Search

The adoption of High Temperature Superconductors (HTS) tapes is becoming a suitable and interesting alternative to copper for windings in transformers, thanks to the improvement in performance and the lowering of costs. Of course, optimized designs, different with respect to usual layouts, must be considered, due to the additional requirements of HTS tapes, such as minimization of orthogonal magnetic fields and consideration of additional losses in the HTS. In the framework of a scientific cooperation among some Italian Universities and private companies, a test model for a 10KVA transformer with HTS secondary windings has been developed, and validated against a demonstrative prototype, manufactured during the project. In the paper, the device model performance is assessed, with particular care to the HTS losses modeling, and some comparisons to the experimental results are presented.

Power transformers based on High Temperature Superconductors (HTS) technology are an appealing promise for several practical applications. The present designs still leave wide margins of possible improvement in terms of both layout optimisation and introduction of new technologies. In the framework of a technical-scientific cooperation among scientific and industrial subjects, a 10 kVA single-phase transformer was designed and manufactured, using copper for primary windings and BSCCO-2223 HTS tape for secondary windings. The layout has been optimized taking into account the particular characteristics of BSCCO tapes, in particular their AC losses, and the usual figures (stray flux, Joule and iron losses, weight and overall footprint) considered in transformers design. The prototype has then been realized and characterized, using general as well as specific tests. The performance of the device has been evaluated and compared with numerical calculation. In the paper, an overview of the device design and manufacturing will be presented, together with a critical comparison between computed and measured performance.

"In order to investigate the quench behavior of conduction cooled MgB2 magnets, a 100 mm inner bore diameter, 0.84 T at 20 K magnet has been built and tested. The test magnet consists of 4 double pancakes wound with the MgB2/Ni-Cu-Fe tape produced by Columbus. Each coil is cooled by means of a 2 mm thick copper disk placed during winding at the middle of the double pancake; after winding the double pancakes have been separately epoxy impregnated. Several experiments have been performed inducing a local transition in the coil by means of a controlled heater placed on the double pancake surface. The propagation of the normal zone has been monitored by means of 8 voltage probes positioned along the tape during coil manufacturing. Each voltage probe detects the voltage drop across a tape length of 30 mm. The experiments have been performed at different temperatures while keeping the magnet current constant during quench. In the paper we report an analysis of the quench propagation velocity measurements and a numerical investigation of the thermal and electrical behavior of the magnet."

We have analyzed the behavior of a 10 cm long straight piece of copper stabilized multifilamentary tape, manufactured by Columbus Superconductors, in a nearly adiabatic condition. The tape was biased at different current values as a function of either the temperature or the induced heat disturbances at fixed temperatures. From the analyses of the voltage increase, recorded from voltage taps placed at known positions, we estimated the highest temperatures which give rise to a quench of the tape at constant values of the current bias. We also measured the minimum energy triggering the quench at fixed values of current bias and temperature. The voltage taps have also been used for homogeneity check of the physical properties which rule the thermal runaway in the tape. The voltage-versus-time curves in different sections of the tape also provide information about the quench propagation speed as a function of the hot spot energy and the temperature. Due to the composite nature of the tape, made of nickel, iron, copper, and , to perform simple FEA computation we modeled the tape with a one dimensional single wire with averaged value of the physical quantities involved in the Joule heat generation and thermal conductivity.

The bulk MgB/sub 2/ material shows very promising characteristics to be applied in many AC devices, i.e. as electric motors, as magnetic field screening apparatus or as variable inductor or resistor for Fault Current Limitation. In all these cases the evaluation of supercurrents persistency and of the AC losses represents the first practical issue to be addressed. We report on the evaluation of these characteristics on very high density MgB/sub 2/ bulk materials, obtained by reactive liquid Mg infiltration of Boron powders preform. Varying the granularity of the original Boron powders, bulks of very different grains size distribution are produced that show different current carrying capability in magnetic field and temperature, indicating a different flux structure and dynamics. Performing AC susceptibility analysis in the low frequency range, the losses have been evaluated on several shapes of the probe materials from the cylindrical one to the slab one type and for several temperatures below Tc. For ring type samples the screening effect of the superconducting currents has been evaluated, in presence of static and variable magnetic fields. The results have been interpreted in term of the critical state model, evidencing the large effects of the thermal instability erasing from the thickness of the samples.

A cryocooler assisted cryostat for a 4 T split-coil NbTi magnet operated in liquid helium bath is under development at the ANSALDO-CRIS. A two stage GM cryocooler (1st stage 15 W @ 45 K, 2nd stage 5 W @ 15 K) is used for the cooling of two radiation shields at temperatures less than 45 K and 15 K respectively. For magnet charge, a couple of 100 A vapor cooled HTS current leads, based on Bi-2223/AgAu tapes, operates between the 1st stage of cryocooler and the helium bath. AlN plates have been used to obtain a good thermal contact electrically insulated between HTS lead and cryocooler. The developed configuration allows to eliminate liquid nitrogen cooled radiation shields and to minimize static liquid helium consumption. Calculated and measured system performances are compared.

High stable trapped fields in bulk superconductors can be useful applied in many electrotechnical applications, as i.e. for motor/generators, for MRI systems or for magnetic separation devices. The HTS bulk materials appear as the matter of choice to this purpose and many attempts have been done on melt textured cuprates tiles, at temperatures up to 77K, applying constant magnetic fields or pulsed magnetization methods. Even if the pulse magnetization is far less effective than the DC magnetization in the trapping capability, it appears as the most friendly technique in practical devices. So we have studied the effects of the pulsed technique, applied to a bulk MgB2 cylinder: a very promising device in view of the feasibility of similar much larger manufacts. To this purpose we have used a copper solenoid to magnetize the MgB2 cylinder, with both cylinder and solenoid conductively cooled by a cryorefrigerator, in the range of temperatures between 5 and 20 K. This simple cryogenic arrangement can be repres...