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ABSTRACT: We present the results of an advanced numerical model for fault current limiter (FCL) based on HTS thin films in which both thermal and electromagnetic aspects are taken into account. This model allows simulating the behavior of FCL in the over-critical current regime and we used it for studying strip lines of a YBCO/Au FCL on sapphire substrate. The electromagnetic and thermal equations have been implemented in finite-element method (FEM) software in order to obtain a model for investigating the comportment of the superconductor when the current exceeds I<sub>c</sub>. In particular, materials equations have been implemented in order to simulate the electrical behavior of superconducting devices with strong over-critical currents. We report results of simulations in voltage source mode where currents largely exceed I<sub>c</sub>. The global behavior of the FCL is compared with measurements, showing a good agreement. The use of FEM simulations offers the advantage to give access to local variables such as current density or temperature. Studies with this model can replace expensive experiments where very high current density might damage or destroy the FCL device.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: The adjunction of constrictions along the meander of a superconducting fault current limiter (FCL) greatly improves its behavior thanks to a better distribution of the dissipative zones at the occurrence of a short circuit. This design works perfectly for symmetrical short circuit (i.e. short circuit at the maximum voltage). However for asymmetrical short circuits (at voltages close to 0), we are facing a problem due to the small number of the initially switched constrictions. To solve this problem, we test the possibility to speed up the transition into the normal state of the whole meander by heating it locally. This thermally assisted transition is realized by growing a gold layer on the backside of the substrate and by patterning it into a meander with its dissipative parts lying just underneath the constrictions of the FCL. This gold meander can be either connected in parallel with the superconducting meander or a capacitor bank can supply the current. In order to confirm the benefit of the thermally assisted transition we have carefully measured the behavior of the FCL during constant current and low voltage pulses as a function of the power injected into the gold line. We present results showing that the response of the FCL to the generated heat is very fast; typically less than 100 mus. Furthermore the distribution of the dissipated power across the wafer, during asymmetrical AC short circuit, is clearly improved.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: In this paper, we present a new numerical model, in which both the thermal and the electromagnetic aspects of the over-critical current regime of HTS materials are taken into account. The electromagnetic and thermal equations have been implemented in finite-element method (FEM) software in order to obtain a novel, closer to reality model for investigating the behaviour of the superconductor when the current exceeds Ic. This model has been applied for studying the behaviour of strip lines of an YBCO/Au FCL with a sapphire substrate. Simulations with currents largely exceeding Ic have been performed, showing that the total current limitation occurs only when the temperature dependence of the electrical parameters is taken into consideration. Such modelling can replace experiments with currents far exceeding Ic which may damage or destroy the studied sample or HTS device.
Journal of Physics Conference Series 07/2006; 43(1):1076.
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ABSTRACT: The magnetic field profile on the surface of Bi-2223/Ag tapes has been dynamically measured across the width of the samples. The experimental technique uses a Hall-probe array with 7 sensors connected to a multiple channel lock-in amplifier especially programmed for fast and synchronous data acquisition measurements. The speed of the system is high enough to measure real-time profiles with 7 probes and 50 Hz sine current through the sample. A numerical method to estimate the current distribution inside the tapes using the measured field profile data is proposed. The inverse problem has been solved using certain assumptions on the current distribution in the superconductor. Validation of the results has been done by comparison with finite element method simulations.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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ABSTRACT: Geometrical aspects of the design of fault current limiters (FCL) have a great impact on their performances. Recently, the University of Geneva have made certain optimizations by splitting the FCL into many small dissipative lengths in order to achieve a distributed transition along the device. For this paper, we have performed new 3D finite element method (FEM) simulations for studying the behavior of strip lines of a YBCO/Au FCL in an AC nominal use (sinusoidal current at industrial frequency) up to 3 I<sub>c</sub>. The very large aspect ratio of the device needs a particular attention to the modeling and meshing process. The numerical results show that presence of sharp corners can influence the performance of the device. Due to the high value of the electric field in these areas, the local losses are much higher than in the case of smooth corners, and this may lead to burning and cracking the wafer. Irreversible damage experiments have confirmed these locations. In this paper we proposed new geometries, taking into account the length of the connecting path and the corners optimization in order to decrease the risk of very high localized losses in the meander.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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ABSTRACT: We propose an equivalent circuit model that describes the behavior of a superconductor viewed by an external user, for whom the global variables voltage and current are of interest. It is used in time-continuous simulations and it treats well both the subcritical and supercritical current regime. The model is based on Maxwell's equations, measurement results as well as on the physical structure of a superconducting tape. The incorporated circuit elements have been described in mathematical expressions: a nonlinear resistance and a nonlinear inductance (superconducting core) in parallel with a linear resistance and a linear inductance (silver sheath or by-pass material). The simplicity of the model makes it fast and easy to apply compared to existing numerical models of superconductors, based on finite element methods. Furthermore, it is wide-ranging and may represent a superconductor in many different applications depending on parameter values. Examples of applied voltage and current with nonstationary waveforms on a HTS tape are given. Calculated are the hysteresis losses (due to flux pinning) and resistive losses (due to flux creep and flux flow) in the tape in the range 0.1 to 1.8 I<sub>c</sub>.
IEEE Transactions on Appiled Superconductivity 07/2003; · 1.04 Impact Factor
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ABSTRACT: For describing the E-J relation of high-Tc superconductors (HTS) in power applications, where the applied current I is generally limited by Ic, the critical state model, a piecewise linear generalization, or a simple power-law of the type E=Ec(J/Jc)^n are most often used. The power-law cannot be used for modelling the E-J relation with I>>I_c due to the unbound exponential increase of the electric field for currents above Ic, while in reality the non-linear HTS resistivity is limited by its normal state value. This paper presents a modified E-J model for describing the V-I characteristic of HTS tapes with applied currents largely exceeding Ic. This model is based on the power-law in combination with a parallel metallic branch and has a limited resistivity - the HTS one in the normal state. It can be used for black-box modelling of superconductors in a unlimited current range, as well as for numerical modelling of superconducting devices, which can be operated at currents far exceeding Ic; for example fault-current limiters or cables with over-critical current excursions. The model has been tested in a simple numerical implementation and the modified power-law has been implemented in finite element method simulations. It is shown that for bulk material with currents above 1.3-2Ic$ (depending on the n-value), the usual power-law results in excessive AC loss estimation.
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ABSTRACT: In this paper we present a numerical model in which both the thermal and the electromagnetic aspects of the over-critical current regime of HTS materials are taken into account. The electromagnetic and thermal equations have been implemented in finite element method (FEM) software in order to obtain a novel, closer to reality model for investigating the behaviour of the materials when the current exceeds Ic. A thermal dependence of the electrical parameters, like the critical current density Jc, has been introduced. This model has been used to analyse the behaviour of strip lines of a YBCO/Au fault current limiter (FCL) on a sapphire substrate. Simulations with currents largely exceeding Ic have been performed, showing that the current limitation phase can be correctly reproduced. Such modelling can be used to study the influence of the geometry on the performance of the FCL. It is designed to replace experiments with currents far exceeding Ic which may damage or destroy HTS samples or devices.
