CN112595357A - Three-phase coaxial high-temperature superconducting cable thermal balance monitoring device and thermal balance optimization method - Google Patents
Three-phase coaxial high-temperature superconducting cable thermal balance monitoring device and thermal balance optimization method Download PDFInfo
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Abstract
The thermal balance monitoring device comprises a three-phase coaxial high-temperature superconducting cable, a cable terminal, a liquid nitrogen circulation monitoring unit, a thermal resistance temperature measuring unit, a thermal resistance, an optical fiber temperature measuring host, an ultralow temperature measuring optical fiber, a three-phase current transformer, a shielding layer current transformer, a thermal balance monitoring unit and a main loop operation parameter adjusting unit, wherein the thermal balance monitoring unit realizes the functions of measuring and protecting all operation parameters of the whole superconducting cable. The invention ensures that the whole set of test system can carry out current carrying and temperature distribution monitoring under the operating state of the three-phase coaxial high-temperature superconducting cable, dynamically adjusts the current carrying of the loop according to the monitoring result, ensures that the superconducting cable operates under the thermal balance state, improves the current carrying capacity and the external thermal disturbance resistance, and ensures the safe and stable operation of the superconducting cable system.
Description
The technical field is as follows:
the 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:
the urban power grid load is rapidly increased, and newly increased or expanded existing power transmission lines face the problems of saturated cable tunnel space, insufficient cable current-carrying capacity, overhigh land acquisition cost of newly increased substation power distribution facilities and the like, so that the distribution capacity of part of urban load centers faces the dilemma that the actual development requirements cannot be met. The superconducting cable has great technical advantages in the aspect of being applied to an underground cable system of an urban load center or realizing large-capacity power transmission under a specific environment. The high-temperature superconducting cable replaces the conventional cable, and can improve the transmission capacity of the underground power grid by times, thereby solving the contradiction between load increase and underground space limitation and breaking the bottleneck of urban power transmission.
Due to the difference of the structures of all phases, the electromagnetic coupling between three-phase conductors is not uniform, and the problem of phase-to-phase imbalance often occurs in the three-phase coaxial superconducting cable. When short-circuit current impact or asymmetric fault occurs to a line, the current of each phase in the three-phase coaxial cable is in a transfer distribution phenomenon due to resistance and heat accumulation caused by superconductor quench, so that the induced current of a shielding layer, the voltage of the cable, equivalent parameters and the like can be changed. For example, under the short circuit action, the three-phase coaxial cable structure can present certain deficiency, and the heat transfer distance is short because the inside phase conductor and the outside phase conductor are close to the cooling medium, can obtain good cooling, even through passing fault current, the heat can also be passed to the cooling medium fast and go. However, because the middle phase conductor is far away from the cooling media on the two sides, once the phase has a single-phase short-circuit fault, the generated heat can be transferred and dissipated only through the other two phases, and a longer time is needed while the other two phases generate a certain temperature rise. When the cryogenic cooling environment and the cooling medium also fail or change, the continued rising temperature will cause the superconducting cable to lose stability, and in severe cases even cause cable damage, due to failure to remove the accumulated heat in a timely manner. At present, no good method is available for monitoring the temperature of a middle phase conductor of a three-phase coaxial superconducting cable structure and the shielding shunt condition under the fault condition, so that the operation parameters of the cable cannot be controlled, and the cable can work under the heat balance stable state with higher efficiency.
The invention content is as follows:
aiming at the defects and the improvement requirements of the prior art, the invention aims to provide a thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable, and also aims to provide a thermal balance dynamic optimization method for the three-phase coaxial superconducting cable.
The purpose of the invention is realized by adopting the following technical scheme:
a thermal balance monitoring device for a three-phase coaxial high-temperature superconducting cable comprises 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 and a thermal balance monitoring unit 10; the method is characterized in that:
the liquid nitrogen circulation monitoring unit 3 is connected with the cable terminal 2 on one side of the three-phase coaxial superconducting cable and is used for providing circulating liquid nitrogen as a cooling medium for the three-phase coaxial high-temperature superconducting cable 1 and the cable terminal 2;
two thermal resistors 5 are respectively arranged on a liquid nitrogen flow-out channel and a liquid nitrogen return channel of the liquid nitrogen circulation monitoring unit 3, and the thermal resistor temperature measuring unit 4 is connected with the thermal resistors 5 and is used for monitoring the temperature of a cooling medium in the flow-out channel and the return channel of the liquid nitrogen circulation monitoring unit 3;
the optical fiber temperature measurement host 6 is connected with the ultralow temperature measurement optical fiber 7 arranged in the three-phase coaxial superconducting cable 1 and is used for simultaneously collecting and analyzing temperature signals of all measurement points in the full-length range of the three-phase coaxial high temperature superconducting cable to obtain the full-length of the cable and the temperature distribution condition between layers;
a three-phase current transformer 8 is arranged on a connecting line of a cable terminal at the other side of the three-phase coaxial superconducting cable and the main loop;
a shielding layer current transformer 9 is arranged at the cable terminal at the other side of the three-phase coaxial superconducting cable and is used for measuring the shunting of the shielding layer of the superconducting cable;
the liquid nitrogen circulation monitoring unit 3, the thermal resistance temperature measuring unit 4, the optical fiber temperature measuring host 6, the three-phase current transformer 8 and the shielding layer current transformer 9 transmit measuring signals to the thermal balance monitoring unit 10 through measuring signal wires 12.
The present invention further includes the following preferred embodiments.
The thermal balance monitoring unit 10 evaluates and judges the operating state of the three-phase coaxial high-temperature superconducting cable 1 by combining all the temperature and current monitoring data.
The three-phase coaxial high-temperature superconducting cable thermal balance monitoring device further comprises a main loop operation parameter adjusting unit 11, and the main loop operation parameter adjusting unit 11 adjusts and executes a new operation strategy according to the operation state of the three-phase coaxial high-temperature superconducting cable 1.
The temperature measuring optical fiber 7 is a temperature measuring optical fiber with the temperature measuring range of-200 to-180 ℃;
the temperature measuring optical fiber 7 is installed between two layers of superconducting strips of conductor layers of each phase of the superconducting cable, and is laid and installed in a wrapping mode together with the semi-conducting layer.
The temperature measuring optical fiber 7 is a bare optical fiber coated with polyimide and other high-performance materials, and the surrounding gap is filled with acrylate adhesive for fixing and protecting.
Detecting the superconducting cable with the length less than or equal to 10m by adopting a distributed optical fiber sensor; and detecting the superconducting cable with the length of more than 10m by using a fiber grating sensor.
If the thermal balance monitoring unit 10 determines that the change of the loop parameter does not affect the normal operation of the three-phase coaxial high-temperature superconducting cable, the main loop operation parameter adjusting 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 of the loop parameter does not cause permanent damage to the superconducting cable system, the main loop operation parameter adjusting unit 11 adjusts the three-phase coaxial high-temperature superconducting cable to enter a derating state to continue monitoring operation;
if the thermal balance monitoring unit 10 determines that the superconducting cable cannot continue to operate due to permanent damage caused by the change of the loop parameters, the main loop operation parameter adjusting 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 from operating.
If the thermal balance monitoring unit 10 determines that the change of the loop parameter does not affect the normal operation of the three-phase coaxial high-temperature superconducting cable, the main loop operation parameter adjusting 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 of the loop parameter does not cause permanent damage to the superconducting cable system, the main loop operation parameter adjusting unit 11 adjusts the three-phase coaxial high-temperature superconducting cable to enter a derating state to continue monitoring operation;
if the thermal balance monitoring unit 10 determines that the superconducting cable cannot continue to operate due to permanent damage caused by the change of the loop parameters, the main loop operation parameter adjusting 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 from operating.
Further, the flow of the thermal balance dynamic optimization method in the cable running state is as follows:
(1) measuring the three-phase current of the circuit, and obtaining the curve of the current along with the time through the ordinary differential equation set of the circuit under the condition of initial temperature;
(2) calculating the heat generation rate of each conductive layer under the initial isothermal condition;
(3) obtaining new temperature distribution by using the obtained heat generation rate and heat conduction equation of each conductor layer;
(4) substituting the new temperature distribution as a temperature load into an ordinary differential equation set of the circuit to calculate a new heat generation rate;
(5) and repeating the steps and repeating the iteration till the set time is over.
Further, the operation strategy after the dynamic optimization of the cable heat balance comprises the operation of lowering the load of the cable loop and the disconnection of the cable loop.
Furthermore, in the operation strategy after the dynamic optimization of the cable heat balance, the priority of the abnormal circulation of the cooling medium is the highest, and when the liquid nitrogen mass flow, the pressure, the refrigeration power and the inlet and outlet temperature monitoring values obtained by the liquid nitrogen circulation monitoring unit have the associated abnormality, the main loop operation parameter adjusting unit directly cuts off the cable loop.
Compared with the prior art, the invention has the following beneficial technical effects:
the thermal balance monitoring device and the optimization method of the three-phase coaxial high-temperature superconducting cable are based on the combined use of the traditional thermal resistance temperature sensor and the optical fiber temperature measurement technology, and the intermediate-phase superconductor temperature monitoring and the full-line distributed temperature monitoring of a superconducting cable system can be innovatively carried out by installing the ultralow temperature sensor into the superconducting cable in advance when the superconducting cable is manufactured; three-phase main loop current and shielding layer shunt monitoring are carried out through a plurality of groups of current transformers to form a complete cable system operation monitoring system. The system can accurately master the temperature distribution along the high-temperature superconducting cable in real time, thereby realizing dynamic analysis and optimal adjustment of the balance state of the cable according to the current-carrying capacity and the operating temperature of the superconducting cable, finding out the cable operating defects related to thermal disturbance and the circuit faults related to overcurrent in time, adjusting the circuit operating mode in time and ensuring the safe operation of the high-temperature superconducting cable. The high-temperature superconducting cable temperature measuring system provided by the invention can be applied to temperature measurement and monitoring protection of high-temperature superconducting cables in a power grid, and has higher stability and reliability.
Description of the drawings:
fig. 1 is a single line schematic diagram of a thermal balance monitoring device for a three-phase coaxial hts cable according to an embodiment of the present invention.
Fig. 2 is a schematic view showing a structure of a three-phase coaxial superconducting cable according to an embodiment of the present invention.
Fig. 3 is a flow chart of the establishment of a new thermal equilibrium state under the interaction of the superconducting cable temperature and current.
Fig. 4 is a flow chart of a balance optimization method of the three-phase coaxial high-temperature superconducting cable based on an electro-magnetic-thermal analysis process.
The specific implementation mode is as follows:
the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. It is specifically stated that the following description is given for the purpose of macroscopic explanation and illustration only and is in no way intended to limit the invention, its application or uses. Unless specifically stated otherwise, the relative arrangement of the components and steps and the numerical expressions and numerical values set forth in the embodiments do not limit the scope of the present invention.
Fig. 1 is a single line schematic diagram of a thermal balance monitoring device for a three-phase coaxial hts cable according to an embodiment of the present invention. In this embodiment, the thermal balance monitoring device for the three-phase coaxial high-temperature superconducting cable comprises 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 balance monitoring unit 10, a main loop operation parameter adjusting unit 11, a measuring signal line 12 and a control signal line 13, wherein the thermal balance monitoring unit 10 is used for realizing the functions of measuring and protecting all operation parameters of the whole superconducting cable. It will be appreciated that in embodiments of the present invention, the test system main circuit is a complete three-phase circuit. The current measured by the three-phase current transformer 8 is the current carrying of each phase conductor, the current measured by the shielding layer current transformer 9 is the current shunting of the shielding layer of the superconducting cable, and the current carrying distribution condition of the three-phase coaxial high-temperature superconducting cable 1 in the running state is tested together.
It is understood that in the embodiment of the present invention, the liquid nitrogen circulation monitoring unit 3 is used for supplying circulating liquid nitrogen as a cooling medium to the three-phase coaxial high-temperature superconducting cable 1 and the cable terminal 2, and ensuring that the three-phase coaxial high-temperature superconducting cable 1 operates below the operating temperature (-196 ℃). It is understood that the thermal resistor 5 is used to monitor the temperature of the cooling medium in the cooling medium outgoing flow path and the cooling medium return flow path of the liquid nitrogen circulation monitoring unit 3, respectively. In addition, other parameter values such as the mass flow, the pressure, the refrigeration power and the like of the liquid nitrogen obtained by the liquid nitrogen circulation monitoring unit 3 are all connected to the thermal balance monitoring unit 10 through a measurement signal line 12.
It will be appreciated that in this embodiment, the thermal resistor 5 is a platinum resistor sensor, which is externally wrapped with a flexible ultra high molecular polyethylene (UPE) protective tube to ensure reliable installation of the sensor and accurate temperature sensing. The platinum resistance sensor adopts a PT100 four-wire system measuring mode, is suitable for monitoring the internal temperature of a shorter superconducting cable sample cable and a cooling terminal, and is respectively provided with a platinum resistance sensor in a liquid nitrogen defluidizing channel and a liquid nitrogen reflux channel. The PT100 is not only widely used for industrial thermometry, but also made as a standard reference gauge. The temperature measuring range of PT100 is-200-650 deg.C, the measuring accuracy can be up to 0.1 deg.C, and it has better stability and faster response speed, so it is an ideal choice for measuring temperature in low-temperature environment. Therefore, the PT100 platinum resistance sensor can be used as a temperature sensor in the present embodiment to monitor the cooling medium temperature.
The temperature measuring optical fibers 7 are three temperature measuring optical fibers of the same type, the same material and the same length, are arranged inside the three-phase coaxial superconducting cable 1 and are used for monitoring the temperature of the three-phase superconducting layer. The optical fiber temperature measurement host 6 is used for simultaneously collecting and analyzing temperature signals of all measurement points in the full-length range of the three-phase coaxial high-temperature superconducting cable 1, and obtaining the temperature distribution conditions of the full length of the cable and the layers.
It is understood that the thermal resistance temperature measuring unit 4 is used for receiving temperature information from the thermal resistance 5; the optical fiber temperature measurement host 6 is used for receiving 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 may be an SRA-D distributed optical fiber sensing temperature measurement analyzer, which is responsible for the functions of signal acquisition, signal processing, data analysis, over-temperature alarm, network transmission, etc. of the whole system, and is composed of an optical frequency generator, a switching power supply, a microprocessor, a network interface, etc.; the thermal resistance temperature measurement unit 4 selects a DM3068 series digital multimeter, supports the direct use of a plurality of temperature sensors such as a TC (thermocouple), an RTD (thermal resistance) and a THERM (thermistor) for measuring the temperature, and meets the international temperature standard ITS-90 standard; the composite temperature measuring component consisting of the optical fiber temperature measuring host 6 and the thermal resistance temperature measuring unit 4 outputs temperature signals to the thermal balance monitoring unit 10, the thermal balance monitoring unit 10 greatly improves the integration of data information, and the flexibility of system use is improved.
The measurement signals of each unit are transmitted to the thermal balance monitoring unit 10 through the measurement signal line 12, the thermal balance monitoring unit 10 evaluates and judges the operation state of the three-phase coaxial high-temperature superconducting cable 1 by combining all the temperature and current monitoring data, adjusts the operation strategy, and controls the main loop operation parameter adjusting unit 11 to execute the adjustment instruction.
If the three-phase operating current does not exceed the critical current, the temperature ranges of the liquid nitrogen defluidizing and refluxing channels are 70-76K, the linear temperature ranges of the phase A and the phase C are 75-78K, the linear temperature range of the phase B is 75.5-78.5K, the three-phase temperature rise is kept in an initial state after the thermal balance iterative calculation, the thermal balance monitoring unit 10 judges that the three-phase coaxial high-temperature superconducting cable is in a normal operating state, and the main loop operating parameter adjusting unit 11 does not adjust the operating state of the three-phase coaxial high-temperature superconducting cable.
If the running current of at least one phase of the three phases exceeds the critical current and the current of the shielding layer is less than 200A, or the average temperature of any phase of the three phases is more than or equal to 81K, the cable can recover the initial stable state again in the derated running state after the thermal balance iterative computation, the thermal balance monitoring unit 10 judges that the circuit parameter change can not cause the permanent damage of the superconducting cable system, the main circuit running parameter adjusting unit 11 adjusts the three-phase coaxial high-temperature superconducting cable to enter the derated state to continue monitoring and running, and sends out an alarm signal.
If the operating current of at least one phase of the three phases exceeds the critical current and the current of the shielding layer is more than or equal to 200A, or the local temperature of any phase of the three phases is more than or equal to 85K, or the inlet temperature of the cooling medium is more than 76K, the thermal balance iterative calculation is not carried out any more, the fault of the superconducting cable body or the circulating cooling system is considered to be the permanent damage of the superconducting cable system and the continuous operation cannot be carried out, the main loop operating parameter adjusting unit 11 cuts off the loop of the three-phase coaxial high-temperature superconducting cable, and the three-phase coaxial high-temperature superconducting cable is.
It can be understood that, in the embodiment of the present invention, the temperature measuring optical fiber 7 with low temperature resistance (-below 196 ℃) is installed inside the three-phase coaxial superconducting cable 1 in advance, the thermal resistor 5 is arranged inside the liquid nitrogen circulation monitoring unit 3, and the thermal balance monitoring unit 10 finally determines the cable running condition according to the temperature signal and the superconducting cable current-carrying distribution, so as to control the main loop running parameter adjusting unit 11 to dynamically optimize and adjust the loop running mode, so that the cable reaches a new thermal balance state again after being disturbed by heat, and the determination is based on the rule that the superconducting cable temperature distribution changes along with the current transfer characteristics of the conductor layer and the shielding layer under the set working condition.
In the preferred embodiment of the present application, if it is determined that the change of the loop parameter does not affect the normal operation of the high temperature superconducting cable system, the operation state of the loop is not adjusted; if the superconducting cable system is judged not to be in fault immediately due to the change of the loop parameters, the cable system is adjusted to enter a derating state to continue monitoring operation; if the superconducting cable is judged to possibly cause permanent damage to the superconducting cable and cannot continue to operate, a loop where the cable system is located is cut off, and the cable system is led out of operation.
Fig. 2 is a schematic view showing a structure of a three-phase coaxial superconducting cable according to an embodiment of the present invention. In the present embodiment, the three-phase coaxial hts cable 1 includes, from outside to inside: a heat insulating layer 101, a shield layer 102, at least one insulating layer 103, a superconducting layer, and a hollow skeleton 107. Each superconducting layer consists of an outer first superconducting tape layer 104, an inner second superconducting tape layer 106, and a wrapped semiconductive layer 105 between the two superconducting tape layers. Liquid nitrogen 108 is filled between the heat insulating layer 101 and the shielding layer 102 and in the hollow framework 107, so that the three-phase coaxial high-temperature superconducting cable 1 works at the operating temperature below (-196 ℃), wherein a liquid nitrogen flow-removing channel is arranged inside the hollow framework 108, and a liquid nitrogen backflow channel is arranged between the heat insulating layer 101 and the shielding layer 102.
It is understood that, in the embodiment of the present invention, the heat insulating layer 101 uses vacuum and multi-layer heat insulating material technology to ensure the liquid nitrogen low temperature heat preservation effect of the incoming and outgoing three-phase coaxial superconducting cable 1.
It is understood that, in the embodiment of the present invention, the shielding layer 102 is a copper shielding layer, belongs to a metal shielding layer, and generally operates in a single-ended grounding manner, and mainly functions to shield an electric field, no current passes through the shielding layer during normal operation, and a shunt function is generated for a fault current in case of a system fault. The hollow framework 107 is a metal corrugated pipe, is mainly used for supporting the winding of the superconducting tape and is also used for a liquid nitrogen pipeline, no current passes through the hollow framework during normal operation, and the hollow framework can generate a shunting effect on fault current under the condition of system fault.
It is understood that the design of the insulating layer 103 depends on the characteristics of the insulating material, the operating voltage, the cable dimensions, and other factors, and the polypropylene laminated paper (PPLP) may be preferred as the low temperature insulating material in this embodiment, in consideration of the electrical properties, the thermal properties, the force properties, and the process difficulty.
In this embodiment, the conductor layers of the respective phases are second-generation high-temperature superconducting tapes, and it can be understood that the second-type high-temperature superconducting tape is also called a YBCO superconducting tape, and the YBCO superconducting tape has a multi-layer structure and mainly comprises a copper stabilizing layer, a silver layer, a YBCO superconducting layer, a cap layer, a seed layer, a buffer layer, an isolation layer, and a hastelloy layer, and the superconducting tapes of different manufacturers may have differences in material and thickness of each layer. Each phase of the superconducting layer contains 2 layers of superconducting strips, self inductance and mutual inductance of each layer are obtained according to an equivalent circuit equation, and winding screw pitch and winding screw angle are obtained through calculation, so that the current sharing design of the electrified conductor can be realized. And a wrapping semi-conducting layer and a filling material are filled between the two layers of superconducting tapes. The temperature measuring optical fiber 7 is arranged between two layers of superconducting tapes and can resist the environment of extremely low temperature (below 196 ℃).
It is understood that, in the embodiment of the present invention, the three-phase coaxial superconducting cable includes three outer insulating layers 103 and three superconducting layers, and the phase C is on the outer side, the phase a is on the inner side, and the phase B is an intermediate phase.
In this embodiment, a temperature measuring optical fiber 7 is further installed inside the three-phase coaxial high-temperature superconducting cable 1, and the temperature measuring optical fiber 7 is installed inside each wrapped semiconductive layer 105, that is, covers each superconducting layer region, and is used for monitoring the temperature of each superconducting layer.
In this embodiment, the temperature measuring fiber 7 may adopt a distributed fiber sensor principle or a fiber grating sensor principle, and may generally adopt a multimode fiber of a quartz system, and may adopt a fiber grating sensor for a superconducting cable sample cable with a medium length (about 5-10m), and the distance between adjacent gratings is not greater than 0.5 m; for longer superconducting cable or engineered superconducting cable products, distributed fiber optic sensors may be employed.
It can be understood that in the embodiment of the present invention, the distributed optical fiber temperature measurement needs a longer pigtail to ensure higher temperature measurement accuracy and spatial resolution, and therefore the temperature measurement optical fiber 7 in the embodiment preferably adopts a cascade fiber grating sensor. When necessary, the Fabry-Perot resonant cavity (F-P resonant cavity) is used for assisting in demodulating the fiber grating sensor, and more fiber 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 arranged in the superconducting layer can not be too large, so that the phenomenon that the performance of the cable is influenced by occupying too much internal space of the superconducting cable is avoided. Therefore, in the present embodiment, the temperature measuring optical fiber 7 installed between the layers is preferably a bare optical fiber coated with a high performance material such as polyimide, and the surrounding gap is filled with an acrylate adhesive for fixation and protection.
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. Generally, when the superconducting cable exceeds the critical current, the superconducting cable is quenched and converted into a conductor with resistance, on one hand, the current capacity is greatly reduced, and on the other hand, larger joule heat is generated, so that the temperature of the superconducting strip and the surrounding cooling medium is increased.
It can be understood that, in the embodiment of the present invention, the present invention pre-sets multiple types of temperature measuring optical fibers 7 (operating below-196 ℃) inside the three-phase coaxial high-temperature superconducting cable 1, and transmits the temperature signal to the thermal balance monitoring unit 10 by the optical fiber temperature measuring host 6 as one of the main parameters for determining the operating condition of the cable (the other parameters also include the temperature of the liquid nitrogen inlet and outlet, the current carrying distribution condition and the balance optimization method), so as to ensure that the three-phase coaxial high-temperature superconducting cable 1 is in the safe operating state.
It is understood that in the embodiment 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 may be installed by being wrapped with the semiconductive layer; the temperature measuring optical fiber 7 arranged in the liquid nitrogen channel inside the three-phase coaxial high-temperature superconducting cable 1 can be laid between the heat insulating layer 101 and the shielding layer 102 and in the hollow framework 107 in a linear or S-shaped laying mode.
It can be understood that, in the embodiment of the present invention, when the loop in which 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 imbalance degree of the phase current; when a loop of a high-temperature superconducting cable system has a fault, the operation mode of the superconducting cable is judged mainly by measuring the temperature change of a superconducting strip passing through a large current and the shunting condition of a shielding layer. When the current of the shielding layer is less than 200A and the average temperature rise of the single phase is less than 3K (the temperature does not exceed 81K), the cable can still recover the initial temperature steady state after derating through the iterative calculation of a heat generation formula, so that the cable does not need to be tripped immediately and can be derated automatically or manually and send out an alarm signal. When the current of the shielding layer exceeds 200A and the average temperature rise of the single phase is greater than 7K (the temperature exceeds 85K), the cable can continuously generate heat and raise the temperature through iterative calculation of a heat generation formula, and the cable can completely enter a quench state, so that the cable must be immediately tripped to protect a high-temperature superconducting cable system. If the change of the loop parameter does not affect the normal operation of the high-temperature superconducting cable system, the operation state of the loop is not adjusted; if the superconducting cable system is judged not to be in fault immediately due to the change of the loop parameters, the cable system is adjusted to enter a derating state to continue monitoring operation; if the superconducting cable is judged to possibly cause permanent damage to the superconducting cable and cannot continue to operate, a loop where the cable system is located is cut off, and the cable system is led out of operation.
It can be understood that in the embodiment of the present invention, in order to improve the current carrying capacity of the superconducting cable, a multi-layer superconducting conducting layer structure is adopted, so that when the superconducting cable carries an alternating current, the current distribution of each layer is uneven, which is generally indicated as that the outer layer current is larger than the inner layer current, and particularly when the total current is increased, the outer layer current is increased very obviously, which causes the outer layer current to reach the critical current first. Therefore, the alternating current loss of the superconducting cable can be increased, the current carrying capacity is reduced, the safety and stability of the operation of the cable can be threatened in serious cases, the operation cost is increased, the distortion of the alternating current carrying can be caused, and the electric energy quality is influenced. In the actual work of the cable, various abnormal working conditions can occur, the superconducting cable needs to ensure the working stability under the fault working conditions to meet the power transmission application, when the situation of exceeding rated current occurs, the current distribution of the superconducting cable is more complex and is related to the temperature of the cable, and the current carrying of the superconducting cable and the response situation of the temperature and the current distribution to time need to be discussed qualitatively in the research.
Fig. 3 is a flow chart of the establishment of a new thermal equilibrium state under the interaction of the superconducting cable temperature and current. A finite element method is used for establishing a compact three-phase coaxial superconducting cable differential equation and analyzing the response of current distribution and temperature distribution under different working conditions. In the process of quenching the superconducting layer, the equivalent resistance can cause the temperature of each layer of the superconducting cable to be increased, and the critical current Ic of the YBCO strip is also influenced by the temperature. Therefore, for the research of the quench process, the temperature rise process including self ac loss heating, equivalent resistance heating conduction and convective heat transfer needs to be considered at the same time. The temperature of the superconducting tape can be changed by the factors, and the change of the temperature of the tape can cause the change of the critical current of the superconducting tape, so that the equivalent resistance of the superconducting layers of the superconducting cable is influenced, and the current distribution of each superconducting layer is further influenced. From the above analysis it can be seen that the conduction process of the superconducting cable is in fact an electro-thermal coupling variation process, and that a change in temperature causes a change in the current distribution, which in turn affects the temperature variation.
Fig. 4 is a flow chart of a balance optimization method of the three-phase coaxial high-temperature superconducting cable based on an electro-magnetic-thermal analysis process. The method comprises the following steps:
(1) measuring three-phase current, and obtaining a current curve along with time through a circuit ordinary differential equation set under the condition of initial temperature;
(2) calculating the heat generation rate of each conducting layer under the initial isothermal condition based on the equivalent resistance of each layer of the superconducting cable;
(3) obtaining the heat generation rate and the heat conduction equation of each conductor layer by using a radial heat conduction differential equation under a cylindrical coordinate system, and obtaining new temperature distribution;
in the formula, rho is density, c is specific heat capacity, r is average radius of a conductor layer, lambda is heat conductivity of the conductor layer, phi' is a cable body heat source, T is time, and T is temperature;
(4) substituting the new temperature distribution as a temperature load into an ordinary differential equation set of the circuit to calculate a new heat generation rate;
where ρ is density, c is specific heat capacity, and r isnIs the average radius of the conductor layer in the nth iteration, Δ r is the difference between the inner and outer diameters of the conductor layer, λ is the thermal conductivity of the conductor layer, T is time, Tn-1、Tn、Tn+1Respectively the temperature distribution obtained by the n-1 th iteration, the n-1 th iteration and the n +1 th iteration, Q'totalIs the heat generation of the cable body heat source in unit volume in unit time;
(5) and (5) repeating the steps (1) to (4), and repeating iteration till the set time is over, wherein the iteration calculation time is usually set to 500 seconds.
It will be appreciated that there are two cable body heat sources: firstly, alternating current loss is generated when the superconducting cable is subjected to current flowing; and secondly, heat leakage from the external environment to the liquid nitrogen backflow channel through the low-temperature thermostat of the cable body is realized. As the liquid nitrogen accumulates heat along the cable, its temperature changes as the length of the cable increases. The temperature of the reflux liquid nitrogen is higher than that of the defluidization liquid nitrogen at any position along the length direction of the superconducting cable.
It can be understood that the rule that the temperature distribution of the superconducting cable changes along with the current transfer characteristics of the conductor layer and the shielding layer under the set working condition can be obtained through the steps. The scheme uses a finite difference method to establish a radial one-dimensional heat conduction model of the superconducting cable, couples the radial one-dimensional heat conduction model with a superconducting cable circuit equation, establishes a one-dimensional radial magnetic-thermal coupling model of the superconducting cable, and can analyze the temperature and current carrying response conditions of the superconducting cable under different current carrying conditions by utilizing the numerical model.
In an embodiment of the invention, the operation strategy for the cable after the dynamic thermal balance optimization comprises the load reduction operation of the cable loop and the cut-off of the cable loop. When the conditions that the operating current of at least one phase of the three phases exceeds the critical current and the current of the shielding layer is more than or equal to 200A, the local temperature of any phase of the three phases is more than or equal to 85K and the inlet temperature of the cooling medium is more than 76K meet the conditions, the thermal balance iterative calculation is not carried out any more, and the superconducting cable body or the circulating cooling system is considered to be in fault. However, the fault priority of the circulating cooling system is highest, and when the correlation abnormality occurs in the liquid nitrogen inlet and outlet temperature monitoring value obtained by the liquid nitrogen circulating monitoring unit and the inlet temperature of the cooling medium is greater than 76K, the main loop operation parameter adjusting unit directly cuts off the cable loop.
While the present invention has been described in terms of exemplary embodiments, it should be understood that the invention is not limited to the exemplary embodiments described above. It will be apparent to those skilled in the art that the above-described exemplary embodiments may be modified without departing from the scope and spirit of the disclosure. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
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