M. Majoros

System Studies of the Superconducting Fault Current Limiter in Electrical Distribution Grids

A superconducting fault current limiter (SFCL) in series with a downstream circuit breaker could provide a viable solution to controlling fault current levels in electrical distribution networks. In order to integrate the SFCL into power grids, we need a way to conveniently predict the performance of the SFCL in a given scenario. In this paper, short circuit analysis based on the Electromagnetic Transient Program was used to investigate the operational behavior of the SFCL installed in an electrical distribution grid. System studies show that the SFCL can not only limit the fault current to an acceptable value, but also mitigate the voltage sag. The transient recovery voltage (TRV) could be remarkably damped and improved by the presence of the SFCL after the circuit breaker is opened to clear the fault.

Anisotropic connectivity and its influence on critical current densities, irreversibility fields, and flux creep in in situ processed MgB2 strands

The anisotropy of the critical current density (Jc) and its influence on the measurement of the irreversibility field (Birr) has been investigated for high quality, in?situ?MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in?situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication and MgB2 formation reactions. Scanning electron micrographs of in?situ processed MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100?nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomly oriented polycrystalline material has a fibrous texture at a larger length scale, with stringers of MgB2 (~60??m long and ~ 5??m in diameter) partially separated by elongated pores?the spaces previously occupied by stringers of elemental Mg. This leads to an interpretation of the differences observed between transport and magnetically determined critical currents, in particular a large deviation between the two at higher fields, in terms of different transverse and longitudinal connectivities within the strand. The different values of the connectivity also lead to different resistive transition widths, and thus irreversibility field values, as measured by transport and magnetic techniques. Finally, these considerations are seen to influence estimated pinning potentials for the strands.

Transport AC losses in YBCO coated conductors

Transport AC loss measurements have been made on YBCO-coated conductors prepared on two different substrate templates—RABiTS (rolling-assisted biaxially textured substrate) and IBAD (ion-beam-assisted deposition). RABiTS samples show higher losses compared with the theoretical values obtained from the critical state model, with constant critical current density, at currents lower than the critical current. An origin of this extra AC loss was demonstrated experimentally by comparison of the AC loss of two samples with different I–V curves. Despite a difference in I–V curves and in the critical currents, their measured losses, as well as the normalized losses, were practically the same. However, the functional dependence of the losses was affected by the ferromagnetic substrate. An influence of the presence of a ferromagnetic substrate on transport AC losses in YBCO film was calculated numerically by the finite element method. The presence of a ferromagnetic substrate increases transport AC losses in YBCO films depending on its relative magnetic permeability. The two loss contributions—transport AC loss in YBCO films and ferromagnetic loss in the substrate—cannot be considered as mutually independent.

Thermal diffusion and quench propagation in YBCO pancake coils wound with ZnO and Mylar insulations

The thermal diffusion properties of several different kinds of YBCO insulations and the quench properties of pancake coils made using these insulations were studied. Insulations investigated include Nomex, Kapton, and Mylar, as well as insulations based on ZnO, Zn2GeO4, and ZnO-Cu. Initially, short stacks of YBCO conductors with interlayer insulation, epoxy, and a central heater strip were made and later measured for thermal conductivity in liquid nitrogen. Subsequently, three different pancake coils were made. The first two were smaller, each using one meter total of YBCO tape present as four turns around a G-10 former. One of these smaller coils used Mylar insulation co-wound with the YBCO tape, the other used YBCO tape onto which ZnO based insulation had been deposited. One larger coil was made which used 12 total meters of ZnO-insulated tape and had 45 turns. The results for all short sample and coil thermal conductivities were ~1-3 Wm-1K-1. Finally, quench propagation velocity measurements were performed on the coils (77 K, self field) by applying a DC current and then using a heater pulse to initiate a quench. Normal zone propagation velocity (NZP) values were obtained for the coils both in the radial direction and in the azimuthal direction. Radial NZP values (0.05-0.7 mm/s) were two orders of magnitude lower than axial values (~14-17 mm/s). Nevertheless, the quenches were generally seen to propagate radially within the coils, in the sense that any given layer in the coil is driven normal by the layer underneath it.

Density effect on critical current density and flux pinning properties of polycrystalline SmFeAsO1 − xFx superconductor

A series of polycrystalline SmFeAs1-xOx bulks was prepared to systematically investigate the influence of sample density on flux pinning properties. Different sample densities were achieved by controlling the pelletizing pressure. The superconducting volume fraction, the critical current densities Jcm and the flux pinning force densities Fp were estimated from the magnetization measurements. Experimental results manifest that: (1) the superconducting volume fraction decreases with the decreasing of sample density. (2) The Jcm values have the similar trend except for the sample with very high density may due to different connectivity and pinning mechanism. Moreover, The Jcm(B) curve develops a peak effect at approximately the same field at which the high-density sample shows a kink. (3) The Fp(B) curve of the high-density sample shows a low-field peak and a high-field peak at several temperatures, which can be explained by improved intergranular current, while only one peak can be observed in Fp(B) of the low-density samples. Based on the scaling behaviour of flux pinning force densities, the main intragranular pinning is normal point pinning.

Experiment Setup for Calorimetric Measurements of Losses in HTS Coils Due to AC Current and External Magnetic Fields

We present a design and details of construction of two calorimetric systems that allow us to measure the total loss in high temperature superconducting coils or linear samples carrying alternating current while exposed to a strong alternating magnetic field. This measurement technique is based on the boil-off of liquid nitrogen. The first system is designed to measure ac losses in superconducting coils in self-field generated by AC transport current. The second system contains a permanent magnet rotor and simulates the environment of an electric motor or generator. The sensitivity of the system is such that it can measure low losses from a few milliwatts to several hundred milliwatts, in either a static or dynamic magnetic field.

Magnetization losses in superconducting YBCO conductor-on-round-core (CORC) cables

Described are the results of magnetization loss measurements made at 77 K on several YBCO conductor-on-round-core (CORC) cables in ac magnetic fields of up to 80 mT in amplitude and frequencies of 50 to 200 Hz, applied perpendicular to the cable axis. The cables contained up to 40 tapes that were wound in as many as 13 layers. Measurements on the cables with different configurations were made as functions of applied ac field amplitude and frequency to determine the effects of their layout on ac loss. In large scale devices such as e.g. Superconducting Magnetic Energy Storage (SMES) magnets, the observed ac losses represent less than 0.1% of their stored energy.

Modeling of Transport AC Losses in Superconducting Arrays Carrying Anti-Parallel Currents

Transport ac losses of parallel arrays of superconducting tapes with an elliptical cross-section carrying mutually anti-parallel currents were modeled numerically by finite element method. Two different configurations-tapes positioned edge to edge (x-array) and tapes positioned face to face (y-array) were considered. It was found that an x-array increases the losses while a y-array decreases them-relative to infinitely distant tapes. The highest ac loss decrease is observed when the tapes are close to each other in y-array. The AC loss of a y-array made of various numbers of tapes was compared with the loss of an array made of round wires. It was found that y-arrays of tapes have lower loss than arrays of round wires. Bringing y-arrays of tapes close to each other in the x direction causes a significant increase in ac loss. The present analysis may be useful if considering fault current limiters made of straight conductors in a meandering configuration. The results are qualitatively valid also for a fault current limiter made of non-inductive (bifilar) windings in the form of several pancake coils, each coil wound non-inductively (i.e. with anti-parallel currents).

Wind and React and React and Wind

Multifllamentary MgB2 wire was used to construct three coils that were tested for transport Jc and magnetic field in the bore between 4.2 K and 30 K. The solenoid coil achieved 85 A at 4.2 K with 3.9 T in a 3.8 cm bore. A racetrack coil was also fabricated with an attached compression plate to test the effects of compressive stress on an MgB2 racetrack coil. At compressive stresses up to 52 MPa, no degradation was observed in transport Jc at 20 K. A react and wind coil with a 56 cm bore and 828 m of MgB2 wire was also tested for transport Jc and magnetic field in the bore to demonstrate the feasibility of the react and wind process. At 5 K, the react and wind coil achieved 168 A and 0.17 T.

{\rm MgB}_{2}

AC applied magnetic field loss measurements have been performed on MgB2 superconducting strands. We measured strands with six untwisted filaments made via an in-situ route with Nb chemical barriers, a Cu-inter-filamentary matrix, and with either non-magnetic (glidcop) or magnetic (monel) outer sheaths. AC losses in magnetic fields applied perpendicular to the wire axis have been measured in fields up to 140 mT (peak) in the frequency range 50-200 Hz in a liquid helium bath at 4.2 K. For samples with no ferromagnetic elements present in their matrix there is a reasonable agreement of the losses with the critical state model in low frequency region. At higher frequencies the losses are dominated by eddy currents in Cu matrix. The samples containing a weakly ferromagnetic matrix showed complex hysteresis loops and AC loss dependencies.