Simulation And Experimental Research On The Second Quenching And Recovery Process Of YBCO Superconducting Tape

Resistive-type superconducting fault current limiters(RSFCL)have broad application prospects for limiting the magnitude of short-circuit current in the power grid in the future.The power grid expects RSFCLs to reclose within 100 ms after the first impact to effectively respond the second short-circuit current,which requires the YBCO superconducting tape to recover quickly in a very short time.However,the disc-type current limiting unit generates a large number of bubbles during the quenching and recovery process and the bubbles will stay in the microporous structure,which cannot be dissipated in a short time.At present,the recovery of the superconducting state is generally used as a reference for the reclosing time,but the trapped bubbles at this time will seriously deteriorate the recovery process of the second impact and affect the performance of the RSFLC.It can be seen that there is an urgent need for a reference standard for reclosing time in practical applications.Therefore,a simulation was conducted in this article to study on the process of the YBCO superconducting tape being impacted by the second impact under the condition that the bubble is trapped and visual experiments were used for verification and comparison.The main work contents are as follows:(1)Through numerical simulation,the changes of various parameters in the recovery process of the first and second impacts are compared and the effectiveness of the liquid nitrogen jet is initially verified.It is pointed out that it is unreasonable to use the superconducting state recovery as a reclosing criterion,which provides theoretical support for determining the reclosing time.A three-dimensional(3D)model of the typical microporous structure of the actual disc current-limiting unit was established and meshed.The VOF model was used to monitor the change of the gas-liquid interface.The temperature,heat flux and gas volume fraction during the recovery process of the first and second impacts were compared and analyzed,which are compared and verified with the experimental results.In the case of restoring to the superconducting state,the recovery performance of the second impact deteriorates severely.Its peak temperature is 143.2 K,which is7.5 K higher than that of the first impact;the maximum temperature difference is 15.5K,which is 4.4 K higher than that of the first impact;the temperature recovery time is about 4.55 s,which is 12.1% longer than that of the first impact.Therefore,the traditionally recognized superconducting state recovery is unreasonable as a standard for reclosing.In addition,the effectiveness of the liquid nitrogen jet to improve the recovery performance of the superconducting tape has been preliminary verified.The results show that the jet can accelerate the cooling of the tape and the dissipation of bubbles,which is of great significance for the power grid's hundreds of millisecondlevel reclosing requirements.(2)The second impact experiment was carried out for three different recovery stages.Combined with the R-T characteristic curve of the superconducting tape,the temperature change during the actual impact and recovery process was obtained.The influence of different bubble states on the temperature change of the second impact was revealed and the recovery of temperature is proposed as one of the criteria for reclosing.A large-current impact visual experiment system was designed and built.Second impact experiments for three different recovery stages of superconducting state unrecovered,temperature unrecovered and bubble retention were conducted.The voltage and current data were obtained,which combined with the R-T characteristics of the superconducting tape to obtain the temperature change curve.The results show that the peak temperature of the second impact is higher and the recovery time of the superconducting state is longer.When the superconducting state is not restored,the recovery performance of the second impact deteriorates the most.The recovery time reaches 3.0 s and it is 77.5% longer than the first impact.The temperature recovery time of the second impact is 2.5 s when the temperature is not restored,which is 47.9%longer than the first impact.Therefore,the complete temperature recovery is proposed as one of the reclosing standards.(3)Images of the recovery process of the first and second impacts were obtained using visualization technology.The influence of different initial states on the dynamic characteristics of the bubble was explored and the temperature recovery time and bubble retention time of the second impact are obtained,which provides a reference for the reclosing time in practical engineering applications.A high-speed camera was used in the first and second impact experiments to obtain the images of bubble generation,dissipation and retention during the impact and recovery processes.The results show that the amount of bubbles in the recovery process of the second impact is significantly more than that of the first impact and the recovery time is also longer.When a small amount of bubbles are retained,the temperature recovery time of the second impact is about 3.0 s,which is 11.1% longer than that of the first impact.In order to improve the safety of the RSFCL system,this paper proposes the recovery of temperature as one of the indexes of reclosing.Since the resistance change is the easiest to measure during the recovery process,the recovery time of the superconducting state is used as a reference for the reclosing time.When the reclosing time is 2-3 times the recovery time of the superconducting state,the temperature recovery time after the second impact is 30% longer than that of the first impact.When the reclosing time is 3-4 times the recovery time of the superconducting state,the temperature recovery time after the second impact is extended by about 10%-20%.In order to ensure that the RSFCL can more effectively deal with the second shortcircuit current impact,it is proposed to use temperature recovery as one of the references for reclosing and the reclosing time of the system is about 4 times the recovery time of superconducting state.