Characterization of the electrical resistance of high temperature superconductor coated conductors at high currents using ultra-fast regulated current pulses

This paper focuses on the experimental determination of the electrical resistance (R) of commercial high temperature superconductor (HTS) coated conductors (CCs) at currents well above the critical current. The major novelty of this work rests on the unique experimental capability of applying constant current pulses in the sample (up to 1000 A) for durations as short as 15 mu s, which allows very precise control of the amount of energy dissipated in the sample (the Joule effect), as well as the resulting temperature rise. By varying the applied current and the duration of the pulses, we show that we can achieve a relatively accurate characterization of R(I, T) simply from the measured dynamical V-I characteristics of the CCs. The resistance model obtained in this way is very important, as R(I, T) is the most fundamental design parameter in many practical HTS applications, especially in fault current limiters.

Optimization Method for Extracting Stabilizer Geometry and Properties of REBCO Tapes

Arooj Akbar, Bertrand Dutoit, Francesco Grilli, Nicolò Riva

A good knowledge of material properties is a critical aspect for modeling high-temperature superconductor (HTS) devices. However, the electrical resistivity of coated conductors above the critical current is limited. The major challenge in characterizing this regime lies in the fact that for I > Ic, heating effects and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. In our previous works we proposed the overcritical current model, obtained by combining fast pulsed current measurements with finite element analysis (Uniform Current (UC) model). In this work, we assessed the impact of the uncertainties of the input parameters on the quantities calculated with the UC model (temperature and current in each layer of the tape). Firstly, sensitivity and uncertainty analyses were performed and it was found that the input parameters that mostly affect the UC model are the electrical resistivity and the thickness of the silver layer. Afterwards, an optimization method to correctly estimate the geometry and the resistivity of the silver layer was developed. This method combined experimental measurements of resistance R(T) of the tape and pulsed current measurement. The development of this strategy allowed us to determine the parameters that significantly impact the UC model and helps to minimize their uncertainties. This can enable a precise estimation of the overcritical current resistivity. 10%

2021

Overcritical Current Resistivity of YBCO-Coated Conductors Through Combination of PCM and Finite-Element Analysis

Bertrand Dutoit, Francesco Grilli, Nicolò Riva

Understanding the electro-thermal behavior of high-temperature superconductor (HTS) materials is a critical aspect for designing efficient and reliable applications. In order to optimize cost and performance, one needs to understand the role played by the various layers of an HTS tape during a quench. On one hand, the electrical and thermal properties of the materials used in the manufacturing of those tapes (e.g., alloy substrate, silver, and copper) are well known. Knowledge of the functional dependence of the superconductor's resistivity rho(J, T) above the critical current in 2G HTS CCs is very limited. In the flux creep or normal state regime, the resistivity can be approximated by empirical laws. In the flux-flow regime, it is difficult to extract rho(J, T) due to the presence of Joule heating. In this contribution, using finite-element analysis, we present a method to retrieve the overcritical current resistivity, by estimating the amount of current, temperature, and heat present in the various layers.

2019

Quench behavior of high-temperature superconductor tapes for power applications: a strategy toward resilience

Nicolò Riva

High-Temperature Superconductors (HTS) can be superconducting in liquid nitrogen 77 K, holding immense promises for our future. They can enable disruptive technologies such as nuclear fusion, lossless power transmission, cancer treatment devices, and technologies for future transportation. In the past years, the numerical models to describe the electrical resistivity of REBCO commercial tapes for devices working near and above the critical current, have been shown to be not accurate or very empirical. The resistivity in this regime, in fact, is not very well known. The lack of this knowledge is a significant issue in developing quality simulation tools. The major challenge in retrieving such properties lies in the fact that when I>Ic, heating effects, and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. Moreover, due to the current sharing between the layers, it is difficult to know the amount of current carried by the superconducting layer and its resistivity. The present work aims to understand better the overcritical current regime combining ultra-fast pulsed current measurements performed on HTS REBCO based coated conductors with Finite Element Modeling. The experimental activities were carried out mostly at EPFL and in part at PM and KIT. The modeling activities were carried out between EPFL and KIT. The major result is a resistivity relationship describing the overcritical current regime to be used in numerical simulations of REBCO tapes. The first part of the thesis illustrates a post-processing method based on the so-called Uniform Current (UC) model to estimate the REBCO material's resistivity in the overcritical from experimental measurements. Pulsed current measurements as short as 15 us and with current magnitude up to 5 Ic were performed in liquid nitrogen bath 77 K on samples from various manufacturers, without damaging the tapes. The second part of the thesis discusses a post-processing method based on regularization of data to treat the experimental measurements extracted in the overcritical current regime. The output of this technique is a look-up table that can be shared with interested partners and used in numerical modeling afterward. The third part of the thesis presents the overcritical current model (rho-\eta\beta): a mathematical relationship of the overcritical current regime based on measurements performed between 77 K and 90 K and in self-field conditions. We compare such models with the power-law model, and we provide a short discussion of the fitting parameters and their typical values. The last part of the thesis discusses the overcritical current model, based on experimental measurements obtained as outlined above. The model was validated experimentally and used to show that for the case of a superconducting fault current limiter when the power-law model is used to model its electro-thermal response, the device quenches faster than with the overcritical model. In conclusion, this work can help optimize the use of superconductors and, consequently, the stabilizer. More interestingly, it opens the study of the overcritical current regime, a new exciting aspect of REBCO commercial tapes. This project has received funding from the Swiss Federal Office of Energy SFOE under grant agreement SI/500193-02.

EPFL2021

Quench behavior of high-temperature superconductor tapes for power applications: a strategy toward resilience

Nicolò Riva

EPFL2021