Development of a Compact Measurement System of Transport Characteristics in HTS Conductors by Using an HTS Current
In order to realize HTS power machines and devices with high performance, it is important to evaluate the characteristics of HTS conductors quantitatively. We developed a compact system to measure the transport characteristics of HTS conductors under various experimental conditions. This system enables us to measure the transport characteristics of straight HTS conductors up to a current capacity of 1 kA class at 77 K, 66 K, and 4.2 K. The measurement of transport current losses is done by the Poynting vector method. The distribution of Poynting vectors around the sample conductor can be obtained by using many sets of both potential leads and pickup coils. An HTS current transformer (HTS-CT) is used to feed current to the sample conductor for the measurement. A secondary current of about 1 kA at 77 K can be induced when the primary current is 40 A. The sample conductor is connected to the secondary coil of this transformer. As a sample conductor, we used a conductor consisting of 16 parallel Ag-sheathed Bi-2223 multifilamentary tapes. The joint method between the HTS-CT and sample conductors is also examined. Based on these results, the usefulness of this measurement system is discussed.
Stability and Protection of Coils Wound With YBCO Bundle Conductor
The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of a low-temperature superconducting (LTS) coil. A HTS conductor has a greater volumetric heat capacity at the operating temperature envisaged for practical applications. Therefore, a HTS coil is much less likely to be quenched than a LTS coil by mechanical disturbances such as the heat generated by the cracking of the impregnation material or by the friction resulting from wire movements. However, the HTS conductor is cyclically subjected to tensile strain because electrical charging and discharging are repeated in real applications involving the Superconducting Magnetic Energy Storage (SMES) system. The superconducting characteristics may locally deteriorate due to this cyclic strain. Therefore, to enhance the reliability and safety of the HTS coil, a quench protection scheme is needed. Because the normal-zone propagation velocity is quite low, detecting a non-recovering normal zone is difficult in HTS coils, and quenching produces excessive overheating that may cause the conductor to melt. In this study, we focus on a coil wound with a YBCO bundle conductor used in SMES applications and investigate the redistribution characteristics of the transport current in and the thermal behavior of the coil during a quench; we use a newly developed computer code based on the finite element method (FEM) and an equivalent circuit. We also discuss a protection scheme to dump the magnetic energy stored in the coils on an external resistance connected in parallel.
A Simple Method for Measurement of AC Losses in Superconducting Wires under Configuration of a Double-Layer Non-Inductive Coil
We measured AC losses in superconducting wires with NbTi fine filaments exposed to a transverse magnetic field of 60 Hz by a simple pickup coil. The samples have configuration of a double-layer non-inductive coil. Potential taps were attached to both terminals of the sample coil. In this configuration, the double-layer sample itself is a pickup coil to measure the AC loss. In this procedure, we can approximately estimate an averaged Poynting’s vector on the side surface between the two layers by means of signals from a pair of the potential taps and a pickup coil for the external magnetic field. For comparison, we measured the AC losses both by a conventional electromagnetic method with coaxial pickup coils and by a calorimetric one. In the latter method, we obtain an equivalent AC loss by calibrating the amount of vaporized helium gas. Experimental results of the AC losses coincide with each other among the three methods in a wide range of the amplitude of external magnetic field. We also measured a transport-current loss in the sample coil and had a good agreement with results from the calorimetric method. Furthermore, we discuss the applicability of the proposed method to more universal types of AC loss measurement.