Luciano Martini

Integral equations for the current density in thin conductors and their solution by the finite-element method

The current density and magnetic field distributions in thin conductors are important for several applications, and they can be computed by solving integral equations. This paper describes the implementation of a one-dimensional (1D) integral equation in a finite-element model. This numerical method does not require the use of ad hoc assumptions to avoid logarithmic divergences of the current density at the conductors edges and, by using a coupling with 2D electromagnetic models, it can be used to solve cases of increasing complexity. With respect to commonly used 2D models, it overcomes the typical problems linked to the mesh of conductors with high aspect ratio, such as the use of large memory and long computing times.

Conceptual Design of a 24 kV, 1 kA Resistive Superconducting Fault Current Limiter

In recent years many large scale demonstrators and prototypes of superconducting fault current limiters have been successfully developed and tested. Within the European Project ECCOFLOW (www.eccoflow.org), it is the first time that a resistive-type superconducting fault current limiter is developed for two different locations and that a permanent installation is foreseen. The limiter has a rating of 20 kV and 1 kA and will be tested in a busbar and transformer feeder application. The paper summarizes the conceptual design of this innovative limiter and reports in detail about the development of the super- conducting limiting elements, their integration into a cryostat and the design of the whole limiter including cooling and grid integration. As a main result it can be summarized that the ECCOFLOW limiter fulfills all requirements according to the two different specifications. Approximately 3 km of 12 mm wide YBCO tape will be used to realize a three phase system.

Design and Production of the ECCOFLOW Resistive Fault Current Limiter

The European project ECCOFLOW aims at a versatile resistive-type superconducting fault current limiter. For the first time, such a device will be tested at two different sites. The project partners have developed a superconducting fault current limiter design based on REBCO tapes with respect to the specifications provided by two hosting utilities. The limiter will operate at 1 kA at a rated voltage of 24 kV, and will be tested in both a busbar and a transformer feeder application. The design started with extensive investigations on the superconductor tape properties, especially with respect to the limitation behavior in all possible scenarios, and subsequent wide-range modeling of its in-grid behavior. The general integrated layout provides a limitation time of up to 1 s. The limiter is cooled using Gifford-McMahon cryocoolers to recondense the evaporated nitrogen. The present status of production and testing will be presented as well.

Modeling High-Temperature Superconducting Tapes by Means of Edge Finite Elements

In this paper we present a new 2D numerical model for AC loss computation in high-temperature superconductors, based on the use of edge finite elements. These elements are curl conforming by construction and involve the continuity of the tangential component of the magnetic field between adjacent elements. In this way they do not require the imposition of the zero divergence of the magnetic field at each time step, which, with standard nodal elements, increases the null space of the matrix and the risks of divergence of the algorithm. The model, implemented in the software package COMSOL, uses the two magnetic field components as state variables. It has been tested for computing the losses of YBCO coated conductor tapes in two cases of practical interest, which cannot be investigated by analytical models.

AC losses in thin superconductors: the integral equation method applied to stacks and windings

In this paper we present a method for computing transport current ac losses in interacting thin superconductors. The method solves the integral equations for the sheet current density distribution and is specifically developed for those configurations where the symmetry of the current density distributions allows writing the equation in a self-consistent form, without the need for using an auxiliary 2D model to describe the interaction between superconducting tapes. This results in very short computation times and therefore the model can be very useful for optimizing the design of superconducting devices. The method has been tested for different cases of practical applications and the ac loss results have been compared with those obtained with analytical models and with experiments.

Large Scale Assembly and Characterization of Bi-2223 HTS Conductors

The powering of the LHC machine requires more than 1000 High Temperature Superconducting (HTS) current leads. These leads contain, at their cold end, HTS conductors made of stacks of Bi-2223 tape with gold-doped silver matrix. CERN specified and purchased 31 km of this material, which was delivered on spools in unit lengths of 100 to 300 m. On reception the tape was inspected, cut into short length and vacuum soldered into stacks. All stacks were characterized in liquid nitrogen using a measuring procedure and set-up specifically developed for this purpose. Contact resistance values and critical currents at different electric field criteria were measured, from which the n-values have been extrapolated from the experimental V-I characteristics. This paper reports on the assembly and electrical characterization (up to 800 A) of more than ten thousand Bi-2223 stacks. Three types of stack were made from tape from two manufacturers. The assembly and soldering procedures and the set-up for the series electrical characterization are described. An analysis of the measured data is presented, with particular regard to the homogeneity of the electrical properties of both the tape and the derived stacks.

Silver-sheathed Bi-2223 tapes: the state of the art

At present, rolled multifilamentary Bi-2223/Ag tapes exhibiting at 77 K are fabricated, making them the most favourable candidates for the fabrication of high-temperature superconductor (HTS) conductors for power applications. In this paper the fabrication process of long Bi-2223/Ag tapes by the powder-in-tube technique is presented and discussed with the aim of surveying the influence of some selected process parameters on the properties of final conductors. limitations in HTSs are discussed and some of the possible applications of Bi-2223/Ag or Bi-2212/Ag tapes are briefly presented.

Test Results on 500 kVA-Class

The growth in generation and the increased interconnection of electrical grids lead to higher fault currents and therefore there is a considerable interest in fault current limiter (FCL) devices. In this work, we refer on the results of electrical testing on resistive-type superconducting FCL prototypes made by Ni-sheathed MgB2, developed in the framework of the Italian Project LIMSAT. This is the first time ever that short circuit testing results are reported on MgB2-based FCL prototypes at 27 K, cooled by a liquid neon bath. The time evolution of limited current and voltage across SFCL prototypes are reported and analysed. Critical aspects associated to the fault event such as the steep HTS tape temperature rise and electrical insulation at cryogenic temperature are also discussed.

{\rm MgB}_{2}

Modeling and design of superconducting power devices (e.g. fault current limiters), based on second generation HTS tapes, requires accurate evaluation and prediction of AC losses. Transport AC losses measurements have been performed on samples of YBCO coated conductors at 77 K, as a function of current. The results have been compared with the classical analytical model for self field AC losses calculation, taking into account the role of the magnetic substrate

-Based Fault Current Limiter Prototypes

Since 2009, Ricerca sul Sistema Energetico S.p.A. has been involved in the development of a resistive-type superconducting fault current limiter (SFCL) for MV applications to be installed in the A2A Reti Elettriche S.p.A. distribution grid in the Milan area. The project started with simulations, design, and testing activities for a single-phase device; this first step paved the way for developing, testing, and live-grid installation at the hosting utility site of the final three-phase SFCL prototype. The result of this research activity is a resistive-type 9 kV/3.4 MVA SFCL device, based on first-generation (1G) BSCCO tapes, nowadays permanently installed as a single-feeder fault protection. This device is the first SFCL successfully installed in Italy. In this paper, we report on installation of the three-phase device and field-testing activity. The next step of the R&D activity will consist in the design of a three-phase SFCL demonstrator 9 kV/15.6 MVA to be installed at the same substation in an incoming feeder to limit the contribution of this feeder to the total short-circuit current of the network. This application is very demanding in terms of SFCL reliability and availability, but also the most interesting one for the A2A hosting utility needs.