applied to the temperature measurement, monitoring, and protection of high-temperature superconducting cables in power grids, and has high stability and reliability. Description of the Drawings:

[0036] Figure 1 It is a single-line schematic diagram of a three-phase coaxial high-temperature superconducting cable thermal balance monitoring device in an embodiment of the present invention.

[0037] Figure 2 Schematic diagram of the structure of a three - phase coaxial superconducting cable in an embodiment of the present invention.

[0038] Figure 3 Flow chart for establishing a new thermal equilibrium state under the interaction of the temperature and current of the superconducting cable.

[0039] Figure 4 Flow chart of the equilibrium optimization method for a three - phase coaxial high - temperature superconducting cable based on the electro - magnetic - thermal analysis process. Specific implementation manner:

[0040] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. It is particularly stated that the following description is essentially only for macroscopic explanation and example illustration, and in no way limits the present invention and its application or use. Unless otherwise specifically stated, the relative arrangements of components and steps described in the embodiments and the numerical expressions and values do not limit the scope of protection of the present invention.

[0041] Figure 1 Single - line diagram of a thermal equilibrium monitoring device for a three - phase coaxial high - temperature superconducting cable in an embodiment of the present invention. In this embodiment, the thermal equilibrium monitoring device for a three - phase coaxial high - temperature superconducting cable includes a three - phase coaxial superconducting cable 1, a cable terminal 2, a liquid nitrogen circulation monitoring unit 3, a thermal resistance temperature - measuring unit 4, a thermal resistance 5, an optical fiber temperature - measuring host 6, a temperature - measuring optical fiber 7, a three - phase current transformer 8, a shielding - layer current transformer 9, a thermal equilibrium monitoring unit 10, a main - circuit operating parameter adjustment unit 11, a measurement signal line 12, and a control signal line 13. The entire superconducting cable's all operating parameter measurement and protection functions are realized by the thermal equilibrium monitoring unit 10. It can be understood that in the embodiment of the present invention, the main circuit of the test system is a complete three - phase circuit. The current measured by the three - phase current transformer 8 is the current carried by each phase conductor, and the current measured by the shielding - layer current transformer 9 is the shunt current of the superconducting cable shielding layer, jointly testing the current - carrying distribution of the three - phase coaxial superconducting cable 1 in the operating state.

[0042] It can be understood that in the embodiment of the present invention, the liquid nitrogen circulation monitoring unit 3 is used to provide circulating liquid nitrogen as a cooling medium for the three - phase coaxial superconducting cable 1 and the cable terminal 2, ensuring that the three - phase coaxial superconducting cable 1 operates below the operating temperature (-196 °C). It can be understood that the thermal resistance 5 is respectively used to monitor the temperature of the cooling medium in the flow - through channel and the return channel of the cooling medium of the liquid nitrogen circulation monitoring unit 3. In addition, other parameter values such as the liquid nitrogen mass flow rate, pressure, and refrigeration power obtained by the liquid nitrogen circulation monitoring unit 3 are all connected to the thermal equilibrium monitoring unit 10 through the measurement signal line 12.

[0043] It can be understood that in this embodiment, the type of the thermal resistor 5 is a platinum resistance sensor, and its exterior is wrapped with a flexible ultra-high molecular weight polyethylene (UPE) protection tube to ensure the reliable installation of the sensor and accurate temperature induction. The platinum resistance sensor adopts a PT100 four-wire measurement method, which is suitable for the internal temperature monitoring of short superconducting cable samples and cooling terminals. A platinum resistance sensor is installed in each of the liquid nitrogen flow-out and flow-back channels. PT100 is not only widely used in industrial temperature measurement but also made into a standard reference instrument. The temperature measurement range of PT100 is -200°C - 650°C, the measurement accuracy can reach 0.1°C, and it has good stability and a fast response speed, making it an ideal choice for temperature measurement in a low-temperature environment. Therefore, the PT100 platinum resistance sensor can be used as a temperature sensor in this embodiment to monitor the temperature of the cooling medium.

[0044] The temperature measurement optical fiber 7 is composed of three optical fibers of the same model, material, and length, and is installed inside the three-phase coaxial superconducting cable 1 for monitoring the temperature of the three-phase superconducting layer. The optical fiber temperature measurement host 6 is used to simultaneously collect and analyze the temperature signals of each measurement point within the full length of the three-phase coaxial high-temperature superconducting cable 1 to obtain the temperature distribution of the entire cable length and between layers.

[0045] It can be understood that the thermal resistor temperature measurement unit 4 is used to receive the temperature information from the thermal resistor 5; the optical fiber temperature measurement host 6 is used to receive the temperature measurement information of each measurement point from the temperature measurement optical fiber 7. More specifically, the optical fiber temperature measurement host 6 used in the high-temperature superconducting cable temperature measurement system of the present invention can be an SRA-D type distributed optical fiber sensing temperature measurement analyzer, which is responsible for functions such as signal acquisition, signal processing, data analysis, over-temperature alarm, and network transmission of the entire system, and is composed of an optical frequency generator, a switching power supply, a microprocessor, a network interface, etc.; the thermal resistor temperature measurement unit 4 selects a DM3068 series digital multimeter, which supports the use of various temperature sensors such as TC (thermocouple), RTD (thermal resistor), and THERM (thermistor) to measure temperature and meets the international temperature scale ITS-90 standard; the composite temperature measurement component composed of the optical fiber temperature measurement host 6 and the thermal resistor temperature measurement unit 4 outputs a temperature signal to the thermal balance monitoring unit 10, and the thermal balance monitoring unit 10 greatly improves the integration of data information and increases the flexibility of system use.

[0046] The measurement signals of each unit are all transmitted to the thermal balance monitoring unit 10 through the measurement signal line 12. The thermal balance monitoring unit 10 evaluates and determines the operating state of the three-phase coaxial high-temperature superconducting cable 1 by combining all temperature and current monitoring data, and adjusts the operating strategy to control the main circuit operating parameter adjustment unit 11 to execute the adjustment instruction.

[0047] If the operating currents of all three phases do not exceed the critical current, the temperature ranges of the liquid nitrogen flow-away and return channels are both 70 - 76K, the temperature ranges along phases A and C are 75 - 78K, and the temperature range along phase B is 75.5 - 78.5K. After thermal equilibrium iterative calculation, if the temperature rise of the three phases remains in the initial state, then the thermal equilibrium monitoring unit 10 determines that the three-phase coaxial high-temperature superconducting cable is in a normal operating state, and the main circuit operating parameter adjustment unit 11 does not adjust the operating state of the three-phase coaxial high-temperature superconducting cable.

[0048] If the operating current of at least one phase among the three phases exceeds the critical current and the shield current is less than 200A, or the average temperature of any one phase among the three phases is greater than or equal to 81K, after thermal equilibrium iterative calculation, the cable will resume the initial stable state under the derated operating state. Then the thermal equilibrium monitoring unit 10 determines that the change in loop parameters will not cause permanent damage to the superconducting cable system, and the main circuit operating parameter adjustment unit 11 will adjust the three-phase coaxial high-temperature superconducting cable to enter the derated state for continued monitoring operation and issue an alarm signal.

[0049] If the operating current of at least one phase among the three phases exceeds the critical current and the shield current is greater than or equal to 200A, or the local temperature of any one phase among the three phases is greater than or equal to 85K, or the inlet temperature of the cooling medium is greater than 76K, no thermal equilibrium iterative calculation will be performed, and it is considered that there is a fault in the superconducting cable body or the circulating cooling system, which is permanent damage to the superconducting cable system and cannot continue to operate. Then the main circuit operating parameter adjustment unit 11 will cut off the loop where the three-phase coaxial high-temperature superconducting cable is located, causing the three-phase coaxial high-temperature superconducting cable to exit the operation.

[0050] It can be understood that in the embodiments of the present invention, the present invention pre-installs low-temperature-resistant (below -196°C) temperature-measuring optical fibers 7 inside the three-phase coaxial superconducting cable 1, arranges thermal resistors 5 inside the liquid nitrogen circulation monitoring unit 3, and finally the thermal equilibrium monitoring unit 10 determines the operating condition of the cable according to the temperature signal and the current distribution of the superconducting cable, so as to control the main circuit operating parameter adjustment unit 11 to dynamically optimize and adjust the loop operating mode, enabling the cable to reach a new thermal equilibrium state again after being thermally disturbed. The determination basis is the change law of the temperature distribution of the superconducting cable under the set working conditions with the current transfer characteristics of the conductor layer and the shield layer.

[0051] In the preferred embodiment of the present application, if it is determined that the change in loop parameters does not affect the normal operation of the high-temperature superconducting cable system, the loop operating state will not be adjusted; if it is determined that the change in loop parameters will not immediately cause a fault in the superconducting cable system, the cable system will be adjusted to enter the derated state for continued monitoring operation; if it is determined that it may cause permanent damage to the superconducting cable and cannot continue to operate, the loop where the cable system is located will be cut off, causing the cable system to exit the operation.

[0052] Figure 2Schematic diagram of the structure of a three-phase coaxial superconducting cable in an embodiment of the present invention. In this embodiment, the three-phase high-temperature superconducting cable 1 includes, from the outside to the inside: an adiabatic layer 101, a shielding layer 102, at least one insulating layer 103, a superconducting layer, and a hollow skeleton 107. Each superconducting layer is composed of an outer first superconducting tape layer 104, an inner second superconducting tape layer 106, and a semi-conductive layer 105 wound between the two superconducting tapes. Liquid nitrogen 108 is filled between the adiabatic layer 101 and the shielding layer 102 and inside the hollow skeleton 107, so that the three-phase high-temperature superconducting cable 1 operates below the operating temperature (-196 °C). The inside of the hollow skeleton 108 is a liquid nitrogen flow-away channel, and the space between the adiabatic layer 101 and the shielding layer 102 is a liquid nitrogen return channel.

[0053] It can be understood that in the embodiment of the present invention, the adiabatic layer 101 uses vacuum and multi-layer insulation material technology to ensure the low-temperature insulation effect of the liquid nitrogen entering and leaving the three-phase coaxial superconducting cable 1.

[0054] It can be understood that in the embodiment of the present invention, the shielding layer 102 is a copper shielding layer, which belongs to a metal shielding layer. Its operation mode is usually single-ended grounding. Its main function is to shield the electric field. There is no current passing through during normal operation, and it will shunt the fault current in the case of a system fault. The hollow skeleton 107 is a metal bellows, whose main function is to support the winding of superconducting tapes and is also used for the liquid nitrogen pipeline. There is no current passing through during normal operation, and it will shunt the fault current in the case of a system fault.

[0055] It can be understood that the design of the insulating layer 103 depends on factors such as the characteristics of the insulating material, the operating voltage, and the cable dimensions. Considering factors such as electrical performance, thermal performance, mechanical performance, and process difficulty, polypropylene laminated paper (PPLP) can be preferably used as the low-temperature insulating material in this embodiment.

[0056] In this embodiment, each phase conductor layer uses second-generation high-temperature superconducting tapes. It can be understood that the second type of high-temperature superconducting tape is also called YBCO superconducting tape. The YBCO superconducting tape has a multi-layer structure and is mainly composed of a copper stabilization layer, a silver layer, a YBCO superconducting layer, a cap layer, a seed layer, a buffer layer, an isolation layer, and a Hastelloy layer. There may be differences in the materials and thicknesses of each layer of superconducting tapes from different manufacturers. Each phase of the superconducting layer contains 2 superconducting tapes. According to the equivalent circuit equation, the self-inductance and mutual inductance of each layer are obtained, and the winding pitch and winding helix angle are calculated to achieve the current sharing design of the energized conductor. A semi-conductive layer and a filling material are filled between the two superconducting tapes. The temperature-measuring optical fiber 7 is installed between the two superconducting tapes and can withstand an extremely low temperature (below -196 °C) environment.

[0057] It can be understood that in the embodiments of the present invention, the three-phase coaxial superconducting cable includes a total of three layers of outer insulation layers 103 and three superconducting layers. The C-phase is located on the outside, the A-phase is located on the inside, and the B-phase is the middle phase.

[0058] In this embodiment, a temperature-measuring optical fiber 7 is further installed inside the three-phase coaxial high-temperature superconducting cable 1. The temperature-measuring optical fiber 7 is installed in each layer of the wrapped semi-conductive layer 105, that is, covering the areas of each superconducting layer, and is used to monitor the temperature of each superconducting layer.

[0059] In this embodiment, the temperature-measuring optical fiber 7 can adopt the principle of distributed optical fiber sensors or fiber Bragg grating sensors inside. Generally, multi-mode optical fibers of quartz series can be used. For superconducting cable samples with a medium length (about 5-10 m), fiber Bragg grating sensors can be used, and the distance between adjacent gratings is not greater than 0.5 m; for longer superconducting cables or engineered superconducting cable products, distributed optical fiber sensors can be used.

[0060] It can be understood that in the embodiments of the present invention, distributed optical fiber temperature measurement requires a longer pigtail fiber to ensure higher temperature measurement accuracy and spatial resolution. Therefore, the temperature-measuring optical fiber 7 in this embodiment is preferably a cascade type fiber Bragg grating sensor. When necessary, the Fabry-Perot resonator (F-P resonator) can be used to assist in demodulating the fiber Bragg grating sensor, and more fiber Bragg grating sensors can be connected in series to improve the temperature monitoring range and accuracy. However, the diameter of the temperature-measuring optical fiber 7 installed in the superconducting layer cannot be too large to avoid occupying too much internal space of the superconducting cable and affecting the cable performance. Therefore, the temperature-measuring optical fiber 7 installed between layers in this embodiment is preferably a bare optical fiber coated with high-performance materials such as polyimide, and the surrounding gaps are filled with acrylate adhesives for fixation and protection.

[0061] It can be understood that monitoring the current-carrying capacity of the superconducting cable and the temperature of the cable system can comprehensively judge the operating state of the superconducting cable. Usually, when the superconducting cable exceeds the critical current, the superconducting cable will quench and turn into a resistive conductor. On the one hand, the current-carrying capacity will be greatly reduced, and a large amount of Joule heat will be generated, causing the temperature of the superconducting tape and the surrounding cooling medium to rise.

[0062] It can be understood that in the embodiments of the present invention, various types of temperature-measuring optical fibers 7 (operating below -196 °C) are pre-installed inside the three-phase coaxial high-temperature superconducting cable 1, and the optical fiber temperature-measuring host 6 transmits the temperature signal to the thermal balance monitoring unit 10, which is one of the main parameters for determining the operating condition of the cable (the other parameters also include the temperature at the liquid nitrogen inlet and outlet, the current-carrying distribution, and the balance optimization method), so as to ensure that the three-phase coaxial high-temperature superconducting cable 1 is in a safe operating state.

[0063] It can be understood that in the embodiments of the present invention, the temperature-measuring optical fiber 7 installed in the superconducting layer inside the three-phase coaxial high-temperature superconducting cable 1 can be laid and installed in a form of being wound together with the semiconductive layer; for the temperature-measuring optical fiber 7 installed in the liquid nitrogen channel inside the three-phase coaxial high-temperature superconducting cable 1, it can be laid in a straight or S-shaped manner between the thermal insulation layer 101 and the shielding layer 102 and inside the hollow skeleton 107.

[0064] It can be understood that in the embodiments of the present invention, when the loop where the high-temperature superconducting cable system is located operates normally, the operation mode of the superconducting cable is mainly determined by monitoring the temperature of the superconducting cable inside the superconducting cable and the phase current unbalance degree; when a fault occurs in the loop where the high-temperature superconducting cable system is located, the operation mode of the superconducting cable is mainly determined by measuring the temperature change when the superconducting tape passes through a large current and the shunt situation of the shielding layer. When the shielding layer current is less than 200 A and the single-phase average temperature rise is less than 3 K (the temperature does not exceed 81 K), after iterative calculation using the heat generation formula, the cable can still return to the initial temperature steady state after derating, so there is no need to trip immediately, and it can operate in automatic or manual derating mode and issue an alarm signal. When the shielding layer current exceeds 200 A and the single-phase average temperature rise is greater than 7 K (the temperature exceeds 85 K), after iterative calculation using the heat generation formula, the cable will continuously generate heat and increase in temperature, and will enter the quench state comprehensively, so it is necessary to trip immediately to protect the high-temperature superconducting cable system. If it is determined that the change in loop parameters does not affect the normal operation of the high-temperature superconducting cable system, the operation state of the loop will not be adjusted; if it is determined that the change in loop parameters will not immediately cause a fault in the superconducting cable system, the cable system will be adjusted to enter the derating state and continue to be monitored for operation; if it is determined that it may cause permanent damage to the superconducting cable and it cannot continue to operate, the loop where the cable system is located will be cut off to make the cable system exit operation.

[0065] It can be understood that in the embodiments of the present invention, in order to improve the current-carrying capacity, the superconducting cable adopts a multi-layer superconducting conductive layer structure, which causes uneven current distribution in each layer when the superconducting cable carries alternating current. Generally, the current in the outer layer is greater than that in the inner layer. Especially when the total current increases, the increase in the outer layer current is very obvious, which will cause the outer layer current to reach the critical current first. This will not only increase the AC loss of the superconducting cable, reduce the current-carrying capacity, seriously threaten the safety and stability of cable operation, increase the operation cost, but also cause distortion of AC current-carrying and affect the power quality. In the actual operation of the cable, there will be various abnormal working conditions. To meet the requirements of power transmission applications, the superconducting cable must ensure the stability of operation under these fault conditions. When the current exceeds the rated current, the current distribution of the superconducting cable is more complex and is related to the temperature of the cable. In the research, it is necessary to qualitatively discuss the response of the current-carrying, temperature, and current distribution of the superconducting cable to time.

[0066] Figure 3Flow chart for the establishment of a new thermal equilibrium state under the interaction of temperature and current in a superconducting cable. Using the finite element method, differential equations of a compact three-phase coaxial superconducting cable are established to analyze the responses of current distribution and temperature distribution under different working conditions. During the quench process of the superconducting layer, the equivalent resistance will cause the temperature of each layer of the superconducting cable to rise, and the critical current Ic of the YBCO tape is also affected by temperature. Therefore, for the study of the quench process, it is necessary to consider the temperature rise process simultaneously, including its own AC loss heating, heat conduction and convection heat transfer of the equivalent resistance. These factors will cause changes in the temperature of the superconducting tape, and the change in the tape temperature will cause changes in the critical current of the superconducting tape, affecting the equivalent resistance of the superconducting layer of the superconducting cable, and further affecting the current distribution of each superconducting layer. From the above analysis, it can be found that the conduction process of the superconducting cable is actually an electro-thermal coupling change process. The change in temperature causes the change in current distribution, and at the same time, the change in current distribution will in turn affect the change in temperature.

[0067] Figure 4 Flow chart of the equilibrium optimization method for a three-phase coaxial high-temperature superconducting cable based on the electro-magnetic-thermal analysis process. The process of this method is as follows:

[0068] (1) Measure the three-phase current. Under the initial temperature condition, obtain the curve of current versus time through the system of ordinary differential equations of the circuit;

[0069] (2) Based on the equivalent resistance of each layer of the superconducting cable, obtain the heat generation rate of each conductive layer under the initial isothermal condition;

[0070] (3) Use the radial heat conduction differential equation in the cylindrical coordinate system to obtain the heat generation rate and heat conduction equation of each conductor layer, and obtain a new temperature distribution;

[0071]

[0072] In the formula, ρ is the density, c is the specific heat capacity, r is the average radius of the conductor layer, λ is the thermal conductivity of the conductor layer, Φ' is the heat source of the cable body, t is the time, and T is the temperature;

[0073] (4) Use the new temperature distribution as the temperature load, substitute it into the system of ordinary differential equations of the circuit, and obtain a new heat generation rate;

[0074]

[0075] In the formula, ρ is the density, c is the specific heat capacity, r n is the average radius of the conductor layer in the nth iteration, Δr is the difference between the inner and outer diameters of the conductor layer, λ is the thermal conductivity of the conductor layer, t is the time, T n-1 、T n 、T n+1 are the temperature distributions obtained in the (n - 1)th, nth, and (n + 1)th iterations respectively, and Q′ totalis the heat generation amount of the heat source of the cable body per unit volume per unit time;

[0076] (5) Repeat steps (1) to (4), and iterate repeatedly until the set time ends. The iteration calculation time is usually set to 500 seconds.

[0077] It can be understood that there are two heat sources in the cable body: one is the AC loss generated when the superconducting cable is energized; the other is the heat leakage from the external environment through the cryostat of the cable body to the liquid nitrogen return channel. As the heat accumulates along the cable, its temperature will change with the increase of the cable length. At any position along the length of the superconducting cable, the temperature of the return liquid nitrogen is higher than that of the outgoing liquid nitrogen.

[0078] It can be understood that through the above steps, the variation law of the temperature distribution of the superconducting cable with the current transfer characteristics of the conductor layer and the shielding layer under the set working conditions can be obtained. This scheme uses the finite difference method to establish a one-dimensional radial heat conduction model of the superconducting cable, couples it with the circuit equation of the superconducting cable, and then establishes a one-dimensional radial magneto-thermal coupling model of the superconducting cable. Using this numerical model, the temperature and current-carrying response of the superconducting cable under different current-carrying conditions can be analyzed.

[0079] In the embodiment of the present invention, the operation strategies after dynamically optimizing the thermal balance of the cable include reducing the load of the cable circuit and cutting off the cable circuit. When any one of the three conditions of "the operating current of at least one phase among the three phases exceeds the critical current and the shielding layer current is greater than or equal to 200A", "the local temperature of any one phase among the three phases is greater than or equal to 85K", and "the inlet temperature of the cooling medium is greater than 76K" is satisfied, the thermal balance iteration calculation is no longer performed, and it is considered that there is a fault in the superconducting cable body or the circulating cooling system. However, the priority of the circulating cooling system fault is the highest. When there is an associated abnormality in the monitored values of the liquid nitrogen inlet and outlet temperatures obtained by the liquid nitrogen circulation monitoring unit and the inlet temperature of the cooling medium is greater than 76K, the main circuit operation parameter adjustment unit will directly cut off the cable circuit.

[0080] Although the present invention has been described by way of example embodiments, it should be understood that the present invention is not limited to the above-described exemplary embodiments. It is obvious to those skilled in the art that the above exemplary embodiments can be modified without departing from the scope and spirit of the present disclosure. The scope of the appended claims should be given the broadest interpretation to cover all such modifications as well as equivalent structures and functions.

Claims

1. A thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable, comprising a three-phase coaxial superconducting cable (1), a cable terminal (2), a liquid nitrogen circulation monitoring unit (3), a thermal resistance temperature measurement unit (4), a thermal resistance (5), an optical fiber temperature measurement host (6), a temperature measurement optical fiber (7), a three-phase current transformer (8), a shield layer current transformer (9), and a thermal balance monitoring unit (10); characterized in that: The liquid nitrogen circulation monitoring unit (3) is connected to the cable terminal (2) on one side of the three-phase coaxial superconducting cable, and is used to provide circulating liquid nitrogen as a cooling medium for the three-phase coaxial high-temperature superconducting cable (1) and the cable terminal (2), and obtain nitrogen mass flow, pressure, and refrigeration power parameter values; Two thermal resistances (5) are respectively arranged on the liquid nitrogen outflow channel and the liquid nitrogen return channel of the liquid nitrogen circulation monitoring unit (3), and the thermal resistance temperature measurement unit (4) is connected to the thermal resistance (5) for monitoring the temperature of the cooling medium in the outflow channel and the return channel of the liquid nitrogen circulation monitoring unit (3); The optical fiber temperature measurement host (6) is connected to the temperature measurement optical fiber (7) installed inside the three-phase coaxial superconducting cable (1) for simultaneously collecting and analyzing temperature signals at each measurement point within the full length of the three-phase coaxial high-temperature superconducting cable, and obtaining the temperature distribution along the full length and between layers of the cable. Among them, the temperature measurement optical fiber (7) is installed in the wrapped semi-conductive layer (105) between two layers of superconducting tapes in each phase; A three-phase current transformer (8) is arranged on the connection line between the cable terminal on the other side of the three-phase coaxial superconducting cable and the main circuit; A shield layer current transformer (9) is arranged at the cable terminal on the other side of the three-phase coaxial superconducting cable for measuring the shunt current of the superconducting cable shield layer; The liquid nitrogen circulation monitoring unit (3), the thermal resistance temperature measurement unit (4), the optical fiber temperature measurement host (6), the three-phase current transformer (8), and the shield layer current transformer (9) all transmit measurement signals to the thermal balance monitoring unit (10) through measurement signal lines (12); When the loop where the high-temperature superconducting cable system is located operates normally, the operation mode of the superconducting cable is determined by monitoring the temperature of the superconducting cable inside the superconducting cable and the phase current unbalance degree; when a fault occurs in the loop where the high-temperature superconducting cable system is located, the operation mode of the superconducting cable is determined by measuring the temperature change when the superconducting tape passes through a large current and the shunt current situation of the shield layer.

2. The thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable according to claim 1, characterized in that: The thermal balance monitoring unit (10) evaluates and determines the operation state of the three-phase coaxial high-temperature superconducting cable (1) by combining all temperature and current monitoring data.

3. The thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable according to claim 2, characterized in that: The thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable further includes a main circuit operation parameter adjustment unit (11), and the main circuit operation parameter adjustment unit (11) adjusts and executes a new operation strategy according to the operation state of the three-phase coaxial high-temperature superconducting cable (1).

4. The thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable according to claim 3, characterized in that: The temperature measurement range of the temperature measurement optical fiber (7) is in the range of -200 to -180 °C; The temperature measurement optical fiber (7) is installed between two superconducting tapes of each phase conductor layer of the superconducting cable, and is laid and installed in a form of being wound together with the semi-conductive layer.

5. The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device according to claim 3, wherein: The temperature measurement optical fiber (7) is a bare optical fiber coated with high-performance materials such as polyimide, and the surrounding gaps are filled with acrylate adhesives for fixation and protection.

6. The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device according to claim 1 or 5, wherein: The temperature measurement optical fiber (7) adopts a distributed optical fiber sensor or a fiber Bragg grating sensor: For superconducting cables with a length less than or equal to 10 m, a distributed optical fiber sensor is used for detection; For superconducting cables with a length greater than 10 m, a fiber Bragg grating sensor is used for detection.

7. The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device according to claim 3, wherein: If the thermal balance monitoring unit (10) determines that the change in loop parameters does not affect the normal operation of the three-phase coaxial high-temperature superconducting cable, the main loop operation parameter adjustment unit (11) does not adjust the operation state of the three-phase coaxial high-temperature superconducting cable; If the thermal balance monitoring unit (10) determines that the change in loop parameters will not cause permanent damage to the superconducting cable system, the main loop operation parameter adjustment unit (11) adjusts the three-phase coaxial high-temperature superconducting cable to enter the derated state and continues to monitor the operation; If the thermal balance monitoring unit (10) determines that the change in loop parameters will cause permanent damage to the superconducting cable and it cannot continue to operate, the main loop operation parameter adjustment unit (11) cuts off the loop where the three-phase coaxial high-temperature superconducting cable is located, so that the three-phase coaxial high-temperature superconducting cable exits the operation.

8. The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device according to claim 7, wherein: The normal operation state of the three-phase coaxial high-temperature superconducting cable means that the three-phase operating currents do not exceed the critical current, the temperature ranges of the liquid nitrogen flow-out and return channels are both 70 - 76 K, and the temperature ranges along the A-phase and C-phase are both 75 - 78 K, and the temperature range along the B-phase is 75.5 - 78.5 K, and after thermal balance optimization iterative calculation, the three-phase temperature rise remains in the initial state; The non-permanent damage state of the superconducting cable system means that at least one phase of the three-phase operating current exceeds the critical current but the shield current is less than 200 A, or the average temperature of any one phase of the three-phase is greater than or equal to 81 K; and after thermal balance optimization iterative calculation, the cable resumes the initial stable state under the derated operation state; The permanent damage state of the superconducting cable system means that at least one phase of the three-phase operating current exceeds the critical current and the shield current is greater than or equal to 200 A, or the local temperature of any one phase of the three-phase is greater than or equal to 85 K, or the cooling medium inlet temperature is greater than 76 K, and no thermal balance optimization iterative calculation is performed, and it is considered that there is a fault in the superconducting cable body or the circulating cooling system.

9. A thermal equilibrium optimization method for a three-phase coaxial high-temperature superconducting cable using the device according to any one of claims 1-8, characterized in that, The thermal balance dynamic optimization method of the same-axis high-temperature superconducting cable includes the following steps: (1) Measure the three-phase current of the circuit, and obtain the curve of current versus time through the ordinary differential equations of the circuit under the initial temperature conditions; (2) Calculate the heat generation rate of each conductive layer under the initial isothermal conditions; (3) Use the obtained heat generation rates of each conductor layer and the heat conduction equation to obtain a new temperature distribution; (4) Use the new temperature distribution as the temperature load, substitute it into the ordinary differential equations of the circuit, and calculate the new heat generation rate; (5) Repeat the above steps (1) to (4), and iterate repeatedly until the set time ends.

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