Status of the European Union Project FASTGRID

High voltage direct current super-grids are one attractive solution for the transmission of bulk power of renewable electricity over long distances. Their protection is still an issue and superconducting fault current limiters (SCFCL) offer attractive perspectives. However, the actual superconducting tapes are not yet properly designed for operation at high voltages (>100 kV): The electric field developed during the current limitation is still too low (approximately 50 V/m for 50 ms) and the limiter requires too long lengths of tape. The European project FASTGRID aims at improving the properties of the REBCO tapes to enhance significantly (by 2-3 times) the electric field limit and so the economical SCFCL attractiveness. We use advanced THEVA tapes. Substantial improvements are also planned on the stabilizer (shunt) layer. Several shunt ways are under investigations. Other improvement on the tape properties will be carried out, namely the increase of the normal zone propagation velocity by at least one order of magnitude, with the help of the innovative current flow diverter architecture. The optimized conductor will be used in a SCFCL module (approximate to 1.5 kA-50 kV) tested at 65 K. We will monitor the temperature along the conductor using an attached optical fibre. FAST-GRID will also develop innovative tapes based on sapphire substrate, which can tolerate very high electric fields, in the range of kilovolts per meter. Validated on the laboratory scale, this game-changing technology needs to be implemented at long lengths with an industrial process. We will provide an overview of the project and its first results.

Modelling of 2G HTS Coated Conductors for Fault Current Limiter Applications

Daniele Colangelo

Thanks to recent scientific achievements, second generation high-temperature superconductor coated conductors (HTS-CCs) can carry enormous current densities with reduced losses. In addition, they are able to switch from a negligible resistivity state to a high resistivity state (quench) in a very fast time. The most promising application for this technology is represented by resistive superconducting fault current limiters (SFCLs). These devices enable us to operate the grid at larger power flows without the need of expensive upgrading interventions. One of the crucial challenge for the design of SFCLs is to deal with the inhomogeneous properties the superconducting materials present. The inhomogeneity of the material properties causes a non uniform transition to the high resistivity state. The consequence is a local heat generation. As HTS-CCs are characterized by low thermal conductivity, the heat does not diffuse longitudinally and the conductors are exposed to local thermal runaway. This Ph.D. thesis focuses on the numerical modelling of (RE)BaCuO-based HTS-CCs for resistive fault current limiter applications. The purpose of numerical modelling is to improve the HTS-CCs thermal stability and to optimize the design of resistive SFCLs. First, we develop a thermal-electrical coupled model that describes the transient response of long length (hundreds of meters) inhomogeneous HTS-CC candidates. The implementation of the inhomogeneous properties is based on Gaussian distributions derived from commercial wires. The electrical part of the model has been tuned to fit experimental measurements made on commercial coated conductors. The thermal part of the model has been validated with the support of finite element methods (FEMs). Then, we extended the modelling of HTS-CCs to SFCL modules. The model of a resistive SFCL has been interfaced with the characteristic parameters of two real medium voltage grids. The effect of the tape inhomogeneity has been analyzed under real fault scenarios. Afterwards, we used FEM calculations to investigate the possibility of improving the thermal stability of commercial HTS-CCs. In this respect, we analyzed the effects of a medium inserted between a HTS-CC and the cooling bath. The benefits of the concept idea has been demonstrated through a simplified 2D electro-thermal model (2D ET). In its turn, the assumptions used to develop the 2D ET model have been validated with a 2D magneto-thermal model (2D MT) and a 3D electro-thermal model (3D ET). The 2D MT model has been used to compare the magnetic and the thermal dynamics of a HTS-CC, whereas the 3D ET model has been used to study the quench propagation across the width and along the length of a HTS-CC.

EPFL2013

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

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

François Roy

EPFL2010

Modelling of 2G HTS Coated Conductors for Fault Current Limiter Applications

Daniele Colangelo

EPFL2013