Manfred Däumling

Critical current of a high Tc Josephson grain boundary junction in high magnetic field

The critical current (Ic) of YBa2Cu3O7−δ grain boundary Josephson junctions was measured up to magnetic fields of 5 T. Magnetic field history dependent Ic values were observed even after correction for self‐field effects stemming from hysteretic shielding currents in the grain adjacent to the boundary. A novel feature observed is an anomalous increase in Ic in high magnetic fields of several Telsa.

Test results of full-scale HTS cable models and plans for a 36 kV, 2 kA/sub rms/ utility demonstration

Cable systems using high-temperature superconducting (HTS) tapes are nearing technical feasibility. Several large-scale demonstrations are under way. This article summarizes the advancements and status of a development project aimed at demonstrating a 36 kV, 2 kA RMS AC cable system through installing a 30 m long full-scale functional model in a power utility substation. The HTS cable line is designed to link two medium-voltage transformer stations in an urban environment. The expected benefits of such a system include reduced energy loss, ease of installation, increased power rating in a small cross section, and insensitivity to the surrounding soil conditions. Results will be presented from tests on several 2 kA-class AC conductors. Electrical losses below 1 W/m at 2 kArms have been obtained in these cable conductors. The cable system consists of terminations, three HTS cables with conventional room-temperature dielectric and stress cones, and a closed-loop circulating cooling system maintaining the temperature between 74 and 84 K. Critical issues before the commercialization of this technology is the improvement of the thermal insulation, the reliability and maintainability of the cooling system, and the reduction of materials costs.

Operation experiences with a 30 kV/100 MVA high temperature superconducting cable system

A superconducting cable based on Bi-2223 tape technology has been developed, installed and operated in the public network of Copenhagen Energy in a two-year period between May 2001 and May 2003. This paper gives a brief overview of the system and analyses some of the operation experiences. The aim of this demonstration project is to gain experience with HTS cables under realistic conditions in a live distribution network. Approximately 50 000 utility customers have their electric power supplied through the HTS cable. The cable system has delivered 226 GW h of energy and reached a maximum operating current of 1157 A. The operation experiences include over-currents of 6 kA due to faults on peripheral lines, commissioning, servicing and failure responses on the cooling system, continuous 24 h, 7 day per week monitoring and performance of the alarm system. The implications of these experiences for the future applications of HTS cable systems are analysed.

The reversible magnetization of oxygen deficient YBa2Cu3O7−δ

The reversible magnetization Mrev of polycrystalline YBa2Cu3O7−δ (0<δ<0.5) was measured. Due to a small grain size a nd low critical current densities in oxygen deficient specimens Mrev could be measured over most of the superconducting regime from 10 K to Tc up to a magnetic field of 5.5 T. The magnetization data is in excellent agreement with calculated values obtained from Ginzburg-Landau (GL) theory. Values for the thermodynamic critical field Hc and the GL parameter κ are obtained. High values of κ ≈ 170–300 (for H∥c) are necessary to fit the magnetization data. Hc(T = 0) decreases strongly for increasing oxygen deficiencies up to δ ≈ 0.15, and then remains nearly constant. Specific heat jumps Δc at Tc were calculated from the data and the fit. Quantitative agreement with data in the literature is obtained. Just like Hc, the specific heat jump is strongly dependent on the oxygen deficiency in the near stoichiometric regime.

First operation experiences from a 30 kV, 104 MVA HTS power cable installed in a utility substation

An HTS cable with a voltage rating of 30 kV and a power rating of 104 MVA, has been installed and operated in the electric grid of Copenhagen Energy in the spring of 2001. This article describes the development phases, the system specifications, and the first experiences of operation under realistic conditions in the substation of Amager (AMK). Approximately 50 000 private and business customers are supplied from this cable. The load can be adjusted from 20% to 100% of the power supplied and the number of branches connected can be altered. This and other early HTS power installations are expected to act as ice-breakers for the HTS technology.

Phase coexistence and critical temperatures of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 Ox phase under partial pressures of oxygen between 10−3 and 0.21 bar with and without additions of silver

We have investigated the stability of the (Bi, Pb)2Sr2Ca2Cu3Ox phase for the stoichiometry (Bi: Pb: Sr:Ca: Cu = 1.72: 0.34: 1.83: 1.97: 3.13), subjecting it to temperatures between 700 and 850 °C under various oxygen partial pressures. A narrow region was found in which Bi, Pb(2223) was the only superconducting phase. This region follows closely the thermal decomposition line. X-ray pure Bi, Pb(2223) will partially decompose if treated outside of the stability region. For a given oxygen partial pressure, the Bi, Pb(2223) phase tends to coexist with the 2201 phase for temperatures above, and the 2212 phase for temperatures below this region. At even lower temperatures an additional lead-rich phase appears. Critical temperatures Tc vary little with treatment and range between 108.5 K and 110.8 K. If 10% silver is added to the starting powder, the phase coexistence regions shift. Silver does not seem to have a significant effect on the absolute values of the critical temperature. The Bi, Pb(2223) thermal decomposition temperature for a given oxygen pressure is lowered by at least 10 K by the presence of Ag.

Overcurrent experiments on HTS tape and cable conductor

Overcurrents in the power grid can have a magnitude of up to 20 times or higher than the rated current. This may cause problems and permanent damage to electrical equipment in the grid. High temperature superconducting (HTS) tapes are known to be sensitive to currents much larger than their critical current. In this light, it is important to investigate the response of HTS tapes and cable conductors to overcurrents several times the critical current. A number of experiments have been performed on HTS tapes and cable conductors, with currents up to 20 times the critical current. During overcurrent experiments, the voltage, and the temperature were measured as functions of time in order to investigate the dynamic behavior of the HTS tape and cable conductor. After each experiment, damage to the superconductors was assessed by measuring the critical current. Preliminary results show that within seconds an HTS tape (critical current=17 A) heats above room temperature with an overcurrent larger than 140 A. Similar overcurrent experiments showed that a HTS cable conductor could sustain damage with overcurrents exceeding 10 times the critical current of the cable conductor.

Alternating current losses of a 10 metre long low loss superconducting cable conductor determined from phase sensitive measurements

The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the resistive (in-phase) voltages over the conductor is correspondingly high. In vectorial representations this results in phase angles between the current and the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifiers due to their limited absolute accuracy. However, by using two lock-in amplifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the results from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measurements were carried out with and without a compensation circuit that could reduce the inductive voltage. The 1 µV cm-1 critical current of the conductor was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was derived to be 0.6±0.15 W m-1. This is, to the best of our knowledge, the lowest value of ac loss of a high temperature superconducting cable conductor reported so far at these high currents.

Measuring AC-loss in high temperature superconducting cable-conductors using four probe methods

Measuring the AC-loss of superconducting cable conductors have many aspects in common with measuring the AC-loss of single superconducting tapes. In a cable conductor all tapes are connected to each other and to the test circuit through normal metal joints at each end. This makes such measurements considerably more complex, especially for samples of laboratory scale (1-5 meters). Here we discuss different measurement configurations using four probe methods and lock-in detection. We conclude that the voltage should be picked up at end of the connecting joints, and we show how the resistive contribution from these joints can be identified and subtracted from the measured data. We also show measurements which indicate that the size of the loop constituted by the voltage leads has no influence on the measurements.

Magnetic field dependence of critical currents of single grain boundary junctions in Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7- delta / superconductor

The authors present data on the magnetic field dependence of critical currents in epitaxial films of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7- delta / containing a single boundary. The data are obtained as a function of temperature, the orientation of the grain boundary, and magnetic fields of up to 5 T. A significant residual critical current is observed which increases with decreasing angle of misorientation at a given field and temperature. The data are in qualitative accord with a model in which the grain boundary comprises a large number of microbridges in parallel.<<ETX>>