| Under the inevitable trend of power system capacity upgrading and DC revolution,further development of protection devices adapted to high-capacity power grids is necessary to ensure power safe.Resistive-type superconducting current limiters（RSFCL）based on YBCO have broad development prospects and application value in limiting high-level fault currents.However,the current research on quench and recovery of RSFCLs is still limited to the field of electricity.The unresolved technical problems such as the uncontrollable and predictable overheat of superconducting tapes,and the long system recovery time which is far from meeting the reclosing requirements,are fundamentally due to the unclear thermal mechanism between the superconducting tapes and liquid nitrogen.Therefore,it is necessary to study the heat transfer characteristics of quench and recovery to provide a reliable theoretical basis for practical engineering applications.In this paper,the main goal is to explore the matching between the recovery performance of the RSFCL and the reclosing requirements.The heat transfer characteristics of the RSFCL during quench and recovery processes are focused on research with the help of CFD simulation and cryogenic visualization experiments.The main research contents and results are as follows:（1）Based on the VOF model,the quench and recovery transient 3D model was established,the local hot spots were located,the bubble dissipation characteristics were revealed,and the mechanism of local hot spot formation was elucidated from insulation structural features and gas phase distribution characteristics.For two typical arrangements（symmetrical/asymmetrical）between the superconducting tape and the PTFE insulation,a three-dimensional model of the single-turn YBCO superconducting tape is established,and the VOF model is used to capture the gas-liquid interface during quench and recovery boiling process.The relative error in the peak temperature calculation is less than 3.2%,and the relative error in the recovery time calculation is less than 10%.Based on the surface temperature distribution of the superconducting tape,the hot spot is located.The hot spot in the horizontal tape with a symmetrical insulation model is most likely to occur at the position where the insulation and the tape are in contact,and the maximum temperature difference is as high as 18.5 K.The hot spot of the horizontal tape with an asymmetrical insulation model is most likely to occur at the position where bubbles are trapped under the tape,and the maximum temperature difference is 8.3 K.The PTFE insulation mainly induces hot spots on the surface of the tape by affecting the bubble dissipation behavior.（2）A cryogenic visualization platform for quench and recovery processes of RSFCL is built,all parameters of the entire quench and recovery processes are captured,and a complete mapping relationship is summarized.The safety of the three reclosing schemes is verified through the second quench experiments.The experimental results show that when the peak temperature of the tape is 151K,it takes about 3.5 s to recover the superconducting state but more than 4.3 s to recover the completely dissipated state of bubbles.There is still a long way to go for the 100-second barrier requirement.The recovery performance of the YBCO tapes is still far from the requirement of 100-second reclosing.The complete mapping relationship between the superconducting transition state and the boiling state of liquid nitrogen is established based on the triple state parameters during quench and recovery processes,including resistance,temperature,and bubbles.Based on the three recovery states,the reclosing criterion is divided in detail,and the second impulse experiment is designed to verify the feasibility of the criterion.The results show that it is inadvisable to use superconducting state recovery as the criterion for reclosing.The peak temperature growth rate is 30.5%,which may cause direct burning of the superconducting tape during the second quench.The reclosing time reference should be based on the comprehensive consideration of the thermal stability of the superconducting tape and breakdown rate in the actual application:it should be considered in the"temperature of LN2 recovery"after reclosing when the thermal stability of the superconducting tape is weak.At this time,the second peak temperature growth rate is within the acceptable range（<5%）;and it is necessary to consider the risk of electrical breakdown caused by bubble retention when the inter-turn insulation environment is weak,reclosing after the"bubble completely recovery"can ensure the safe and stable operation of the system.（3）Based on the connected domain image algorithm,the statistical analysis of the bubble group is realized,and the mechanism of the recovery inhomogeneity of the superconducting coil is revealed.The experimental correlation between the peak temperature and the accumulated heat is proposed,which solves the problem that the temperature rise of the overall superconducting coil cannot be accurately predicted.The inhomogeneity quench and recovery will occur in the vertical disc-type coil,the local recovery time difference of this type of coil is as high as 1.418 s,and the local instantaneous maximum temperature difference is as high as 10.8 K.Compared with the vertical disc-type coil,the uniformity of quench and recovery processes of the horizontal disc-type coil is better,the local recovery time difference of the coil is only 0.043 s,and the instantaneous maximum temperature difference is only 1.2 K.More importantly,the horizontal disc-type coils are less prone to bubble retention.From the perspective of bubble breakdown,the reference threshold of reclosing time for the horizontal disc-type coils can be greatly shortened,making it more advantageous in engineering applications.The most important finding is that the terminal temperature has no relation to the layout form of the insulations,placement position and gravity direction,but shows a linear correlation with the accumulated heat in the quench process.Based on the correlation formula,the temperature rise prediction of superconducting components can be realized according to the short-circuit level,and the maximum input power it can withstand can be deduced according to the temperature threshold inversion,which provides data prediction experience support for power grid line protection.This paper can provide theoretical guidance and technical support for the heat transfer research of quench and recovery processes in superconducting current limiters,the transient heat transfer characteristics of liquid nitrogen pool boiling,and the dynamic characteristics of bubble dissipation behavior. |
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