Mike Staines

Development of a 1 MVA 3-Phase Superconducting Transformer Using YBCO Roebel Cable

We present design details for the planned construction of a 3-phase 1 MVA 11 kV/415 V transformer using YBCO Roebel cable (YBCO-Yttrium Barium Copper Oxide second generation high temperature superconducting coated conductor). The YBCO Roebel cable is a promising technology for carrying high currents. It simplifies manufacture of the winding while managing AC loss. We present the transformer winding design which follows a simple layout with the low-voltage windings utilizing a 15 strand × 5 mm (15/5) Roebel cable in a single-layer 20 turn solenoid. The target rated current capacity of the cable is 1500 A rms at 77 K. The high-voltage winding will be in the form of a stack of double pan cake coils arranged on a composite former outside the low-voltage windings. The coils will be immersed in sub-cooled liquid nitrogen with a target maximum operating temperature of 70 K. The development methodology is described along with the results of experiments and modeling to validate the performance characteristics of the windings.

Transport AC loss measurement of a five?strand YBCO Roebel cable

Transport AC loss in a 5/2 YBCO Roebel cable (five 2 mm wide strands) with a pitch length of 90 mm is measured over a range of frequencies from 59 to 354 Hz. Five rectangular voltage loops are arranged from pairs of voltage taps attached at one pitch length separation on each strand. There are significant differences in the apparent transport AC loss measured from the different voltage loops at low frequency due to phase shifts in the current in each strand with respect to the phase of the total cable current. The difference in transport AC losses measured from different voltage loops becomes small with increasing frequency, because current is more equally distributed in each strand due to the higher reactance at high frequency. At 354 Hz, results measured with different voltage loops agree well with each other. As theoretically expected, the transport AC losses at different frequencies calculated from the mean value of in-phase voltages measured from the five different voltage loops agree well with each other. At It/Ic = 0.85, the transport AC loss in the 5/2 Roebel cable normalized by the number of strands and square of critical current per strand is around 2.9 times that in a single strand compared to the prediction of five times for a close bundle composed of five conductors. This difference may be due to the transposition of the strands.

Verification Testing for a 1 MVA 3-Phase Demonstration Transformer Using 2G-HTS Roebel Cable

We present results from testing to verify the performance of major subsystem components for a 1-MVA three-phase transformer demonstration project. The transformer utilizes a 15-strand Roebel cable for the low-voltage windings and a 4-mm-wide superconductor for the high-voltage windings. Both windings use a YBCO conductor. The winding assemblies will be housed in three individual vacuum insulated glass-fiber composite cryostats and will be cooled by circulated liquid nitrogen and subcooled to a target operating temperature of 70 K. Results from thermal insulation performance tests on a sample composite cryostat are presented. AC loss test results of a low-voltage winding assembly are also presented. The Roebel cable is proven to exhibit low AC loss operation at high current. We conclude that AC loss is not a fundamental obstacle to HTS transformer commercialization.

The dependence of AC loss characteristics on the spacing between strands in YBCO Roebel cables

Transport AC loss in a short length of 9/2 YBCO Roebel cable, (i.e. with 9 × 2 mm width strands), with 0.25 mm spacers between the strands is measured. The frequency varies from 59 to 354 Hz. The result is compared with the loss for a cable without spacers between the strands. Transport AC loss is decreased by the presence of the spacers. The AC loss reduction due to the extra spacing is more significant when the amplitude of the cable current is small compared to the cable Ic. The losses in the cable with spacers normalized by the square of the cable currents plotted against It/Ic approximately agree with those in a cable without spacers. Electromagnetic modelling was carried out for 9/2 and 8/2 cables, modelled as parallel stacks, to assist in understanding the above experimental results. The 8/2 configuration allows the greater use of symmetry to speed computations. Reasonable agreement between the numerical results and the measured results was obtained. This supports suggestions made in previous publications that the transport AC loss in a Roebel cable is roughly equivalent to the loss in two parallel stacks carrying the same current in each tape. The electromagnetic analysis in the 8/2 stacks shows the flux lines are more perpendicular to the strand face when the vertical space between strands is smaller, and this leads to a larger induced electrical field and larger AC loss. At small current amplitudes, the modelling shows the spacing has a strong effect on the AC loss in the surfaced part of the strands.

Numerical Computation of AC Losses and Flux Profiles in High-Aspect-Ratio Superconducting Strips in Perpendicular AC Magnetic Field

We present the results of finite-element modeling of a YBCO thin-film superconductor deposited on a nonmagnetic substrate and on a weakly ferromagnetic substrate. The model was implemented using commercial software to calculate the ac loss in the presence of a sinusoidal external magnetic field applied perpendicular to the surface of the superconducting tape. A strategy is demonstrated to overcome the difficulties in the finite-element method due to the high aspect ratio of the YBCO film by using the computed values of ac loss for thicker samples to extrapolate the results to the actual physical thickness of the superconductor ( ~ 1 mum). The effect of the width of the weekly ferromagnetic substrate upon the ac loss of the superconductor film has also been studied.

Biaxially textured YBCO coated tape prepared using dynamic magnetic grain alignment

A new magnetic grain alignment technique has been applied to produce biaxially aligned YBCO coated tapes. A biaxially aligned dispersion of orthorhombic Y2Ba4Cu7O15 (Y-247) powder was settled on untextured silver substrates. The Y-247 tapes were then melt processed to achieve high critical current YBa2Cu3O7 (Y-123) tapes with CuO as a secondary phase. The biaxial alignment is preserved after the densification process and a clear enhancement of Jc relative to identically prepared untextured or uniaxially textured samples is obtained. Critical current densities of up to 5000 A cm-2 at 77 K in self-field and 1500 A cm-2 in 0.5 T magnetic field at 65 K were obtained in films from 20 to 40 µm thick. Problems were experienced in achieving fully densified thick films while retaining biaxial texture. The initial grain size distribution was found to have a major influence on the final microstructure. Provided significant improvements in Jc can be obtained this method offers an alternative to coated tape processes based on epitaxial growth which has the advantage that it does not require textured substrates. The biaxial alignment technique described here intrinsically acts on the bulk material rather than at surfaces. This offers the possibility of texturing without thickness limitations.

Ac loss modelling and measurement of superconducting transformers with coated-conductor Roebel-cable in low-voltage winding

Power transformers using a high temperature superconductor (HTS) ReBCO coated conductor and liquid nitrogen dielectric have many potential advantages over conventional transformers. The ac loss in the windings complicates the cryogenics and reduces the efficiency, and hence it needs to be predicted in its design, usually by numerical calculations. This article presents detailed modelling of superconducting transformers with Roebel cable in the low-voltage (LV) winding and a high-voltage (HV) winding with more than 1000 turns. First, we model a 1 MVA 11 kV/415 V 3-phase transformer. The Roebel cable solenoid forming the LV winding is also analyzed as a stand-alone coil. Agreement between calculations and experiments of the 1 MVA transformer supports the model validity for a larger tentative 40 MVA 110 kV/11 kV 3-phase transformer design. We found that the ac loss in each winding is much lower when it is inserted in the transformer than as a stand-alone coil. The ac loss in the 1 and 40 MVA transformers is dominated by the LV and HV windings, respectively. Finally, the ratio of total loss over rated power of the 40 MVA transformer is reduced below 40% of that of the 1 MVA transformer. In conclusion, the modelling tool in this work can reliably predict the ac loss in real power applications.

Transport AC Loss Measurements in Single- and Two-Layer Parallel Coated Conductor Arrays With Low Turn Numbers

We measure transport ac losses in planar one-layer four-turn parallel coated conductor arrays (1 × 4) and in two-layer (2L) four-turn parallel coated conductor arrays (2 × 4) with a frequency up to 1 kHz. The horizontal separation between the conductors, i.e., gh, and the vertical separation between the neighboring superconducting layers, i.e., gv, were varied to investigate the transport ac loss dependence on gh and gv. In 2L arrays, the tapes in the top layer sit either aligned with the tapes in the bottom layer or aligned with the gaps between the tapes in the bottom layer. We show that the losses differently scale in arrays with low turn numbers to the scaling expected with an infinite array of tapes.

Total AC loss measurements in a six strand Roebel cable carrying an AC current in an AC magnetic field

We measured the total AC loss in a 6/2 (i.e. 6 strands of 2 mm width) Roebel cable by incorporating a transport measurement method for Roebel cable into a total AC loss measurement system. The amplitude and frequency of the external magnetic field and applied transport current, and the field angle (the angle between the field and the normal vector to the cable surface) were varied. The results for 60 scale with the perpendicular magnetic field component, and increase with increasing transport current. In the high magnetic field region, the total AC loss values for a current amplitude I Ic agree with the Brandt model for a 2 mm-wide Roebel strand, not for a thin strip with the Roebel cable width. This shows the advantage of a Roebel cable over equivalent stacks composed of wider conductors. The total AC losses in a parallel magnetic field become more independent of the field amplitude with increasing current amplitude due to the increased dominance of the transport AC losses in the total AC losses. No frequency dependence was observed in the total AC loss data. A maximum entropy model was successfully constructed for the total AC loss results in a perpendicular magnetic field. (Some figures may appear in colour only in the online journal)

Magnetic and Transport AC Losses in HTS Roebel Cable

We present results for magnetic losses in a five strand HTS Roebel cable with 2 mm wide insulated strands. The loss as a function of field is compared to the isolated strands and an un-insulated cable. The loss as a function of the angle of the applied field to the cable normal is found to scale simply with the normal component of field. The dependence of the loss on the frequency of the applied field is presented and a small intrinsic frequency dependence of the superconductor is observed. The frequency dependence of the loss in a cable coupled by copper bridges is presented. The coupling loss is found to follow the Debye form at low fields. Transport AC losses in HTS Roebel cable with different strand insulation thicknesses are found to vary with the vertical separation of strands.