Applied Superconductivity Key Lab,Institute of Electrical Engineering,Chinese Academy of Sciences
Institute of Electrical Engineering,Chinese Academy of Sciences
University of Chinese Academy of Science
Abstract:
The direct current(DC)short-circuit fault current endangers the operation of the power gird seriously,while the resistive superconducting fault current limiter(RSFCL)could effectively limit the increase of the short-circuit fault current and reduce the requirements for the breaking capacity and breaking time of the DC breaker.In recent years,with remarkable progress in the preparation technology,yttrium barium copper oxide(YBCO)superconducting tape has become the main material for developing RSFCL at home and abroad.However,it is still difficult to model the resistance characteristics of YBCO tape under short-time DC impact current accurately.Therefore,this paper proposes a modeling method to predict the resistance of YBCO tape under DC current impact based on LM neural network.The study of DC impact characteristics of YBCO tapes and the modeling analysis based on neural network were carried out in this paper.In order to study the resistance characteristics of YBCO tape used in RSFCL under short-time DC impact current,a high-voltage DC impact platform was established according to the fault current characteristics.By adjusting the inductance value,capacitance value,resistance value and capacitor charging voltage,the platform can realize the short-time DC impact process with different current peak values and impact time.The resistance changes of YBCO tape were measured under different impact currents whose current peak at 1000 to 3000 A and impact time at 1.9 to 8 ms.The results would be used in the analysis and establishment of the model.The experimental results showed that when the peak of DC impact current was small,the resistance of YBCO tape increased with the increase of current and decreased to zero with the decrease of current,but when the peak of DC impact current was large,the resistance of YBCO tape did not decrease with the decrease of current,it just kept getting higher.The difference between the two situations was whether or not the resistance curve crosses an inflection point.The inflection point was about 0.043 Ω·m-1.According to the theoretical model based on the three possible states for a superconductor,this was because the transition between superconducting state and flux-flow state was easier in the early stage of DC impact.As a result,the change of the resistance was sensitive to the impact current before turning to normal state,and the speed of quench and recovery was very fast.However,the increase was slowed down after turning to normal state.Besides,with the resistance of the superconducting layer increasing,the current gradually transferred from the YBCO layer to the buffer layer,the substrate layer and the stainless steel layer.Therefore,the experimental measurement of the resistance of the tape after crossing the inflection point of 0.043 Ω·m-1 showed the resistance change of the tape except the superconducting layer with the temperature rise.The interaction of electric field,magnetic field and thermal field during the quench process of YBCO superconducting tape was difficult to describe accurately with simple mathematical formula.To establish the accurate prediction model of DC impact characteristics of YBCO tape,the neural network method based on Levenberg-Marquardt(LM)algorithm was used.First,two neural networks were designed separately with analysis result of the experiments to fit the data before and after the inflection point.It was due to the different stages of YBCO tape quenching showed different patterns of change.The LM algorithm was selected as the training algorithm,because its high optimization efficiency.Besides,it had both the local convergence of Gauss Newton method and the global characteristic of gradient descent,which meant the training effect would be better.Next,the parameters of the two networks were trained repeatedly and the frameworks were modified and optimized,including the input layers,output layers,number of hidden layers and other training parameters.After training,the coefficient of determination(R2) of the two neural networks reached 0.9998 and 0.9996,which suggested that this model had fit the experimental data well.Finally,the model of DC impact characteristics of YBCO tape was established by the trained neural networks and was use to predict its resistance and voltage under DC impact.For the case that the YBCO tape's resistance did not cross the inflection point and for the case that the YBCO tape overshoot resistance did cross the inflection point,the simulation results were verified by the experimental data.The results showed that the predicted voltage error was all less than 5.1 V.The comparison of simulation and experiment suggested that the simulation results were in good agreement with the experimental results under different impact conditions.It could be inferred that,this accurate modeling method based on LM neural network was feasible and accurately reflects the resistance change characteristics of YBCO superconducting tapes under DC impact.This research could be helpful to the study of resistive superconducting fault current limiter.
Keyword:
YBCO tape;direct current impact;resistance characteristic;LM neural network;