Superconductors for power applications: an executable and web application to learn about resistive fault current limiters

Bertrand Dutoit, Francesco Grilli, Nicolò Riva
2021
Journal paper

Abstract

High-temperature superconductors (HTS) can be superconducting in liquid nitrogen (77 K) at atmospheric pressure, which holds immense promises for our future such as nuclear fusion, compact medical devices and efficient power applications. In a power system, high short-circuit currents can exceed the operational current by more than ten times, putting at risk many parts of the system. Superconducting fault current limiters (SFCL) can limit the prospective fault current without disconnecting the power system, and are thus becoming increasingly attractive for future grids. With a growing interest in modeling and commercializing SFCL, the question of how to teach and to explain their operation to students has arisen. In order to help students visualize the potential use and benefits of a SFCL, we created an executable and a web application using COMSOL Multiphysics. This executable allows students to investigate the electro-thermal response of a resistive SFCL. The executable solves a 1D electro-thermal model of the SFCL under AC fault conditions, evaluating important figures of merit such as the limited current, the prospective current and the maximum temperature reached within the tape. Finally, the geometrical parameters as well as the superconducting properties of the device can be modified. The importance of the amount of silver stabilizer necessary to protect the device from over-heating occurring during a fault current can be investigated. In addition, the effects of having a sharp nonlinear transition from the superconducting to the normal state (intrinsic property of the superconductor) to obtain a current limitation can be well explored. The executable allows the users to learn about the consequences of superconductors in real-life applications, without the prerequisite of extensive modeling or experimental setup. The executable can be downloaded from the HTS modeling website and run on the most commonly used operating systems.

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Bertrand Dutoit, Francesco Grilli, Nicolò Riva
2021
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/282947?ln=en

About this result

Related concepts (15)

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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

The Superconducting Fault Current Limiter (SFCL) appears to be a device of great interest to efficiently build the electrical grid of tomorrow. With the recent progress made by the superconducting wires manufacturers, there are needs coming from the industry to evaluate the potential of such devices. In the present thesis work, the behavior under external field and transport current of the last generation of wire is investigated. This study is conduct both experimentally and numerically in order to link the physics occurring at the wires level to the design of SFCLs as a whole. From the nature of the material, the resistance appears non-uniformly in high temperature superconductors. For the purpose of building SFCLs it is important to obtain a fast and uniform resistive transition (quench) when a fault occurs through those conductors. This in order to reduce the local heat generation that may damage the device. This fast quenching property is related to the Normal Zone Propagation Velocity (NZPV). In this work the NZPV is measured using a localized magnetic field to initiate quenches in commercial coated conductors. Those velocities have been measured to be larger than 14 cm/s for pulsed currents above the critical value. The NZPV experiments have demonstrated that the superconductor non-uniformity (generated by the localized field) helps to reduce the initial delay before the quench initiation for transport currents in the range of the critical value. However, for larger transport currents the effect of the non-uniformity on the delay is less important since, with increasing transport current amplitudes, the normal state transition has shown to occur more as a consequence of the heat generated in the stabilizer than as the unique consequence of the advancement of the normal zone in the superconductor. From the experimental measurements, it has been shown that a reduction of the liquid-nitrogen temperature (subcoooled) increases the NZPV. This effect has been observed taking into account of the increase of the critical current associated with the temperature reduction. Nevertheless, it is not clear if it is the heat transfer or the estimation of the critical current that is responsible for this effect. In order to validate the numerical models, time-resolved voltage traces obtained from the experiments have been compared to the outputs of the models. Those are based on the thermal- and electrical-diffusion equations. From the simulations, it has been demonstrated that the NZPV can be increased by three methods: by using a thick diffusive substrate, by inserting a resistive interface between the superconductor and the stabilizer as well as by increasing the heat generation in the stabilizer. In light of those results, it seems that the insertion of a resistive layer is the most promising approach to improve the NZPV in coated conductors. As a matter of fact, a resistive interface increases the normal-zone size and keeps an acceptable temperature level along the conductor during quenches. The present work allowed to simulate the flux-flow regime in coated conductors. Comparing those simulations to experimental data have shown that the power-law may be inappropriate to simulate this regime under weak external magnetic fields. In addition, it appears that the role of transient heat transfer with the surroundings needs to be studied in more details to determine the specifications of a prospected SFCL made of coated conductors.

EPFL2010

Hysteresis Modelling of High Temperature Superconductors

Marten Sjöström

The present dissertation considers the capabilities, limitations and possible extensions of modelling the hysteresis that is exhibited by type-II superconductors, especially those with high critical temperature. Superconductors of type-II, including high temperature superconductors, are partially penetrated by magnetic flux. The tubes, through which the flux passes the superconductor, are `pinned' to certain locations due to impurities in the crystal structure of the material, and they must be forced by an external magnetic field or a transport current in order to move. Thus, the pinning of flux tubes constitutes a memory that gives rise to a hysteresis with corresponding losses. The critical state model is a well-known, macroscopic model that describes well this partial flux penetration. Furthermore, the flux tubes can start to flow due to the Lorenz-force when a large transport current flows in the superconductor. This produces an additional resistive voltage. The concept of hysteresis and its main properties are discussed, and a number of various models describing this phenomenon are presented. An emphasis is made on the classical Preisach model of hysteresis, which is a weighted superposition of relay operators. A hysteretic system produces higher harmonics, just as most nonlinear systems. An investigation reveals under what conditions the Preisach model generates only odd harmonics, and also when all odd harmonics are present, as well as how this knowledge can be utilised. A parameterised Preisach model is proposed, which always applies when the critical state model is an acceptable approximation of the superconductor hysteresis. Its capabilities and limitations as a hysteresis model for superconductors are investigated. It is demonstrated how the parameters can be estimated from different electric measurements on high temperature superconductors and that the output and the losses can quickly and accurately be computed for an arbitrary signal, once the parameter identification is made. Moreover, the structure of the parameterised model is such that an inverse model can easily be obtained. It is also shown that the hysteresis saturation, which occurs when the flux flow starts, can be modelled by introducing limiting functions in the model. A generalised equivalent circuit has been proposed that takes into account both the hysteretic and resistive behaviour, the latter being due to flux flow. This extended model describes the global electric behaviour of a superconducting device, which can be applied either when the superconductor is part of a larger system or when it stands alone. A few examples of its application are given.

2001