Didier Buzon

Current limitation with bulk Y-Ba-Cu-O

The fault current limiter is a very attractive device for electric networks. Meander pattern conductors cut from bulk melt textured YBCO were studied for this application. The meanders are put in series and/or in parallel to match the required current and voltage. The YBCO materials are attractive because they show a very effective limitation with a relative low volume (high engineering current density and normal state resistivity). However, they are sensitive to hot spots. To avoid these destructive hot spots the operating temperature is chosen very close to the critical temperature (above 90 K). This temperature range is reached using a pressurised liquid nitrogen bath. Working close to Tc has two major advantages. The first is reduced values of Jc which limit the power dissipation. The Jc can be matched by changing the pressure on the nitrogen bath. The second is that the proximity of the normal state is favourable for homogeneous quenches along the whole meander as it can be experimentally recorded. Results obtained on single meanders and on the whole assembly are reported under steady state operation as well as during current limitation. Forty three meanders in series limited the current to 740 A (11 000 A unlimited value) under 1 kV.

Superconducting fault current limiter with bulk materials

Abstract Superconducting materials offer the unique possibility to limit fault currents by their self-triggering quench to a resistive state above a given threshold current. Their use in power networks would improve the power quality, which is a real need today. YBCO or BSCO bulk materials are good candidates for current limitation. Their behaviours are rather different, in particular the electric field versus current characteristics. BSCO compounds are protected by their homogeneous quench due to the numerous defects (grain boundaries) distributed along the material. An assembling of 60 sintered Bi bars was tested under 1 kV. The current limitation is effective (a 5000 A short-circuit has been limited to 1080 A) but it occurs at 24 times the critical current. The increase of the critical current density of Bi compounds by their texturation reduces the current excursion above the critical value. In YBCO bulk elements, the defects are localized and make YBCO very sensitive to hot spots. Those are unavoidable and the operating conditions should be adapted in order to make them non-destructive. That is the self-protection concept. To fulfil it, we chose to operate near the critical temperature (above 90 K) to reduce the critical current density. The small difference with T c is very favourable for the quench. A fault current limiter based on 43 YBCO meanders working at 90.5 K in a pressurized liquid nitrogen bath is presented. The meanders are cut in 45 mm in diameter single domain pellets elaborated using the top-seeding technique. Supplied under a maximum voltage of 1 kV, the current was limited at 740 A instead of the theoretical value of 6500 A without quench.

Current limitation based on bulk YBaCuO meanders

Abstract This work aims at testing the performance of melt textured YBaCuO to limit fault currents. To do so, a 1 kV resistive fault current limiter (FCL) has been built and tested. The basic elements of this demonstrator were meanders cut out of textured pellets. Preliminary tests on meanders showed that the operating temperature of the limiter had to be above 90 K in order to obtain a homogeneous quench. The upper limiting capabilities of YBaCuO single bars and meanders were measured at 90.5 K. Bars reached a maximum electric field of 500 V/m and an average temperature of 280 K after a 20 ms short-circuit, while meanders attained values of 320 V/m and 210 K. The 1 kV demonstrator consisted of 43 meanders connected in series, with a shunt resistor added in parallel to each meander. The operating temperature was 90.5 K and the nominal current 100 A peak . Although this demonstrator succeeded to limit a fault current of 5.1 kA peak at a value of 730 A, several of the meanders broke during limitation.

c-Axis YBCO domains for current limiting applications

Abstract A superconducting fault current limiter using YBCO single domains requires long elements cut along the ab -planes or along the c -axis. These elements have to be assembled in series in order to produce a limited fault current after the quench to the resistive state under the network voltage. The superconductor total length is much smaller with c -axis elements because their high resistivity after transition contributes to a very high electric field during the current limitation. The critical current density has to be decreased in order to be sure that the Joule effect related to the limiting current does not increase too much the material temperature before its interruption by a current breaker. The inhomogeneously oxygenated YBCO elements limit the current even if the only surface layer is superconducting and produce an electric field up to 5000 V/m. A homogeneous oxygenation without macrocrack is obtained when the thickness along the ab -planes is thin. Several elements have been tested by applying a 20 ms duration fault current in a liquid nitrogen bath at T =77 K and above.

Current limitation properties of YBa2Cu3O7??/Au thin films: application to transformer connection

In view of the development of superconducting fault current limiters, the properties of switching and recovering of YBa2Cu3O7−δ/Au (YBCO/Au) thin films are studied at 77 K and 50 Hz for overcurrents. The bilayers present an abrupt transition to a high dissipative state leading to a current limitation at a maximum value of about 2.5 times the critical current Ic, and allow the development of electric fields of 3 kV m−1 without any damage. After the overcurrent regime, the recovery of the superconducting state under the rated current In is studied as a function of overcurrent parameters. These results show clearly the strong potential of YBCO/Au thin films which can recover their superconducting state under nominal mode. This last point is crucial for transformer connection, as experimentally shown in this paper.

Current limitation with bulk YBaCuO meanders

Forty three meanders were mounted and put in series to build a resistive FCL prototype. The nominal current and the temperature were respectively fixed at 100 A and 90.5 K. The voltage and the mean electric field respectively reached 1 kV and 106 V/m during 20 ms. The current was limited to 730 A whereas it would have reached 5.1 kA without limitation. The superconducting parameters were adjusted so as to reduce the temperature (self-protection) reached by the samples. Bulk YBCO proved in our experiment their strong ability to limit the current. However, meanders suffered large electromechanical and thermal stresses during the transition. In this article, we analyse the prototype results.

Effects of thermal exchanges on the YBCO transition and the superconductivity recovery

The influence of the current slope (dI/dt) and thermal exchanges on the magneto-thermal transition of textured bulk YBCO are analyzed within the framework of the resistive Fault Current Limiter (FCL). This study was carried out at 90.5 K both in pressurized liquid nitrogen, in vacuum and when the YBCO sample is in contact with alumina plates. The superconductor U(I) response is governed by a power law, depends on dI/dt and is influenced by the thermal surroundings. These phenomena can be justified if the dissipated energy during the transition is concentrated in a small volume of the conductor. The second part of this paper deals with the thermal recovery of bulk materials. Even in the case of film boiling where the exchanges are poor, a thermal gradient due to the weak thermal conductivity of HTc materials appears in the section of the conductor. This point and its consequences are described and discussed by using a finite element software.

Thermal effects in YBCO/metal bilayers at high current densities

Abstract We report on the thermal aspects associated with the switching properties at high current densities of YBCO thin films shunted by a metallic layer. The role of the shunt is first detailed in a current ramp mode experiment under various conditions of temperature and magnetic field. We discuss specifically a regime associated with an almost total current diversion by the metallic layer, observed when the critical current density is reduced at temperatures close to the critical temperature T c or under magnetic field. This regime is associated to a thermal anomaly, as seen in the temperature signal measured on the rear side of the substrate, which can explain the partial recovery of the superconductive state under magnetic field. Under current limitation conditions at 50 Hz, 77 K and self-field, the temperature reaches 260 K at the rear side of the sapphire substrate after 0.4 s without any damage to the superconducting device.

Transition into dissipative state of superconducting YBa2Cu3O7−x/Au films and fault current limitation: switching and recovering properties

Abstract Properties of the transition into dissipative states of YBa 2 Cu 3 O 7− x /Au thin films are studied during sweep current and fault current limitation (FCL) experiments. Dissipation mechanisms associated with power laws behaviour E ≈( J / J c ) n are observed and lead to a transition into the normal state by a thermal runaway. FCL experiments show that bilayer devices limit the current at about 2.5 J c and allow one to develop electric fields of about 3 kV/m without any damage. After the default regime, a recovery of the superconducting state under nominal current I n is for the first time reported. These results demonstrate the strong potential of YBCO/Au thin films as the superconducting fault current limiter elements that could be used autonomously in case of transient over-currents.