N. J. Long

Transport AC loss characteristics of a nine strand YBCO Roebel cable

Transport AC loss in a short length of 9/2 YBCO Roebel cable (nine 2xa0mm wide strands) is measured. The AC loss data are compared with those in a 5/2 YBCO Roebel cable (five 2xa0mm wide strands) as well as that in a single strand. All the strands composing the cables and the single strand are insulated and cut from the same stock material. The validity of the measurement method was reconfirmed by results at a range of frequencies. At a wide range of It/Ic, the normalized AC losses in the Roebel cable were around 6.2–6.7 times of those in the single strand. This is less than the nine times predicted for a tight bundle of nine conductors. The normalized transport AC losses in the 5/2 Roebel cable are much smaller than those in the 9/2 Roebel. This should be due to larger superposition of magnetic field in the 9/2 Roebel. The Ic of the 9/2 and 5/2 Roebel cables is determined by serial connection of the strands. This eliminates the effect where differing resistances in the current terminations cause uneven current sharing between strands when the strands are connected in parallel.

Frequency dependence of magnetic ac loss in a Roebel cable made of YBCO on a Ni–W substrate

We have investigated the frequency dependent contributions to the magnetic ac loss in axa010 strand Roebel cable with 2xa0mm wide non-insulated strands and a transposition length of 90xa0mm. This cable is made from 40xa0mm wide YBCO coated conductor tape manufactured by AMSC and stabilized by electroplating 25xa0µm thick copper on either side prior to the mechanical punching of the cable strands. The measurements were carried out in both perpendicular and parallel field orientation, at frequencies in the range of 30–200xa0Hz. While the loss in the perpendicular orientation is predominantly hysteretic in nature, we observe some frequency dependence of the loss when the cable approaches full flux penetration at high field amplitudes. The magnitude is consistent with eddy current losses in the copper stabilization layer. This supports the fact that the inter-strand coupling loss is not significant in this frequency range. In the parallel field orientation, the hysteresis loss in the Ni–W alloy substrate dominates, but we see an unusually strong frequency dependent contribution to the loss which we attribute to intra-strand current loops.

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.

AC loss measurements in pancake coils wound with 2G tapes and Roebel cable: dependence on spacing between turns/strands

AC loss measurements in five single pancake coils wound with 4xa0mm wide commercial high temperature superconductor wires were carried out to investigate the dependence of coil AC loss on separation between the superconductor layers in the neighbouring coil turns (g) and coil turn number (N) for a given number of ampere-turns NI. The highest frequency was set at approximately 1xa0kHz. The AC losses measured at different frequencies agreed well with each other. AC loss in the coils with the same N increases with decreasing g. When g is increased to 1.5 times the tape width (6xa0mm), the loss level is similar to that in an isolated wire. Transport AC loss per unit length in the pancake coils increases with increasing turn number. However, when g is increased to 1.5 times the tape width, the loss level in the coils with different turn number is almost the same. This indicates that, at the same NI, a coil with greater N is advantageous, even considering the conductor length difference. Therefore to achieve a given level of NI for a coil and minimize AC loss, we should favour coils with more turns. Coil AC loss can be measured using voltage taps attached to copper blocks outside a coil with reasonable accuracy. This is important for measuring AC loss in a coil with a complicated structure where voltage taps are not able to be attached inside the coil. AC loss in a nine-turn pancake coil wound with a 5xa0m long 9/2 Roebel cable (i.e.xa0with 9xa0mmxa0×xa02xa0mm width strands) was measured using voltage loops arranged in each Roebel strand in the central turn of the coil. The AC loss in the coil was compared with two straight 9/2 Roebel cables with and without spacing between the strands. The combination of inter-strand spacing and turn spacing is an effective way to reduce AC loss in a single pancake coil wound with a Roebel cable.

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.

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.

Stability, Inter-Strand Contact Resistance, and AC Losses in YBCO Roebel Cables

A dc transport current was applied to the strands of a Roebel cable at 77 K in liquid nitrogen bath. The inter-strand contact resistance was measured. It was modified either by applying a pressure on the cable at 77 K in liquid nitrogen bath or using different soldering patterns between the strands of the cable. Magnetization ac losses were measured in frequency range 50-200 Hz in applied magnetic field 4-70 mT perpendicular to the broader face of the cable to test the inter-strand contact resistance effect. High stability and very low level of coupling losses were observed in the cables even with the lowest inter-strand resistances.