Zhenan Jiang

An experimental method for total AC loss measurement of high Tc superconductors

The authors have developed a new electromagnetic method to measure the total AC loss in high Tc superconductors carrying AC transport current in an AC transverse magnetic field with arbitrary orientation. While simultaneously applying an AC transverse magnetic field and AC transport current, we measure the magnetization loss using a linked pick-up coil (LPC), and the transport loss using voltage taps with a spiral loop. Then we calculate the two to obtain the total AC loss. The LPC can be calibrated both theoretically and experimentally. One advantage of the LPC is that the error due to the variation in field orientation is negligible even if the sample superconducting tape is wide like a typical coated conductor. A precise non-inductive shunt resistor is used as the phase reference for both the magnetization loss and transport loss measurements. By combining an internal compensation coil and a non-inductive shunt resistor, we can reduce the error in the measured loss, which comes from the inductive component of the output voltage of the LPC and a phase error, to a sufficiently low level. No mechanically tunable compensation coil is required to reduce the inductive component in either the LPC output voltage or the tap voltage. An experimental system was incorporated to measure the total AC loss, and the measured total AC loss is compared with theoretical values.

Dynamic resistance of a high-Tc superconducting flux pump

Superconducting flux pumps enable large currents to be injected into a superconducting circuit, without the requirement for thermally conducting current leads which bridge between the cryogenic environment and room temperature. In this work, we have built and studied a mechanically rotating flux pump which employs a coated conductor high-Tc superconducting (HTS) stator. This flux pump has been used to excite an HTS double pancake coil at 77 K. Operation of the flux pump causes the current within the superconducting circuit to increase over time, before saturating at a limiting value. Interestingly, the superconducting flux pump is found to possess an effective internal resistance, Reff, which varies linearly with frequency, and is two orders of magnitude larger than the measured series resistance of the soldered contacts within the circuit. This internal resistance sets a limit for the maximum achievable output current from the flux pump, which is independent of the operating frequency. We attribute this ...

AC loss characteristics of multifilamentary YBCO coated conductors

One of the methods to reduce magnetization loss of YBCO coated conductors in a perpendicular magnetic field is subdividing the YBCO layer into filaments and twisting them as a whole. A 10 mm wide multifilamentary YBCO coated conductor with 200 /spl mu/m wide filaments was prepared by striation using the laser ablation technique. The number of filaments was 40. The sample length was varied from 100 mm to 25 mm, and their magnetization loss was measured at various frequencies. The measured magnetization losses were scaled using sample length, frequency, and field amplitude. This clarifies the magnetization loss characteristics of multifilamentary YBCO coated conductors and obtains empirical expressions for the magnetization loss. The measured loss was compared with the loss calculated numerically using a two dimensional FEM model. The experimentally confirmed effect of striation to reduce the magnetization loss was compared with theoretical predictions.

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.

Magnetization losses in multifilament coated superconductors

We report the results of a study of the magnetization losses in experimental multifilament, as well as control (uniform) coated superconductors exposed to time-varying magnetic fields of various frequencies. Both the hysteresis loss, proportional to the sweep rate of the applied magnetic field, and the coupling loss, proportional to the square of the sweep rate, have been observed. A scaling is found that allows us to quantify each of these contributions and extrapolate the results of the experiment beyond the envelope of accessible field amplitude and frequency. The combined loss in the multifilament conductor is reduced by about 90% in comparison with the uniform conductor at full field penetration at a sweep rate as high as 3 T/s.

Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

HTS synchronous generators, in which the rotor coils are wound from high-T c superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads.

Multifilament YBa2Cu3O6+x-coated conductors with minimized coupling losses

We report an experimental approach to making multifilament coated conductors with low losses in applied time-varying magnetic field. Previously, the multifilament conductors obtained for that purpose by laser ablation suffered from high coupling losses. Here we report how this problem can be solved. When the substrate metal in the grooves segregating the filaments is exposed to oxygen, it forms high resistivity oxides that electrically insulate the stripes from each other and from the substrate. As the result, the coupling loss has become negligible over the entire range of tested parameters (magnetic field amplitudes B and frequencies f).

Total AC losses in twisted and untwisted multifilamentary Bi-2223 superconducting tapes carrying AC transport current in AC longitudinal magnetic field

In some electrical apparatuses, superconducting tapes are exposed to the longitudinal magnetic field. In this work, AC losses were measured in twisted and untwisted Bi-2223 tapes carrying AC transport current in the AC longitudinal magnetic field. In twisted tapes, the transport, magnetization and total losses depend on the relative direction of the longitudinal magnetic field to the direction of the transport current, while the field direction does not influence the AC loss characteristics in untwisted tapes. In the Z-twisted tapes, the total AC loss is larger in the longitudinal magnetic field that is anti-parallel to the transport current than in the longitudinal magnetic field of another direction. Numerical analysis shows that this field direction dependence of the total AC loss results from the change in the current distribution. In the longitudinal magnetic field that is anti-parallel to the transport current, the total AC loss in the Z-twisted tape is more than that in the untwisted tape. This dependence on the field direction is reversed in S-twisted tapes. It is to be noted that the twist increases the total AC loss in a longitudinal magnetic field of a certain direction, while it reduces the AC loss in the transverse magnetic field.

Impact of flux gap upon dynamic resistance of a rotating HTS flux pump

HTS flux pumps enable superconducting currents to be directly injected into a magnet coil without the requirement for thermally inefficient current leads. Here, we present results from an experimental mechanically rotating HTS flux pump employing a coated-conductor stator and operated at 77 K. We show the effect of varying the size of the flux gap between the rotor magnets and coated conductor stator from 1 to 7.5 mm. This leads to a corresponding change in the peak applied perpendicular magnetic field at the stator from approximately 350 to 50 mT. We observe that our experimental device ceases to maintain a measurable output at flux gaps above 7.5 mm, which we attribute to the presence of screening currents in the stator wire. We show that our mechanically rotating flux pump is well described by a simple circuit model which enables the output performance to be described using two simple parameters, the open-circuit voltage V oc and the internal resistance, R d. Both of these parameters are found to be directly proportional to magnet-crossing frequency and decrease with increasing flux gap. We show that the trend in R d can be understood by considering the dynamic resistance experienced at the stator due to the oscillating amplitude of the applied rotor field. We adopt a literature model for the dynamic resistance within our coated-conductor stator and show that this gives good agreement with the experimentally measured internal resistance of our flux pump.

Numerical calculation of AC losses in multi-layer superconducting cables composed of coated conductors

Two types of four-layer cables composed of coated conductors, with practical cross-sectional geometries and various critical currents, were designed. The AC losses in the cables were numerically calculated using a numerical model employing a one-dimensional FEM (1D model). The AC loss of each layer in the four-layer cable with w = 4.0 mm was compared with Norriss analytical values, QNS, for a wide range of It/Ic. The AC loss in the first (innermost) layer was smaller than its QNS, whereas the losses in the outer layers were close to, or much larger than, their QNS. These results indicate that, in multi-layer cables, the AC loss in an outer layer increases because of the magnetic field generated by the current in the inner layers, and the entire AC loss of the cable is dominated by the AC losses in the outer layers. The AC losses in the four-layer cables with w = 3.3 mm, which had smaller gaps between the conductors, were smaller than those for the cable where w = 4.0 mm. These results showed the effectiveness of using narrower conductors and decreasing the gaps to reduce AC loss in multi-layer cables composed of coated conductors. The AC losses of the four-layer cable with w = 3.3 mm and Icc/w = 500 A cm−1 at 1 and 3 kArms were much smaller than the measured AC losses in cables fabricated with YBCO-coated conductors or BSCCO tapes. The AC loss in a four-layer cable with practical cross-sectional geometries should approximately be within an order of magnitude of their QNS.