The 'Garber Current Pattern': An Additional Contribution to AC Losses in Helical HTS Cables?

Conductors made of high-temperature (HTS) wires helically wound in one or more layers on round tubes (CORT) are compact, flexible, and can carry a large amount of current. Although these conductors were initially developed for DC applications, e.g. in magnets, it is worth considering their use for AC, e.g. in underground cables for medium voltage grids and with currents in the kA-range. In these cases, the major challenge is reducing AC losses. In contrast to a straight superconducting wire, in a helical arrangement, due to superconducting shielding, the current does not follow the direction of the wires, but takes a non-trivial zig-zag path within the individual HTS wires (Garber pattern). This includes current components across the thickness of the superconducting layers, so that the often used thin-shell approximation does not hold. In this contribution, we studied a one-layer three-wire CORT by means of fully three-dimensional simulations, based on the H-formulation of Maxwell's equations implemented in the commercial software package COMSOL Multiphysics. As a result of our simulations, the peculiar current profiles were confirmed. In addition, the influence of current, pitch angle, and frequency on the AC losses was studied. We found an optimum for the pitch angle and that the current profiles strongly depend on frequency

New coil configurations with 2G-HTS and benefits for applications

A scalable concept to prepare high current density windings with high-temperature superconducting material is introduced. The concept covers miniaturized high-current windings and large coils for applications in energy technology as well. The principle is based on a circular disk-up-down-assembly (\u27DUDA\u27) and extended to rectangular coils. First measurements on the assemblies (≈40 turns) in liquid nitrogen are presented as a proof-of-concept. Centre fields of ≈40 mT and ≈340 mT are measured during steady operation and pulsed operation respectively. Operating the assemblies at lower temperatures will strongly increase the current and field performance. Lowering the contact resistance might lead to further improvements. Due to the homogenous structure in the radial direction, shear stresses are minimized and large winding heights can be realized in principle. The DUDA concept of coil windings can be used to build even more sophisticated magnetic arrangements, e.g. Halbach-arrays, or complex windings for new stator configurations of rotating machines. The optional miniaturization offers compact and powerful magnets, e.g. for accelerators too

Project FASTGRID - Tests on 2G HTS for its Application in DC Resistive SC FCL

HVDC (High Voltage Direct Current) super-grids could become a future solution for the long-distance power-transmission. The Superconducting Fault Current Limiter (SCFCL) is a necessary facility to protect such transmission lines. In the framework of the project FASTGRID dedicated HTS wires for resistive type DC-SCFCL are under development. To reduce the cost per switching capacity: Reduction of the amount of HTS shall be achieved by increasing the allowed electrical field and the critical current density at operating conditions. A wire with an additional laminated 500 µm Hastelloy® shunt is the basic solution for FASTGRID. This work shows experiments on this prospective solution, compared with tests on bare coated conductors at lower E-field, once applied in ECCOFLOW SCFCL. The goal of this work is the validation of the HTS conductor for an electric field higher than 130 V/m for a fault clearing time of 50 ms

Test of a DC-HTS Busbar Demonstrator for Power Distribution in Hybrid-Electric Propulsion Systems for Aircraft

In the framework of the German project TELOS (Thermo-Electrically Optimised Aircraft Propulsion Systems) a high-temperature superconducting 40 MVA DC demonstrator busbar for hybrid-electric propulsion systems for aircraft has been developed. The design current for a temperature below 25 K is 13.3 kA and the rated voltage is 3 kV. The 2-pole busbar contains 2 stacks of REBCO coated conductors which are supported by a 3D-printed structure allowing compensation of thermal length changes of the superconductor. It fits in a cryostat tube with an inner diameter of 25 mm. A special focus has been put on low-resistive joints that are necessary to connect single elements of the busbar system. The special layout of the joints allows an effective current redistribution between the different tapes in a stack. We present results for the test of the DC busbar demonstrator in liquid nitrogen at 77 K. The design current for this temperature is 3.3 kA which corresponds to a rated power of 10 MW. We applied currents up to 3.5 kA and measured the I-V characteristics and contact resistances of 90° and 180° joints in a virgin and in a strained state thus simulating thermal length changes. We also present results of Lorentz-Force tests with short AC current pulses up to 20 kA to demonstrate the viability of the design for application with currents up to 13.3 k

Simulation of the Thermal Performance of HTS Coated Conductors for HVDC SFCL

Within the EU-funded project FastGrid a resistive superconducting fault current limiter for DC-application is actually under development. To reduce substantially the amount of deployed superconducting tape an increase of voltage per length in the limitation case is needed. Moreover, the resistive transition (quench) should be fast and uniform. In the present work, the thermal behavior of three different architectures of coated conductors are investigated by means of transient simulations. The first architecture is composed by an additional Hastelloy shunt layer, which is supposed to avoid overheating of the conductor during fault limitation. The other architectures include layers of silver and copper as stabilizers. Our parameter study including the geometries of the different layers and the inhomogeneity of the critical current density of the superconducting tape shall lead to an optimized tape layout. First comparisons with experiment indicate, that, with the first architecture, electric field strength up to 100 V/m should be within reach

Efficient HTS DC- Cable for Power Distribution in Hybrid-Electric Aircraft

With respect to a substantial reduction of greenhouse gas emission, noise reduction and combustible consumption in aircraft an increasing interest in hybrid-electric propulsion systems has emerged in the last years. In the frame of a German research project we actually develop a HTS superconducting busbar system for DC currents able to join the different components as generator, motors and battery systems on the plane. Main features of this busbar system are large currents at moderate voltages and in particular low ohmic contacts, including T-type, Y-type and cross connections between system segments, this with a minimization of outer dimensions and weight. The two-pole cable consists of two stacks of REBCO tapes. Compensation of Lorentz forces between the two poles, compensation of thermal length changes and sufficient electric insulation are the major challenges. We will present details of the cable design and first test results on a lab-scale cable demonstrator

Design and Performance of a Conduction-Cooled HTS Magnet in the Radio-Blackout Experiment COMBIT

In the framework of the Helmholtz-Russia Joint Research Group (HRJRG) „COMBIT“ we developed a conduction-cooled HTS magnet to provide a high magnetic field for a radio blackout mitigation experiment in the arc-heated wind tunnel L2K at the German Aerospace Center in Cologne. The radio blackout phenomenon is well-known since the early days of space exploration. During hyper-sonic flights or during reentry in a planet\u27s atmosphere a dense plasma layer can form at the surface of the space vehicle leading to mitigation or reflection of radio waves. As a consequence voice communication with ground stations and GPS data telemetry can be disturbed. The goal of “COMBIT” was to demonstrate that the radio blackout can be mitigated by a local reduction of the plasma density in the vicinity of senders and antennas by magneto-hydrodynamic effects using crossed electric and magnetic fields. In order to generate a high magnetic field in the plasma we developed a conduction-cooled HTS magnet and a cryogenic system that is able to withstand the high temperatures in the plasma. The HTS magnet was made with RE-Ba-Cu-O coated conductors and has an outer diameter of only 70 mm. Despite the small size which is a consequence of the experimental boundary conditions the magnet was able to generate a high and variable magnetic field outside the cryostat in the plasma. In several measurement campaigns, the magnetic field reached up to 2 T in the plasma, corresponding to a maximum magnetic field of 5.16 T at the conductor. Mitigation of the radio blackout could be demonstrated successfully. After an introduction to the radio blackout phenomenon we present the design of the conduction-cooled HTS magnet and the cryogenic system and discuss their performance during the experimental campaigns