Thermal-Hydraulic Analysis of the Low-T-c Superconductor (LTS) Winding Pack Design Concepts for the DEMO Toroidal Field (TF) Coil

Three design concepts, namely, WP#1, WP#2, and WP#3, for the low-T-c superconductor DEMO toroidal field coil was proposed in 2014. This paper is focused on the thermal-hydraulic analysis of all the designs using simplified models, which includes: 1) hydraulic analysis-calculation of the mass flow rates in each conductor at operating conditions during the dwell time; 2) heat removal analysis-calculation of the steady-state mass flow rates and temperature profiles in each conductor as functions of the nuclear heat deposition rate; and 3) estimation of the maximum quench temperature and pressure in each conductor assuming the extreme scenario for the maximum pressure. The detailed analysis of the WP#1 design in burn and in quench conditions using the THEA code was also carried out. It took into account the expected electromagnetic and heat load maps along each conductor and different quench initiation scenarios leading to the maximum hot spot temperature or to the maximum pressure. Our analysis shows that the temperature margin in the WP#1 andWP#3 conductors is sufficiently large, but it is too small in some WP#2 conductors. The hot spot temperatures in some WP#1 cables calculated with THEA are too high. These results provide information for further improvements of the conductors' design.

Magneto-thermal finite element modeling of 2nd generation HTS for FCL design purposes

Bertrand Dutoit, Francesco Grilli, François Roy, Frédéric Sirois

Coated conductors are very promising for the design of novel and e±cient Fault Current Limiter (FCL). However, before considering using them in a power grid, their thermal and electromagnetic behaviors in the presence of over-critical currents need to be investigated in details. In this context, we performed ¯nite element magneto-thermal modeling of coated conductors under over-critical current on several geometries. Accordingly, we have investigated the substrate electrical connectivity and thermal properties on the HTS-FCL behavior. All simulations were performed using in COMSOL Multiphysics, a commercial finite element package, which has a built-in coupling between the thermal and electrical equations, allowing us to compute both quantities simultaneously during the solving process. Our simulations allowed us to formulate thresholds for the current density usable in coated HTS as well as limitation capability of a device made of these new conductors.

Iop Publishing Ltd, Dirac House, Temple Back, Bristol Bs1 6Be, England2008

Modeling and Characterization of Coated Conductors Applied to the Design of Superconducting Fault Current Limiters

François Roy

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

Evaluation of the Applicability of Phenomenological HTS Models for Numerical Analysis of Quenches in Coated Conductors: Simulations vs. Experiments

Bertrand Dutoit, François Roy, Frédéric Sirois

The electrical resistivity of coated conductors is strongly related to the inter-dependent set of parameters (J, H, T), respectively current density, magnetic field and temperature. On the one hand, it is difficult to isolate the contribution of each of these parameters on the resistivity measured experimentally. On the other hand, numerical methods, which may allow this separation, require a good knowledge of the fundamental laws governing the superconducting transition which are, up to now, derived from curve fitting with experimental data. In this paper, we investigate the influence of phenomenological formulas on the outputs of a recently developed finite element model. The outputs are compared against experimental voltage curves, which have been obtained under pulsed transport currents between 80 and 160 A and external magnetic fluxes of 0 to 350 mT. The comparisons indicate that the numerical models may reproduce well the measurements, using the right set of phenomenological laws parameters. Nevertheless, the solution may still be inaccurate at low field values and high current amplitudes, where the curvature of the simulated E-J curves is more pronounced, indicating that further refinement is required in order to obtain models valid over a wider range of parameters.

2011

Modeling and Characterization of Coated Conductors Applied to the Design of Superconducting Fault Current Limiters

François Roy

EPFL2010