Stability and quench development in a HTSC magnet have been experimentally studied with the transport current in the magnet being below or above the “thermal quench current” level. The magnet was tested at both cryocooler cooling and liquid nitrogen cooling, with and without background magnetic field (up to 4 T). The temperature and electrical voltages in different sections of the magnet were measured using 20 thermocouples and 24 potential taps embedded in the winding. In this paper, the experimental procedure and the results are described. The results are compared with those obtained earlier in the experiments with the smaller HTSC specimens and are analysed by using the scaling theory of the thermal quench.

We have studied a method to evaluate electric field (E) vs. current density (J) characteristics of high temperature superconductors as a basis for their power device application. The E-J characteristics of Bi-2223 wires were measured in a wide range of magnetic field and temperature. Those characteristics are discussed by the theory of critical-current-density distribution and by the scaling law of the pinning force.

Temperature and magnetic field dependences of pinning parameters connected with Jc distribution are obtained from measurements on electric field-current density characteristics. A scaling law of the pinning force is confirmed. The temperature dependences of the normalization field which is the glass-liquid transition field and that of the maximum pinning force are discussed. The results suggest the existence of two phase structure in Bi-2223 and Bi-2212 samples.

It is very important to know electric field (E) vs. current density (J) characteristics on temperature (T) and magnetic field (B) in practical wire for High T c superconductor (HTS) application. We obtained the scaling characteristics of E-J in Bi-2223/Ag wires by parameters of Weibull function expressing the J c distribution, or J cm and J 0, instead of the scaling by using conventional J c value for a specific electric field criterion. Scaling characteristics of pinning force in wide range of T and B for B-directions of parallel and perpendicular to sample surfaces are also shown.

We present here the outline of the development of a 1kWh/1MW module-type SMES (ESK; Experimental SMES of Kyushu Electric Power Co., Inc.) as a full system to be tested in power line. Total system of ESK have been completed early this year, and is now being in test at Imajuku substation in Fukuoka. ESK is originally planned as a 1st step in scaling up SMES technology to that of practical one. Its main characteristics are; toroidal type with 6 coils connected each 3 in series as having 2 independent modules with one converter and 3 coils for each. Cooling test, energizing tests, and fundamental tests have already been taken place. As a preliminary test for power system control, some controllability test for load fluctuation was made. Details of those results will be shown.

The influence of flux pinning on the vortex glass-liquid (GL) phase transition and irreversibility in HTSs has been examined on the basis of systematic measurements of electric field (E) versus current density (J) curves in a YBCO thin film. It has been shown that the GL transition is identical to the thermal depinning determined by the minimum value of the macroscopic pinning strength. The isothermal scaling of the E-J characteristics is directly related to the nature of the pinning. The irreversibility line can be described as an iso-J line governed by the statistical pin distribution. If we determine the criteria for the irreversibility line, we can describe the irreversibility line based on the depinning line. The essential physical quantity, therefore, can be attributed to the flux depinning.

In spite of rather high general stability of high temperature superconducting (HTSC) Bi-based magnets, catastrophic thermal quench (TQ) may appear in them under certain circumstances. It happens because of non-linearity of voltage–current characteristics in HTSC superconductors. Starting with small samples in our previous works, we continue to study the TQ with large samples. We prepared a highly instrumented HTSC pancake coil. It is wound using the Bi-2223-based tape. We attached many potential taps to the tape and installed in the winding 10 cryogenic thermocouples (TC) and two heaters. Quench development in the coil was measured under different temperatures, different magnetic fields and different cooling conditions. In this paper, the experimental details and the results obtained are presented. The results are discussed from the point of view of scaling theory for quenching in HTSC devices.

Quench dynamics in a YBCO HTSC film and a Bi-based small HTSC coil have been studied. While the stability margin of HTSC against a local disturbance was very large, quench current was limited by a catastrophic temperature rise originated from the nonlinear characteristic of Joule heating in HTSC. The crucial parameter for the quench becomes the nonlinear resistance in HTSC as a function of temperature and transport current. It has been shown that the dynamic characteristics of the quench in both the film and the coil can be described quantitatively by the simplified one-dimensional heat balance equation even though the time scales are different by more than six orders, i.e., several hundreds micro seconds for the film and several hundreds seconds for the tape coil.

A full system of 1 kWh/1 MW superconducting magnetic energy storage (SMES) has been completed early this year. This SMES is the first step to the realization of practical SMES system for power line stabilization. Main points in its design are two module type arrangement of six coils having three coils and one converter as one unit of module, modified-D-shaped coils with mechanical supports, liquid helium vessel type cooling of coils, and high-temperature superconducting current leads. The first test experiment was carried out on the site recently. The above design points were examined. A preliminary test for power line control was also made in the distribution line at the site. Satisfactory results were obtained.

We have already reported on a plan for development of an experimental SMES system for kyushu (ESK)1). This SMES system is now being installed at substation in Fukuoka and is going to be tested in order to confirm the stabilizing effect for actual electric power system. This coil system is consisted of 6 modified D shaped coils. We manufactured 2 coils and have finished shop assembly and test of each coils. We confirmed that these coils have inductance almost corresponding to design value, and that these coils can be charged at rated current and under the condition of actual electromagnetic force that is generated in case of charging 6 coils in stable.

Using a Bi-based small high Tc superconductor (HTSC) coil, we have studied its stability against a local disturbance and current-induced quench in the helium gas cooling condition. While the stability margin of HTSC coil against a local disturbance was very large, quench current was limited by a catastrophic temperature rise which originated from the nonlinear characteristic of the Joule heating. The crucial parameter for the quench becomes the nonlinear resistance in HTSC as a function of temperature and transport current. It has been shown that the dynamic characteristics of the quench can be described quantitatively by the simplified one-dimensional heat balance equation.

Our experimental SMES of 1kWh/1MW named "ESK" (Experimental SMES of Kyushu Electric Power Co., Inc.) is going to be completed in 1997. It has a toroidal coil system with 6 elementary coils (similar to each 1/6 kWh) made by 3 manufactures independently, each of which has its own type of coil and conductor: Compacted Rutherford type for conductors, and pancake and layer-winding type for coils. The newly designed coil shape "modified D" has two straight parts in-between D parts.Measurements were made for each of the 3 coil types in the 6 elementary coils. The results are: coils were stable up to the current corresponding to the rated 1000A of the toroidal arrangement. Losses were measured for the condition of repeated charging and discharging with high ramp rate were within the designed value. Strains have also been measured on the support structures of each coil mechanical characteristics of each coil are compared.

We propose a new approach to formulate and predict the nonlinear resistance in high T-c superconductors quantitatively in a wide range of magnetic field and temperature based on a study on stochastic distribution of critical current density and scaling of pinning force densities. True superconductive region as well as the nonlinear resistive region, which is governed by a percolative partial flux flow, have been shown.

For pt.I see ibid., vol.7, no.2, p.684-7 (1997). We have been developing HTS current leads for a 1 kWh/1 MW module type SMES system (which we call "ESK" for Experimental SMES of Kyushu Electric Power Co., Inc.). Each module of a module type SMES system has a pair of current leads. For the purpose of reducing the heat load from the current leads, we have employed a bulk HTS. As a step in the preparation of HTS current leads for ESK, we trial produced HTS current leads and tested them to evaluate their characteristics. Our test results indicated that the heat load in a steady state at a rated current of 1 kA and with a rated flow rate of 0.05 g/s was 0.035 W, well below the specified value of 0.1 W. Also, our results indicated that the HTS current leads can be satisfactorily energized for pulsing operation at a rated current for ESK of 500 A-1 kA. The heat load under such conditions was 0.025 W, approximately 80% of that under normal 1 kA operating conditions. It was also verified that operation could be continued for 15 minutes even when the coolant flow was stopped.

We are developing high-temperature-superconductor (HTS) current loads for a 1-MW/1-kWh modular SMES system. In the modular SMES system, a pair of current leads would be installed in each module so that the heat load to the low-temperature end through the leads is minimized. Design study of the lead configuration including the safety lead and its thermal characteristics is presented herein. The safety lead composed of stainless steel is placed in parallel to the bulk HTS. A test unit for verification of safe operation has been developed and the simulated bulk HTS quench test has been demonstrated. The maximum temperature of the safety lead reached 200 K and the terminal voltage of the safety lead was only 1.2 V in the case of a 1000-A quench test. This temperature rise was well below the designed value. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(1): 23–32, 1997

Combining the four-probe method and the magnetization method, we have measured electric-field vs. current-density (E-J) characteristics in YBa2Cu3O7-delta thin films over more than 13 decades of electric-fields. It has been shown that the EJ curves obtained by the both methods lie on a continuous line each other. Furthermore, it has been pointed out that the scaling of EJ curves is valid only in a limited range of electric-field. That is to say, S-like-shape InE-lnJ curve can be observed even in the so-oiled glass regime.

Depinning properties in a random pin medium under the influence of thermal agitation have been studied by the Monte-Carlo method. It has been shown that the depinning probability is scaled on a power function in the vicinity of the depinning threshold, that is the minimum value of critical current density. The simulation results have been compared with the measurements in a Y1Ba2Cu3O7-delta thin film, then the scaling based on the statistic feature of critical current density is confirmed. The scaling is consistent with that of the well-known glass-liquid transition within the measured electric field region. A new analytical expression describing the current-voltage curves has also been derived then the origin of the extremely wide scaling regime is discussed.

We have been developing high-temperature superconducting (HTS) current leads for a 1 kWh/1 MW module-type SMEs. Each module of a module-type SMES requires a pair of current leads. Therefore, we employed bulk HTS in order to reduce the heat load of the current leads. It is important that HTS current leads for SMES be reliable. The HTS current leads described in this paper have been designed to minimize the heat load and to maintain a high level of reliability. The HTS current leads are designed to hold the heat load at the cold-end terminal to less than 0.1 W. They are also designed with safety leads to bypass current in the event the HTS is quenched and with metal superconductors to assure the continuation of SMES operation even if the HTS should fail or deteriorate in performance. This paper describes an optimal design and the results of a heat load evaluation of HTS current leads for SMES.