Anisotropy in the transport properties of type II superconducting films with periodic pinning

Using numerical simulations, we analyze the anisotropy effects in the critical currents and dynamical properties of vortices in a thin superconducting film submitted to hexagonal and Kagomé periodical pinning arrays. The calculations are performed at zero temperature, for transport currents parallel and perpendicular to the main axis of the lattice, and parallel to the diagonal axis of the rhombic unit cell. We show that the critical currents and dynamic properties are anisotropic for both pinning arrays and all directions of the transport current. The anisotropic effects are more significant just above the critical current and disappear with higher values of current and both pinning arrays. The dynamical phases for each case and a wide range of transport forces are analyzed. © 2012 Springer Science+Business Media, LLC.

The effects of pinning on critical currents of superconducting films

Using molecular dynamics simulations, we analyze the effects of artificial periodic arrays of pinning sites on the critical current of superconducting thin films as a function of vortex density. We analyze two types of periodic pinning array: hexagonal and Kagomé. For the Kagome pinning network we make calculations using two directions of transport current: along and perpendicular to the main axis of the lattice. Our results show that the hexagonal pinning array presents higher critical currents than the Kagomé and random pinning configuration for all vortex densities. In addition, the Kagomé networks show anisotropy in their transport properties. © 2012 Springer Science+Business Media, LLC.

Electrical and Superconducting Properties in Lap Joints for YBCO Tapes

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Characterization of superconducting BSCCO/CaSiO3 and BSCCO/CaZrO3 Ag PIT wires

The addition of two compounds, calcium silicate and calcium zirconate was tested in the preparation of Bi: 2212 silver sheathed wires by powder-in-tube method. The wires were treated in an atmosphere of O-2/Ar using partial melting method. The characterizations were structural and on their electrical and magnetic properties. It was found that the addition of calcium silicate or zirconate promoted higher transition temperatures, up to 116 K for BSCCO with 1wt.% CaSiO3. The critical current densities determined by transport and magnetization measurements were improved in comparison with the wires without any addition.

Characterization of superconducting BSCCO/CaSiO3 and BSCCO/CaZrO3 Ag PIT wires

The addition of two compounds, calcium silicate and calcium zirconate was tested, in the preparation of Bi: 2212 silver sheathed wires by powder-in-tube method, which were successfully tested previously in processing chips. The wires were treated in an atmosphere of O2/Ar using partial melting method. The characterizations were structural and on their electrical and magnetic properties. As the results, transition temperatures were higher than the expected for this stage, ranged from 105K (BSCCO880) to 116K (+Si883). The critical current densities encountered in transport and magnetization measurements were improved in comparison with the wires without addition.

Experimental Analysis and Numerical Simulation of Quench in Superconducting HTS Tapes and Coils

The quench characteristics of second generation (2 G) YBCO Coated Conductor (CC) tapes are of fundamental importance for the design and safe operation of superconducting cables and magnets based on this material. Their ability to transport high current densities at high temperature, up to 77 K, and at very high fields, over 20 T, together with the increasing knowledge in their manufacturing, which is reducing their cost, are pushing the use of this innovative material in numerous system applications, from high field magnets for research to motors and generators as well as for cables. The aim of this Ph. D. thesis is the experimental analysis and numerical simulations of quench in superconducting HTS tapes and coils. A measurements facility for the characterization of superconducting tapes and coils was designed, assembled and tested. The facility consist of a cryostat, a cryocooler, a vacuum system, resistive and superconducting current leads and signal feedthrough. Moreover, the data acquisition system and the software for critical current and quench measurements were developed. A 2D model was developed using the finite element code COMSOL Multiphysics R . The problem of modeling the high aspect ratio of the tape is tackled by multiplying the tape thickness by a constant factor, compensating the heat and electrical balance equations by introducing a material anisotropy. The model was then validated both with the results of a 1D quench model based on a non-linear electric circuit coupled to a thermal model of the tape, to literature measurements and to critical current and quench measurements made in the cryogenic facility. Finally the model was extended to the study of coils and windings with the definition of the tape and stack homogenized properties. The procedure allows the definition of a multi-scale hierarchical model, able to simulate the windings with different degrees of detail.

Integrated Geophysical Investigation of the St. James Fault Complex: A Case Study

We noninvasively detected the characteristics and location of a regional fault in an area of poor bedrock exposure complicated by karst weathering features in the subsurface. Because this regional fault is associated with sinkhole formation, its location is important for hazard avoidance. The bedrock lithologies on either side of the fault trace are similar; hence, we chose an approach that capitalized on the complementary strengths of very low frequency (VLF) electromagnetic, resistivity, and gravity methods. VLF proved most useful as a first-order reconnaissance tool, allowing us to define a narrow target area for further geophysical exploration. Fault-related epikarst was delineated using resistivity. Ultimately, a high-resolution gravity survey and subsequent inverse modeling using the results of the resistivity survey helped to further constrain the location and approximate orientation of the fault. The combined results indicated that the location of the fault trace needed to be adjusted 53 m south of the current published location and was consistent with a north-dipping thrust fault. Additionally, a gravity low south of the fault trace agreed with the location of conductive material from the resistivity and VLF surveys. We interpreted these anomalies to represent enhanced epikarst in the fault footwall. We clearly found that a staged approach involving a progression of methods beginning with a reconnaissance VLF survey, followed by high-resolution gravity and electrical resistivity surveys, can be used to characterize a fault and fault-related karst in an area of poor bedrock surface exposure.

Two-dimensional Lattice Gauge Theories with Superconducting Quantum Circuits

A quantum simulator of U(1) lattice gauge theories can be implemented with superconducting circuits. This allows the investigation of confined and deconfined phases in quantum link models, and of valence bond solid and spin liquid phases in quantum dimer models. Fractionalized confining strings and the real-time dynamics of quantum phase transitions are accessible as well. Here we show how state-of-the-art superconducting technology allows us to simulate these phenomena in relatively small circuit lattices. By exploiting the strong non-linear couplings between quantized excitations emerging when superconducting qubits are coupled, we show how to engineer gauge invariant Hamiltonians, including ring-exchange and four-body Ising interactions. We demonstrate that, despite decoherence and disorder effects, minimal circuit instances allow us to investigate properties such as the dynamics of electric flux strings, signaling confinement in gauge invariant field theories. The experimental realization of these models in larger superconducting circuits could address open questions beyond current computational capability.

New selective earth faults only current directional method for isolated neutral systems

In order to achieve total selectivity at electrical distribution networks it is of great importance to analyze the defect currents at ungrounded power systems. This information will help to grant selectivity at electrical distribution networks ensuring that only the defect line or feeder is removed from service. In the present work a new selective and directional protection method for ungrounded power systems is evaluated. The new method measures only defect currents to detect earth faults and works with a directional criterion to determine the line under faulty conditions. The main contribution of this new technique is that it can detect earth faults in outgoing lines at any type of substation avoiding the possible mismatch of traditional directional earth fault relays. This detection technique is based on the comparison of the direction of a reference current to the direction of all earth fault capacitive currents at all the feeders connected to the same bus bars. This new method has been validated through computer simulations. The results for the different cases studied are remarkable, proving total validity and usefulness of the new method.

Role of superconducting shields in electrodynamic propulsion

An electrodynamic tether can propel a spacecraft through a planetary magnetized plasma without using propellant. In the classical embodiment of an electrodynamic tether, the ambient magnetic fleld exerts a Lorentz force on the current along the tether, the ambient plasma providing circuit closure for the current A suggested propulsion scheme would hypothetically eliminate tether performance dependence on the plasma density by using a full wire loop to close the current circuit, and a superconductor to shield a loop segment from the external uniform magnetic fleld and cancel the Lorentz force on that segment. Here, we use basic electromagnetic laws to explain how such a scheme cannot produce a net force. Because there is no net current in the superconducting shield, the circulation of the magnetic field along a closed line outside the full cross section, in its plane, is just due to the current flowing in the loop segment. The presence of the superconducting shield simply moves the Lorentz force from the shielded loop segment to the shield itself and, as a result, the total magnetic force, acting on full loop plus shield, remains zero.

Low Voltage Ride-through in DFIG Wind Generators by Controlling the Rotor Current without Crowbars

Among all the different types of electric wind generators, those that are based on doubly fed induction generators, or DFIG technology, are the most vulnerable to grid faults such as voltage sags. This paper proposes a new control strategy for this type of wind generator, that allows these devices to withstand the effects of a voltage sag while following the new requirements imposed by grid operators. This new control strategy makes the use of complementary devices such as crowbars unnecessary, as it greatly reduces the value of currents originated by the fault. This ensures less costly designs for the rotor systems as well as a more economic sizing of the necessary power electronics. The strategy described here uses an electric generator model based on space-phasor theory that provides a direct control over the position of the rotor magnetic flux. Controlling the rotor magnetic flux has a direct influence on the rest of the electrical variables enabling the machine to evolve to a desired work point during the transient imposed by the grid disturbance. Simulation studies have been carried out, as well as test bench trials, in order to prove the viability and functionality of the proposed control strategy.

Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets

Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as are available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e. holes) arrays, vortex pinning is the main mechanism for field induced oscillations in resistance R(H), critical current Ic(H), magnetization M(H) and ac-susceptibility χ ac(H) in a broad temperature range. Due to the coherence length divergence at Tc, a crossover to wire network behaviour is experimentally found. While pinning occurs in a wide temperature range up to Tc, wire network behaviour is only present in a very narrow temperature window close to Tc. In this temperature interval, contributions from both mechanisms are operational but can be experimentally distinguished.

Computer workstation vetting by supply current monitoring

It is our goal within this project to develop a powerful electronic system capable to claim, with high certainty, that a malicious software is running (or not) along with the workstations’ normal activity. The new product will be based on measurement of the supply current taken by a workstation from the grid. Unique technique is proposed within these proceedings that analyses the supply current to produce information about the state of the workstation and to generate information of the presence of malicious software running along with the rightful applications. The testing is based on comparison of the behavior of a fault-free workstation (established i advance) and the behavior of the potentially faulty device.

A novel ground fault non-directional selective protection method for ungrounded distribution networks

This paper presents a new selective and non-directional protection method to detect ground faults in neutral isolated power systems. The new proposed method is based on the comparison of the rms value of the residual current of all the lines connected to a bus, and it is able to determine the line with ground defect. Additionally, this method can be used for the protection of secondary substation. This protection method avoids the unwanted trips produced by wrong settings or wiring errors, which sometimes occur in the existing directional ground fault protections. This new method has been validated through computer simulations and experimental laboratory tests.

A Novel Ground Fault Identification Method for 2 x 5 kV Railway Power Supply Systems

The location of ground faults in railway electric lines in 2 × 5 kV railway power supply systems is a difficult task. In both 1 × 25 kV and transmission power systems it is common practice to use distance protection relays to clear ground faults and localize their positions. However, in the particular case of this 2 × 25 kV system, due to the widespread use of autotransformers, the relation between the distance and the impedance seen by the distance protection relays is not linear and therefore the location is not accurate enough. This paper presents a simple and economical method to identify the subsection between autotransformers and the conductor (catenary or feeder) where the ground fault is happening. This method is based on the comparison of the angle between the current and the voltage of the positive terminal in each autotransformer. Consequently, after the identification of the subsection and the conductor with the ground defect, only the subsection where the ground fault is present will be quickly removed from service, with the minimum effect on rail traffic. This method has been validated through computer simulations and laboratory tests with positive results.